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N-Annulated perylene diimide (PDI) dimers offer added advantages over conventional (non-annulated) dimers or S/Se-annulated dimers due to the two extra positions of functionalization that they prov...

Femtosecond and nanosecond transient absorption spectroscopies combined with theoretical calculations were performed to investigate the formation mechanisms of quinone methides (QMs) from 4-(2-hydroxyphenyl)pyridine (1). In acetonitrile (ACN), the singlet excited state of 1 (1(S1)) with the cis-form underwent a thermodynamically favorable and ultrafast ESIPT to produce the singlet excited state QM, which could either relax first into highly vibrational states of its ground state followed by hydrogen transfer to return to the starting compound or alternatively may undergo a dehydrogenation to produce a radical species (1-R). In ACN-H2O, 1(S1) interacted with water molecules to form a solvated species, which induced water-assisted ESPT to the pyridine nitrogen to generate the singlet excited state QM in a concerted asynchronous manner that was initiated by deprotonation of the phenolic OH. These results provide deeper insights into the formation mechanisms of QMs in different solvent environments, which is important in the application of QMs in biological and chemical systems as well as in the design of molecules for efficient QM formation.

The excited-state and luminescence properties of a cyclometalated Ir(III) complex with two C∧N ligands (C∧N = 2-(2,4-difluorophenyl)pyridine, F2ppy) and one acyclic diamino carbene (ADC) ancillary ...

Summary In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).

The relationship between the photoexcitation dynamics and the structures of semi-aliphatic polyimides (3H-PIs) was investigated using ultrafast fluorescent emission spectroscopy at atmospheric and increased pressures of up to 4 GPa. The 3H-PI films exhibited prominent fluorescence with extremely large Stokes shifts (Δν > 10 000 cm-1) through an excited-state intramolecular proton transfer (ESIPT) induced by keto-enol tautomerism at the isolated dianhydride moiety. The incorporation of bulky -CH3 and -CF3 side groups at the diamine moiety of the PIs increased the quantum yields of the ESIPT fluorescence owing to an enhanced interchain free volume. In addition, 3H-PI films emitted another fluorescence at shorter wavelengths originating from closely packed polyimide (PI) chains (in aggregated forms), which was mediated through a Forster resonance energy transfer (FRET) from an isolated enol form into aggregated forms. The FRET process became more dominant than the ESIPT process at higher pressures owing to an enhancement of the FRET efficiency caused by the increased dipole-dipole interactions associated with a densification of the PI chain packing. The efficiency of the FRET rapidly increased by applying pressure up to 1 GPa owing to an effective compression of the interchain free volume and additionally gradually increased at higher pressures owing to structural and/or conformational changes in the main chains.

Despite the outstanding relevance of proton transfer reactions, investigations of the solvent dependence on the elementary step are scarce. We present here a probe system of a pyrene-based photoacid and a phosphine oxide, which forms stable hydrogen-bonded complexes in aprotic solvents of a broad polarity range. By using a photoacid, an excited-state proton transfer (ESPT) along the hydrogen bond can be triggered by a photon and observed via fluorescence spectroscopy. Two emission bands could be identified and assigned to the complexed photoacid (CPX) and the hydrogen-bonded ion pair (HBIP) by a solvatochromism analysis based on the Lippert-Mataga model. The latter indicates that the difference in the change of the permanent dipole moment of the two species upon excitation is ∼3 D. This implies a displacement of the acidic hydrogen by ∼65 pm, which is in quantitative agreement with a change of the hydrogen bond configuration from O-H···O to -O···H-O+.

Organic semiconductors are formed by coupling organic molecules with π-conjugated structure through van der Waals interactions. Different from traditional inorganic semiconductors, such as silicon, the core element of organic semiconductors is carbon atom. Generally, in a π-conjugated organic molecule, carbon atoms are polymerized by sp or sp2 hybridization, forming small molecules or polymers due to the molecular size. Taking the polymer cis -polyacetylene as an example, each carbon atom has three 2sp2 hybrid orbitals in a plane and one 2p z orbital orthogonal to this plane. The 2sp2 orbitals give rise to three σ-bonds, two of which are formed with neighboring carbons and one with a hydrogen. The electron in the 2p z orbital of one carbon atom will be paired with the electron in the neighboring carbon’s 2p z orbital, such that this electron can be delocalized along the chain, denoted as π-electron. For such an organic molecule, according to the Peierls instability, it will be dimerized and form a single-double bond alternation lattice structure, behaving in a semiconductor character. Through doping, the conductivity of organic molecules can be greatly improved and become good conductors. This is the reason why the functional properties of organic molecules first attracted great attention in the 1970s, by which, Heeger, MacDiarmid and Shirakawa won the 2000 Nobel Prize in Chemistry. When organic molecules form solids, the packing of molecules will ultimately determine their functional properties and applications, such as organic polymer films and organic small molecule crystals. Over the past few decades, due to the flexibility and ease of processing, organic semiconductors have become very promising materials in functional devices, such as field-effect transistors, light-emitting diodes, solar cells, as well as organic spin valves. In general, the functional processes of these organic devices are closely related to the quantum dynamics of the formed excited states, including soliton, polaron, bipolaron, exciton, biexciton, charge transfer state, and trion, etc. These excited states are spatially localized due to the self-trapping effect of organic molecules, which arises from their strong electron-lattice interactions. Especially, these excited states behave in abundant and unusual charge-spin relations. For these reasons, the functional processes in organic devices are usually more unique and complex compared with conventional inorganic devices. To date, although many kinds of organic functional devices have been successfully developed, there are still many unsolved questions in their functional mechanisms. The excited state research is the key to solve these questions, and thus the core of organic semiconductor physics and device research. In view of this, through the combination of theoretical and experimental methods, we studied the quantum effects and modulation mechanisms of different excited states in organic semiconductors and their hetero structures. The main contents included are as follows: Quantum dynamics and field modulations for the photoexcitation, interfacial charge separation, and resonance energy transfer in organic photovoltaic systems; spin injection and transport in organic semiconductors, organic magnetoresistance effect, excited ferromagnetism, spin current and organic chiral spin effects. On the one hand, these studies greatly enrich the understanding for the physical phenomena and mechanisms in organic semiconductors; on the other hand, they should greatly promote the designs and applications of organic functional devices.

