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Ionic liquids (ILs) have attracted increasing attention since last few decades due to their high molecular design abilities and wide applications in different fields. In this study, four novel fluorescent isoquinolino [2,1-a]quinoxalin-5-ium ILs were designed and synthesized via a two-step process including a simple dual Schiff's base formation and a subsequent [RhCp∗Cl2]2-catalyzed oxidative C–H activation/annulation reaction. The as-synthesized ILs were extensively characterized using FT-IR, 1H-NMR, 13C-NMR, 19F-NMR, HSQC-NMR, HMBC-NMR and HR-MS. Their photophysical properties were determined by steady-state fluorescence spectroscopy. The results demonstrate that all these ILs showed dual or triple emissions, large stokes shift (90 nm) and mechanochromic behaviors. Basing on solvatochromism and titration experiments, it is thought that the emission bands of the ILs are raised from their local excited states, charge transfer states or excited state proton transfer of cations, while the substitute effect of these quinoxaline derived ILs on their stokes shifts is negligible.

Upconversion nanosensitizers have been widely considered to have important applications in the treatment of major diseases such as tumors and the utilization of solar energy. Majority of the efforts so far have been focused on improving the efficiency of energy transfer (ET) between upconversion nanoparticles (UCNPs) and the anchored sensitizers with premise that high ET efficiency will lead to high acceptor efficacy. This premise is, however, proved by our current work to be invalid for commonly used load. Interaction between adjacent sensitizing molecules was found to be critical which undermines the amount of excited monomer sensitizers and thus fades the efficacy. Here NaYF4:Yb3+,Er3+ UCNPs and rose bengal (RB) photosensitizer molecules were used as the model energy donors and acceptors, respectively. Contrary to monotonous increase of the ET efficiency from UCNPs to RB species with increasing RB loading, acceptor efficacy characterized by the reactive oxygen species, as well as the RB fluorescence, exhibits bizarre dependence on the RB loading. The underlying mechanism was well studied by the steady-state and time-resolved spectroscopy of a series of samples. RB aggregates are believed to be responsible for the severe deviation between the ET efficiency and acceptor efficacy. The conclusion was validated by in vitro test where the photodynamic therapy with the most monomer RB in UCNPs-RB nanosensitizers kills 35.8% more cells than that with the highest RB loading. This understanding sheds light on construction of new ET based nanosystems for broad applications, such as medicine, solar energy utilization and optical storage.

Luminescent metal–organic frameworks (MOFs) are of interest for sensing, theranostics, dosimetry, and other applications. The use of lanthanoids in MOF metal nodes allows for intrinsic met-al-based luminescence. In this work, a facile route for modulat-ing the photoluminescent and radioluminescent properties of Tb(III)-based MOFs is reported. By using Tb(III)-cluster nodes as X-ray attenuators, and organic linkers with varying excited state energies as sensitizers, MOFs with metal-based, linker-based, and metal+linker-based photo- and radioluminescence are reported.

Preliminary explorations of the annihilation electrogenerated chemiluminescence (ECL) of mixed metal complexes have revealed opportunities to enhance emission intensities and control the relative intensities from multiple luminophores through the applied potentials. However, the mechanisms of these systems are only poorly understood. Herein, we present a comprehensive characterisation of the annihilation ECL of mixtures of tris(2,2'-bipyridine) ruthenium(II) hexafluorophosphate ([Ru(bpy)(3)](PF6)(2)) and fac-tris(2-phenylpyridine)iridium(III) ([Ir(ppy)(3)]). This includes a detailed investigation of the change in emission intensity from each luminophore as a function of both the applied electrochemical potentials and the relative concentrations of the two complexes, and a direct comparison with two mixed (Ru/Ir) ECL systems for which emission from only the ruthenium-complex was previously reported. Concomitant emission from both luminophores was observed in all three systems, but only when: (1) the applied potentials were sufficient to generate the intermediates required to form the electronically excited state of both complexes; and (2) the concentration of the iridium complex (relative to the ruthenium complex) was sufficient to overcome quenching processes. Both enhancement and quenching of the ECL of the ruthenium complex was observed, depending on the experimental conditions. The observations were rationalised through several complementary mechanisms, including resonance energy transfer and various energetically favourable electron-transfer pathways. Refereed/Peer-reviewed

Here, we report a finding on light-mediated CO2-responsiveness. It is found on the microgels that are made of side-chain type metallopolymers containing metalla-aromatics. Turbidity and laser light scattering studies on dilute aqueous dispersion of these microgels in dark indicate high CO2-responsivity, but poor reversibility upon N2 purge, which can be improved by exposing to light. This light-mediated CO2-responsiveness can be elucidated by the loss of aromaticity from initial photoexcitation and concurrent formation of a less reactive, antiaromatic excited state of relatively low CO2 binding affinity, and by subsequent relief of antiaromaticity that can enhance the CO2 removal. The finding is also checked by CO2 uptake-release experiments on the microgels, which enables both CO2 capture of high capacity and CO2 removal of good reversibility under a mild condition, allowing effective and reversible response to dilute CO2.

Naphthalene diimides (NDIs) are a promising material for n-type polymer acceptors in thin-film devices such as organic light-emitting diodes. The intersystem crossing (ISC) pathway of NDIs has been...

Herein we demonstrate the synthesis and investigation of organic D-A type fluorophores, named Np-TPA and Np-2TPA, comprising of twisted triphenylamine (TPA) as electron-donor and planar 1,8-naphthalenimide (Np) as electron-acceptor. These fluorophores emit strong fluorescence not only in dilute solutions but also in the solid state with quantum yields up to 0.74 and 0.92, respectively. From theoretical and structural analysis, the conformation planarization of Np-TPA and Np-2TPA in the excited state could effectively diminish the molecular motions thus allowing intense emission in solution, while highly restricted π-π stacking permits their bright emission in crystalline state. Efficient yellow bioimaging in the cytoplasm of multiple of various cells can be achieved after the incubation with even extremely low concentrations of Np-2TPA (about 30 nM). All the results imply that the dual-state emissive fluorophore represents a new paradigm for bioimaging.

