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This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry Letters, copyright © 2021 American Chemical Society after peer review and technical editing by the publisher. We demonstrate that spin-vibronic coupling is the most significant mechanism in vibrational coherence transfer (VCT) from the singlet (S1) to the triplet (T1) state of the [Pt2(P2O5H2)4]4– complex. Our time-dependent correlation function-based study shows that the rate of intersystem crossing (kISC) through direct spin–orbit coupling is negligibly small, making VCT vanishingly small due to the ultrashort decoherence time (2.5 ps). However, the inclusion of the spin-vibronic contribution to the net kISC in selective normal modes along the Pt–Pt axis increases the kISC to such an extent that VCT becomes feasible. Our results suggest that kISC for the S1 →T2 (τISC = 1.084 ps) is much faster than the S1 → T1 (τISC = 763.4 ps) and S1 → T3 (τISC = 13.38 ps) in CH3CN solvent, indicating that VCT is possible from the low-lying excited singlet (S1) to the triplet (T1) state through the intermediate T2 state. This is the first example where VCT occurs solely due to spin-vibronic interactions. This finding can pave the way for new types of photocatalysis.

DFT calculations on the photoisomerization of hydrazones of 1,2,4-oxadiazole derivatives to 1,2,5-triazoles have been performed showing that the reaction occurred through the first excited singlet state. The Z isomer gave the reaction through a hydrogen atom transfer of the hydrazonic nitrogen atom to the nitrogen atom in four position on the oxadiazole ring. In this case, the isomerization was a concerted reaction. The E isomer could undergo the same reaction. However, it could not be a concerted reaction but required the presence of a ring opening intermediate.

Reactions of electronically excited singlet oxygen (1O2) with the radical cations of guanine (9HG•+), 9-methylguanine (9MG•+), 2′-deoxyguanosine (dGuo•+), and guanosine (Guo•+) were studied in the ...

The permanent dipole moments of 2-cyanoindole (cyanoindole = CNI) in its ground and lowest excited singlet states have been determined from rotationally resolved electronic Stark spectroscopy under jet-cooled conditions. From the orientation of the transition dipole moment and the geometry changes upon electronic excitation the lowest excited singlet state could be shown to be of Lb-symmetry. The general statement, that the La-state has the larger permanent dipole moment of the two lowest excited singlet states, will be challenged in this contribution. On the basis of the different electronic nature of the first excited singlet state the behavior of 2-, 3-, 4- and 5-CNI is discussed. The excited state lifetime of isolated 2-CNI in the gas phase has been determined to be 9.4 ns. This value is compared to the excited state lifetime in ethyl acetate solution of 2.6 ns, which was quantified with a Strickler–Berg analysis. Using water as solvent shortens the 2-CNI lifetime to

Manipulating the dynamics of dark excited states (DESs), such as higher excited singlet or excited triplet states with no or small radiative decay, are of both fundamental and practical interests, an important application being photoactivated diagnosis and therapy (phototheranostics), which include photoacoustic (PA) imaging, photodynamic therapy (PDT), and photothermal therapy (PTT). However, the current understanding of DESs in organic structures is rather limited, thus making any rational manipulation of DES in organic materials very challenging.A DES decays primarily by radiationless transition through two pathways: (i) singlet-to-triplet intersystem crossing (ISC) and (ii) internal conversion (IC) relaxation. The deactivation of a DES via ISC can generate cytotoxic reactive oxygen species (ROS) for PDT, while IC could convert photons into heat for PA imaging and PTT. In this Account, we highlight our research on developing a fundamental understanding of structure-property relationships for manipulation of DESs in organic materials in relation to phototheranostic applications. We describe the application of femtosecond transient absorption (fs-TA) spectroscopy for obtaining valuable insights into the DES dynamics. Afterward, we present our work on DESs in nonrigid molecules that revealed greatly enhanced ISC through geometry twisting, which leads to an innovative pathway to develop organic materials exhibiting external stimuli-responsive reversible switching of ISC. We introduce the concept of smart PDT where highly efficient ISC imparted by geometry twisting in the acidic environment specific to tumors leads to very efficient and highly localized PDT, thus leaving surrounding healthy tissues at a different pH unaffected. This insightful understanding of ISC can lead to the development of more advanced photosensitizers for PDT. Two other emergent concepts from our work presented here are (1) significantly enhanced IC producing strong local heating by combining two-photon absorption with excited state absorption for cumulative multiphoton absorption, thus greatly increasing the strength of the PA signal for nonlinear PA imaging, and (2) shown by an example of an organic molecule, BODIPY, nanoscale charge-transfer state mediated strong IC in aggregate nanoparticles resulting in exceptionally high photothermal conversion efficiency of 61% for both PA and PTT. Some