The dynamics of photodissociation and recombination in heme proteins represent an archetypical photochemical reaction widely used to understand the interplay between chemical dynamics and reaction environment. We report a study of the photodissociation mechanism for the Fe(II)-S bond between the heme iron and methionine sulfur of ferrous cytochrome c. This bond dissociation is an essential step in the conversion of cytochrome c from an electron transfer protein to a peroxidase enzyme. We use ultrafast X-ray solution scattering to follow the dynamics of Fe(II)-S bond dissociation and 1s3p (Kβ) X-ray emission spectroscopy to follow the dynamics of the iron charge and spin multiplicity during bond dissociation. From these measurements, we conclude that the formation of a triplet metal-centered excited state with anti-bonding Fe(II)-S interactions triggers the bond dissociation and precedes the formation of the metastable Fe high-spin quintet state., The dissociation mechanism of the heme axial ligand in heme proteins is not yet fully understood. The authors investigate the photodissociation dynamics of the bond between heme Fe and methionine S in ferrous cytochrome c using femtosecond time-resolved X-ray solution scattering and X-ray emission spectroscopy, simultaneously tracking electronic and nuclear structure changes.

The novel photosensitizer [Ru(S–Sbpy)(bpy)2]2+ harbors two distinct sets of excited states in the UV/Vis region of the absorption spectrum located on either bpy or S–Sbpy ligands. Here, we address the question of whether following excitation into these two types of states could lead to the formation of different long-lived excited states from where energy transfer to a reactive species could occur. Femtosecond transient absorption spectroscopy identifies the formation of the final state within 80 fs for both excitation wavelengths. The recorded spectra hint at very similar dynamics following excitation toward either the parent or sulfur-decorated bpy ligands, indicating ultrafast interconversion into a unique excited-state species regardless of the initial state. Non-adiabatic surface hopping dynamics simulations show that ultrafast spin–orbit-mediated mixing of the states within less than 50 fs strongly increases the localization of the excited electron at the S–Sbpy ligand. Extensive structural relaxation within this sulfurated ligand is possible, via S–S bond cleavage that results in triplet state energies that are lower than those in the analogue [Ru(bpy)3]2+. This structural relaxation upon localization of the charge on S–Sbpy is found to be the reason for the formation of a single long-lived species independent of the excitation wavelength., We investigate selective excitation in two types of ligands present in [Ru(S−Sbpy)(bpy)2]2+. Femtosecond transient absorption spectroscopy identifies the formation of the same final state within 80 fs for both excitation wavelengths. Surface hopping simulations reveal ultrafast mixing of singlet and triplet states within less than 50 fs. Energy lowering due to S−S bond cleavage is identified as the driving factor for convergence to a single final state.

Accurate and efficient calculations of absorption spectra of molecules and materials are essential for the understanding and rational design of broad classes of systems. Solving the Bethe–Salpeter equation (BSE) for electron–hole pairs usually yields accurate predictions of absorption spectra, but it is computationally expensive, especially if thermal averages of spectra computed for multiple configurations are required. We present a method based on machine learning to evaluate a key quantity entering the definition of absorption spectra: the dielectric screening. We show that our approach yields a model for the screening that is transferable between multiple configurations sampled during first principles molecular dynamics simulations; hence it leads to a substantial improvement in the efficiency of calculations of finite temperature spectra. We obtained computational gains of one to two orders of magnitude for systems with 50 to 500 atoms, including liquids, solids, nanostructures, and solid/liquid interfaces. Importantly, the models of dielectric screening derived here may be used not only in the solution of the BSE but also in developing functionals for time-dependent density functional theory (TDDFT) calculations of homogeneous and heterogeneous systems. Overall, our work provides a strategy to combine machine learning with electronic structure calculations to accelerate first principles simulations of excited-state properties., Machine learning can circumvent explicit calculation of dielectric response in first principles methods and accelerate simulations of optical properties of complex materials at finite temperature.

Transient absorption data of [FeII(tpy)(CN)3]- reveals spectroscopic signatures indicative of 3MLCT with a ∼10 ps kinetic component. These data are supported by DFT and TD-DFT calculations, which show that excited state ordering is responsive to the number of cyanide ligands on the complex.

Four trinuclear ruthenium(II) polypyridyl complexes were synthesized, and a detailed investigation of their excited-state properties was performed. The tritopic sexi-pyridine bridging ligands were obtained via para or meta substitution of a central 2,2′-bipyridine fragment. A para connection between the 2,2′-bipyridine chelating moieties of the bridging ligand led to a red-shifted MLCT absorption band in the visible part of the spectra, whereas the meta connection induced a broadening of the LC transitions in the UV region. A convergent energy transfer from the two peripheral metal centers to the central Ru(II) moiety was observed for all trinuclear complexes. These complexes were in thermal equilibrium with an upper-lying 3MLCT excited state over the investigated range of temperatures. For all complexes, deactivation via the 3MC excited state was absent at room temperature. Importantly, the connection in the para position for both central and peripheral 2,2′-bipyridines of the bridging ligand resulted in a trinuclear complex (Tpp) that absorbed more visible light, had a longer-lived excited state, and had a higher photoluminescence quantum yield than the parent [Ru(bpy)3]2+, despite its red-shifted photoluminescence. This behavior was attributed to the presence of a highly delocalized excited state for Tpp.

Luminescence thermometry, an alternative to thermal imaging using the thermovision technique, requires the development of new approaches and a thorough understanding of the physical phenomena involved, in order to improve the temperature readout parameters. A phenomenon that has recently been shown to cause an extremely strong increase in the emission intensity for the temperature elevation is the thermally induced excited state absorption. This work demonstrates that taking advantage of the strong thermal dependence of the thermally induced excited state absorption process, the limitation associated with the two thermally coupled excited levels usually involved in the ratiometric temperature readout can be overcome, improving the thermometric properties of the luminescent thermometer. The same excitation wavelength was used to induce the emission resulting from the thermally induced excited state absorption of the Tb3+ ions and ground-state absorption of the other type of co-dopant ions causing the opposite nature of the thermal dependence of their emission intensities. Moreover, thanks to the strong color changes exhibited by the phosphors, it was possible to demonstrate the applicability of the proposed approach for through-object 2D thermal imaging of a microelectronic printed circuit board covered with a glass plate using an ordinary commercial digital camera, where the thermovision camera fails.