Carbon dots (CDs) are the unique class of luminescent nanomaterials consist of various chromophoric units heterogeneously distributed throughout the nanoparticle, resulting intriguing multistate emissive properties. Herein, we have critically investigated the specific locations of the blue and green-emitting centers inside dual emissive CDs by steady-state and time-resolved polarized emission study. It is further clarified by a temperature-dependent fluorescence study for both the emitting domains. Results suggest that the blue chromophoric units are located at the interior part of CDs, while green units are mostly at the exterior region. Furthermore, we have investigated the solvent-dependent inter-chromophoric interactions between the two emissive domains by the Time-Resolved Area Normalized Emission Spectroscopy (TRANES). Results suggest that at polar aprotic solvent acetone, time-dependent positive evolution of green-emitting states and negative evolution of blue emissive domains have been observed. This reversible emitting properties evolve due to the excited state energy migration from blue emissive domains to green emissive domains at polar aprotic medium, while in the case of polar protic solvent water, this phenomenon is missing. This switchable inter-chromophoric interaction are correlated further with the inter-particle interactions of CDs.

Near-infrared (NIR) fluorophores are emerging tools for biophotonics because of their reduced scattering, increased tissue penetration and low phototoxicity. However, the library of NIR fluorophores is still limited. Here, we report the NIR fluorescence of two benzene-fused oligo-BODIPYs in their hexameric (H) and octameric (O) forms. These dyes emit bright NIR fluorescence (H: maxima 943/1075 nm, O: maxima 976/1115 nm) that can be excited in the NIR (H = 921 nm, O = 956 nm) or non-resonantly over a broad range in the visible region. The emission bands of H show a bathochromic shift and peak sharpening with increasing dye concentration suggesting the presence of J-aggregates. Furthermore, the emission maxima of both H and O shift up to 20 nm in solvents of different polarity. These dyes can be used as NIR ink and imaged remotely on the macroscopic level with a stand-off distance of 20 cm. We furthermore demonstrate their versatility for biophotonics by coating microscale beads and performing microrheology via NIR video particle tracking (NIR-VPT) in biopolymer (F-actin) networks. No photodamaging of the actin filaments takes place, which is typically observed for visible fluorophores and highlights the advantages of these NIR dyes.

The synthesis, structural characterization and photophysical properties of a series of platinum(II) complexes of benzannulated pincer-type diarylamido ligands are described. The ligands all contain tricyclic phenanthridine (3,4-benzoquinoline) rings as donor arms, which were elaborated into N∧N−∧O-coordinating β-enaminoketonato chelates via partial condensation with acetylacetone. The proligands are easily deprotonated, and metallation can be achieved under mild conditions using simple Pt(II) salts and Ag2O as a base. The resulting Pt(II) complexes exhibit strong metal-to-ligand charge-transfer absorptions in the region of ∼450–575 nm and are phosphorescent in solution at room temperature, emitting bright orange light (λmax ∼ 600 nm) with quantum yields of up to 16% and excited-state lifetimes on the order of ∼20 μs, representing significant improvements to these photophysical properties compared with many previously reported N∧N∧O or N∧N∧N-ligated systems. Computational modelling reveals that the lowest-lying triplet state is populated efficiently thanks to strong coupling between singlet and triplet excited state manifolds, as in cyclometallated compounds of Pt(II). Substituents (CH3, tBu, or CF3) in the 2-position of the phenanthridinyl unit are found to have little influence on the optical properties, but the emission is severely quenched when a methyl substituent is introduced ortho to the coordinating nitrogen. Molecular distortions in the excited state are shown to be primarily responsible for the quenching in this complex.

In this work, a series of acenaphthene derivatives (AC-derivatives) were designed and synthesized by incorporating imidazole as an electron deficient moiety and triphenylamine (TPA) as an electron rich moiety with different functional groups at the N1 position of imidazole. All the dyes were structurally characterized by spectroscopic methods (1H NMR, 13C NMR, IR, and Mass) and some of the dyes were characterized by single crystal X-ray diffraction. The detailed thermal, photophysical and electrochemical properties were systematically investigated. All the dyes showed good thermal stability with high thermal decomposition (Td) temperatures ranging from 400 to 430 °C. All the dyes had similar absorption behaviour, with peak wavelengths at 240 nm and 350 nm assigned to the π → π* transitions of the aromatic segments. All the dyes exhibited a broad bluish-white to yellowish-orange emission with the emission maximum in the range of 520–600 nm. Electrochemical analysis was performed to discover the HOMO and LUMO energy levels of the dyes, which were verified using DFT calculations. Theoretical investigations, such as optimization, singlet–triplet and HOMO and LUMO energy levels calculations, were calculated using DFT and TD-DFT analysis. The excited singlet–triplet energy gaps of the dyes were estimated and correlated to the structural and energetic characteristics of the known host materials. In addition, examinations were carried out to find the optical, electronic, charge transport and stability properties of dyes for use as emissive and charge transport materials in organic light emitting devices (OLEDs). All the dyes are good emissive materials with their hole transporting nature more favourable than the electron transporting nature. The theoretical investigations clearly indicate that AC-mCF3 has more potential as an emitter for OLED applications.

Organic room temperature phosphorescent materials have attracted a lot of research interest due to their wide applications. Herein, a series of intramolecular-locked triphenylamine derivatives have been synthesized with adjustable room temperature phosphorescence (RTP) properties, in which the RTP lifetime ranged from 30 to 288 ms and the emission color varied from sky blue to green. These results are mainly related to the substituent effect, which can tune the intramolecular charge transfer and intermolecular interactions, contributing to different intersystem crossing (ISC) processes, and the stability and energy levels of excited triplet states.

