Excited‐State Intramolecular Proton Transfer: Photoswitching in Salicylidene Methylamine Derivatives

The effect of chemical substitutions on the photophysical properties of the salicylidene methylamine molecule (SMA) (J. Jankowska, M. F. Rode, J. Sadlej, A. L. Sobolewski, ChemPhysChem, 2012 , 13, 4287–4294) is studied with the aid of ab initio electronic structure methods. It is shown that combining π‐electron‐donating and π‐electron‐withdrawing substituents results in an electron‐density push‐and‐pull effect on the energetic landscape of the ground and the lowest excited ππ* and nπ* singlet states of the system. The presented search for the most appropriate SMA derivatives with respect to their photoswitching functionality offers an efficient prescreening tool for finding chemical structures before real synthetic realization.     

Absorption Spectra and Photoreactivity of Para-aminobenzophenone by Time-dependent Density Functional Theory

The absorption spectral properties of para-aminobenzophenone (p-ABP) were investigated in gas phase and in solution by time-dependent density functional theory. Calculations suggest that the singlet states vary greatly with the solvent polarities. In various polar solvents, including acetonitrile, methanol, ethanol, dimethyl sulfoxide, and dimethyl formamide, the excited S1 states with charge transfer character result from π→π* transitions. However, in nonpolar solvents, cyclohexane, and benzene, the S1 states are the result of n→π* transitions related to local excitation in the carbonyl group. The excited T1 states were calculated to have ππ* character in various solvents. From the variation of the calculated excited states, the band due to π→π* transition undergoes a redshift with an increase in solvent polarity, while the band due to n→π* transition undergoes a blueshift with an increase in solvent polarity. In addition, the triplet yields and the photoreactivities of p-ABP in various solvents are discussed.     

Photophysical and Photochemical Studies of Pyridoxamine

The absorption and fluorescence emission of pyridoxamine were studied as function of pH and solvent properties. In the ground state, pyridoxamine exhibits different protonated forms in the range of pH 1.5–12. Fluorescence studies showed that the same species exist at the lowest singlet excited state but at different pH ranges. The phenol group is by ca. 8 units more acidic in the excited state than in the ground state. On the other hand, the pyridine N‐atom is slightly more basic in the lowest excited state than in the ground state. Excitation spectra and emission decays in the pH range of 8–10 indicate the protonation of the pyridine N‐atom by proton transfer from the amine group, in the ground and singlet excited states. Spectroscopic studies in different solvents showed that pyridoxamine in the ground or excited states exhibits intramolecular proton transfer from the pyridine N‐atom to the phenol group, which is more favorable in solvents of low hydrogen‐bonding capacity. The cationic form with the protonated phenolic group, which emits at shorter wavelength, is the dominant species in nonprotic solvents, but, in strong proton‐donor solvents, both forms exist. The fluorescence spectra of these species exhibit blue shift in protic solvents. These shifts are well‐correlated with the polarity and the H‐donor ability of the solvent.     

Oxygen Quenching of nπ* Triplet Phenyl Ketones: Local Excitation and Local Deactivation

We have studied the charge‐transfer‐induced deactivation of nπ* excited triplet states of benzophenone derivatives by O₂(³Σ), and the charge‐transfer‐induced deactivation of O₂(¹Δ_g) by ground‐state benzophenone derivatives in CH₂Cl₂ and CCl₄. The rate constants for both processes are described by Marcus electron‐transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have ππ* configuration. The results demonstrate that deactivation of the locally excited nπ* triplets occurs by local charge‐transfer and non‐charge‐transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable Franck‐Condon factors for the deactivation of ketone‐oxygen complexes to the ground‐state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O₂(¹Δ_g) formation observed with nπ* excited ketones. Compared with the deactivation of ππ* triplets, the non‐charge‐transfer process is largely enhanced, and charge‐transfer interactions are less important. The deactivation of singlet oxygen by ground‐state benzophenone derivatives proceeds via interactions of O₂(¹Δ_g) with the Ph rings.     

Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited‐State Proton Transfer and Deactivation via Conical Intersections

We employed the complete active space self‐consistent field (CASSCF) and its multistate second‐order perturbation (MS‐CASPT2) methods to explore the photochemical mechanism of 2‐hydroxyazobenzene, the molecular scaffold of Sudan I and Orange II dyes. It was found that the excited‐state intramolecular proton transfer (ESIPT) along the bright diabatic ¹ππ* state is barrierless and ultrafast. Along this diabatic ¹ππ* relaxation path, the system can jump to the dark ¹nπ* state via the ¹ππ*/¹nπ* crossing point. However, ESIPT in this dark state is largely inhibited owing to a sizeable barrier. We also found two deactivation channels that decay ¹ππ* keto and ¹nπ* enol species to the ground state via two energetically accessible S₁/S₀ conical intersections. Finally, we encountered an interesting phenomenon in the excited‐state hydrogen‐bonding strength: it is reinforced in the ¹ππ* state, whereas it is reduced in the ¹nπ* state. The present work sets the stage for understanding the photophysics and photochemistry of Sudan I–IV, Orange II, Ponceau 2R, Ponceau 4R, and azo violet.     