The development of smart materials, in particular those exhibiting highly sensitive mechanochromic luminescence (MCL) is desirable, but challenging since the MCL internal mechanism and the structure–performance relationship still remain unclear. Herein, we report a new MCL material, BN benzo[f]tetraphene, synthesized using a molecular-level design strategy by introducing a BN unit to a π-conjugated system. By investigating BN benzo[f]tetraphene (5) and its analogue, it was found that the introduction of the boron-nitrogen unit is the key to tailoring the molecular dipole moment and intermolecular interactions, which can therefore form an easily deformable molecular stacking pattern and endow 5 with wonderful MCL properties. Theoretical calculations confirmed that inherent energies like excited singlet (S) and highly sensitive triplet (T) states exist in the MCL process, and the formation and fracture of ordered molecular aggregates have significant effect on radiative and nonradiative transitions. The material also shows high-contrast and self-reversible properties related to thermal-and force-stimulus, which makes it a promising candidate for security ink, optical recording applications. This work possibly opens up a new way to develop efficient organic smart materials, and therefore, trigger the discovery of new functions and properties of azaborine compounds.

Vibronic couplings between the two lowest excited singlet states of the neurotransmitter serotonin are studied both experimentally and theoretically. Using rotationally resolved electronic spectroscopy, we observe a mode-dependent rotation of the S 1 transition dipole moment (TDM). This is rationalized as a coupling of the S 1 ( 1 L b ) to the S 2 ( 1 L a ) state, lying about 3300 cm − 1 higher in energy.

[Image: see text] Singlet fission (SF) involves the conversion of one excited singlet state into two lower excited triplet states and has received considerable renewed attention over the past decade. This Perspective highlights recent developments and emerging concepts of SF in solid-state crystalline materials. Recent experiments showed the crucial role of vibrational modes in speeding up SF, and theoretical modeling has started to define an optimal energetic landscape and intermolecular orientation of chromophores for highly efficient singlet fission. A critical analysis of these developments leads to directions for future research to eventually find singlet fission chromophores with excellent optoelectronic properties.

The photoreaction of 2,3,4',5-tetrahydroxystilbene-2-O-β-d-glucoside (TSG) has been investigated. Water-assisted/water-accelerated photodimerization of trans-TSG favored the formation of syn-head-to-tail [2 + 2] photocyclobutane under 365 nm irradiation as a result of hydrophobic association and a fluorescent solute-solute aggregate from their excited singlet states. In contrast, irradiation with 254 nm led to [2 + 2] photocycloreversion. The two cyclobutane dimers were first obtained through straightforward photoreaction and identified as multiflorumiside A and multiflorumiside C through the detailed analysis of high-resolution electrospray ionization mass spectrometry and one- and two-dimensional nuclear magnetic resonance. Therefore, trans-TSG should be protected from light and water.

This work provides a comprehensive DFT study on the conversion mechanism and photoisomerization of the effective and commonly used nonsteroidal anti-inflammatory medicine fenoprofen. The results obtained exhibit that the rearrangement procedure of (R)-fenoprofen to its (S)-enantiomer occurs in [1,3]-hydrogen shifts with inversion of configuration at chiral center C8. According to the computed energies, we can find out that the excitation of (R)-fenoprofen methyl ester in its primary form to the first excited singlet state S1 at λ = 240 nm is the rate-limiting step of photoisomerization of fenoprofen. In order to obtain further insight into the isomerization process occurring upon excitation, this process was studied by scanning the C8-H11 distance for the excited singlet. Our calculations revealed that the isomerization process requires passing a barrier of approximately 84 kcal/mol. Furthermore, the more photostability of (S)-fenoprofen methyl ester than that of its related (R)-enantiomer can be attributed to the − 1.34 kcal/mol of thermodynamic stability of the (S)-fenoprofen methyl ester.