The ability to access panchromatic absorption and long-lived charge-transfer (CT) excited states is critical to the pursuit of abundant-metal molecular photosensitizers. Fe(II) complexes supported by benzannulated diarylamido ligands have been reported to broadly absorb visible light with nanosecond CT excited state lifetimes, but as amido donors exert a weak ligand field, this defies conventional photosensitizer design principles. Here, we report an aerobically stable Fe(II) complex of a phenanthridine/quinoline diarylamido ligand, Fe(ClL)2, with panchromatic absorption and a 3 ns excited-state lifetime. Using X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Fe L-edge and N K-edge, we experimentally validate the strong Fe-Namido orbital mixing in Fe(ClL)2 responsible for the panchromatic absorption and demonstrate a previously unreported competition between ligand-field strength and metal-ligand (Fe-Namido) covalency that stabilizes the 3CT state over the lowest energy triplet metal-centered (3MC) state in the ground-state geometry. Single-crystal X-ray diffraction (XRD) and density functional theory (DFT) suggest that formation of this CT state depopulates an orbital with Fe-Namido antibonding character, causing metal-ligand bonds to contract and accentuating the geometric differences between CT and MC excited states. These effects diminish the driving force for electron transfer to metal-centered excited states and increase the intramolecular reorganization energy, critical properties for extending the lifetime of CT excited states. These findings highlight metal-ligand covalency as a novel design principle for elongating excited state lifetimes in abundant metal photosensitizers.

Excited-state intramolecular proton transfer (ESIPT) dyes typically show strong solid-state emission, but faint fluorescence intensity is observed in the solution state owing to detrimental molecular motions. This article investigates the influence of direct (hetero)arylation on the optical properties of 2-(2'-hydroxyphenyl)benzoxazole ESIPT emitters. The synthesis of two series of ESIPT emitters bearing substituted neutral or charged aryl, thiophene, or pyridine rings is reported herein along with full photophysical studies in solution and solid states, demonstrating the dual solution-/solid-state emission behavior. Depending on the nature of substitution, several excited-state dynamics are observed: quantitative or partially frustrated ESIPT process or deprotonation of the excited species. Protonation studies revealed that pyridine substitution triggered a strong increase of quantum yield in the solution state for the protonated species owing to favorable quinoidal stabilization. These attractive features led to the development of a second series of dyes with alkyl or aryl pyridinium moieties showing strong tunable solution/solid fluorescence intensity. For each series, ab initio calculations helped rationalize and ascertain their behavior in the excited state and the nature of the emission observed by the experimental results.

Criegee intermediates make up a class of molecules that are of significant atmospheric importance. Understanding their electronically excited states guides experimental detection and provides insight into whether solar photolysis plays a role in their removal from the troposphere. The latter is particularly important for large and functionalized Criegee intermediates. In this study, the excited state chemistry of two small Criegee intermediates, formaldehyde oxide (CH2 OO) and acetaldehyde oxide (CH3 CHOO), was modeled to compare their specific dynamics and mechanisms following excitation to the bright ππ* state and to assess the involvement of triplet states to the excited state decay process. Following excitation to the bright ππ* state, the photoexcited population exclusively evolves to form oxygen plus aldehyde products without the involvement of triplet states. This occurs despite the presence of a more thermodynamically stable triplet path and several singlet/triplet energy crossings at the Franck-Condon geometry and contrasts with the photodynamics of related systems such as acetaldehyde and acetone. This work sets the foundations to study Criegee intermediates with greater molecular complexity, wherein a bathochromic shift in the electron absorption profiles may ensure greater removal via solar photolysis.

Excited state intramolecular proton transfer (ESIPT) has drawn much attention for its important applications in a variety of areas. Here, the steady-state and time-resolved absorption spectroscopic experiments as well as DFT/TD-DFT calculations are employed to study the photophysical properties and photochemical reaction mechanisms of 2-(2'-hydroxyphenyl) benzoxazole (HBO) and selected derivatives (compounds 1-3). Because of their larger π-conjugation framework, compounds 1-3 display red-shifted absorbance but blue-shifted fluorescence compared with HBO. A fast ESIPT process is observed directly for HBO while compound 3 has an enol/keto equilibrium type of ESIPT that exhibits dual emission. Interestingly, only the emission of the enol form is observed for compounds 1 and 2 which suggests that the ESIPT process is strongly inhibited. These results indicate the decoration with electron-withdrawing groups such as thiadiazol and pyrazine on the hydroxyphenyl ring (compounds 1 and 2) apparently suppresses the proton-transfer processes in their excited states. Whereas the ESIPT process is rarely increased for compound 3 that modified with the phenanthrol ring, because the effective conjugation is reduced for compound 3 compared with HBO. The work here provides fundamental insights that may be useful for designing novel ESIPT molecules in the future.

A series of functional platinum(II) complexes (Pt1-Pt3), which present high activity in four-photon absorption, in vivo imaging, and precise cancer therapy, as previously reported by the experimental work of Zhang et al. (Inorg. Chem.2021, 60, 2362-2371), are computationally investigated in the article. We find that after the complex goes through four-photon absorption to the S1 state, it undergoes intersystem crossing to the T2 state and eventually reaches the T1 state through internal conversion. On the T1 state, both radiative and nonradiative decay to S0 exit. The radiative decay forms the basis for the phosphorescence imaging in tissues as reported in the original paper. In addition, the nonradiative decay can simultaneously generate cytotoxic singlet oxygen by the excited energy transfer process, also known as triplet oxygen's quenching of triplet states. We conclude that the phosphorescence property as well as the photosensitizer character jointly bring high activity of in vivo imaging and photodynamic therapy to these complexes.

Substituted tetralones such as 3,3,6,8-tetramethyl-1-tetralone undergo photoenolization to produce a photoenol excited state with a lifetime around ~ 3 μs, which involves the carbonyl triplet state of the ketone (τ ~ 1.9 ns), as a precursor; the excited photoenol also has biradical character and is useful for the fast synthesis of gold nanostructures. In the case of excited photoenols like this one, if metal ion trapping fails, they return to the original ketone precursor and remain available for future events that can lead to the target nanoparticles. This study includes the characterization of the photochemistry of the substituted tetralone, and the dual behavior of reaction intermediates, as biradicals and excited states, in energy and electron transfer processes.

