Photodynamics and time-resolved fluorescence of azobenzene in solution: a mixed quantum-classical simulation

We have simulated the photodynamics of azobenzene by means of the Surface Hopping method. We have considered both the trans → cis and the cis → trans processes, caused by excitation in the n → π* band (S(1) state). To bring out the solvent effects on the excited state dynamics, we have run simulations in four different environments: in vacuo, in n-hexane, in methanol, and in ethylene glycol. Our simulations reproduce very well the measured quantum yields and the time dependence of the intensity and anisotropy of the transient fluorescence. Both the photoisomerization and the S(1) → S(0) internal conversion require the torsion of the N═N double bond, but the N-C bond rotations and the NNC bending vibrations also play a role. In the trans → cis photoconversion the N═N torsional motion and the excited state decay are delayed by increasing the solvent viscosity, while the cis → trans processes are less affected. The analysis of the simulation results allows the experimental observations to be explained in detail, and in particular the counterintuitive increase of the trans → cis quantum yield with viscosity, as well as the relationship between the excited state dynamics and the solvent effects on the fluorescence lifetimes and depolarization.     

Picosecond spectroscopy of phenylnaphthylethylenes

S_n←S₁ transitions in the trans isomers of 1-phenyl-2-(1-naphthyl)ethylene (Ph-1N) and 1-phenyl-2-(2-naphthyl)ethylene (Ph-2N) peaked at 510 nm are assigned by picosecond absorption spectroscope. The S₁ state lifetimes of both conformers of Ph-2N are shown to depend upon solvent viscosity, demonstrating the singlet mechanism of trans—cis photoisomerization and the stilbene-like behavior of Ph-2N.     

Molecular mechanisms of the photostability of indigo

The photophysics of indigo as well as of bispyrroleindigo, the basic chromophore of indigo, has been investigated with ab initio electronic-structure calculations. Vertical electronic excitation energies and excited-state potential-energy profiles have been calculated with the CASSCF, CASPT2 and CC2 methods. The calculations reveal that indigo and bispyrroleindigo undergo intramolecular single-proton transfer between adjacent N-H and C=O groups in the (1)ππ* excited state. The nearly barrierless proton transfer provides the pathway for a very efficient deactivation of the (1)ππ* state via a conical intersection with the ground state. While a low-lying S(1)-S(0) conical intersection exists also after double-proton transfer, the latter reaction path exhibits a much higher barrier. The reaction path for trans→cis photoisomerization via the twisting of the central C=C bond has been investigated for bispyrroleindigo. It has been found that the twisting of the central C=C bond is unlikely to play a role in the photochemistry of indigo, because of a large potential-energy barrier and a rather high energy of the S(1)-S(0) conical intersection of the twisted structure. These findings indicate that the exceptional photostability of indigo is the result of rapid internal conversion via intramolecular single-proton transfer, combined with the absence of a low-barrier reaction path for the generation of the cis isomer via trans→cis photoisomerization.     

Influence of solvent polarity and medium acidity on the UV-Vis spectral behavior of 1-methyl-4-[4-amino-styryl] pyridinum iodide

Electronic absorption, and excitation spectra of 1-methyl-4-[4-aminostyryl] pyridinum iodide (M-NH2) were measured in solvents of different polarity. The (M-NH2) dye exhibits negative solvatochromism, i.e. a hypsochromic band shift as the solvent polarity increases. The fluorescence quantum yield is also sensitive to the polarity and viscosity of the medium. The ground and excited state protonation constants were calculated and amount to 3.35 and 0.62, respectively. The effect of micellization on the emission spectrum of (M-NH2) are also studied in sodium dodecyl sulphate (SDS). The fluorescence intensity increases as the concentration of SDS increases with an abrupt change at cmc. The quantum yield of the cis trans photoisomerization is also determined in aqueous buffer solution of pH 1.1.     

