Photochemical properties of 1-(9-Anthryl)-2-(2-Quinolyl)ethylene

Spectral and photochemical properties of 1-(9-anthryl)-2-(2-quinolyl)ethylene (9A2QE) in neutral and protonated forms have been studied experimentally and by quantum-chemical methods. It has been found that the quantum yield of trans-cis photoisomerization (φ_tc) has values of φ_tc < 0.5 typical of the diabatic photoisomerization for both forms of 9A2QE. A comparison of this data with the results of the study of other aza-diarylethylenes containing the 2-styrylquinoline (2SQ) moiety has led to the general conclusion that the increase in the π-system in 2SQ upon fusion of the benzene rings results in the disappearance of the α-effect, which lies in the fact that the quantum yield increases upon going from the neutral to protonated form up to the values φ_tc > 0.5, which exceeds the limiting value for the diabatic photoisomerization.     

Photochemical Properties of 1-(9-Phehanthryl)-2-(2-Quinolyl)ethylene

Spectral and photochemical properties of 1-(9-phehanthryl)-2-(2-quinolyl)ethylene (9Ph2QE) in neutral and protonated forms have been investigated. It has been found that both isomers of 9Ph2QE are photoactive. The quantum yield of trans–cis photoisomerization (? _tc = 0.47) in the neutral form is typical of the diabatic photoisomerization; on passing to the protonated form, ? _tc increases up to 0.70. Thus, the double annelation of the 2-styrylquinoline phenyl group to form 9Ph2QE makes it possible to conserve the α-effect, which consists in an increase in the quantum yield to ? _tc > 0.5 on passing from the neutral to protonated form, whereas the effect disappears for other types of annelation (naphthylquinolylethylenes, 1-(9-anthryl)-2-(2-quinolyl)ethylene).     

Photochemical properties of amino and nitro derivatives of 2-and 4-styrylquinolines and their hydrochlorides

The trans-cis and cis-trans photoisomerization reactions of a double bond in nitro and amino derivatives of 2-and 4-styrylquinolines in the neutral and protonated (hydrochloride) forms were studied. Protonation in nitrostyrylquinolines was shown to have no effect on the photoisomerization quantum yields. In aminostyrylquinolines, the photoisomerization reaction is “switched off” for the monocationic form as a result of the competitive process of intramolecular charge transfer and is “switched on” again for the dication. In the latter case, the quantum yield of trans-cis photoisomerization decreases by a factor of 2–2.5 and the quantum yield of cis-trans photoisomerization remains practically unchanged as compared to that of the neutral compound. Upon long-term irradiation of 4-(4′-nitrostyryl)quinoline, the photocyclization reaction of the cis-isomer was observed.     

A quantum-chemical study of aza derivatives of (<Emphasis Type="Italic">E</Emphasis>)-1-styrylnaphthalene in the ground and lowest electronically excited states

The structures of (E)-1-styrylnaphthalene (1SN) and its aza derivatives 1-styrylisoquinoline (1SiQ) and 4- and 8-styrylquinolines (4SQ and 8SQ, respectively) in the neutral and protonated forms were calculated by the semiempirical (PM3) and DFT (B3LYP/6-31G*) methods. It follows from the DFT data that, in the ground state (S0), 1SiQ and 8SQ are planar, whereas 1SN, neutral 4SQ, and all protonated azastyrylnaphthalenes are nonplanar with aromatic cores twisted by 5 to 40° out of the plane of the double bond and with linear correlation between the torsion angles of the two cores. The calculated adiabatic excitation energy (E _ad) varies within 61–64 kcal mol^?1 for the neutral compounds and decreases for the protonated forms to 48, 45, and 33 kcal mol^?1 for 1SiQH⁺, 4SQH⁺, and 8SQH⁺, respectively. The lower E _ad value for 8SQH⁺ is in qualitative agreement with a lower photoisomerization quantum yield for this compound as compared with that for other protonated azastyrylnaphthalenes.     

