Modeling the structure,absorption spectra,and cis-trans isomerization of thiacarbocyanine dyes

The relative stability of the trans-and cis-isomers of 3,3′-diethylthiacarbocyanine (Dye1) and 3,3′-diethyl-9-methylthiacarbocyanine (Dye2)¹, as well as sections of the potential energy surfaces along the internal coordinate of the isomerization reaction, were studied using the density functional theory. Calculation of the minimum energy pathway for the isomerization reaction showed that the barrier for rotation about the C₈–C₉ bond is higher for Dye1 than for Dye2. Local minimums were found for the singlet excited state of the 8,9-cis-and trans-isomers of the dyes. In the case of the trans-isomers, substantial changes in the dye structure do not occur and the local minimum of the excited state corresponds to the geometry of the starting trans-isomers, which favors efficient fluorescence. A search for the nearest local minimum of the singlet excited state of the 8,9-cis-isomers leads to structures, which differ significantly from the starting structures, and the intensity of the S₁ → S₀ transition in those structures appears to be practically zero. The results are in agreement with experimental data on the absorption, fluorescence, and fluorescence excitation spectra of the dyes.     

Photochemistry and photophysics of stilbene dendrimers and related compounds

The syntheses and reactions of photoresponsive dendrimers are described. Dendrimers with photoreversible stilbene cores undergo mutual cis–trans isomerization in organic solvents to give photostationary state mixtures of cis- and trans-isomers. Even stilbene dendrimers with molecular weights as high as 6500 underwent mutual cis–trans isomerization within the lifetime of the excited singlet state. The large dendron group surrounding the photoreactive core may affect the excited state properties of the core to induce the efficiency of photoisomerization and/or reduce the fluorescence efficiency. The photochemistry of stilbene dendrimers, with various types of dendron groups, azobenzene dendrimers and other photoresposive dendrimers is discussed.     

Quantum mechanical study of energetics and mechanisms of cis–trans isomerization of some four-membered heterocycles

Ab initio quantum mechanical calculations using density functional (B3LYP) method and 6-311G** basis set have been performed on two cis and trans conformers of 2,4-diphenyl thietane dioxide (DPTD), 2,4-diphenyl thietane (DPT), 2,4-diphenyl azetidine (DPA) and 2,4-diphenyl oxetane (DPO). The calculated stability energy for cis–trans isomerization in gas phase and in different solvents such as benzene, DMSO, water and methanol indicated that the cis conformer is more stable than trans in all above-mentioned compounds about 11–2 kcal mol^?1. In the next step, a transition states for cis–trans inter-conversion for all four-membered heterocycles (DPTD, DPT, DPA and DPO) were proposed in methanol as solvent. Thermodynamic functions such as standard enthalpies of isomerization (?Hº_iso), standard entropy of isomerization (?Sº_iso) and standard Gibbs free energy of isomerization (?Gº_iso) for all studied compounds were also evaluated. The calculation showed that the conversion of trans to cis isomer is exothermic and spontaneous. In all calculations, solvent effects were considered using a polarized continuum model.     

One-way isomerization and internal rotation in anthrylethylenes in the excited triplet state

1-(2-Anthryl)-2-(bromophenyl)ethylene and 1-(2-anthryl)-2-(4-methoxyphenyl)ethylene undergo cis→trans one-way isomerization in the excited triplet state through an adiabatic process from the cis-triplet to the trans-triplet states. The trans-isomers of these compounds undergo one-way internal rotation in the excited triplet state with an activation barrier of }7 kcal mol^?1 and a frequency factor of }10¹² s^?1, while no internal rotation takes place in the excited singlet state.     

Photoisomerization of 2-styrylquinoline in neutral and protonated forms

The quantum yields of the trans-cis and cis-trans photoisomerization of 2-styrylquinoline (2SQ) and its several derivatives were measured in neutral, protonated, and quaternized forms. It was shown that electron-donor substituents in the styryl moiety increase the quantum yield of trans-cis photoisometization ?_tc in the neutral form as a result of stabilization of the intermediate zwitterionic perpendicular conformer. On passing from the neutral to the positively charged forms (protonated or quaternized), an increase in the quantum yields to ?_tc > 0.5 was observed, thus suggesting in terms of the classical diabatic mechanism of photoisomerization via the perpendicular conformer the shift of the minimum on the potential energy surface (PES) of the S ₁ state relative to the maximum of the S ₀ state PES to the cis-isomer or a possible contribution of the adiabatic route to the photoisomerization of the 2SQ cations.     

Trans-cis photoisomerization and photoinduced electron transfer as competitive reactions of 9-etylthiacarbocyanine in binary mixtures

The effect of the composition of the dioxane-water mixture on the ability of 9-ethylthiacarbocyanine to participate in competitive reactions of trans-cis photoisomerization and photoinduced electron transfer was studied. An increase in the dioxane content in the range 0–50 vol % leads to a shift of the equilibrium between the dye dimers and monomers toward the monomers (cis-monomers), which is accompanied by a drop in the yield of the triplet dimer molecules, which appear under the action of a laser flash, and electron-transfer products that are formed via the triplet state of dimers in the presence of methylviologen. With growing the dioxane content in the range 50–80 vol %, a shift of the equilibrium between the cis-and trans-monomers toward the trans-monomers occurs, which is accompanied by an increase in the fluorescence intensity. At the dioxane content above 80 vol %, a further shift of the equilibrium toward the trans-monomers occurs, which is accompanied by a substantial increase in the fluorescence intensity and an appearance under the action of a laser flash of the band of the triplet-triplet absorption of the trans-monomer and the absorption band of the cis-monomer as a result of trans-cis photoisomerization. The trans-monomers in the triplet state participate in the electron-transfer reaction with methylviologen. The intersystem crossing process competes with fluorescence and the trans-cis isomerization reaction, which occurs via the excited singlet state of the trans-monomers.     