Probing highly efficient photoisomerization of a bridged azobenzene by a combination of CASPT2//CASSCF calculation with semiclassical dynamics simulation

Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S(0) and S(1) potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S(0) and S(1) states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S(1) → S(0) decay funnel. The S(1) state lifetime is ～30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S(1) relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S(0) state via CI, formation of the trans product occurs as the downhill motion on the S(0) surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.     

Photoisomerization dynamics of N-1-methyl-2-(tolylazo) imidazole and the effect of complexation with Cu(ii)

Azo-compounds containing an imidazole moiety have the potential to photoregulate biofunctions, such as gene-expression and enzymatic action. Photoinduced isomerization of the azo-backbone is the vital process for such applications, but the photoisomerization dynamics of azo-imidazole compounds has not been well explored. We investigated the photoisomerization dynamics of trans-N-1-methyl-2-(tolylazo) imidazole (trans-MTAI) using femtosecond transient absorption spectroscopy following photoexcitation to the S(2) state. Time evolution of the transient electronic spectra and the global analysis of the temporal profiles reveal a three state relaxation (S(2) → S(1) → S(0)) process in different kinds of solvents. The lifetime of the S(2) state is independent of the viscosity of solvent, whereas that of the S(1) state becomes longer with increasing solvent viscosity. This observation clearly indicates that the large amplitude motion that leads to the trans → cis isomerization occurs only in the S(1) state and relaxation of the S(2) state is not associated with the isomerization process. We have also investigated the excited state dynamics of [Cu(trans-MTAI)(2)]Cl(2) to examine the effect of complexation with the metal ion on the isomerization dynamics of trans-MTAI. It is observed that the photoinduced isomerization of the azo-backbone in trans-MTAI is completely inhibited upon complexation with Cu(ii).     

Cyclooctatetraene computational photo- and thermal chemistry: a reactivity model for conjugated hydrocarbons

We use ab initio CASSCF and CASPT2 computations to construct the composite multistate relaxation path relevant to cycloocta-1,3,5,7-tetraene singlet photochemistry. The results show that an efficient population of the dark excited state (S(1)) takes place after ultrafast decay from the spectroscopic excited state (S(2)). A planar D(8)(h)-symmetric minimum represents the collecting point on S(1). Nonadiabatic transitions to S(0) appear to be controlled by two different tetraradical-type conical intersections, which are directly accessible from the S(1) minimum following specific excited-state reaction paths. The higher-energy conical intersection belongs to the same type of intersections previously documented in linear and cyclic conjugated hydrocarbons and features a triangular -(CH)(3)- kink. This point mediates both cis --> trans photoisomerization and cyclopropanation reactions. The lowest energy conical intersection has a boat-shaped structure. This intersection accounts for production of semibullvalene or for double-bond shifting. The mapping of both photochemical and thermal reaction paths (including also Cope rearrangements, valence isomerizations, ring inversions, and double-bond shifting) has allowed us to draw a comprehensive reactivity scheme for cyclooctatetraene, which rationalizes the experimental observations and documents the complex network of photochemical and thermal reaction path interconnections. The factors controlling the selection and accessibility of a number of conjugated hydrocarbon prototype conical intersections and ground-state relaxation channels are discussed.     

Population branching in the conical intersection of the retinal chromophore revealed by multipulse ultrafast optical spectroscopy

The branching ratio of the excited-state population at the conical intersection between the S(1) and S(0) energy surfaces (Φ(CI)) of a protonated Schiff base of all-trans retinal in protic and aprotic solvents was studied by multipulse ultrafast transient absorption spectroscopy. In particular, pump-dump-probe experiments allowed to isolate the S(1) reactive state and to measure the photoisomerization time constant with unprecedented precision. Starting from these results, we demonstrate that the polarity of the solvent is the key factor influencing the Φ(CI) and the photoisomerization yield.     