Photoisomerization and photocyclization of 4-styrylquinoline derivatives

The photochemical properties of 4-styrylquinoline and its derivatives with fluorine and chlorine atoms and nitro group in the para-position of the styryl moiety were studied. Under irradiation in neutral alcohol solutions, these compounds undergo reversible trans-cis photoisomerization with quantum yields close to 0.5, and the cis-isomers undergo cyclization to give benz[i]phenanthridines with quantum yields that do not exceed 0.01. For several compounds, the primary product of photocyclization, dihydrobenz[i]phenanthridine, was spectrally detected. On passing from the neutral to protonated forms, the photoisomerization quantum yields of these styrylquinolines changed insignificantly, and the photocyclization did not occur at all.     

Spectral and photochemical properties of isomeric styrylbenzoquinolines

Photochemical properties of the annelated 2-styrylquinoline (2SQ) derivatives 2-styryl-benzo[g]quinoline (2SBQ) and 3-styrylbenzo[f]quinoline (3SBQ) and 3-styrylacridine (3SA) have been investigated by experimental and quantum-chemical methods. It has been found that the trans-isomer of 3SBQ undergoes the photoisomerization reaction with a quantum yield of φ_tc= 0.19, which increases to 0.44 for the protonated form, with the fluorescence quantum yield decreasing from 0.74 to 0.09. In contrast, the trans-isomers of 2SBQ and 3SA are photochemically inert in both neutral and protonated forms, φ_tc < 10^?3. Quantum-chemical calculation by the PM3 method show that the difference in photochemical properties of 2SBQ and 3SBQ is due to the difference in the structure of frontier molecular orbitals, which, in turn, results in the difference of the shape of the potential energy surface of excited S1 states. Comparison of the properties of 2SBQ and 3SBQ with those of 2SQ reveals size and topological effects in the photochemistry of aza-diarylethylenes.     

Quantum-chemical study of the photoisomerization and photocyclization reactions of styrylquinolines: Potential energy surfaces

The cross sections of potential energy surfaces (PES) for the S₀ and S₁ states were calculated by the semiempirical PM3 and PM3-CI (8 × 8) methods, respectively, along the reaction coordinate of the isomerization and cyclization of 2- and 4-styrylquinolines (SQ). The PES of the S₀ state exhibits three minima separated by the transition-state barriers of isomerization and cyclization corresponding to three isomeric SQ forms, the E- and Z-isomers and the dihydrogenated cyclic product. On the PES of the S₁ state, the “perpendicular minimum” at dihedral angle values of ～ 90° corresponds to the transition state of the isomerization reaction and the pericyclic minimum with a distance of 1.7–2.0 Å between the atoms involved in cyclization corresponds to the transition state of the cyclization reaction. With simultaneous scanning of the PES of the S₁ state along the isomerization and cyclization reaction coordinates, the minimal-energy path was found for 4SQ, which makes it possible to explain the formation of the photocyclization product in the single-photon process upon irradiation of the E-isomer. It was found that the PM3 method overestimates the stability of the structures in which the aromatic ring is oriented perpendicular to the plane of the molecule, resulting in virtual minima on the PES of the S₁ states.     

Photochemical properties of the naphthol-styrylquinoline dyad in neutral and ionic forms

The photochemical properties of the naphthol-styrylquinoline dyad 2-(E)-{4-[4-(3-hydroxynaphthalen-2-yloxy)butoxy]styryl}quinoline (SQ4Np) have been investigated. It has been found that the excited-state acidity of the styrylquinoline (SQ) moiety is reduced by six orders of magnitude and that of the naphthol (Np) moiety increases by four orders of magnitude. As part of the dyad in the neutral and protonated forms, the SQ moiety retains a high photoisomerization quantum yield characteristic of model styrylquinoline. Deprotonation of the Np moiety of the dyad reduces the SQ photoisomerization quantum yields, presumably because of the formation of intramolecular complexes (exciplexes) or energy transfer to the Np anion.     

Spectral and photochemical properties of hydroxylated 2-styrylquinoline derivatives

The spectral and photochemical properties of 2-(4-hydroxystyryl)quinoline 1 and 2-(2-hydroxystyryl)quinoline 2 have been studied. The trans-cis photoisomerization quantum yield for 1 and 2 in the neutral form is 0.2–0.5, and it is reduced by a factor of 6–7 by protonation, presumably, owing to the competitive process of hydroxyl group deprotonation in the excited state and the formation of quinoid tautomers. The quantum yield of the back cis-trans photoisomerization in the protonated form varies insignificantly. The neutral, protonated, and deprotonated forms of 1 have been used for to simulate a molecular logic gate, the half-adder with chemical inputs and absorbance as outputs.     