The complex Ag(I)(flavonolate)(PPh3)2 has been prepared and characterized including the crystal structure. This compound shows in degassed solution two emission bands, a fluorescence at λ max=505 nm and a phosphorescence at λ max=780 nm. Both emissions originate from lowest-energy flavonolate intraligand excited singlet and triplet states, respectively. In the presence of oxygen, the phosphorescence disappears and the complex becomes light-sensitive. The flavonolate ligand undergoes a photooxidation by oxygen.

Organic electro-optic (EO) polymers have attracted much attention to their potential use of the optical interconnection for faster data communication. Photochemical stability is a crucial problem to be solved for using in commercial systems. We investigated the photochemical stability of the EO polymers under irradiation of a laser at the O-band (1310 nm) to reveal the factors of photodegradation and to obtain a good estimate for the operating lifetime of the devices. Based on the results obtained, we will discuss the effects of excited singlet oxygen on the photochemical stability of the EO polymers.

The question investigated in this study is which functional/s of the twelve tested density functionals in combination with PCM-LR can reproduce the absorption and emission spectra of DFTP dye in three solvents with different polarity. DFTP is an abbreviation of 1-(2,5-dimethylfuran-3-yl)-3-(2,4,5-trimethoxyphenyl)prop-2-en-1-one, and it is with D-π-A structure. The twelve functionals range from standard pure to hybrid and long-range corrected functionals. The simulation of the experimental data included the ability to reproduce the maximum wavelength/transition energy of the absorption and emission spectra as well as the Stokes shift in CCL4, ethanol and DMSO solvents. The geometrical parameters and dipole moments of the ground and first excited singlet states were compared and discussed too. The PBE0 functional appears as the best one in reproducing the absorption, emission and Stokes shifts.

1-Hydroxy-2-acetonaphthone, compound 1b is known to undergo excited-state intramolecular proton transfer (ESIPT) in the lowest excited singlet state in solution, giving weak fluorescence with a large Stokes-shift. We converted compound 1b to the corresponding difluoroboronated (BF2) complex 2b, and investigated the photophysical features in solution and the solid state along with those of the related compound 1d. The other compounds 1a, 1c and 1d were unsuccessful in forming the corresponding BF2 complexes. Blue fluorescence from compounds 1a, 1b and 1d and yellow one from compound 2b in the solid state were recorded. Compounds 1a, 1b and 1d moderately fluoresced with quantum yields of ca. 0.2 in both solution and solid state. Compound 2b showed fluorescence in the solid state with a quantum yield of 0.18 and in solution with a quantum yield of 0.65. The present study introduces potentials of ESIPT molecules, the related compounds and the corresponding difluoroboronated complexes to solid-state fluorophores.

sEDA, pEDA, and cSAR descriptors of the substituent effect were determined for >30 monosubstituted benzenes in the first excited singlet S1 state at the LC-ωB97XD/aug-cc-pVTZ level. It was found that in the S1 state, the σ- and π-valence electrons are a bit less and a bit more affected, respectively, than in the S0 state, but basically, the effect in both states remains the same. In the S0 and S1 states, the d(C–X) distances to the substituent’s first atom and the ring perimeter correlate with the sEDA and pEDA in the appropriate states, respectively. The energies and the gap of the frontier orbitals in the two states are linearly correlated and for the HOMO(S1), LUMO(S1), and HOMO(S1)–LUMO(S1) gap correlate also with the pEDA(S1) and cSAR(S1) descriptors. In all studied correlations, three similar groups of substituents can be distinguished, for which correlations (i) are very good, (ii) deviate slightly, and (iii) deviate significantly. Comparison of the shape of the HOMO(S0) and HOMO(S1) orbitals shows...