Twisted internal charge transfer (TICT) states are of fundamental importance during the photo-physical processes of dyes and sensors. In this contribution, excited-state dynamics of an Al3+ fluorescence sensor 1-{[(2-hydroxyphenyl)-imino]methyl}naphthalen-2-ol based on the turn-on signal is clarified. Two different dark TICT states are observed by exploring the excited-state potential energy surface. With the twist of the C2-N bond, the two dark states can be reached facilely, which induce the experimentally observed weak fluorescence of the sensor. The sensing mechanism is then uncovered by investigating the electronic coupling between the sensor and analyte. Al3+ is proved to form strong coordination bonds with the sensor, which restricts the motion of the C2-N bond. Consequently, the TICT states are eliminated, which generate the turn-on signal. This sensing mechanism is trustworthy and intrinsically different from the previously proposed one, which would shed some light on the design of turn-on sensors.

By utilizing the bipolarity of 1,2-diphenylphenanthroimidazole (PPI), two types of asymmetrical tripartite triads (PPI-TPA and PPI-PCz) were designed with triphenylamine (TPA) and 9-phenylcarbazole (PCz). These triads are deep-blue luminescent materials with a high fluorescence quantum yield of nearly 100 %. To trace the photophysical behaviors of these triads, their excited-state evolution channels and interchromophoric interactions were investigated by ultrafast time-resolved transient absorption and excited-state theoretical calculations. The results suggest that the electronic nature, asymmetrical tripartite structure, and electron-hole distance of these triads, as well as solvent polarity, determine the lifetime of intramolecular charge transfer (ICT). Interestingly, PPI-PCz triads show anti-Kasha ICT, and the charge-transfer direction among the triads is adjustable. For the PPI-TPA triad, the electron is transferred from TPA to PPI, whereas for the PPI-PCz triad the electron is pushed from PPI to PCz. Exploration of the excited-state ICT in these triads may pave the way to design better luminescent materials in the future.

Diradical character and excited-state aromaticity serve as guidelines to identify molecules that show nonlinear optical properties. Cumulenes are known to have small singlet-triplet gaps resulting in significant diradical character. Herein, we report a computational investigation on the electronic structure and excited-state properties of cumulenes of different lengths and with various terminal carbene groups. Intriguingly, cumulenes with an even number of cumulative double bonds, which barely have been studied experimentally, are predicted to be thermodynamically more stable than their odd counterparts. We propose that this is due to the stabilizing effect of electron delocalization in the helical Möbius-type frontier orbitals. Accordingly, we delineate how to control the energies of the excited states by the choice of carbene and length of the cumulene. We find that π-acceptor carbenes decrease the diradical character, whereas donors as well as captodative substitution or potentially a biscationic charge leads to an open-shell ground state. We also predict that bent allenes are better in stabilizing organic radicals than carbenes. Eventually, we identify suitable candidates for experimental endeavors toward new singlet fission molecules.

As one of the most promising nonfullerene acceptors for organic photovoltaics, perylene diimide (PDI)-based multibranched molecules with twisted or three-dimensional (3D) geometric structures have been developed, which effectively increase the power conversion efficiency (PCE) of organic solar cells. Understanding the structure-property relationships in multichromophoric molecular architectures at molecular and ultrafast time levels is a crucial step in establishing new design principles in organic electronic materials. For this, photodriven excited-state symmetry-breaking charge separation (SB-CS) of PDI-based multichromophoric acceptors has been proposed to improve the PCE by reducing the self-aggregation of the planar PDI monomer. Herein, we investigated the intramolecular excited-state SB-CS and charge recombination (CR) dynamics of two symmetric phenyl-methane-based PDI derivatives, a twist dimer PM-PDI2 (phenyl-methane-based PDI dimer) and a 3D configuration tetramer PM-PDI4 (phenyl-methane-based PDI tetramer), in different solvents using ultrafast femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical calculations. The quantum chemical calculations and steady-state spectra show that the two PDI derivatives undergo conformational changes upon excitation, leading to their emission states that have the characteristics of partial charge-transfer (CT) exciton in all solvents. Based on the evolution of the fs-TA data, it is observed that the evolution from the CT state to SB-CS state is disfavored in a weak polar solvent, whereas clear SB-CS spectroscopic signatures of cationic and anionic PDI are observed in polar solvents. Faster CS and slower CR processes of PM-PDI4 are observed in comparison to those of PM-PDI2. The crowded space in the 3D structure shortens the distance between the branches, leading to a stronger electronic coupling at the lowest excited state and a larger negative Gibbs free energy change of PM-PDI4 relative to that of PM-PDI2, which benefits the charge separation among PDI units in PM-PDI4. Besides, the 3D structure of PM-PDI4 also restricts rotation to a surface crossing region between the excited state and ground state, thus inhibiting nonradiative CR process and increasing the CS state lifetime. Our results suggest that the kinetics of CS and CR processes are strongly related to the molecular geometric structure, and the excited-state symmetry breaking in the 3D structure acceptor has superior photogenerated charge and photovoltaic properties from the perspective of ultrafast dynamics.

Subtle ligand modifications on RuII -polypyridyl complexes may result in different excited-state characteristics, which provides the opportunity to tune their photo-physicochemical properties and subsequently change their biological functions. Here, a DNA-targeting RuII -polypyridyl complex (named Ru1) with highly photosensitizing 3 IL (intraligand) excited state was designed based on a classical DNA-intercalator [Ru(bpy)2 (dppz)]⋅2 PF6 by incorporation of the dppz (dipyrido[3,2-a:2',3'-c]phenazine) ligand tethered with a pyrenyl group, which has four orders of magnitude higher potency than the model complex [Ru(bpy)2 (dppz)]⋅2 PF6 upon light irradiation. This study provides a facile strategy for the design of organelle-targeting RuII -polypyridyl complexes with dramatically improved photobiological activity.