Electronic structure of the pyrimidine and purine components of nucleic acids in their ground and lower excited singlet and triplet states

Quantum-chemical calculation of the energies of the electronic transitions and the electronic structures of the neutral and ionic species of the nucleic acids components in their ground and lower excited singlet and triplet ππ* and nπ* states has been carried out in the all-valence-electron approximation CNDO /S . The results of the calculation allow one to identify the most photoreactive sites of the molecules and to consider the dependence of the location of these sites on the ionic state of the molecules. The calculated data are compared with our previous results obtained in a π-electron approximation. The individual absorption spectra of various ionic and tautomeric species of the nucleic acids components obtained by us earlier have been decomposed into bands corresponding to separate electronic transitions. As a rule, there is a good agreement between the calculated data in the two approximations and the experimental results.     

Proton transfer in phenol–amine complexes: Phenol electronic effects on free energy profile in solution

Free energy profiles for the proton transfer reactions in hydrogen‐bonded complex of phenol with trimethylamine in methyl chloride solvent are studied with the reference interaction site model self‐consistent field method. The reactions in both the electronic ground and excited states are considered. The second‐order Møller‐Plesset perturbation (MP) theory or the second‐order multireference MP theory is used to evaluate the effect of the dynamical electron correlation on the free energy profiles. The free energy surface in the ground state shows a discrepancy with the experimental results for the related hydrogen‐bonded complexes. To resolve this discrepancy, the effects of chloro‐substitutions in phenol are examined, and its importance in stabilizing the ionic form is discussed. The temperature effect is also studied. In contrast to the ground state, the ππ^* excited state of phenol–trimethylamine complex exhibits the proton transfer reaction with a low barrier. The reaction is almost thermoneutral. This is attributed to the reduction of proton affinity of phenol by the ππ^* electronic excitation. We further examine the possibility of the electron–proton–coupled transfer in the ππ^* state through the surface crossing with the charge transfer type πσ^* state. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Photolysis of the azo-precursors of 2,3- and 1,8-naphtoquinodimethane. Preliminary communication

A novel oxidation method for the synthesis of labile azo-alkanes is reported. Matrix-isolated 2,3-naphthoquinodimethane is obtained by photolysis of 1,4-dihydrobenzo[g]-phthalazine in a rigid matrix (EPA glass) at 77 K and the electronic structure of its ground and lowest excited states is discussed. Nitrogen elimination from 1,4-dihydronaphtho[1,8-de][1,2]-diazepine to yield acenaphthene occurs exclusively upon ππ* excitation while irradiation in the n-π* absorption region induces cis/trans isomerization of the azo-moiety. Neither ns flash nor low-temperature photolysis provide evidence for the occurrence of 1,8-naphthoquinodimethane as an intermediate in the formation of acenaphthene.     

Electronic structure of benzaldehyde: A comparative study of the lowest excited singlet π* ← π and π* ← n states

VE-PPP, CNDO/2, and CNDO/s-CI methods have been used to investigate the electronic spectrum and structure of benzaldehyde. Electronic charge distributions and bond orders in the ground and lowest excited singlet π* ← π and π* ← n states of the molecule have been studied. The molecule has been shown to be nonplanar in the lowest π* ← n excited singlet state, in agreement with the conclusions drawn from the study of vibrational spectra. Dipole moments in both excited states have been shown to be larger than the ground-state value. Thus, the ambiguity in the experimental result for the π* ← π n excited singlet state dipole moment has been resolved. It has been shown that the n orbital is mainly localized on the CHO group. Furthermore, charge distributions, dipole moments, and molecular geometries are shown to be very different in the excited singlet π* ← π and π* ← n states.     