Stairway to the conical intersection: a computational study of the retinal isomerization

The potential-energy surface of the first excited state of the 11-cis-retinal protonated Schiff base (PSB11) chromophore has been studied at the density functional theory (DFT) level using the time-dependent perturbation theory approach (TDDFT) in combination with Becke's three-parameter hybrid functional (B3LYP). The potential-energy curves for torsion motions around single and double bonds of the first excited state have also been studied at the coupled-cluster approximate singles and doubles (CC2) level. The corresponding potential-energy curves for the ground state have been calculated at the B3LYP DFT and second-order M?ller-Plesset (MP2) levels. The TDDFT study suggests that the electronic excitation initiates a turn of the beta-ionone ring around the C6-C7 bond. The torsion is propagating along the retinyl chain toward the cis to trans isomerization center at the C11=C12 double bond. The torsion twist of the C10-C11 single bond leads to a significant reduction in the deexcitation energy indicating that a conical intersection is being reached by an almost barrierless rotation around the C10-C11 single bond. The energy released when passing the conical intersection can assist the subsequent cis to trans isomerization of the C11=C12 double bond. The CC2 calculations also show that the torsion barrier for the twist of the retinyl C10-C11 single bond adjacent to the isomerization center almost vanishes for the excited state. Because of the reduced torsion barriers of the single bonds, the retinyl chain can easily deform in the excited state. Thus, the CC2 and TDDFT calculations suggest similar reaction pathways on the potential-energy surface of the excited state leading toward the conical intersection and resulting in a cis to trans isomerization of the retinal chromophore. According to the CC2 calculations the cis to trans isomerization mechanism does not involve any significant torsion motion of the beta-ionone ring.     

Transient Raman studies of the cis—trans photoisomerization processes in stilbene and retinal

This article reviews the present status of the application of transient or time-resolved Raman spectroscopy to the mechanistic studies of the cis—trans photoisomerization in conjugated molecules. Attention is focused on the vibrational information about the molecular structure of the electronically excited intermediates which are involved in the process of photoisomerization. Two examples, the trans to cis photoisomerization of stilbene through the lower excited singlet state and the cist to trans isomerization of retinal through the lowest excited triplet state, are included.     

Excited state behaviour of some trans-stilbene analogues bearing thiophene rings

The effect of replacing the phenyl group(s) of trans-stilbene with thienyl groups, or polycyclic groups containing a thienyl moiety, on the relaxation properties of the lowest excited states was studied by fluorometric, photochemical and laser flash photolysis techniques, as well as by theoretical calculations, for four trans compounds in non-polar and polar solvents. In some cases, a larger contribution of intersystem crossing and, consequently, a triplet mechanism to trans → cis photoisomerization, with respect to the parent hydrocarbons, was found. Although the compound with a single thienyl group, 2-styrylthiophene, shows reactive relaxation in the singlet manifold as in the case of stilbene, the presence of two heteroaromatic rings in di(2-thienyl)ethene enhances the spin-orbit coupling, thus leading to a mixed singlettriplet mechanism in non-polar solvents. The presence of polycondensed rings in dibenzothienylethene and thienyl-naphthothienylethene reduces the isomerization yield due to an increase in the torsional barrier for twisting in the singlet manifold. Therefore these compounds deactivate mainly through fluorescence emission and intersystem crossing, which leads to a predominant triplet mechanism for trans → cis photoisomerization. Polar solvents reduce the activation barrier to twisting, thus favouring isomerization in the singlet manifold.     

Inertial effects on the intramolecular vibrational energy redistribution and nonadiabatic photoisomerization of a 2,3-substituted 1,3-butadiene: a quasi-classical CASSCF dynamics study

Quasi-classical CASSCF trajectory calculations have been carried out on s-cis-1,3-butadiene and substituted 2,3-dideuterio-1,3-butadiene (DDB) to assess the inertial effect on the ultrafast nonadiabatic deactivation of their first singlet excited states. Calculations indicate that even this modest increase in the mass of the 2,3-substituents noticeably affects the photodynamics of cis --> trans isomerization, by reducing the efficiency of the vibrational energy leakage between the initial relaxation and subsequent nonadiabatic decay modes. In qualitative agreement with experimental findings on related 1,3-dienes, the slowing down of the intramolecular vibrational energy redistribution (IVR) upon substitution results in extended excited-state lifetimes and reorients the photoregioselectivity toward cis rotamers and cyclic products.     