Recently, highly emissive neutral copper halide complexes have received much attention. Here, a series of four-coordinate mononuclear Cu(i) halide complexes, [CuX(dpqu)(dpna)] (dpqu = 8-(diphenylphosphino)quinoline, dpna = 1-(diphenylphosphino)naphthalene, X = I (1), Br (2) and Cl (3)), were synthesized, and their molecular structures and photophysical properties were investigated. These complexes exhibit near-saturated red emission in the solid state at room temperature and have peak emission wavelengths at 669-691 nm with microsecond lifetimes (τ = 0.46-1.80 μs). Small S1-T1 energy gaps in the solid state indicate that the emission occurs from a thermally activated excited singlet state at ambient temperature. The emission of the complexes 1-3 mainly originates from MLCT transition. The solution-processed devices of complex 1 exhibit stable red emission with a CIE(x, y) of (0.62, 0.38) for a doped device and (0.63, 0.37) for a non-doped device.

Summary The concept of excited-state aromaticity is receiving much attention in that completely reversed aromaticity in the excited state (so-called aromaticity reversal) provides crucial insight into photostability, photoreactivity, and its application to the photosynthetic mechanism and photoactive materials. Despite this significance, experimental elucidation of excited-state aromaticity is still unsolved, particularly for the excited singlet state. Here, as an unconventional approach, time-resolved IR (TRIR) spectroscopy on aromatic and anti-aromatic hexaphyrin congeners shed light on excited-singlet-state aromaticity. The contrasting spectral features between the Fourier transform IR and TRIR spectra reveal the aromaticity-driven structural changes, corroborating aromaticity reversal in the excited singlet states. Our paradigm for excited-state aromaticity, the correlation of IR spectral features with aromaticity reversal, provides another fundamental key to understanding the role of (anti)aromaticity in the stability, dynamics, and reactivity in the excited singlet state of π-conjugated molecular systems.

Two kinds of enediyne-cored polyphenylene dendrimer (mGn, pGn) were prepared as pure cis and trans isomers. In comparison to parent enediyne with no dendron substituents, the dendrimers exhibited highly efficient fluorescence emission with mutual cis–trans isomerization. Along with an increase in the generation of dendrons, decrease of Φiso and shortening of τS were observed, suggesting that dendrons affect cis–trans photoisomerization and deactivation from excited singlet state.

Kinetics of singlet fission in organic semiconductors, in which the excited singlet state (S_1) spontaneously splits into a pair of triplet (T) excitons, is known to be strongly influenced by back geminate annihilation of TT-pairs. We show that this influence can be properly described only by taking into account the diffusive exciton migration. The migration effect is treated in the model of two kinetically coupled states: the intermediate state of interacting TT-pairs and the state of migrating excitons. Within this model the singlet fission (including magnetic field effects) is studied as applied to the fluorescence decay kinetics (FDK) I_{S_1}(t) for S_1-state. The analysis shows that migration strongly affects the FDK resulting, in particular, in the universal long-time dependence I_{S_1}(t) \sim t^{-3/2}. The model accurately describes the FDK, recently observed for a number of systems. Possible applications of the considered model to the analysis of mechanisms of migration, using experimentally measured FDK, are briefly discussed., 7 pages, 2 figures, misprint in Eq.(1) and some misprints in the text of the paper are corrected

To obtain even qualitatively correct results of potential energy surfaces (PESs) of the ground and two lowest-lying excited singlet states of C2 is a difficult task due to the strong geometry-dependent closeness of these PESs leading to real and avoided crossings. Our IVO-SSMRPT method in which the nondynamical correlation is simplified with IVO-CASCI has been applied to these states. IVO-SSMRPT provides comparable results with reference theoretical and experimental data indicating all components of the approach work in harmony for a correct representation of the surfaces including the locations of the challenging crossing points between the states.

We report a new approach to search for structures of minimum energy conical intersection (MECIs) automatically. Gradient projection (GP) method and single component artificial force induced reaction (SC-AFIR) method were combined in the present approach. As case studies, MECIs of benzene and naphthalene between their ground and first excited singlet electronic states (S 0 /S 1 -MECIs) were explored. All S 0 /S 1 -MECIs reported previously were obtained automatically. Furthermore, the number of force calculations was reduced compared to the one required in the previous search. Improved convergence in a step in which various geometrical displacements are induced by SC-AFIR would contribute to the cost reduction.

