A theoretical study of the excited‐state decay of acylhydrazones

Acylhydrazones is a novel yet underexploited class of molecular switches. In the present paper, we investigated the excited‐state decay of three model systems of acylhydrazones in the gas phase by a combination of electronic structure calculations and Tully's surface hopping dynamic simulations. Our computational results demonstrated that the S₂(n_Nπ*) state decay of the three model systems leads to both the imine‐like photo‐isomerization through the S₁(n_Nπ*)/S₀ intersection and population of the S₁(n_Oπ*) state that will cross to the triplet manifold. The position of phenyl substituent was found to have an effect on the ratio of the two S₁ states. The present theoretical work provides some understandings of the intramolecular mechanism for de‐population of the excited electronic states of acylhydrazones.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

Unveiling the Hidden,Dark, and Short Life of a Vibronic State in a Boron Difluoride Formazanate Dye

Boron difluoride (BF₂) formazanate dyes are contenders for molecular species that exhibit a large Stokes shift and bright red emission. Excitation of 3‐cyanoformazanate complexes with 10 μs wide pulses of specific wavelengths resulted in strong luminescence at 663 nm at both room temperature in solution and at 77 K in a frozen solution. Analysis of the short‐lived excitation spectrum from this luminescence shows that it arises from a vibronic manifold of a higher‐lying excited state. This dark state relaxes to the emitting state over 10 μs. TD‐DFT calculations of the two lowest‐energy excited states show that the relaxed geometries are planar for S₁ but highly distorted in S₂. The specific time‐ and wavelength‐dependence of the excitation profile provides a unique optical encryption capability through the comparison of emission intensities between adjacent vibronic bands only accessible in the 0–12 μs time domain.     

Unveiling the Hidden,Dark, and Short Life of a Vibronic State in a Boron Difluoride Formazanate Dye

Boron difluoride (BF₂) formazanate dyes are contenders for molecular species that exhibit a large Stokes shift and bright red emission. Excitation of 3‐cyanoformazanate complexes with 10 μs wide pulses of specific wavelengths resulted in strong luminescence at 663 nm at both room temperature in solution and at 77 K in a frozen solution. Analysis of the short‐lived excitation spectrum from this luminescence shows that it arises from a vibronic manifold of a higher‐lying excited state. This dark state relaxes to the emitting state over 10 μs. TD‐DFT calculations of the two lowest‐energy excited states show that the relaxed geometries are planar for S₁ but highly distorted in S₂. The specific time‐ and wavelength‐dependence of the excitation profile provides a unique optical encryption capability through the comparison of emission intensities between adjacent vibronic bands only accessible in the 0–12 μs time domain.     

Channels to Singlet and Triplet Phenylcarbenes in Phenyldiazomethane: A CASSCF and MRCI Study

Diazo compounds such as phenyldiazomethane (C₆H₅C(H)N₂) exhibit intriguing phenomena including the ultrafast formation of singlet carbene and the excited‐state rearrangement reaction (RIES). In this work, we have used multi‐reference configuration interaction with single and double excitations (MRCI‐SD) and complete active space self‐consistent field (CASSCF) methods to study the photodissociation dynamics of C₆H₅C(H)N₂. The equilibrium structures, transition states in the lowest three electronic states (S₁, T₁, and S₀), and S₁/S₀ and T₁/S₀ minimum‐energy crossing points both in the Franck–Condon region and on the pathway of the CN bond dissociation have been optimized. On the basis of the calculated S₁, T₁, and S₀ potential energy surfaces, we have uncovered the most efficient pathways to the lowest singlet and triplet phenylcarbenes (C₆H₅CH) in irradiated C₆H₅C(H)N₂.     