Spectral and photochemical properties of covalently linked dyads based on 2-styrylquinoline and 6-hydroxy-2-naphthoic acid

Photophysical and photochemical properties of bichromophoric covalently linked SnN dyads (n = 3, 5, 9), in which the 2-styrylquinoline (SQ) and 6-hydroxy-2-naphthoic acid (Np) moieties are linked by the dioxypolymethylene bridge–O–(CH₂)_n–O–have been studied. The properties of the dyads have been compared with those of the model compounds 2-(4-methoxystyryl)quinoline and methyl 6-hydroxy-2-naphthoate. Inductive-resonance (Förster) energy transfer (FRET) from the Np to the SQ unit with an efficiency up to 99.6% is observed in the S₁ state of the dyads. The Np unit in the neutral form does not affect the photoisomerization of the SQ unit regardless of the form of the latter, neutral or protonated (cationic). The Np moiety in the anionic (deprotonated) form hinders the photoisomerization of the SQ moiety, presumably, as a result to a combined action of several factors.     

Proton-controlled photoisomerization of 1-(2-pyridyl)-2-(2-quinolyl)ethylene

Quantum-chemical calculations (B3LYP/6-31G*) predict the formation of intramolecular hydrogen bond (IMHB) in the monoprotonated Z-isomer of 1-(2-pyridyl)-2-(2-quinolyl)ethylene (2P2Q), with this bond stabilizing the isomer relative to its E-counterpart. An experimentally observed increase in the quantum yield of trans-cis photoisomerization (φ_tc) by more than an order of magnitude (from 0.033 to 0.42 in acetonitrile) on passing from the neutral to the monoprotonated form of 2P2Q can be associated with IMHB, which manifested itself in the spectral properties of the Z-isomer. The IMHB breaks in the diprotonated form, and the value of φ_tc decreases back to the initial value. In addition to the photoisomerization, the photoreduction and photoaddition reactions of solvent molecules have been observed in an ethanol solution of 2P2Q.     

Does one‐photon photocyclization of trans‐diarylethylenes involve adiabatic trans‐to‐cis photoisomerization? potential energy surface calculations for 1‐styrylnaphthalene

Potential energy surface (PES) for 1‐styrylnaphthalene was calculated by PM3 method for the S₀ state and PM3‐CI(2x2) method with configuration interaction for the S₁ state. Scanning PES along both isomerization and cyclization reaction coordinates enabled to reveal the minimum energy path (MEP) with low barriers on the S₁ PES from E‐isomer to dihydrocyclophotoproduct (DHP). This is consistent with formation of the photocyclization product in one‐photon process during irradiation of E‐isomer. Additionally, the MEP was found to bypass the coordinate region of Z‐isomer, i.e. one‐photon E‐isomer‐to‐DHP photocyclization does not demand participation of the excited Z‐isomer. Therefore, adiabatic trans‐to‐cis isomerization is likely not an intermediate stage on the E‐isomer photocyclization pathway, and experimentally observed one‐photon formation of the DHP from the E‐isomer is likely not an evidence for adiabatic trans‐to‐cis photoisomerization, as it is usually assumed. According to the results obtained, two photochemical reactions of E‐isomer, photoisomerization to Z‐isomer and photocyclization to DHP, are not consecutive but parallel reactions with branching at perpendicular conformer on the S₁ PES. © 2012 Wiley Periodicals, Inc.     

Photoisomerization of naphthylquinolylethylenes in neutral and protonated forms

The quantum yield of the trans-cis photoisomerization of 1-(1-naphthyl)-2-(2-quinolyl)ethylene and 1-(2-naphthyl)-2-(2-quinolyl)ethylene is close to the theoretical limit (0.5) for diabatic photoisomerization and does not change on passing from the neutral to the protonated form. The data obtained indicate the absence of the α-effect for the test compounds, which consists in an increase in the trans-cis photoisomerization quantum yield to values of >0.5 upon protonation of some azadiarylethylenes with the nitrogen atom in the α-position to the ethylene group.     