The cluster structures of hydrated aminopyrazines, APz-(H2O)n=2-4, in supersonic jets have been investigated measuring the size-selected electronic and vibrational spectra and determined with the aid of quantum chemical calculations. The APz-(H2O)2 structure is assigned as a cyclic N1 type where a homodromic hydrogen-bond chain starts from the amino group and ends at the 1-position nitrogen atom of the pyrazine moiety, corresponding to 2-aminopyridine-(H2O)2. On the other hand, APz-(H2O)n=3,4 has a linear hydrogen-bond network ending at the 4-position one (N4), which resembles 3-aminopyridine-(H2O)n=3,4. The hydrogen-bond network switching from the N1 type to the N4 one provides the accompanying red shifts of the S1-S0 electronic transition that are entirely consistent with those of the corresponding 2-aminopyridine and 3-aminopyridine clusters and also shows the drastically strengthened fluorescence intensity of origin bands in the electronic spectrum. The significant change in the excited-state dynamics is explored by the fluorescence lifetime measurement and the time-dependent density functional theory (TD-DFT) calculation. It is suggested that the drastic elongation of fluorescence lifetimes is due to the change in the electronic structure of the first excited state from nπ* to ππ*, resulting in the decreasing spin-orbit coupling to T1 (ππ*).

Resonance Raman scattering (RRS) spectra of the complexes of 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin with Co(II) and Cu(II) (CoIITMpyP4 and CuTMpyP4) in various solvents and in complexes with DNA are studied. Under nanosecond pulsed excitation, additional lines have been found in the RRS spectra of CoIITMpyP4 in a dimethylformamide (DMF) solution containing formic acid as an impurity and in a complex with DNA. At the same time, such lines are absent in the spectra of CoIITMpyP4 in pure DMF, dimethyl sulfoxide, water, and alcohols under the same excitation conditions. To interpret the experimental data, we have calculated the structure and vibrations of the solvate complexes of CoIITMpyP4 and CuTMpyP4 with water, methanol, and formic acid in the ground and in excited states. Based on the obtained data, additional lines in the Raman spectra are assigned to the dd excited state corresponding to the d(z2) → d(x2 – y2) transition, the lifetime of which increases with the formation of complexes of CoIITMpyP4 with formic acid. According to the calculation results, this correlates with a decrease in rate constant kic of the internal conversion due to an increase in energy gap ΔE between the ground and dd states of CoIITMpyP4. A decrease in the kic value in the case of binding of CoIITMpyP4 and CuTMpyP4 to DNA is caused by the additional interaction of the extra-ligand water molecule with one of its bases directly or via several water molecules immobilized on DNA surface. This complicates the conformational rearrangement of the water molecule in the dd excited state, which gives rise to an increase in the ΔE value.

The electronic communication between two ferrocene groups in the electron-deficient expanded aza-BODIPY analogue of zinc manitoba-dipyrromethene (MB-DIPY) was probed by spectroscopic, electrochemical, spectroelectrochemical, and theoretical methods. The excited-state dynamics involved sub-ps formation of the charge-separated state in the organometallic zinc MB-DIPYs, followed by recovery of the ground state via charge recombination in 12 ps. The excited-state behavior was contrasted with that observed in the parent complex that lacked the ferrocene electron donors and has a much longer excited-state lifetime (670 ps for the singlet state). Much longer decay times observed for the parent complex without ferrocene confirm that the main quenching mechanism in the ferrocene-containing 4 is reflective of the ultrafast ferrocene-to-MB-DIPY core charge transfer (CT).

Ultraviolet (UV) irradiation is known to be responsible for DNA damage. However, experimental studies in DNA oligonucleotides have shown that UV light can also induce sequence-specific self-repair. Following charge transfer from a guanine adenine sequence adjacent to a cyclobutane pyrimidine dimer (CPD), the covalent bond between the two thymines could be cleaved, recovering the intact base sequence. Mechanistic details promoting the self-repair remained unclear, however. In our theoretical study, we investigated whether optical excitation could directly lead to a charge-transfer state, thereby initiating the repair, or whether the initial excited state remains localized on a single nucleobase. We performed conformational sampling of 200 geometries of the damaged DNA double strand solvated in water and used a hybrid quantum and molecular mechanics approach to compute excited states at the complete active space perturbation level of theory. Analysis of the conformational data set clearly revealed that the excited-state properties are uniformly distributed across the fluctuations of the nucleotide in its natural environment. From the electronic wavefunction, we learned that the electronic transitions remained predominantly local on either adenine or guanine, and no direct charge transfer occurred in the experimentally accessed energy range. The investigated base sequence is not only specific to the CPD repair mechanism but ubiquitously occurs in nucleic acids. Our results therefore give a very general insight into the charge locality of UV-excited DNA, a property that is regarded to have determining relevance in the structural consequences following absorption of UV photons.

Zinc(II) bis(dipyrrin) complexes, which feature intense visible absorption and efficient symmetry breaking charge transfer (SBCT) are outstanding candidates for photovoltaics but their short lived triplet states limit applications in several areas. Herein we demonstrate that triplet excited state dynamics of bis(dipyrrin) complexes can be efficiently tuned by attaching electron donating aryl moieties at the 5,5'-position of the complexes. For the first time, a long lived triplet excited state (τT =296 μs) along with efficient ISC ability (ΦΔ =71 %) was observed for zinc(II) bis(dipyrrin) complexes, formed via SBCT. The results revealed that molecular geometry and energy gap between the charge transfer (CT) state and triplet energy levels strongly control the triplet excited state properties of the complexes. An efficient triplet-triplet annihilation upconversion system was devised for the first time using a SBCT architecture as triplet photosensitizer, reaching a high upconversion quantum yield of 6.2 %. Our findings provide a blueprint for the development of triplet photosensitizers based on earth abundant metal complexes with long lived triplet state for revolutionary photochemical applications.

Under excess illumination, photosystem II of plants dissipates excess energy through the quenching of chlorophyll fluorescence in the light harvesting antenna. Various models involving chlorophyll quenching by carotenoids have been proposed, including (i) direct energy transfer from chlorophyll to the low-lying optically forbidden carotenoid S1 state, (ii) formation of a collective quenched chlorophyll-carotenoid S1 excitonic state, (iii) chlorophyll-carotenoid charge separation and recombination, and (iv) chlorophyll-chlorophyll charge separation and recombination. In previous work, the first three processes were mimicked in model systems: in a Zn-phthalocyanine-carotenoid dyad with an amide linker, direct energy transfer was observed by femtosecond transient absorption spectroscopy, whereas in a Zn-phthalocyanine-carotenoid dyad with an amine linker excitonic quenching was demonstrated. Here, we present a transient absorption spectroscopic study on a Zn-phthalocyanine-carotenoid dyad with a phenylene linker. We observe that two quenching phases of the phthalocyanine excited state exist at 77 and 213 ps in addition to an unquenched phase at 2.7 ns. Within our instrument response of ∼100 fs, carotenoid S1 features rise which point at an excitonic quenching mechanism. Strikingly, we observe an additional rise of carotenoid S1 features at 3.6 ps, which shows that a direct energy transfer mechanism in an inverted kinetics regime is also in effect. We assign the 77 ps decay component to excitonic quenching and the 3.6 ps/213 ps rise and decay components to direct energy transfer. Our results indicate that dual quenching mechanisms may be active in the same molecular system, in addition to an unquenched fraction. Computational chemistry results indicate the presence of multiple conformers where one of the dihedral angles of the phenylene linker assumes distinct values. We propose that the parallel quenching pathways and the unquenched fraction result from such conformational subpopulations. Our results suggest that it is possible to switch between different regimes of quenching and nonquenching through a conformational change on the same molecule, offering insights into potential mechanisms used in biological photosynthesis to adapt to light intensity changes on fast time scales.