Luminescence properties and optimized structural conformations of the S0, S1 and T1 states of a tetranuclear formamidinate complex based on AuI and AgI metal ions

N,N′‐Bis(pyridin‐4‐yl)formamidine (4‐pyfH) was reacted with Au^I and Ag^I metal salts to form a novel tetranuclear complex, tetrakis[μ‐N,N′‐bis(pyridin‐4‐yl)formamidinato]digold(I)disilver(I), [Ag₂Au₂(C₁₁H₉N₄)₂] or [Au_xAg_4–x(4‐pyf)₄] (x = 0–4), 1 , which is supported by its metallophilicity. Due to the potential permutation of the coordinated metal ions, six different canonical structures of 1 can be obtained. Complex 1 shows an emission at 501 nm upon excitation at 375 nm in the solid state and an emission at 438 nm upon excitation at 304 nm when dispersed in methanol. Time‐dependent density functional theory (TD‐DFT) calculations confirmed that these emissions can be ascribed to metal‐to‐ligand charge transfer (MLCT) processes. Moreover, the calculations of the optimized structural conformations of the S₀ ground state, and the S₁ and T₁ excited states are discussed and suggest a distorted planar conformation for the tetranuclear Au₂Ag₂ complex.     

Multiple‐State Emissions from Neat,Single‐Component Molecular Solids: Suppression of Kasha's Rule

Three rigid and structurally simple heterocyclic stilbene derivatives, (E)‐3H,3′H‐[1,1′‐biisobenzofuranylidene]‐3,3′‐dione, (E)‐3‐(3‐oxobenzo[c] thiophen‐1(3H)‐ylidene)isobenzofuran‐1(3H)‐one, and (E)‐3H,3′H‐[1,1′‐bibenzo[c] thiophenylidene]‐3,3′‐dione, are found to fluoresce in their neat solid phases, from upper (S₂) and lowest (S₁) singlet excited states, even at room temperature in air. Photophysical studies, single‐crystal structures, and theoretical calculations indicate that large energy gaps between S₂ and S₁ states (T₂ and T₁ states) as well as an abundance of intra and intermolecular hydrogen bonds suppress internal conversions of the upper excited states in the solids and make possible the fluorescence from S₂ excited states (phosphorescence from T₂ excited states). These results, including unprecedented fluorescence quantum yields (2.3–9.6 %) from the S₂ states in the neat solids, establish a unique molecular skeleton for achieving multi‐colored emissions from upper excited states by “suppressing” Kasha's rule.     

Mechanism and Nature of the Different Viscosity Sensitivities of Hemicyanine Dyes with Various Heterocycles

A series of hemicyanine derivatives are excellent fluorescent viscosity sensors in live cells and in imaging of living tissues due to their low quantum yields in solution but large fluorescence enhancements in viscous environments. Herein, three carbazole‐based hemicyanine dyes with different heterocycles are studied. They have different background quantum yields, and hence different sensitivities to viscosity detection, large Stokes shifts, and high sensitivity. Better understanding of the structure–property relationships for viscosity sensitivity could benefit the design of improved dyes. Computational studies on these dyes reveal the mechanism of viscosity sensitivity of fluorescent molecular rotors and the nature of the difference in viscosity sensitivity of the three dyes. The results show that the greatly raised HOMO and greatly lowered LUMO in the S₁ state compared with the S₀ state are responsible for the large Stokes shift of the three dyes. The heterocyclic moieties have the primary influence on the LUMO levels of the three hemicyanine dyes. Rotation about the C? C bond adjacent to the carbazole moiety of the three dyes drives the molecule toward a small energy gap between the ground state and the first excited state, which causes mainly nonradiative deactivation. The oscillator strengths in the lowest singlet excited state drop rapidly with increasing rotation between 0 and 95°, which leads to a dark state for these dyes when fully twisted at 95°. We draw a mechanistic picture at the molecular level to illustrate how these dyes work as viscosity‐sensitive fluorescent probes. The activation barriers and energy gaps of C? C bond rotation strongly depend on the choice of heterocycle, which plays a major role in reducing fluorescence quantum yield in the free state and provides high sensitivity to viscosity detection in viscous environments for the carbazole‐based hemicyanine dyes.     

Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

Compared with green fluorescence protein (GFP) chromophores, the recently synthesized blue fluorescence protein (BFP) chromophore variant presents intriguing photochemical properties, for example, dual fluorescence emission, enhanced fluorescence quantum yield, and ultra‐slow excited‐state intramolecular proton transfer (ESIPT; J. Phys. Chem. Lett., 2014 , 5, 92); however, its photochemical mechanism is still elusive. Herein we have employed the CASSCF and CASPT2 methods to study the mechanistic photochemistry of a truncated BFP chromophore variant in the S₀ and S₁ states. Based on the optimized minima, conical intersections, and minimum‐energy paths (ESIPT, photoisomerization, and deactivation), we have found that the system has two competitive S₁ relaxation pathways from the Franck–Condon point of the BFP chromophore variant. One is the ESIPT path to generate an S₁ tautomer that exhibits a large Stokes shift in experiments. The generated S₁ tautomer can further evolve toward the nearby S₁/S₀ conical intersection and then jumps down to the S₀ state. The other is the photoisomerization path along the rotation of the central double bond. Along this path, the S₁ system runs into an S₁/S₀ conical intersection region and eventually hops to the S₀ state. The two energetically allowed S₁ excited‐state deactivation pathways are responsible for the in‐part loss of fluorescence quantum yield. The considerable S₁ ESIPT barrier and the sizable barriers that separate the S₁ tautomers from the S₁/S₀ conical intersections make these two tautomers establish a kinetic equilibrium in the S₁ state, which thus results in dual fluorescence emission.     

New Squaraine‐based two‐component initiation systems for UV‐blue light induced radical polymerization: Kinetic and time‐resolved laser spectroscopy studies

In this article, the ability of two‐component photoinitiator systems for efficient polymerization of 2‐ethyl‐2‐(hydroxymethyl)?1,3‐propanediol triacrylate was presented. The photophysics and photochemistry of squaraine dyes in the presence of an electron donor as well as an electron acceptor was investigated, and it was found that the photosensitizer in an excited state might act as an electron acceptor or an electron donor. The excited states of squaraines may be quenched by tetramethylammonium n‐butyltriphenylborate ( B2 ), diphenyliodonium chloride ( I1 ), and N‐methoxy‐4‐phenylpyridinium tetrafluoroborate ( NO ). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 471–484     

Moderately high resolution fluorecence spectrum of the S1 → S0 transition of azulene

We report an Ar/Kr ion laser induced spectrally resolved S₁ → S₀ emission from an azulene in a host naphthalene crystal observed at the helium λ-point and at 77 K. Less well resolved S₁ → S₀ emission from crystalline azulene dispersed in a KBr pellet at 300 K is also reported. For the 6471 Å excitation the emission from the azulene in naphthalene system is analyzed in terms of three components: a resonance enhanced Raman emission originating from a nonstationary laser photon energy state 800 cm^?1 above the S₁ origin, a partially relaxed fluorecence originating from the 665 cm^?1 vibrational level of S₁ and a totally relaxed fluorecence from the S₁ origin (14651 cm^?1). The interpretation of the spectral lines is based on totally symetric vibrational modes (406, 679, 825,902, 1203, 1269, 1401, and 1586 cm^?1) the most prominent of which is the progression forming 825 cm^?1 mode. On the basis of both energies and intensities, correlations are made between ground and excited state vibrations and are compared with earlier results. Based on our results, a discussion is given on a plausible relaxation scheme for our system including the influence of Franck—Condon factors on the observability of unrelaxed emission.     

Theoretical study on electronic structure of (CNC)Fe_2N2 and its N2 elimination mechanism