A new cadmium/6-aminonicotinate-based coordination polymer (CP) with an unprecedented multicolored and long-lasting emission is reported. This material shows a blue fluorescence which rapidly turns to green persistent phosphorescence with a lifetime of nearly 1 s. Time-dependent density functional theory calculations revealed that electronic transitions arising from both first excited singlet and triplet states involving ligand-centered and ligand-to-metal charge-transfer mechanisms are responsible for such behavior.

A series of cyclo-meta-phenylene congeners with a variation of interphenylene bridges was synthesized by adopting concise synthetic routes to investigate the structure–fluorescence relationships of...

Coumarin C-2 was reported (Signore et al., J. Am. Chem. Soc., 2010, 132, 1276 and Brancato et al., J. Phys. Chem. B, 2015, 119, 6144) to break Kasha’s rule. However, the two lowest excited singlet ...

According to Hund’s rule, the lowest triplet state (T1) is lower in energy than the lowest excited singlet state (S1) in closed-shell molecules. The exchange integral lowers the energy of the tripl...

When a molecular electron donor interacts with multiple electron acceptors, quantum coherence can enhance the electron transfer (ET) rate. Here we report photodriven ET rates in a pair of donor–acceptor (D-A) compounds that link one anthracene (An) donor to one or two equivalent 1,4-benzoquinone (BQ) acceptors. Subpicosecond ET from the lowest excited singlet state of An to two BQs is about 2.4 times faster than ET to one BQ at room temperature, but about 5 times faster at cryogenic temperatures. This factor of 2 increase results from a transition from ET to one of two acceptors at room temperature to ET to a superposition state of the two acceptors with correlated system–bath fluctuations at low temperature.

The rotationally resolved electronic spectrum of 4-cyanoindole and some N-D and C-D deuterated isotopologues has been measured and analyzed. Dipole moments in the ground and electronically excited state have been determined, using electronic Stark spectroscopy. From the geometry changes upon excitation, orientation of the transition dipole moment, and the values for the permanent dipole moments, the lowest excited singlet state could be shown to be of La symmetry. The excited state lifetime of isolated 4-cyanoindole has been determined to be 11 ns, while for the ringdeuterated isotopologues lifetimes between 5 and 6 ns have been found. The different behavior of 3-, 4-, and 5-cyanoindole is discussed on the basis of the different electronic nature of the electronically excited singlet states.

Time-resolved fluorescence of [Au(CN)2−]3 in water reveals a coherent vibration of 74 cm-1. A massive non-Condon effect, proved unambiguously by time-resolved fluorescence spectra, demonstrates that the wave packet motion arises from the coherent bond formation.

Dissociation of a Zn3O3 ring has been theoretically studied in the present work. For this, the complete active space self-consistent field (CASSCF) method with many-body perturbation simulation has been performed. Scan between two atoms from 1.4 A to 2.5 A was taken for analyzing active and CASSCF energies with total system energy. This work reveals the theoretical behavior in an absorption corresponding to the first excited singlet and triplet states concerning the dissociation of the ring. The excited states reporting the changes and the change in the gap between them as an effect of breaking the shield type ring.

The changes of the structure of 4-cyanoindole upon electronic excitation have been determined from a combined Franck-Condon and rotational constants fit. The geometry changes were fit to simultaneously reproduce the intensities of 72 lines in the fluorescence emission spectra, obtained via excitation of six different vibronic bands in the first excited singlet state, along with the three rotational constants from rotationally resolved electronic spectra.

The dynamics of ferulic acid (3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid) and caffeic acid (3-(3,4-dihydroxyphenyl)-2-propenoic acid) in acetonitrile, dioxane and water at pH 2.2 following photoexcitation to the first excited singlet state are reported. These hydroxycinnamic acids display both strong ultraviolet absorption and potent antioxidant activity, making them promising sunscreen components. Ferulic and caffeic acids have previously been shown to undergo trans-cis photoisomerization via irradiation studies, yet time-resolved measurements were unable to observe formation of the cis-isomer. In the present study, we are able to observe the formation of the cis-isomer as well as provide timescales of relaxation following initial photoexcitation.