In electron-transfer initiated photosensitization processes, molecular oxygen (O2 ) is not involved in the first bimolecular event, but almost always participates in subsequent steps giving rise to oxygenated products. An exception to this general behavior is the photosensitized dimerization of tyrosine (Tyr), where O2 does not participate as a reactant in any step of the pathway yielding Tyr dimers (Tyr2 ). In the pterin (Ptr) photosensitized oxidation of Tyr, O2 does not directly participate in the formation of Tyr2 and quenches the triplet excited state of Ptr, the reactive species that initiates the process. However, O2 is necessary for the dimerization, phenomenon that we have named as the oxygen paradox. Here, we review the literature on the photosensitized formation of Tyr2 and present results of steady-state and time resolved experiments, in search of a mechanistic model to explain the contradictory role of O2 in this photochemical reaction system.

Electron transfer dissociation (ETD) is an analytically useful tool for primary structure interrogation of intact proteins, but its utility is limited by higher-order reactions with the products. To inhibit these higher-order reactions, first-generation fragment ions are kinetically excited by applying an experimentally tailored parallel ion parking waveform during ETD (ETD-PIP). In combination with subsequent ion/ion proton transfer reactions, precursor-to-product conversion was maximized as evidenced by the consumption of more than 90% of the 21 kDa Protein G precursor to form ETD product ions. The employment of ETD-PIP increased sequence coverage to 90% from 80% with standard ETD. Additionally, the inhibition of sequential electron transfers was reflected in the high number of complementary ion pairs from ETD-PIP (90%) compared to standard ETD (39%).

Integrating multiple photosensitive properties into an “all-in-one” photosensitizer (PS) shows great promise for the treatment of cancers owing to synergistic effect among them. However, the development of such PSs, especially those that need a single laser source, remains a challenge. Herein, we report an orchestration of electron donors and acceptors in a propeller-like pentad, PBI-4Cz, where four carbazole (Cz) units are covalently linked to the ortho-positions of the perylene bisimide (PBI) core. Strong intramolecular donor-acceptor interaction significantly quenches the luminescence and largely extends the absorption spectra to near-infrared region. Excited-state dynamics investigated via femto- and nano-second transient absorption spectroscopy revealed exclusive charge separation of the PBI-4Cz within initial 0.5 ps when photoexcited regardless of which intermediate is involved. Energy dissipation of the resulting charge-separated state (PBI•−-4Cz•−) is subjected to the toggle between intersystem-crossing toward excited triplet states and charge recombination toward ground states. Relative importance of the two pathways can be tuned by micro-environmental polarity, which endows PBI-4Cz remarkable performances of singlet-oxygen generation (>90.0%) in toluene and photothermal conversion (∼28.6%) in DMSO. Harnessing intrinsic photostability and excited-state processes of heavy-atom-free PBI derivatives not only holds a promise for multifunctional phototheranostics, but also provides a prototype for designing high-performance PSs with tunable photoconversion pathways.

Strategic incorporation of a meta dimethylamino (-NMe2 ) group on the conformationally locked green fluorescent protein (GFP) model chromophore (m-NMe2 -LpHBDI) has drastically altered molecular electronic properties, counterintuitively enhancing fluorescence of only the neutral and cationic chromophores in aqueous solution. A ~200-fold decrease in fluorescence quantum yield of m-NMe2 -LpHBDI in alcohols (e.g., MeOH, EtOH, and 2-PrOH) supports this GFP-derived compound as a fluorescence turn-on water sensor, with large fluorescence intensity differences between H2 O and ROH emissions in various H2 O/ROH binary mixtures. A combination of steady-state electronic spectroscopy, femtosecond transient absorption, ground-state femtosecond stimulated Raman spectroscopy (FSRS), and quantum calculations elucidates an intermolecular hydrogen-bonding chain between a solvent -OH group and the chromophore phenolic ring -NMe2 and -OH functional groups, wherein fluorescence differences arise from an extended hydrogen-bonding network beyond the first solvation shell, as opposed to fluorescence quenching via a dark twisted intramolecular charge transfer state. The absence of a meta-NMe2 group twisting coordinate upon electronic excitation was corroborated by experiments on control samples without the meta-NMe2 group or with both meta-NMe2 and para-OH groups locked in a six-membered ring. These deep mechanistic insights stemming from GFP chromophore scaffold will enable rational design of organic, compact, and environmentally friendly water sensors.

Newly designed push-pull ligands (L1 and L2) with bithiophene (bth) as a donor and phenazine (phz) or quinoxalino[2,3-b]quinoxaline (qxq) as acceptors were synthesized and also incorporated with a bipyridyl Ru(II) complex to give Ru1 and Ru2, respectively. The ultrafast photophysical dynamics of the ligand and their respective Ru(II) complexes were well-characterized using time-resolved spectroscopies and quantum chemical calculations. Photoinduced charger transfer (CT) and intersystem crossing (ISC) processes were directly observed for L1 and L2. In addition, the interplay of three different triplet excited states was directly observed in the related Ru(II) complexes. The lowest-lying triplet excited states of the ligands and their respective Ru(II) complexes were both attributed to the CT transitions from donor (bth) to acceptor (phz or qxq) and result in 3ICT (intramolecular charge transfer) and 3ILCT (intraligand charge transfer) excited states, respectively. The lifetimes of the lowest-lying triplet excited states of L1, L2, Ru1, and Ru2 were measured to be 21.3, 50.4, 2.75, and 4.16 μs, respectively.