The density function theory (DFT) is to elucidate the electronic structure of bis(dinitrogen) Fe(0) complex, (CNC)Fe_2N₂, and its N₂ elimination mechanism. (CNC)Fe_2N₂ has a low‐spin singlet (S = 0) ground state with a distorted square pyramidal structure. Fragment orbital interaction analysis yields total occupancy of π* orbitals (LUF₍₄₎O and LUF₍₄₎O−1) of apical N3 N4 is 0.188 while that of basal N1 N2 is 0.187 in ^S0(CNC)Fe_2N₂, suggesting nearly the same activation extent for both basal and apical N₂ ligands. The lowest‐lying triplet state T₁ (3‐A′) has a repulsive potential energy surface along the Fe N3 bong length by PBE functional, while a minimum on T₂ state (3‐A″) with higher energy is found by B3LYP functional. The nonadiabatic N₂ elimination mechanism of (CNC)Fe_2N₂ involves an S₀‐T₁ states crossing, which lowers the activation energy to 9.7 kcal/mol and produces high‐spin intermediate (CNC)Fe N₂. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Length of Soliton Wave in the Excited State in the Cations of Symmetrical Pyridocyanines and Their Carbo and Hetero Analogs

Quantum-chemical calculations in the AM1 approximation were undertaken for the optimized geometry of the cations of polymethine dyes [R⁺—(CH=CH) _n —CH=R], where R represents phenyl substituents or pyridinium, pyrylium, and thiopyrylium groups and n = 1-6, in the ground state and for the electron density distribution in the Frank – Condon excited state. It was found that excitation of the molecule by a light quantum leads to a decrease in the length of the charge wave (soliton). It was shown that the length of the soliton depends on the electron-donating character of the terminal groups R and on the length of the conjugated chain, while shortening of the soliton in the thiopyrylocyanines leads to disruption of its symmetry.     

TDDFT modeling of the CO‐photolysis of Fe2(S2C3H6)(CO)6, a model of the [FeFe]‐hydrogenase catalytic site

Upon irradiation with ultraviolet wavelengths, Fe₂(S₂C₃H₆)(CO)₆, a simple model of the [FeFe]‐hydrogenase active site, undergoes CO dissociation to form the unsaturated Fe₂(S₂C₃H₆)(CO)₅ species and successively a solvent adduct at the vacant coordination site. In the present work, the CO‐photolysis of Fe₂(S₂C₃H₆)(CO)₆ was investigated by density functional theory (DFT) and time‐dependent DFT (TDDFT). Trans Fe₂(S₂C₃H₆)(CO)₅ form and the corresponding trans heptane or acetonitrile solvent adducts are the lowest energy ground state forms. CO dissociation barriers computed for the lowest triplet state are roughly halved with respect to those for the ground state suggesting that some low‐lying excited potential energy surface (PES) could be loosely bound with respect to Fe? C bond cleavage. The TDDFT excited state PESs and geometry optimizations for the excited states likely involved in the CO‐photolysis suggest that the Fe? S bond elongation and the partial isomerization toward the rotated form could take place simultaneously, favoring the trans CO photodissociation. © 2014 Wiley Periodicals, Inc.     

Photoisomerization Mechanism of Ruthenium Sulfoxide Complexes: Role of the Metal‐Centered Excited State in the Bond Rupture and Bond Construction Processes

Phototriggered intramolecular isomerization in a series of ruthenium sulfoxide complexes, [Ru(L)(tpy)(DMSO)]ⁿ⁺ (where tpy=2,2’:6’,2’’‐terpyridine; DMSO=dimethyl sulfoxide; L=2,2’‐bipyridine (bpy), n=2; N,N,N’,N’‐tetramethylethylenediamine (tmen) n=2; picolinate (pic), n=1; acetylacetonate (acac), n=1; oxalate (ox), n=0; malonate (mal), n=0), was investigated theoretically. It is observed that the metal‐centered ligand field (³MC) state plays an important role in the excited state S→O isomerization of the coordinated DMSO ligand. If the population of ³MC_S state is thermally accessible and no ³MC_O can be populated from this state, photoisomerization will be turned off because the ³MC_S excited state is expected to lead to fast radiationless decay back to the original ¹GS_S ground state or photodecomposition along the Ru²⁺?S stretching coordinate. On the contrary, if the population of ³MC_S (or ³MC_O) state is inaccessible, photoinduced S→O isomerization can proceed adiabatically on the potential energy surface of the metal‐to‐ligand charge transfer excited states (³MLCT_S→³MLCT_O). It is hoped that these results can provide valuable information for the excited state isomerization in photochromic d⁶ transition‐metal complexes, which is both experimentally and intellectually challenging as a field of study.     