Electrostatic control of the regioselectivity in the photoisomerization of trans,trans-1-fluoro-2,4-hexadiene: evidence for competing conical intersections

trans,trans-1-Fluoro-2,4-hexadiene (EE-FHD) was used to explore the excited potential energy surfaces of acyclic 1,3-dienes. Our investigations show that the regioselectivity of double-bond photoisomerization is primarily controlled by the charge-stabilizing characteristic of substituents on the double bonds of the diene. Furthermore, changes in the photoregioselectivity with solvent polarity suggest that the excited unsymmetrical diene returns to the ground state by competing pathways which traverse polarized 1Bu states and drop into two different conical intersections     

Tetrathiafulvalenenaphthalenophanes: planar chirality and cis/trans photoisomerization

A cyclophane incorporating one 1,5-dioxynaphthalene ring system and one tetrathiafulvalene (TTF) unit bridged by [SCH(2)CH(2)O] linkages has been synthesized. In this cyclophane, the TTF unit can adopt either cis or trans configurations. In addition, the 1, 5-dioxynaphthalene ring system imposes one element of planar chirality on this cyclophane. A second element of planar chirality is introduced by the trans form of the TTF unit. Thus, the cyclophane exists in diastereoisomeric forms as three pairs of enantiomers. The enantiomeric pairs associated with the cis form of the TTF unit, as well as one of those associated with the trans form, have been isolated by crystallization, and their structures assigned in the solid state by single-crystal X-ray analyses. In solution, cis/trans isomerization occurs when either the cis or the trans form of the cyclophane is exposed to light. The photoisomerization reaction can be followed by (1)H NMR and UV-vis spectroscopies, as well as by HPLC. The photoisomerization quantum yield has been measured at two different excitation wavelengths (406 and 313 nm). In both cases, the trans --> cis process (Phi = 0.20 at 406 nm) is much more efficient than the reverse cis --> trans process (Phi = 0.030 at 406 nm). Since the absorption spectra of the trans and cis isomers are different and the quantum yield of the trans --> cis photoisomerization reaction depends on the excitation wavelength, the mole fraction of the two diastereoisomers present at the photostationary state depends on the wavelength of the exciting light. No isomerization occurs when the solutions, regardless of the mole fraction of the two diastereoisomers, are stored in the dark.     

A multireference configuration interaction investigation of the excited-state energy surfaces of fluoroethylene (C2H3F)

Multireference configuration interaction with singles and doubles (MR-CISD) calculations has been performed for the optimization of conical intersections and stationary points on the fluoroethylene excited-state energy surfaces. For the planar ground state geometry, the vertical spectrum including 3s and 3p Rydberg states was calculated. From this geometry, a rigid torsion around the CC bond strongly reduces the energy gap between S0 and S1 states. Furthermore, a search for the minimum of the crossing seam shows that there exists a conical intersection close to the twisted structure and two additional ones for cis and trans pyramidalized structures. These three intersections are connected by the same seam. We have shown that the Hula-Twist process is an alternative way to the direct CC twisting in order to reach this part of the seam. Other conical intersections were also located in the CH3CF and CH2FCH, H-migration, and C(3v) structures. The photodynamics of the system is discussed based on topological features of these intersections.     