Femtosecond time-resolved absorption measurements were carried out for the dark and signaling states of a BLUF (Blue Light Using FAD) protein, PixD, from the cyanobacterium Synechocystis. When the dark state was excited, FAD semiquinone radical (FADH•) was produced from the S1 state, and FADH• led to the signaling state. On the other hand, photoexcitation of the signaling state generated FADH• and FAD anion radical (FAD•-), and they decayed back to the original signaling state. In both cases, FADH• was formed and decayed with a proton-coupled electron transfer (PCET) via the hydrogen-bond network that involves FAD, Gln50, and Tyr8, and hence the kinetics of FADH• directly reflects the hydrogen-bond structure in the FAD-binding sites. It was found that the formation rate of FADH• was significantly different between the dark and signaling states, whereas the decay rate was the same. This indicates that the hydrogen-bond network of FAD-Gln50-Tyr8 in the dark and signaling states is initially different but it becomes indistinguishable after FADH• is formed, implying that the FAD-Gln50-Tyr8 hydrogen-bond network is rearranged during the PCET to generate FADH•. The present results best agree with the model in which the Gln tautomerizes without rotation in the signaling-state formation.

Substituted fluorene structures have demonstrated unusual photochemical properties. Previous reports on the substituted fluorene Schiff base FR0-SB demonstrated super photobase behavior with a ΔpK(b) of ~14 upon photoexcitation. In an effort to understand the basis for this unusual behavior, we have examined the electronic structure and relaxation dynamics of the structural precursor of FR0-SB, the aldehyde FR0, in protic and aprotic solvents using time-resolved fluorescence spectroscopy and quantum chemical calculations. The calculations show three excited singlet states in relatively close energetic proximity. The spectroscopic data are consistent with relaxation dynamics from these electronic states that depend on the presence and concentration of solvent hydroxyl functionality. These results underscore the central role of solvent hydrogen bonding to the FR0 aldehyde oxygen in mediating the relaxation dynamics within this molecule.

A theoretical study was carried out on the coumarin derivatives substituted at positions 3 and 6 by means of the two quantum methods: DFT and time-dependent density functional theory (TDDFT). The compound 3-fluorophenyl 2-oxo-2H-1-benzopyran-3-carboxylate is specific. The mesomeric donor effect of fluorine creates a strong delocalization of charge generating a twisted structure with a very high dipole moment. This compound show fluorescence at 446 nm with Stokes shift of 90 nm in ethanol. The presence of two transitions of the σ–σ* type in this compound leads to the breakdown of the carbon–oxygen bond and generates the phenolate ion, a fluorescence quenching agent. Whatever the nature and position of the substituent, all the excited states of the 3,6-substituted derivatives have a high value of the dipole moment and HOMO/LUMO electronic transitions of charge transfer nature.

A preliminary study on photoinduced ligand loss from [Ru(bpy)2(py)2]2+ has been made. The photolysis of cis-[Ru(bpy)2(py)2]2+ in the presence of trifluoroacetic acid, using broad band, white light excitation, leads to a shift in λmax from 458 nm to 472 nm, consistent with a substitution reaction of the pyridine by THF. A flash photolysis study was also performed to investigate the excited state changes of [Ru(bpy)2(py)2]2+ in THF.

Nitroaromatic compounds are inherently nonfluorescent, and the subpicosecond lifetimes of the singlet excited states of many small nitrated polycyclic aromatic hydrocarbons, such as nitronaphthalenes, render them unfeasible for photosensitizers and photo-oxidants, despite their immensely beneficial reduction potentials. This article reports up to a 7000-fold increase in the singlet-excited-state lifetime of 1-nitronaphthalene upon attaching an amine or an N-amide to the ring lacking the nitro group. Varying the charge-transfer (CT) character of the excited states and the medium polarity balances the decay rates along the radiative and the two nonradiative pathways and can make these nitronaphthalene derivatives fluoresce. The strong electron-donating amine suppresses intersystem crossing (ISC) but accommodates CT pathways of nonradiate deactivation. Conversely, the N-amide does not induce a pronounced CT character but slows down ISC enough to achieve relatively long lifetimes of the singlet excited state. These paradigms are key for the pursuit of electron-deficient (n-type) organic conjugates with promising optical characteristics.

Energy transfer plays a special role in photocatalysis by utilizing the potential energy of the excited state through indirect excitation, in which a photosensitizer determines the thermodynamic feasibility of the reaction. Bioinspired by the energy-transfer ability of natural product cercosporin, here we developed a green and highly efficient organic photosensitizer HiBRCP (hexaisobutyryl reduced cercosporin) through structural modification of cercosporin. After structural manipulation, its triplet energy was greatly improved, and then, it could markedly promote the efficient geometrical isomerization of alkenes from the E-isomer to the Z-isomer. Moreover, it was also effective for energy-transfer-mediated organometallic catalysis, which allowed realization of the cross-coupling of aryl bromides and carboxylic acids through efficient energy transfer from HiBRCP to nickel complexes. Thus, the study on the relationship between structural manipulation and their photophysical properties provided guidance for further modification of cercosporin, which could be applied to more meaningful and challenging energy-transfer reactions.

We designed two near-infrared (NIR) lanthanide complexes [(L)2 -Nd(NO3 )3 ] (L=TPE2 -BPY for 1, TPE-BPY for 2) by employing aggregation-induced emission (AIE)-active tetraphenylethylene (TPE) derivatives as sensitizers, which possessed matched energy to NdIII , prevented competitive deactivation under aggregation, even shifted the excitation window toward 600 nm by twisted intramolecular charge transfer. Furthermore, benefiting from the 4 f electron shielding effect and antenna effect, the enhanced excitation energies of the AIE-active sensitizers by structural rigidification transferred into the inert NdIII excited state through 3 LMCT, affording the first aggregation-induced phosphorescence enhancement (AIPE)-active discrete NIR-emitting lanthanide complexes. As 1 equipped with more AIE-active TPE than 2, L→Nd energy transfer efficiency in the former was higher than that in the latter under the same conditions. Consequently, the crystal of 1 exhibited one of the longest lifetimes (9.69 μs) among NdIII -based complexes containing C-H bonds.