Photochemical reactions of tris(salicylideneaniline) were explored by theoretical investigation of relevant potential energy profiles using ab initio methods. A photophysical mechanism was proposed, in which an optical excitation leads to the formation of several tautomeric species in the first excited singlet state, allowing emission over a broad range of wavelengths.

A hybrid Bethe–Salpeter/time-dependent density-functional-theory method is described that aims at improving the performance of the GW/Bethe–Salpeter-equation (GW/BSE) method in general and for excited triplet states in particular. The static screened exchange W used in the BSE is combined with the correlation kernel of the underlying density functional in a manner that retains a proven feature of the BSE, that is, the correct description of charge–transfer excitations. The performance of the new method, labeled cBSE, is assessed using G0W0 or evGW quasiparticle energies, and an improved performance is observed. The cBSE approach shows nearly equal performance for excited singlet and triplet states, rivaling coupled-cluster theory (in the CC2 approximation) in accuracy at a computational cost that is at least one order of magnitude smaller.

Single crystals of 4-(diisopropylamino)benzonitrile (DIABN) undergo an intramolecular charge transfer (ICT) reaction in the excited singlet state. At 300 K, the fluorescence consists of emissions from the locally excited (LE) and from the ICT state. Upon cooling to 5 K, the ICT fluorescence intensity gradually decreases relative to that of the LE emission and is absent below 60 K. With crystalline 4-(dimethylamino)benzonitrile (DMABN), in contrast, only LE emission is found over the entire range from 300 to 5 K. The phosphorescence spectra of the DIABN and the DMABN crystals do not present any evidence for an additional ICT emission, showing that ICT does not occur in the triplet state. An activation energy E-a of similar to 4 kJ/mol is determined for the LE -> ICT reaction of DIABN crystals, from the temperature dependence of the fluorescence decay times tau(2) and tau(1) E-a is attributed to changes in the molecular conformation of DIABN other than a full rotation of the large diisopropylamino group with respect to the benzonitrile moiety. In a comparison with crystal and solution data, literature results from transient vibrational and absorption spectra are discussed and it is concluded that they cannot be employed to favor the TICT (perpendicular twist) over the PICT (planar) model for DIABN and DMABN.

2-(4-(9,9-Dimethylacridin-10(9H)-yl)phenyl)-5-phenyl-1,3,4-oxadiazole has an energy difference between the lowest excited singlet and triplet states (ΔEST) of ca. 0.24 eV. Introduction of two electronegative fluorine atoms onto the acceptor portion of the molecule to give 2-(4-(9,9-dimethylacridin-10(9H)-yl)-3,5-difluorophenyl)-5-phenyl-1,3,4-oxadiazole lowers the energy of the singlet emission with a negligible effect on the corresponding triplet energy, leading to a donor–acceptor compound with decreased ΔEST of ca. 0.13 eV that displays thermally activated delayed fluorescence. Organic light-emitting diodes fabricated using the latter compound display high EQEmax of 21.9% at a luminance of 10 cd/m2 and sky-blue emission, however, they suffer from a large efficiency roll-off at increased luminance.

The bent bond/antiperiplanar hypothesis (BBAH) is used to propose a mechanism-based orbital model for the facial selectivity of sigmatropic hydrogen shifts under both thermal and photochemical conditions. The BBAH analysis of these concerted rearrangements invokes transient vibrationally excited singlet diradicals in both 4 n and 4 n+2 polyenes.

Experimental studies of reactions with vibrationally excited singlet oxygen have been performed using pulse laser technique. The rate constant for reaction of vibrationally excited singlet oxygen with oxygen atom was estimated to be in the range 10-12 - 10-11 cm3/s.

The influence of phenyl linkage and donor strength on the photophysical properties of new derivatives of quinoxaline-containing iminodibenzyl and iminostilbene moieties is studied. The donor-acceptor derivatives showed dual thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) despite a large energy gap between the excited singlet and triplet states (ca. 0.5 eV). This extremely rare observation is explained by the twisted and rigidified structure of the iminodibenzyl moiety.