Singlet excited-state dynamics of 5-fluorocytosine and Cytosine: an experimental and computational study

The photophysics of singlet excited 5-fluorocytosine (5FC) was studied in steady-state and time-resolved experiments and theoretically by quantum chemical calculations. Femtosecond transient absorption measurements show that replacement of the C5 hydrogen of cytosine by fluorine increases the excited-state lifetime by 2 orders of magnitude from 720 fs to 73 +/- 4 ps. Experimental evidence indicates that emission in both compounds originates from a single tautomeric form. The lifetime of 5FC is the same within experimental uncertainty in the solvents ethanol and dimethyl sulfoxide. The insensitivity of the S(1) lifetime to the protic nature of the solvent suggests that proton transfer is not the principal quenching mechanism for the excited state. Excited-state calculations were carried out for the amino-keto tautomer of 5FC, the dominant species in polar environments, in order to understand its longer excited-state lifetime. CASSCF and CAS-PT2 calculations of the excited states show that the minimum energy path connecting the minimum of the (1)pi,pi state with the conical intersection responsible for internal conversion has essentially the same energetics for cytosine and 5FC, suggesting that both bases decay nonradiatively by the same mechanism. The dramatic difference in lifetimes may be due to subtle changes along the decay coordinate. A possible reason may be differences in the intramolecular vibrational redistribution rate from the Franck-Condon active, in-plane modes to the out-of-plane modes that must be activated to reach the conical intersection region.     

Photochemical dimerization of acephenanthrylene and the heavy atom effect

The solution-phase photodimerization of acephenanthrylene (1) in cyclohexane, chloroform, dibromomethane, and ethyl iodide with visible light (lambda = 419 nm) occurs to form cis (2) and trans (3) photodimers in ratios that vary with the solvent and with the presence or absence of oxygen. The photodimerization exhibits a significant heavy atom effect and also is sensitive to selective quenching by dissolved oxygen, in which only cis stereoisomers are produced. Irradiation of the dimers (lambda = 254 nm) shows that the trans dimers cleave more rapidly than the cis dimers. This is attributed to stabilization of the excited state in the cis dimers.     

Photophysical behaviour of some pyrenylethylene derivatives and its implication on trans → cis photoisomerisation reactions

The fluorescence properties of some pyrenylethylene derivatives capable of exhibiting twisted intramolecular charge transfer phenomenon havebeen studied as a function of solvent polarity. The solvent polarity dependent change in the fluorescence quantum yield has been interpreted in terms of intramolecular retational relaxations. The trans → cis photoisomerization data of the compounds indicate that the twist around the ethylenic double bond is accelerated in polar solvents.     

Synthesis, characterization, and photochemical properties of azobenzene-conjugated Ru(II) and Rh(III) bis(terpyridine) complexes

We synthesized azobenzene-conjugated bis(terpyridine) Ru(II) and Rh(III) mononuclear and dinuclear complexes and investigated their photochemical properties on excitation of the azo pi-pi band upon 366 nm light irradiation. The Ru mononuclear complex underwent trans-to-cis photoisomerization to reach the photostationary state with only 20% of the cis form, while the Ru dinuclear complex did not isomerize at all photochemically. On the other hand, the mononuclear and dinuclear Rh complexes showed almost complete trans-to-cis photoisomerization behavior. Cis forms of the Rh complexes thermally returned to the trans form at a much slower rate than those of organic azobenzenes, but they did not isomerize photochemically. The reduction potential of the cis forms was 80 mV more negative than that of the trans forms. The photoisomerization quantum yields of the Rh complexes were strongly dependent on the polarity, viscosity, and donor site of the solvents as well as the size of the counterions. We investigated the photoisomerization process of these complexes using femtosecond absorption spectroscopy. For the Rh complexes, we observed S(n) <-- S(2) and S(n) <-- S(1) absorption bands similar to those of organic azobenzenes. For the Ru complexes, we observed very fast bleaching of the MLCT band of the Ru complex, which indicated that the energy transfer pathway to the MLCT was the primary cause of the depressed photoisomerization. The electronic structures, which were estimated from ZINDO molecular orbital calculation, supported the different photochemical reaction behavior between the Ru and Rh complexes.