The photochemical release of nitric oxide (NO) from a NO precursor is advantageous in terms of spatial, temporal, and dosage control of NO delivery to target sites. To realize full control of the quantitative NO administration from photoactivated NO precursors, it is necessary to have detailed dynamical information on the photodissociation of NO from NO precursors. We synthesized two new water-soluble Roussin's red esters (RREs), [Fe2(μ-N-acetylcysteine)2(NO)4] and [Fe2(μ-N-acetylpenicillamine)2(NO)4], which have five times longer lifetime than the well-known [Fe2(μ-cysteine)2(NO)4]. The photodissociation dynamics of NO from these RREs in water were investigated over a broad time range from 0.3 ps to 10 μs after excitation at 310 and 400 nm using femtosecond time-resolved infrared (IR) spectroscopy. When these RREs are excited, they either release one NO, producing a radical species deficient in one NO (R), [Fe2(μ-RS)2(NO)3], or relax into the ground state without photodeligation via an electronically excited intermediate state (M). R appears immediately after photoexcitation, suggesting that one NO is photodissociated faster than 0.3 ps. A certain fraction of R undergoes geminate recombination (GR) with NO with a time constant of 7-9 ps, while the remaining R competitively binds to the solvent. Solvent-bound R eventually bimolecularly recombines with NO with a rate constant of (1.3-1.6) × 108 M-1 s-1. For a given RRE molecule, the fractional yield of M (0.62-0.76) depends on the excitation wavelength (λex); however, the relaxation time of M (6 ± 1 ns) is independent of λex. Although the primary quantum yield of NO photodissociation (Φ1) was found to be 0.24-0.38, the final yield of NO suitable for other reactions (Φ2) was reduced to 0.14-0.29 due to the picosecond GR of the dissociated NO with R. Detailed photoexcitation dynamics of RRE can be utilized in the quantitative control of NO administration at a specific site and time, promoting pin-point usage of NO in chemistry and biology. We demonstrate that femtosecond IR spectroscopy combined with quantum chemical calculations is a powerful method for obtaining detailed dynamic information on photoactivated NO precursors such as Φ1 and Φ2, the GR yield, and secondary reactions of the nascent photoproducts, which are essential information for the design of efficient photoactivated NO precursors and their quantitative utilization.

In the past decades, tremendous efforts have been invested into organic molecules involved in the excited-state intramolecular proton transfer (ESIPT) reaction due to their enormously Stokes-shifted fluorescence and distinctive photophysical properties. The alterations of the environmental medium can effectively adjust the luminous performance of ESIPT molecules, which inspires us to unravel the solvent effect on the ESIPT mechanism. Here, we report the solvent-dependent excited-state properties of two new seven-membered ring pyrrole-indole ESIPT molecules, g-PPDBI and e-PPDBI, by steady-state spectra, picosecond transient fluorescence spectra, femtosecond transient absorption spectra, and theoretical calculations. The bathochromic-shifted normal fluorescence and the negligibly shifted tautomer fluorescence suggest the occurrence of an excited-state intramolecular proton-coupled charge transfer reaction. Thus, the solvent effect plays a vital role in stabilizing the intramolecular charge transferred state, resulting in a higher ESIPT reaction barrier in more polar solvents. Additionally, the observation of the slight dynamic difference between PPDBIs with different π-conjugation positions provides a new strategy to adjust the performance of ESIPT molecules.

Furocoumarins are the useful derivatives of coumarin, and they act as skin photosensitizing material due to their exciting exciplex states. Dendrimers also form exciplexes when they contain coumarin as core and used for light harvesting. A number of ingredients in human nutrition contain coumarins. The apiaceae family is the most common example of this class that contains carrots, celery and parsley etc. Rutaceae family is also an important food source that contains coumarins and provides citrus food. Besides fruits and vegetables, beverages like coffee, wine, black and green tea also incorporates coumarin derivatives. Few coumarin dyes show absorption and one emission peak but it can show a double amplified spontaneous emission band due to the generation of super exciplexes in coumarins. Exciplex formation of different coumarin derivatives including 7-methoxy coumarin, 7-hydroxy coumarin show wide application in synthesis of various other derivatives and they can be used as dye due to their prominent fluorescence properties. Exciplexes and excimers of coumarin are widely used in lasers. Both singlet and triplet state exciplexes are considered. Thus a comprehensive review of excited state complexes of coumarin derivatives is discussed here with emphases on the interaction and overlap of the energy levels.

In this study, we investigated the solvent effects on the photophysical properties of α,ω-di(4-pyridyl)polyenes 1–5 having 1–5 double bonds. The solution photoproperties depend strongly on conjugation chain length. The absorption maximum (λa) of dipyridylethylene 1 is observed at around 290 nm and only slightly redshifts as the solvent polarity increases, whereas its fluorescence maximum (λf) redshifts from 368 nm in hexane to 403 nm in acetonitrile. Although 1 is a centrosymmetric molecule, its fluorescence energy linearly correlates with the Onsager solvent polarity function f(e) − f(n2) = (e − 1)/(2e + 1)−(n2 − 1)/(2n2 + 1), indicating that the emission originates from an intramolecular charge transfer (CT) excited state. Exceptionally, λf in methanol is largely blue-shifted to 341 nm from those in other aprotic solvents. The fluorescence solvatochromism of longer polyenes 2–5 is much less significant than that of 1. Upon protonation and N-alkylation, both the absorption and fluorescence spectra of all five compounds are red-shifted in methanol. The largest shifts in λa and λf on protonation are observed for pentaene 5 (73 nm) and diene 2 (57 nm), respectively.