Photochemistry of azidostyrylquinolines: 1. Quantum-chemical study of the structure in the ground and the lower excited singlet states

The structures of isomeric 2-and 4-azidostyrylquinolines and their protonated forms in the ground (S ₀) and the lowest excited singlet (S ₁) states were calculated by the PM3 semiempirical method and the density functional theory (DFT) using the B3LYP/6-31G^* basis set. It was shown that the σ _NN ^* molecular orbital, which is localized on the azide group and is antibonding for the N-N₂ bond, is populated in the S₁ state of these azides in both neutral and protonated forms. Based on this result, it was assumed that the test azides would be photoactive in both forms, i.e., would have a photodissociation quantum yield of φ > 0.1. The calculation of absorption spectra by the TD B3LYP/6-31G^* method showed that the long-wavelength absorption bands of the protonated forms are shifted to visible spectral region, thus suggesting that azidostyrylquinolines in the protonated form will be sensitive to visible light.     

Characterization of the Excited States of Indigo Derivatives in their Reduced Forms

A comprehensive characterization of the electronic spectral and photophysical properties of the leuco (reduced) form of several indigo derivatives, including indigo and Tyrian Purple, with di‐, tetra‐, and hexa‐substitution, was obtained in solution. The characterization involves absorption, fluorescence, and triplet–triplet absorption spectra, together with quantitative measurements of quantum yields of fluorescence, ?_F (0.46–0.04), intersystem crossing, ?_T (0.013–0.034), internal conversion, ?_IC, and the corresponding lifetimes. The position and degree of substitution promote differences in the spectral and photophysical properties displayed by the investigated leuco derivatives. The ?_F values are about two orders of magnitude higher than those previously obtained for the corresponding keto forms. Also in contrast with the behavior found for the keto forms, the S₁～～→T₁ intersystem crossing is an efficient route for the excited‐state deactivation channel. These findings strengthen the fact that, in contrast to keto indigo where the internal conversion dominates the deactivation of the excited‐state, with leuco indigo (and derivatives), the excited state deactivation involves competition between internal conversion, triplet state formation, and fluorescence. A time‐resolved investigation of one of the compounds in glycerol showed the presence of a photoisomerization process.     

Complex excited dynamics around a plateau on a retinal-like potential surface: chaos, multi-exponential decays and quantum/classical differences

We investigate the classical and quantum dynamics on the plateau of an excited potential energy surface (PES) whose shape mimics the PES driving the photoisomerization of the protonated Schiff base of retinal (PSBR). We adopt a two-dimensional analytical model of the PES, and perform an extended study by varying the potential parameters, revealing a scenario whose interest goes beyond the relevance for the specific case of PSBR. In fact, we document cases with net differences among classical and quantum dynamical predictions, for barrierless PESs. Classical trajectories released on the PES display the signature of chaos and partial trapping on the plateau, whose origin is purely dynamical, since no barrier exists. At variance, on the same barrierless PESs, quantum dynamics does not predict any trapping, always showing a complete depletion of the excited population according to an approximate mono-exponential law. The plateau on the PES promotes complex and unusual dynamical features, and it is sufficient to introduce a very small barrier along the cis–trans torsional mode to give rise to a multi-exponential decay, also at quantum level. Our results are of general interest because plateaux are often found in the excited states of conjugated chromophores.     

The N‐Arylamino Conjugation Effect in the Photochemistry of Fluorescent Protein Chromophores and Aminostilbenes