Localization function study of excitation processes in a set of small isoelectronic molecules

Electron localization function (ELF) theory is used to characterize changes that occur upon excitation from ground singlet to first excited triplet states in a series of isoelectronic 16‐electron molecules including H₂CCH₂, HNCH₂, H₂CO, HNNH, HNO, and O₂ (ground triplet to excited singlet). ELF allows one to visualize lone pair or nonbonding electrons, and in these cases the π→π* or n→π excitation processes involved lead to an effective 90° rotation of the electronic structure about one heavy atom center and consequent distortion towards pyramidal symmetry about both heavy atom centers. The heavy atom bond lengths change very little in those cases where effectively two‐center three‐electron bonds can be formed (HNNH, HNO, and O₂) while a significant lengthening occurs in those cases where hydrogen atoms prevent such interactions (H₂CCH₂, HNCH₂, and H₂CO). It is shown that both ELF basin populations and atoms‐in‐molecules (AIM) delocalization indices reflect expected bond orders for conventional single and double bonds provided one compares the ratio of the molecular quantities rather than their absolute magnitudes. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1702–1711, 2001     

Some remarks on the semiempirical one-center electron repulsion integrals

The identification of the stage of ionization for various kinds of one-center electron repulsion integrals, occurring when nonbonding or lone-pair electrons are considered explicitly as well as π-electrons, is discussed for conjugated organic molecules containing heteroatoms N. It is concluded that the value for the negative ions should be used for (π_Cπ_C | π_Cπ_C) in all the states but for (π_Nπ_N | π_Nπ_N) only in the π-π states. In the n-π states, the appropriate value of (π_Nπ_N | π_Nπ_N) is that of the neutral atom if the molecule contains only one N atom. If more than one N atom is involved in the molecule, some weighted mean of the values for the negative ion and for the neutral atom should be used. The value for the neutral atom is most adequate for one-center repulsion integrals other than the (ππ | ππ) type in both the π-π and the n-π states. The method of determining these integrals is also discussed. It is concluded that they are to be determined from the consideration of appropriate electron-transfer reactions except for exchange integrals. The exchange integrals are shown to have to be determined from the Slater–Condon parameters derived from the analysis of the experimental atomic energy levels. Illustrative calculations are given for the lower singlet levels of the formaldehyde, pyrazine, pyridine, and the p-benzoquinone molecule. It is found that the calculated energies of the n-π transitions become much too low unless the (ππ | ππ) values of the heteroatoms in the molecule are chosen differently in the n-π states and in the π-π states as pointed out theoretically in this article.     

ENERGY TRANSFER-ENHANCED CHEMILUMINESCENCE OF ADAMANTANONE (n,π*) AND ESTER (π,π*) SINGLET AND TRIPLET EXCITED STATES IN THE THERMOLYSIS OF SILYLOXYARYL-SUBSTITUTED SPIROADAMANTYL DIOXETANES†

Abstract— The thermal generation of singlet and triplet excited states from silyloxyaryl-substituted spiroadamantyl dioxetanes lab and the adamantylidineadamantane dioxetane (1c) was investigated by direct and enhanced chemiluminescence (CL). 9,10-Diphenylanthracene (DPA) and 9-fluorenone were used as energy acceptors in the singlet-singlet (S-S), naphthalene and europium chelate Eu(TTA)₃Phen (TTA = thenoyltrifluoroacetone, Phen = 1,10-phenanthroline) in the triplet-triplet (T-T) and 9,10-di-bromoanthracene (DBA) in triplet-singlet (T-S) energy transfer experiments. The direct chemiluminescence observed in the thermolysis of dioxetanes lab consisted of fluorescence derived from the singlet-excited adamantanones 2a,b. In the presence of naphthalene, selective T-S energy transfer with DBA (napthalene as quencher) displayed the adamantanone triplets 2a,b and with Eu(TTA)₃Phen (naphthalene as mediator) also the silyloxyaryl ester 3 triplets. From the Stern-Volmer constants (k_TNT_T⁰) the triplet lifetimes t⁰_t of these triplet state products were assessed. By using the Hastings-Weber standard, the total triplet excitation yield (φ^t) was estimated to be ca 20%. The energies of the first excited singlet and triplet states of the adamantanones 2a,b and the silyloxyaryl ester 3, the products of the thermally induced decomposition of dioxetanes la-c , were determined by semiempirical calculations (AMI-based configuration interaction), which included explicitly solvent effects on the excitation energies in terms of a self-consistent reaction field approach. The calculations revealed that the first excited singlet and triplet states of the adamantanones 2a,b are expectedly n,π*-type excitations while the silyloxyaryl ester 3 possesses π,π* character. The semiempirical computations suggest that excitation of the adamantanones 2a,b as well as the silyloxyaryl ester 3 is feasible in the thermolysis of the spiroadamantyl dioxetanes lab , which has been confirmed by the experimental energy transfer studies.     