Photoinduced charge transfer and separation events in a newly synthesized azobenzene‐bridged perylenediimide‐dimer (PDI‐dimer) are demonstrated. Trans‐to‐cis conversion (∼50 % efficiency) from the initial trans PDI‐dimer by 355 nm pulsed laser light, and its reversal, cis‐to‐trans, process by 435 nm laser light irradiation has been possible to accomplish. Efficient fluorescence quenching in the PDI‐dimer, more so for the cis isomer was witnessed, and such quenching increased with increasing solvent polarity. DFT‐calculated geometry and electronic structures helped in visualizing the charge transfer in the PDI‐dimer in both isomeric forms, and also revealed certain degree of participation of the azobenzene entity in the charge transfer events. Femtosecond transient absorption spectral studies confirmed occurrence of both charge transfer followed by charge separation in the studied PDI‐dimer in both trans and cis forms in polar solvents, and the evaluated time constants from Global target analysis revealed accelerated events in the cis PDI‐dimer due to proximity effects. The present study offers key insights on the role of the azobenzene bridge, and the dimer geometry in governing the excited state charge transfer and separation in symmetrically linked PDI dimer., Excited state charge transfer and charge separation events in a newly synthesized azobenzene bridged PDI‐dimer are demonstrated. Photoconversion of trans to cis‐isomer accelerated the photoevents to an appreciable level.

The fluorescent turn-on detection of metal ions is highly desirable for public health and environmental security. Herein, we report a rationally designed fluorescent probe (1) for the detection of Cu2+ synthesized by integrating 2-hydrazinylbenzothiazole with 3-acetyl-7-hydroxycoumarin. The probe alone is non-fluorescent due to the isomerization of C=N in the excited state. The addition of Cu2+ can cause a delayed fluorescence enhancement. A comparative study of 1 and its analogues indicated that the turn-on fluorescence response was thanks to the sulfur atom coordinating to Cu2+. The response delay of 1 in sensing Cu2+ was ascribed to the gradual transition from N-coordination to S-coordination (N and S in thiazole moiety). The proposed new function of S-donor would provide a new approach for the turn-on fluorescence sensation of Cu2+.

Bioluminescence of a number of marine organisms is conditioned by Ca2+-regulated photoprotein (CaRP) with coelenterazine as the reaction substrate. The reaction product, coelenteramide, at the first singlet excited state (S1) is the emitter of CaRP. The S1-state coelenteramide is produced via the decomposition of coelenterazine dioxetanone. Experiments suggested that the neutral S1-coelenteramide is the primary emitter species. This supposition contradicts with theoretical calculations showing that the anionic S1-coelenteramide is a primary product of the decomposition of coelenterazine dioxetanone. In this study, applying molecular dynamic (MD) simulations and the hybrid quantum mechanics/molecular mechanics (QM/MM) method, we investigated a proton-transfer (PT) process taking place in CaRP obelin from Obelia longissima for emitter formation. Our calculations demonstrate a concerted PT process with a water molecule as a bridge between anionic S1-coelenteramide and the nearest histidine residue. The low activation barrier as well as the strong hydrogen-bond network between the proton donor and the proton acceptor suggests a fast PT process comparable with that of the lifetime of excited anionic S1-coelenteramide. The existence of the PT process eliminates the discrepancy between experimental and theoretical studies. The fast PT process at emitter formation can also take place in other CaRPs.

Investigation of temperature-dependent photoluminescent properties of potassium perylene-3,4,9,10-tetracarboxylate (K4PTC), a molecule with no internal rotational degrees of freedom, shows aggregation-induced enhanced emission at room temperature. The different excitonic emission processes are dependent of temperature, some of which quenches in an intermediate temperature range (from 50 to 150 K). The exciton excited states switching phenomenon from "dark" to "bright" states is observed and its explained using Herzberg-Teller selection rule. K4PTC is a molecule comparable to the size of its precursor, perylene-3,4,9,10-tetracarboxylic anhydride (PTCDA) and is highly soluble in water, contrary to PTCDA, which is poorly soluble in most solvents. Powder x-ray diffraction measurements corroborate a lesser degree of ordering of bulk K4PTC compared to bulk PTCDA. The green luminescent molecule could, in principle, be used as a biomarker, or in photodynamic therapy, if further studies show relatively low toxicity.

The design of robust boron acceptors plays a key role in the development of boron-based thermally activated delayed fluorescence (TADF) emitters for the realization of efficient and stable blue organic light-emitting diodes (OLEDs). Herein, we report a set of donor (D)-acceptor (A)-type blue TADF compounds (1-3) comprising triply bridged triarylboryl acceptors, the so-called B-heterotriangulenes, which differ depending on the identity of one of the bridging groups: methylene (1), dimethylmethylene (2), or oxo (3). The X-ray crystal structures of 2 and 3 reveal a highly twisted D-A connectivity and a completely planar geometry for the B-heterotriangulene rings. All compounds exhibit blue emissions with the unitary photoluminescence quantum yields and small singlet-triplet energy splitting (

Complexes with ligand-to-metal charge-transfer (LMCT) excited states involving d0 metals represent a new design for photocatalysts. Herein, the photochemistry and photophysics of d0 titanocenes of the type Cp2Ti(C2R)2, where C2R = ethynylphenyl (C2Ph), 4-ethynyldimethylaniline (C2DMA), or 4-ethynyltriphenylamine (C2TPA), have been investigated. Cp2Ti(C2Ph)2 and Cp2Ti(C2DMA)2 have also been characterized by single-crystal X-ray diffraction. The two aryl rings in Cp2Ti(C2DMA)2 are nearly face-to-face in the solid state, whereas they are mutually perpendicular for Cp2Ti(C2Ph)2. All three complexes are brightly emissive at 77 K but photodecompose at room temperature when irradiated into their lowest-energy absorption band. The emission wavelengths and photodecomposition quantum yields are as follows: Cp2Ti(C2Ph)2, 575 nm and 0.65; Cp2Ti(C2TPA)2, 642 nm and 0.42; Cp2Ti(C2DMA)2, 672 nm and 0.25. Extensive benchmarking of the density functional theory (DFT) model against the structural data and of the time-dependent DFT (TDDFT) model against the absorption and emission data was performed using combinations of 13 different functionals and 4 basis sets. The model that predicted the absorption and emission data with the greatest fidelity utilized MN15/LANL2DZ for both the DFT optimization and the TDDFT. Computational analysis shows that absorption involves a transition to a 1LMCT state. Whereas the spectroscopic data for Cp2Ti(C2TPA)2 and Cp2Ti(C2DMA)2 are well modeled using the optimized structure of these complexes, Cp2Ti(C2Ph)2 required averaging of the spectra from multiple rotamers involving rotation of the Ph rings. Consistent with this finding, an energy scan of all rotamers showed a very flat energetic surface, with less than 1.3 kcal/mol separating the minimum and maximum. The computational data suggest that emission occurs from a 3LMCT state. Optimization of the 3LMCT state demonstrates compression of the C-Ti-C bond angle, consistent with the known products of photodecomposition.