Efficient photomolecular motors will be critical elements in the design and development of molecular machines. Optimisation of the quantum yield for photoisomerisation requires a detailed understanding of molecular dynamics in the excited electronic state. Here we probe the primary photophysical processes in the archetypal first generation photomolecular motor, with sub‐50 fs time resolved fluorescence spectroscopy. A bimodal relaxation is observed with a 100 fs relaxation of the Franck‐Condon state to populate a red‐shifted state with a reduced transition moment, which then undergoes multi‐exponential decay on a picosecond timescale. Oscillations due to the excitation of vibrational coherences in the S1 state are seen to survive the ultrafast structural relaxation. The picosecond relaxation reveals a strong solvent friction effect which is thus ascribed to torsion about the C−C axle. This behaviour is contrasted with second generation photomolecular motors; the principal differences are explained by the existence of a barrier on the excited state surface in the case of the first‐generation motors which is absent in the second generation. These results will help to provide a basis for designing more efficient molecular motors in the future., Generation conflict? The excited state dynamics of first‐generation molecular motors are studied by means of ultrafast time resolved fluorescence. A 100 fs structural reorganization in the excited state is followed by a slower excited state relaxation, which is influenced by solvent viscosity. The decay is accompanied by coherent vibrational excitation of modes in the excited electronic state. The observed behaviour is contrasted with that of the second‐generation motors.

Recently, the potential use of organic π-radicals and related spin systems has been expanded to modern technological applications. The unique excited-state dynamics of organic π-radicals can be useful to improve the stability of photochemically unstable organic compounds, make the polarization transfer applicable to information technology, and achieve effective up-conversion of interest for luminescence bioimaging, among others. Furthermore, highly luminescent stable π-radicals have been recently reported, which are especially interesting for application in organic light-emitting devices owing to their potential to provide an internal quantum efficiency of 100 %. Thus, the excited-state nature of stable π-radicals as well as the control of their excited-state spin dynamics are emerging topics both in terms of fundamental science and related technological applications. In this minireview, we focus on the excited-state dynamics of both photostable non(weakly)-luminescent and luminescent π-radicals, which are opposites of each other. In particular, we cover the following topics: 1) effective generation of high-spin photoexcited states and control of the excited-state dynamics by using non-luminescent π-radicals, 2) unique excited-state dynamics of luminescent π-radicals and radical excimers, and 3) applications utilizing excited-state dynamics of π-radicals.

Although amazing progress has been made since the genesis of quantum mechanics in modeling the ground state wave functions and energies of electronic states, modeling the excited electronic states with similar accuracy is still a difficult challenge. In this dissertation orbital optimization of Multi-Slater Jastrow wave functions and its coupling to various Quantum Monte Carlo (QMC) features are used to determine the optical gaps of various systems. The new features being coupled to orbital optimization include: an excited state targeting function, configuration selection for QMC wave functions, a variance matching scheme for optical gaps, and a modified guiding function for sampling within QMC. After a study of the utility of these features, the success of optical gap prediction on both gas phase molecules (aperiodic systems), and condense phase materials (periodic systems) is explored.For aperiodic systems we found that our QMC optical gap workflow produces predictions on par in terms of accuracy with other standard techniques (e.g. MRCI+Q, CASSCF, CASPT2, EOM-CCSD) for small molecules (e.g. Formaldimine, Thioformaldehyde). In addition, for cases like [C3N2O2H4Cl] in which state-averaged orbitals (between the ground and first excited state) heavily compromises the accuracy of these states, and multiple reasonable active-space choices lead to very different state energies, our workflow can be advantageous to use.For periodic systems we found that our QMC optical gap workflow produces predictions on par in terms of accuracy with other standard techniques (e.g. DFT, G0W0) for simple bulk materials (e.g. MgO, Trans-Polyaceylene). For more challenging systems, such as bulk transition metal oxides (FeO and MnO) we found that QMC orbital optimization provides the advantage of allowing one to bypass difficult parameterization (e.g. +U value, choice of functional).

Spectral-luminescence properties of a hybrid compound containing a coumarin-type spiropyran and an azomethinocoumarin fragment in toluene-acetonitrile solution in the presence of Li+, Ca2+, Zn2+ and Mg2+ ions are reported. Two excited state proton transfers can occur in the hybrid compound—the transfer of a proton from the OH group of the 7-hydroxy coumarin tautomer to the N atom of the C=N bond of the azomethine fragment leading to green ESIPT fluorescence with a maximum at 540 nm and from the OH group of the 7-hydroxy coumarin tautomer to the carbonyl group of the pyrone chromophore, which leads to the formation of the 2-hydroxyl-tautomer T of coumarin with blue fluorescence with a maximum at 475 nm. Dependence of these excited state proton transfers on the metal nature and irradiation with an external UV source is discussed.

Here, the synthesis of photochromic hydrogen bond-assembled [2]rotaxanes using bis-fumarate as a thread for the first time is reported. In fact, photochromic 1,3-diazabicyclo[3.1.0]hex-3-ene moieties were used as stoppers and two-atom spacers managed good binding sites for the tetralactam macrocycles in clipping reactions. Moreover, the yields of photochromic [2]rotaxanes highly depended on the NO2 substituent stoppers. While the thread with a para –NO2 substituent as stopper units was shown to be an excellent template for the synthesis of photochromic [2]rotaxanes. The structures of the [2]rotaxanes are established clearly in solution by chemical shifts of the 1H 13C NMR signals and UV–Vis spectra. A pronounced bathochromic shift was occurred in the excitation wavelength of photochoromic [2]rotaxanes compared with the absorption band of photochromic threads. Therefore, these organizations can be applied in light-driven molecular switches and motors. The reversible transformation of trans and cis geometric photoisomers under UV radiation was identified. In other efforts, the possibility of the process of trans to cis interconversion of the fumarate linker under UV irradiation has been examined computationally and it has appeared that it may cause the transverse of the bis-fumarate linker inside the tetralactam macrocycle to some extent.