Squaraine Rotaxane as a Reversible Optical Chloride Sensor

A mechanically interlocked squaraine rotaxane is comprised of a deep‐red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl^? binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl^? is removed from the solution. Steady‐state fluorescence and excited‐state lifetime measurements show that this reversible machine‐like motion modulates several technically useful optical properties, including a three‐fold increase in deep‐red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time‐correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl^? binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 μs. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18‐coated, reverse‐phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen‐fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM ) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl^? levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick’s color slowly fades at a rate that depends on the amount of Cl^? in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl^? to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C₄O₂ core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.     

Synthesis and Electronic Properties of 1,2‐Hemisquarimines and Their Encapsulation in a Cucurbit[7]uril Host

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2‐hemisquarimines (1,2‐HSQiMs), through the microwave‐assisted condensation of aniline derivatives with the 1,2‐squaraine core is reported. In CH₃CN, 1,2‐HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530 nm, as well as an emission band centered at about λ=574 nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state. Encapsulation of the chromophore into a cucurbit[7]uril host revealed fluorescence enhancement and increased photostability in water. The redox characteristics of 1,2‐HSQiMs indicate that charge injection into TiO₂ is possible; this opens up promising perspectives for their use as photosensitizers for solar energy conversion.     

A CASSCF study on the lowest π→π* excitation of urocanic acid

The photochemical cis/trans isomerization of urocanic acid (UCA, (E)‐3‐(1′H‐imidazol‐4′‐yl)propenoic acid) was investigated using complete active space SCF (CASSCF) ab initio calculations. The singlet ground state and the triplet and the singlet manifolds of the lowest‐lying π→π* (HOMO→LUMO) excitation of the neutral and the anionic UCA were calculated using the 6‐31G* and the 6‐31+G* basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T₁ and S₁ excited states of the neutral UCA and in the T₁ but not in the S₁ excited state of the anionic UCA, as compared to the S₀ state of the respective protonation form. The cis‐isomer of both the neutral and the anionic UCA is lower in energy than the trans‐isomer in the S₀, T₁, and S₁ states. This energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis‐UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans‐to‐cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S₀ state of the neutral UCA as well. The cis‐to‐trans isomerization is suggested to proceed only in the S₀ state of the neutral UCA. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 25–37, 1999     

Luminescence Properties of C‐Diazaborolyl‐ortho‐Carboranes as Donor–Acceptor Systems

Seven derivatives of 1,2‐dicarbadodecaborane (ortho‐carborane, 1,2‐C₂B₁₀H₁₂) with a 1,3‐diethyl‐ or 1,3‐diphenyl‐1,3,2‐benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low‐energy fluorescence emissions with large Stokes shifts of 15100–20260 cm^?1 and quantum yields (Φ_F) of up to 65 % in the solid state. The low‐energy fluorescence emission, which was assigned to a charge‐transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C? C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660–18090 cm^?1 for the CT bands and 1960–5540 cm^?1 for the high‐energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ_F values between 8–32 %. Two distinct excited singlet‐state (S₁) geometries, denoted S₁(LE) and S₁(CT), were observed computationally for the benzodiazaborolyl‐ortho‐carboranes, the population of which depended on their orientation (ψ). TD‐DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C‐diazaborolyl‐ortho‐carboranes were viewed as donor–acceptor systems with the diazaborolyl group as the donor and the ortho‐carboranyl group as the acceptor.     

A novel spectroscopy using ultrafast two-pulse excitation of large polyatomic molecules

A first infrared pulse at frequency ν₁ interacts with vibrational states in S₀ and a second visible pulse at ν₂ promotes the excited molecules to the S₁ state from where they fluoresce. Tuning the frequency ν₂ over 600 cm^?1 allows the observation of a detailed spectrum which gives information on vibrational states in S₀ and on vibronic states in S₁ together with corresponding Franck—Condon factors. The spectra differ drastically from the common broad and featureless absorption and fluorescence bands.