Photophysics of Push–Pull Distyrylfurans,Thiophenes and Pyridines by Fast and Ultrafast Techniques

Time‐resolved transient absorption and fluorescence spectroscopy with nano‐ and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet–triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge‐transfer process, which is operative even in a non‐polar solvent. Both the thiophene and pyridine moieties enhance the S₁→T₁ rate with respect to the furan one. This is due to the heavy‐atom effect (thiophene compounds) and to the ¹(π,π)*→³(n,π)* transition (pyridine compounds), which enhance the spin‐orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push–pull compounds: in fact, a particularly fast ¹LE*→¹CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non‐polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet‐state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.     

Excited‐State Dynamics of the Metal String Complex Co3(dpa)4(NCS)2 from Femtosecond Transient Absorption Spectra

Transient absorption spectroscopy is used to study the excited‐state dynamics of Co₃(dpa)₄(NCS)₂, where dpa is the ligand di(2‐pyridyl)amido. The ππ*, charge‐transfer, and d–d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low‐lying metal‐centered d–d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand‐centered ππ* and the charge‐transfer states to this metal‐centered d‐d state within 100 fs. The excited molecule then crosses to a second d–d state within the ligand‐field manifold, with a time coefficient of 0.6–1.4 ps. Because the ground‐state bleaching band recovers with a time coefficient of 10–23 ps, we propose that an excited molecule crosses from the low‐lying d–d state either directly within the same spin system or with spin crossing via the state ²B to the ground state ²A₂ (symmetry group C₄). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.     

Photophysical properties of the isomorphic emissive RNA nucleobase analogues and effect of water solution,ribose, and base pairing: A theoretical study

In the present work, a comprehensive theoretical investigation on the excited state properties of the isomorphic emissive RNA nucleobase analogues, namely ^tzA, ^tzG, ^tzC, and ^tzU, was performed. Vertical transition energies are determined with the time‐dependent density functional theory method at both the B3LYP and CAM‐B3LYP levels using the 6‐311++G(d,p) basis set. The nature of the low‐lying singlet excited states is discussed and the results are compared with the findings from experiment and those for thieno analogues and natural bases. In gas phase, it was found that the S₁ state is ππ* in nature for all the tz‐bases except for ^tzA, for which the S₁ state is predicted to be nπ* in nature with the ππ* state being the S₂. While in water solution, the S₁ state for all tz‐bases are predicted to be ππ* dominated by the configuration HOMO→LUMO. Compared with natural bases, the lowest ππ* states are about 0.85–1.22 eV red‐shifted. When compared with thieno analogues, it is interesting to note that the S₁ state (ππ*) transition energies of the two counterparts from the two alphabets are nearly equal due to the very little differences of their HOMO‐LUMO gaps. In addition, it was found that the hydration + PCM model can perfectly reproduce the photophysical properties of the tz‐bases since the calculated excitation maxima and fluorescence are in good agreement with the experimental data. The microenvironment effects of linking to ribose, base pairing, and further hydration of base pairs were also studied.     

The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD‐DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet nπ* state, ¹A_u, which is conventionally regarded dark due to the dipole‐forbidden ¹A_u←¹A_g transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring nπ* ¹B_3u and ππ* ¹B_2u states through vibronic coupling. The scans on potential‐energy surfaces further indicated that the ¹A_u state intersects with the ¹B_2u states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.     

UV‐induced DNA Damage: The Role of Electronic Excited States

The knowledge of the fundamental processes induced by the direct absorption of UV radiation by DNA allows extrapolating conclusions drawn from in vitro studies to the in‐vivo DNA photoreactivity. In this respect, the characterization of the DNA electronic excited states plays a key role. For a long time, the mechanisms of DNA lesion formation were discussed in terms of generic “singlet” and “triplet” excited state reactivity. However, since the beginning of the 21^st century, both experimental and theoretical studies revealed the existence of “collective” excited states, i.e. excited states delocalized over at least two bases. Two limiting cases are distinguished: Frenkel excitons (delocalized ππ* states) and charge‐transfer states in which positive and negative charges are located on different bases. The importance of collective excited states in photon absorption (in particular in the UVA spectral domain), the redistribution of the excitation energy within DNA, and the formation of dimeric pyrimidine photoproducts is discussed. The dependence of the behavior of the collective excited states on conformational motions of the nucleic acids is highlighted.     

A Time‐Dependent Picture of the Ultrafast Deactivation of keto‐Cytosine Including Three‐State Conical Intersections

Using mixed quantum–classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto‐cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S₁ and S₂ states, with minor contributions from the S₃. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto‐cytosine to the ground state. Non‐adiabatic transfer between the ππ* and nπ* excited states and deactivation via three‐state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S₂ state. The latter two events can be connected to the fs and ps transients observed experimentally.