The one‐electron oxidizing capacity of 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) improves significantly by visible‐light excitation. The exited‐state DDQ (DDQ*) even converts benzene, fluoroarenes, heteroarenes, benzyls and olefins into their corresponding radical cations as well as chloride and other anions into their radicals. These reactive intermediates have been utilized for the generation of C‐C and C‐X (N, O, or Cl) bonds in the synthesis of valuable organic compounds and natural products. This mini review provides an overview of such DDQ*‐initiated organic transformations with their scope and limitations and discusses the proposed reactions mechanisms.

An unprecedented and previously unknown photochemical reactivity of 1,3-dicarbonyl compounds is observed with amino-alkenes leading to dihydropyrans. This novel photochemical reactivity changes the established paradigm related to the De Mayo reaction between 1,3-dicarbonyl compounds and alkenes. This new reaction allows convenient access to the Marmycin core in a single step from commercially available reactants. The origin and scope of this new photoreaction is detailed with preliminary photophysical and mechanistic investigations.

TLD1433 is the first Ru(II) complex to be tested as a photodynamic therapy agent in a clinical trial. In this contribution we study TLD1433 in the context of structurally-related Ru(II)-imidozo[4,5-f][1,10]phenanthroline (ip) complexes appended with thiophene rings to decipher the unique photophysical properties which are associated with increasing oligothiophene chain length. Substitution of the ip ligand with ter- or quaterthiophene changes the nature of the long-lived triplet state from metal-to-ligand charge-transfer to (3)ππ* character. The addition of the third thiophene thus presents a critical juncture which not only determines the photophysics of the complex but most importantly its capacity for (1)O(2) generation and hence the potential of the complex to be used as a photocytotoxic agent. ENTRY FOR THE TABLE OF CONTENTS: A low-lying triplet intraligand state ((3)IL) determines the properties of the long-lived excited states in a series of Ru(II) complexes. The (3)IL state can be accessed by increasing the length of an oligothiophene chain. The (3)IL state is extremely efficient at generating (1)O(2) and thus enhances the potency of the complexes as PDT agents. [Image: see text]

This paper describes reversible "on-off" switching of the photoluminescence (PL) intensity of CdSe quantum dots (QDs), mediated by photochromic furylfulgide carboxylate (FFC) molecules chemisorbed to the surfaces of the QDs. Repeated cycles of UV and visible illumination switch the FFC between "closed" and "open" isomers. Reversible switching of the QDs' PL intensity by80% is enabled by different rates and yields of PL-quenching photoinduced electron transfer (PET) from the QDs to the respective isomers. This difference is consistent with cyclic voltammetry measurements and density functional calculations of the isomers' frontier orbital energies. This work demonstrates fatigue-resistant modulation of the PL of a QD-molecule complex through remote control of PET. Such control potentially enables applications, such as all-optical memory, sensing, and imaging, that benefit from a fast, tunable, and reversible response to light stimuli.

In this work, two carbazole- and benzo[d]oxazole-based novel multifunctional materials with a hybridized local and charge-transfer (HLCT) characteristic, namely, OCI and OCT, which could act as deep-blue fluorophors and phosphorescent hosts, were first designed and synthesized. Both OCI and OCT display meritorious thermal stability with a glass transition temperature (Tg) as high as 139 and 137 °C, respectively. The two compounds also reveal excellent bipolar carrier transport ability and emit deep-blue emission with photoluminescence quantum yields (PLQY) of 34.5% and 14.0%, respectively. Used as blue fluorophors, OCI- and OCT-based non-doped devices exhibit a very low turn-on voltage (Von) of 3.0 V and realize deep-blue emission with peaks located at 438–453 nm, and the optimized OCI-based device achieve the maximum external quantum efficiency (ηext,max) reaching 3.19%. Remarkably, doped blue devices show even deeper blue emission at 424 and 428 nm and the optimized OCI-based device achieves a significantly improved ηext,max of 5.06% with CIEy < 0.04, which could rank among the deepest blue HLCT fluorophors. Additionally, green phosphorescent OLEDs hosted by OCI and OCT reveal a low Von of 2.7 V with maximum current efficiencies (ηc,max) of 49.43 cd A−1 (ηext,max of 14.11%) and 52.81 cd A−1 (ηext,max of 15.57%), respectively, which are superior to that of CBP (Von of 3.0 V, ηc,max of 51.76 cd A−1 and ηext,max of 12.88%). The superb electroluminescence (EL) performance of OCI and OCT as deep-blue HLCT emitters and phosphorescent hosts demonstrates that they have great potential for industrial applications.

Donor–spacer–acceptor (D–π–A) materials CAPI and CCAPI, with hybridized local and charge transfer (HLCT) emissive states, have been synthesized. The twisting D–π–A architecture promotes the partial separation of HOMO and LUMO, leading to an enhanced % CT component, and the anthracene moiety in CAPI and CCAPI increases the conjugation length, leading to an enhanced % LE component. The non-doped device with CCAPIb shows the blue emission (450 nm) with maximum current efficiency (ηc), power efficiency (ηp), and external quantum efficiency (ηex) of 16.83 cd A−1, 15.32 lm W−1, and 12.0%, respectively, as well as exciton utilization efficiency (EUE) of 95% with a luminance of 32 546 cd m−2 and a roll-off efficiency of 0.53%. The new design strategy has great potential for developing high-performance blue electroluminescent materials.