To understand the nonradiative decay mechanism of fluorescent protein chromophores in solutions, a systematic comparison of a series of (Z)‐4‐(N‐arylamino)benzylidene‐2,3‐imidazolinones (ABDIs: 2P , 2PP , 2OM , and 2OMB ) and the corresponding trans‐4‐(N‐arylamino)‐4′‐cyanostilbenes (ACSs: 1P , 1PP , 1OM , and 1OMB ) was performed. We have previously shown that the parameter Φ_f+2 Φ_tc, in which Φ_f and Φ_tc are the quantum yields of fluorescence and trans→cis photoisomerization, respectively, is an effective probe for evaluating the contribution of twisted intramolecular charge transfer (TICT) states in the excited decays of trans‐aminostilbenes, including the push–pull ACSs. One of the criteria for postulating the presence of a TICT state is Φ_f+2 Φ_tc?1.0, because its formation is decoupled with the C?C bond (τ) torsion pathway and its decay is generally nonradiative. Our results show that the same concept also applies to ABDIs 2 with the parameter Φ_f+2 Φ_ZE in which Φ_ZE is the quantum yield of Z→E photoisomerization. We conclude that the τ torsion rather than the C? C bond (φ) torsion is responsible for the nonradiative decays of ABDIs 2 in aprotic solvents (hexane, THF, acetonitrile). The phenyl‐arylamino C? N bond (ω) torsion that leads to a nonradiative TICT state is important only for 2OM in THF and acetonitrile. If the solvent is protic (methanol and 10–20 % H₂O in THF), a new nonradiative decay channel is present for ABDIs 2 , but not for ACSs 1 . It is attributed to internal conversion (IC) induced by solvent (donor)–solute (acceptor) hydrogen‐bonding (HB) interactions. The possible HB modes and the concept of τ torsion‐coupled proton transfer are also discussed.     

About the intrinsic photochemical properties of the 11-cis retinal chromophore: computational clues for a trap state and a lever effect in Rhodopsin catalysis

CASPT2//CASSCF/6-31G* computations are used on the singlet S ₁ and S ₂ states to map the photoisomerization process of the 11-cis retinal protonated Schiff base in vacuo and to characterize its optical properties. It is shown that the spectroscopic observations recorded in Rhodopsin are reproduced quite well, calling for a substantially neutral effect of the protein. Furthermore, a rationale is proposed for the unreactive population recently observed in Rhodopsin, which is here addressed to the accessible S ₂ state, behaving as a trap. The experimental transient absorption and (absorption-wavelength dependent) emission are discussed and interpreted under the light of this novel model. Finally, a planarization of the β-ionone ring is observed on S ₁, which may cause a steric lever effect into the protein pocket, thus assisting photoisomerization catalysis. The reported results constitute a solid reference for further studies aimed to rationalize the effect of the environment on the photochemical reactivity of retinal chromophores. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.     

Double resonance spectroscopy of different conformers of the neurotransmitter amphetamine and its clusters with water

In this paper the conformational landscape of amphetamine in the neutral ground state is examined by both spectroscopy and theory. Several spectroscopic methods are used: laser-induced fluorescence (LIF), resonance-enhanced two-photon ionization (R2PI), dispersed fluorescence and IR/R2PI hole burning spectroscopy. The latter two methods provide for the first time vibrationally resolved spectra of the neutral ground state of dl-amphetamine and the amphetamine–(H₂O)_1,2 complexes. Nine stable conformers of the monomer were found by DFT (B3LYP/6-311++G(d,p)) and ab initio (MP2/6-311++G(d,p)) calculations. For conformer analysis the vibrations observed in the IR/R2PI hole burning and dispersed fluorescence spectra obtained from single vibronic levels (SVLF) of a selected conformer were compared with the results of an ab initio normal mode analysis. By this procedure three S₀ → S₁ transitions in the R2PI spectrum were assigned to three different conformer structures. Another weak transition earlier attributed to another conformer could be assigned to a vibronic band of one of the three conformers. Furthermore spectra of amphetamine–(H₂O)_1,2 are tentatively assigned.     

Discrimination of s-cis/s-trans conformers of jet-cooled methyl cinnamate by population labelling spectroscopy

The S₁−S₀ electronic spectrum of methyl cinnamate in a supersonic jet has been investigated to discriminate the transitions of the s-cis/s-trans conformers. Population labelling spectroscopy was applied to the conformer discrimination, and vibronic bands belonging to each conformer were identified. The relative population was estimated from the fluorescence quantum yields obtained by lifetime measurements. It was found that both conformers exist almost equally in a supersonic jet. The conformer identification of the vibronic bands was carried out based on the difference of the red-shift of their hydrogen-bonded complexes with methanol.