The synthesis, characterization and photoluminescent properties of four cyclometalated (C N)-type gold(III) complexes bearing a bidentate diacetylide ligand, tolan-2,2'-diacetylide (tda), are reported. The complexes exhibit highly tunable excited state properties and show photoluminescence (PL) across the entire visible spectrum from sky-blue (λPL =493 nm) to red (λPL =675 nm) with absolute PL quantum yields (PLQY) of up to 75 % in solution, the highest PLQY found for any monocyclometalated Au(III) complex in solution. As a consequence of the use of the strongly rigidifying diacetylide bidentate ligand, a significant increase in the excited state lifetimes (τ0 =16-258 μs) was found in solution and in thin films. The complexes showed remarkable singlet oxygen generation in aerated solution with absolute singlet oxygen quantum yield (ϕ1Δ ) values reaching up to 7.5×10-5 and singlet oxygen lifetimes (τ01Δ ) in the range of 66-95 μs. Furthermore, the radiative and non-radiative rates of singlet oxygen were determined using the ϕ1Δ and τ01Δ values and correlations are drawn between the formation of singlet oxygen and its interaction with cyclometalated (C N) gold(III) complexes.

Panchromatic absorbers have potential applications in molecular-based energy-conversion schemes. A prior porphyrin-perylene dyad (P-PMI, where "MI" denotes monoimide) coupled via an ethyne linker exhibits panchromatic absorption (350-700 nm) and a tetrapyrrole-like lowest singlet excited state with a relatively long singlet excited-state lifetime (τS) and increased fluorescence quantum yield (Φf) versus the parent porphyrin. To explore the extension of panchromaticity to longer wavelengths, three arrays have been synthesized: a chlorin-terrylene dyad (C-TMI), a bacteriochlorin-terrylene dyad (B-TMI), and a perylene-porphyrin-terrylene triad (PMI-P-TMI), where the terrylene, a π-extended homologue of perylene, is attached via an ethyne linker. Characterization of the spectra (absorption and fluorescence), excited-state properties (lifetime, yields, and rate constants of decay pathways), and molecular-orbital characteristics reveals unexpected subtleties. The wavelength of the red-region absorption band increases in the order C-TMI (705 nm) < PMI-P-TMI (749 nm) < B-TMI (774 nm), yet each array exhibits diminished Φf and shortened τS values. The PMI-P-TMI triad in toluene exhibits Φf = 0.038 and τS = 139 ps versus the all-perylene triad (PMI-P-PMI) for which Φf = 0.26 and τS = 2000 ps. The results highlight design constraints for auxiliary pigments with tetrapyrroles to achieve panchromatic absorption with retention of viable excited-state properties.

The excited state proton transfer (ESPT) reaction from the photoacid 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS or pyranine) to an acetate molecule has been investigated in explicit aqueous solution via excited state ab initio molecular dynamics simulations based on hybrid quantum/molecular mechanics (QM/MM) potentials. In all the trajectories, the direct proton transfer has been observed in the excited state within 1 ps. We find that the initial structural configuration extracted from the ground state distribution strongly affects the ESPT kinetics. Indeed, the relative orientation of the proton donor-acceptor pair and the presence of a water molecule hydrogen bonded to the phenolic acid group of the pyranine are the key factors to facilitate the ESPT. Furthermore, we analyze the vibrational fingerprints of the ESPT reaction, reproducing the blue shift of the acetate CO stretching (COac), from 1666 to 1763 cm-1 testifying the transformation of acetate to acetic acid. Finally, our findings suggest that the acetate CC stretching (CCac) is also sensitive to the progress of the ESPT reaction. The CCac stretching is indeed ruled by the two vibrational modes (928 and 1426 cm-1), that in the excited state are alternately activated when the proton is shared or bound to the donor/acceptor, respectively.

Herein, we present a new strategy for the development of efficient heavy-atom free singlet oxygen photosensitizers based on rigid borafluorene scaffolds. Physicochemical properties of borafluorene complexes can be easily tuned through the choice of ligand, thus allowing exploration of numerous organoboron structures as potent 1O2 sensitizers. The singlet oxygen generation quantum yields of studied complexes vary in the range of 0.55-0.78. Theoretical calculations reveal that the introduction of the borafluorene moiety is crucial for the stabilization of a singlet charge transfer state, while intersystem crossing to a local triplet state is facilitated by orthogonal donor-acceptor molecular architecture. Our study shows that quantitative oxidation of selected organic substrates can be achieved in 20-120 min of irradiation with only 0.05 mol % loading of a photocatalyst.

Irradiation of dilute mixtures of NH3 and O2 dispersed in solid Ne at 3.6 K with light of wavelengths 200, 192.5, 173, and 143 nm from a synchrotron initiated photochemical dissociation of those precursors and the production of N, NH, NH2, O, O3, HO2, NO, NO2, N2O, H2O, HONO in both s-cisoid and s-transoid rotational isomers, and HONO2, detected according to their spectra in mid-infrared absorption from 450 to 4000 cm−1 and in absorption and emission from 200 to 1100 nm.