Nitrated aromatic molecules have unique photoinduced channels. Due to the presence of oxygen-centered non-bonding orbitals, they can undergo sub-picosecond intersystem crossing showing one of the strongest couplings between the singlet and triplet manifolds among organic molecules. Several nitroaromatic compounds also have a distinctive nitric oxide photodissociation channel which occurs through a complex sequence of atom rearrangements and state changes. These remarkable processes have stimulated the attention of several research groups over the last few years who have applied modern femtosecond spectroscopies and new computational methods to these topics. Nitroaromatic molecules also have demonstrated their value as case-studies, where they can serve to understand the influence of torsional motions between the nitro substituent and the aromatic system in the conversions between states. In this contribution we highlight several of the recent results in this area. Due to the importance of the atmospheric photochemistry of nitrated compounds and their accumulating applications as nitric oxide release agents, continued research about the effects of the different state orderings, substitution patterns, and solvent effects is central to the development of future applications and for a better understanding of their environmental pathways. From this analysis, several pending issues are highlighted, which include the nature of the dominant singlet state involved in intersystem crossing, the role of the formation of charge-transfer states, the yield of the internal conversion channel to the electronic ground state, and a more generalized understanding of the sequence of steps which lead to nitric oxide dissociation.

Stimulus-responsive supramolecular architectures have become an attractive alternative to conventional ones for many applications in sensing, drug-delivery and switchable memory systems. Herein, we used an anion receptor (H: host) as a hydrostatic-pressure-manipulatable fluorescence foldamer and halide anions as chiral (binaphthylammonium) and achiral (tetrabutylammonium) ion pairs (SS or RR·X and TBA·X; X = Cl, Br), and then investigated their (chir)optical properties and molecular recognition behavior under hydrostatic pressures. The conformational changes and optical properties of H in various organic solvents were revealed by UV/vis absorption and fluorescence spectra and fluorescence lifetimes upon hydrostatic pressurization. The anion-recognition abilities of H upon interactions with SS or RR·X and TBA·X at different pressure ranges were determined by hydrostatic-pressure spectroscopy to quantitatively afford the binding constant (Kanion) and apparent reaction volume changes . The results obtained indicate that hydrostatic pressure as well as solvation plays significant roles in the dynamic control of the present supramolecular system in the ground and excited states. This work will provide a new guideline for further developing hydrostatic-pressure-responsive foldamers and supramolecular materials., Hydrostatic pressure can control interactions of chiral countercations with helical receptors containing anions, causing remarkable chiroptical changes.

Epsilon-near-zero (ENZ) properties have been reported in organic molecular films. In particular, cyanine and squaraine films have been shown to exhibit ENZ properties in the visible spectral region with a strong 3rd order nonlinear optical response near the ENZ spectral region. Noting both cyanine and squaraine belong to the polymethine family, a series of six curcuminoid borondifluoride (Curc) derivatives were developed to examine whether such a polymethine character is positively correlated with the ENZ property of the organic films. Those Curc derivatives possess a Donor–Acceptor–Donor (D–A–D) architecture with acceptor, AcacBF2, located at the molecular center. The backbone of Curc is designed such that the donor strength can be tuned to transit between charge transfer (CT) and polymethine character. This balance between CT and polymethine character of the Curc series is examined based on the Lippert–Mataga plot. As donor strength in the D–A–D structure increases, CT character is less marked resulting in a more dominant polymethine character. The structural and optical properties of the Curc films with a thickness in the order of 30 nm were examined to correlate the polymethine character with the ENZ response. The results obtained in isotropic Curc thin films demonstrate that an increase of polymethine character associated with a stronger donor strength leads to an appearance/enhancement of the ENZ property in the visible spectrum range from 500 to 670 nm. Overall, this study provides useful guidelines to engineer new organic materials showing ENZ properties in a desired spectral range.

Manipulating the molecular orbital properties of excited states and the subsequent relaxation processes can greatly alter the emission behaviors of luminophores. Herein we report a vivid example of this, with luminescence conversion from thermally activated delayed fluorescence (TADF) to ultralong room-temperature phosphorescence (URTP) via a facile substituent effect on a rigid benzothiazino phenothiazine tetraoxide (BTPO) core. Pristine BTPO with multiple heteroatoms shows obvious intramolecular charge transfer (ICT) excited states with small exchange energy, featuring TADF. Via delicately functionalizing the BTPO core with peripheral moieties, the excited states of the BTPO derivatives become a hybridized local and charge transfer (HLCT) state in the S1 state and a local excitation (LE) dominated HLCT state in the T1 state, with enlarged energy bandgaps. Upon dispersion in a polymer matrix, the BTPO derivatives exhibit a persistent bright green afterglow with long lifetimes of up to 822 ms and decent quantum yields of up to 11.6%., The decoration of a BTPO core results in a change in the luminescence nature from TADF to URTP. The phosphors in an amorphous PMMA matrix showed monomeric URTP with phosphorescence lifetimes of up to 822 ms and quantum yields of up to 11.6%.

A systematic theoretical study of the regulating effect of the substituent position on the photoinduced deactivation process of the benzyluracil systems has been performed based on the high-level static electronic structure calculations and on-the-fly full-dimensional excited-state dynamics simulations. Similarities and differences coexist for the two systems by comparative studies on the photoinduced deactivation process of the 5-benzyluracil (5-BU) and 6-benzyluracil (6-BU) systems. They both obey an S2 → S1 → S0 two-step decay pattern, and the decay coordinates of the S2 → S1 and S1 → S0 processes are mainly driven by the elongation of the bridging bond and the out-of-plane ring deformation motion, respectively. However, the puckering motion occurring at the C2 atom in the uracil fragment dominates the decay pathway of the 5-BU system. On the contrary, the puckering motion at the C5 atom in the benzene fragment mainly drives the decay coordinate of the 6-BU system. Therefore, the substituent position could play significant roles in the deactivation process of the benzyluracil systems. Moreover, the S1 → S0 decay process of the 6-BU system consists of five pathways, possessing a more complex deactivation picture than the 5-BU system. The fitted time scale of the puckering motion is compatible with the experimentally observed lifetimes. This work provides a fundamental understanding of the photophysical and photochemical properties of the benzyluracil systems and can give rational suggestions to further design or regulate the bionic molecular systems.

Electronic structures of four pyrene-derived N-heterocyclic carbene complexes of platinum(II) and gold(III) bearing cyclometallated 2,6-diphenylpyridine have been investigated theoretically. The structures of all complexes in gas phase have been first optimized. Different functionals and basis sets have been tested to find the best theoretical method. The results show that B3PW91 (LANL2DZ/6-31G*) combination give the best description of the molecules as compared with experimentally determined structural features. Using the calculation data from that best functional and mixed basis set combination, frontier molecular orbitals of the complexes have been examined. To gain understanding on the nature of electronic transitions in the UV-vis absorption spectra of the compounds, TD-DFT calculations have been carried out. A details analysis of the vertical excitation reveals that the nature of all the vertical electronic transitions in the complexes. Notably, the lowest energy transitions are πcb + dPt → $$\pi _{{{\text{cb}}}}^{{\text{*}}}$$ (for Pt1 and Pt2) and πCNC → dAu + $$\pi _{{{\text{CNC}}}}^{{\text{*}}}$$ (for Au1 and Au2) charge transfer in nature. A sharp contrast in lowest energy triplet excited state for d8‑squared planar Au1 and Pt1 has been demonstrated. While no contribution from gold(III) d orbital to the SOMO and SOMO – 1 orbital of Au1, there is significant contribution of platinum(II) dyz orbital in the SOMO – 1 orbital of Pt1, suggesting possible interaction of this singly occupied orbital with radical of π-symmetry substrates.

Five cationic iridium(III) complexes with fluorinated cyclometalating tetrazole ligands [Ir(dfptrz)2L]+, where Hdfptrz = 5-(2,4-difluorophenyl)-2-methyl-2H-tetrazole and L = 2,2′-bypiridine (1F), 4,4′-ditert-butyl-2,2′-bipyridine (2F), 1,10-phenantroline (3F), 4,4′-bis(dimethylamino)-2,2′-bipyridine (4F), and tert-butyl isocyanide (5F), were prepared following a one-pot synthetic strategy based on a bis-cyclometalated solvato complex obtained via silver(I)-assisted cyclometalation, which was then reacted with the proper ancillary ligand to get the targeted complexes. The X-ray crystal structures of 2F and 4F were determined, showing that the tetrazole ligands are in a trans arrangement with respect to the iridium center. Electrochemical and photophysical properties, along with density functional theory calculations, allowed a full rationalization of the electronic properties of 1F-5F. In acetonitrile solution at 298 K, complexes 1F-3F, equipped with bipyridine and phenanthroline ligands, exhibit strong vibronically structured luminescence bands in the blue region with photoluminescence quantum yields (PLQYs) in the range 56-76%. This behavior is radically different from the nonfluorinated analogues reported previously, which emits in the green region from 3MLCT excited states. 4F shows relatively strong emission (PLQY = 40%) of charge transfer character centered on the amino-bipyridine ancillary ligand, whereas the emission of 5F is very weak (PLQY = 0.6%), further blue-shifted and attributed to the lowest ligand-centered (3LC) triplet state of the tetrazolyl cyclometalated moiety. A similar photophysical behavior is observed in PMMA at 298 K, whereas in a 77 K matrix, all of the compounds are strong emitters. This novel fluorinated phenyl-tetrazole cyclometalating ligand provides the corresponding iridium(III) complexes with a combination of excited-state energy and redox potentials that make them very promising as photoredox catalysts.

The triple hydrogen-bonded triquinolonobenzene (TQB) molecule is investigated for its excited-state dynamics and proton transfer (ESIPT) mechanism in different solvents in this work. Through insights into electrostatic potential surface (EPS), reduced density gradient, and isosurfaces of gradient, we confirm that three intramolecular hydrogen bonds are formed for the TQB molecule. Exploring geometrical parameters involved in hydrogen bonds, infrared (IR) vibrational spectra, and bond energy via atoms in molecules (AIM) analyses, it could be verified that hydrogen bonds are strengthened in the first (S

2-(6'-Hydroxy-2'-pyridyl)benzimidazole (BI), having double functional groups donating protons, is investigated theoretically with an aim to determine the excited-state proton transfer (ESPT) mechanism in different solvents. We demonstrate that the ESPT reaction can take place with the assistance of protic solvents (water and ethanol). At the same time, the vitally important role of bridges of water and ethanol for the ESPT reaction is confirmed by the disappearance of the ESPT reaction when we replaced the protic solvent with aprotic solvent acetonitrile (ACN). We regulate the ESPT reaction of BI via solvents successfully. A different ESPT mechanism from the one proposed previously (the proton of BI transfers from benzimidazole NH to pyridyl nitrogen in ethanol) is proposed. Our simulated potential energy barriers indicate that the ESPT reaction of BI can occur only between the hydroxyl proton and pyridyl nitrogen with the assistance of water or ethanol molecules. We further verify that the water- or ethanol-assisted ESPT reaction of BI is stepwise, and the concerted mechanism is unambiguously ruled out. This systematic investigation into the ESPT mechanism of BI is significant in designing and constructing the desirable supramolecular architectures, which can provide potential supramolecular recognition sites and supramolecular inter- or intra-H-bonding interactions.

In this work, DFT and TDDFT approaches have been executed to make a detailed exploration about the excited state luminescent properties as well as excited state intramolecular proton transfer (ESIPT) mechanism for the novel 2-(1H-imidazol-2-yl)-phenol (PI) compound. Optimized geometrical parameters of primary chemical bonds and infrared (IR) spectra prove that the O37-H38···N26 hydrogen bond of PI should be strengthened in the S1 state. Insights into frontier molecular orbitals (MOs), we infer charge redistribution and charge transfer (ICT) phenomena motivate ESIPT trend. Via probing into potential energy curves (PECs) in related electronic states, we come up with the ultrafast ESIPT behavior due to low potential barrier. Furthermore, we search the reaction transition state (TS) and simulate intrinsic reaction coordinate (IRC) path; the ultrafast ESIPT behavior and mechanism of PI compound can be re-confirmed. We sincerely wish that this work could play roles in further developing novel applications based on PI compound and in promoting new ratiometric fluorescence probes in the future.

We demonstrate that, rather than resorting to high-cost dynamic correlation methods, qualitative failures in excited-state potential energy surface predictions can often be remedied at no additional cost by ensuring that optimal molecular orbitals are used for each individual excited state. This approach also avoids the weighting choices required by state-averaging and dynamic weighting and obviates their need for expensive wave function response calculations when relaxing excited-state geometries. Although multistate approaches are of course preferred near conical intersections, other features of excited-state potential energy surfaces can benefit significantly from our single-state approach. In three different systems, including a double bond dissociation, a biologically relevant amino hydrogen dissociation, and an amino-to-ring intramolecular charge transfer, we show that state-specific orbitals offer qualitative improvements over the state-averaged status quo.