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     

An ab initio study of the electronic structure of diimide

Ab initio calculations on the structure and geometry of the three isomers of N₂H₂ (trans-diimide, cis-diimide, and 1,1-dihydrodiazine) were performed both on HF and CI level using gaussian basis sets with polarization functions. The trans and cis isomers have singlet ground states; the trans isomer is found to be lower in energy than the cis isomer by 6.9 kcal/mol (HF) and 5.8 kcal/mol (CI), respectively. The barrier for the trans-cis isomerization is predicted to be 56 (HF) and 55 (CI) kcal/mol. H₂ N=N has a triplet ground state with a non-planar equilibrium geometry and a rather long NN bond of 1.34 Å. Its lowest singlet state, however, is planar with an NN double bond of 1.22 Å; it is found to lie about 3 kcal/mol above the triplet and 26 kcal/mol above the singlet ground state of trans-diimide.     

Study of <Emphasis Type="Italic">cis</Emphasis>–<Emphasis Type="Italic">trans</Emphasis> isomerization mechanism of 3,3′-azobenzene disulphonate in the lowest singlet and triplet electronic states by density functional theory

Abstract  

The cis–trans isomerization pathways of 3,3′-azobenzene disulphonate in the S₀ and T₁ states are studied by DFT method at the B3LYP/6-31G(d,p) level. In the S₀ state, the cis–trans isomerization concerns the complex pathway that is characterized by the inversion of one NNC angle combined with rotation around the NC bond, and the three sequential transition states are also found on the potential energy profile. Therefore, the cis–trans isomerization of 3,3′-azobenzene disulphonate can be understood in terms of a pathway involving successive rotation, inversion, and rotation processes. The energy barrier of the S₀ state is 22.79 kcal mol⁻¹. In the T₁ state, the isomerization mainly concerns the rotational pathway around the NN double bond, and the two isomers are connected through only one transition state. The isomerization of the T₁ state is related to a lower energy barrier, 5.02 kcal mol⁻¹, but requires a change in spin-multiplicity.     

The di-π-methane rearrangement of systems with simple vinyl moieties : Mechanistic and exploratory organic photochemistry

3,3-Dimethyl-1,1-diphenyl-1,4-pentadiene and two 5-substituted derivatives were synthesized and studied. The regioselectivity, stereochemistry, quantum efficiency, multiplicity, and excited state reaction rates were studied in each case. The parent hydrocarbon, 5-MeO-derivative, and 5-cyano-diene—all rearranged on direct irradiation to give vinylcyclopropanes. The first compound led to 3,3-dimethyl-2,2-diphenyl-1-vinylcyclopropane. The second afforded 3,3-dimethyl-2,2-diphenyl-1-(2'-methoxyvinyl)cyclopropane. The last gave 1-cyano-3,3-dimethyl-2-(2',2'-diphenylvinyl)cyclopropane. Thus, the vinyl and methoxyvinyl groups survive in the products intact, while the cyanovinyl group is incorporated in the three-ring. In the two substituted dienes, cis-reactant gave cis-product and trans-reactant gave trans-product, both where the substituent was on the vinyl group of the product and where it became a ring substituent. The substituted di-π-methane systems underwent only cis-trans isomerization on sensitization, while the parent, unsubstituted diene led to di-π-methane product on sensitized as well as direct photolysis. While the quantum yields for the hydrocarbon diene were the same at room temperature for the direct and sensitized runs, only the sensitized runs showed a temperature dependence of efficiency with a dramatic, 5-fold increase on a 46° temperature increase. Thus, evidence was obtained for a singlet rearrangement in all cases and a triplet process only in the case of the unsubstituted diene. A sizable activation energy was seen for the triplet but not for the singlet. The room temperature quantum yields in the direct irradiations were: φ(parent diene)=0.011, φ(trans-methoxydiene)=0.051, φ(cis-methoxy-diene)= 0.050, φ(trans-cyanodiene)=0.36, and φ(cis-cyano-diene) = 0.20. A competing side reaction was cis-trans isomerization but these quantum yields were lower. Single photon counting was employed to obtain excited singlet reaction and decay rates at low temperature (i.e. 77°K) and the method of magic multipliers was used to obtain room temperature rates. These were: k_r(parent diene) = 4.7 × 10⁸ sec^?1, k_r(trans-cyano-diene)= 1.5 ×10¹⁰ sec^?1, k_r(cis-cyano-diene)= 8.0 × 10⁹sec^?1, and k_r(trans-methoxy-diene) = 1.9 × 10⁹ sec^?1. The results are discussed in terms of excited state molecular structure.An SCF-CI molecular orbital treatment of the reaction was developed. This used a cyclopropyldicarbinyl diradical species, with Walsh cyclopropane basis orbitals, as representing the half-reacted species. The energy of formation of this species from vertical excited state reactant was calculated for all three dienes and an excellent correlation with observed excited singlet rates was obtained. Similarly, dissection of the excited diradical energy into bond components led to a correlation between regioselectivity and weakness of the three-ring bond broken in the regioselectivity-determining step. Evidence was adduced for localization of the excitation energy in S₁ of reactant in the diphenylvinyl chromophore with migration of electronic excitation into the cyclopropyldicarbinyl diradical moiety during the vinyl-vinyl bridging process. A general method for quantitatively partitioning excitation energy was developed and applied to the case in hand. Finally, there was predicted a greater probability of di-π-methane three-ring fission in the excited state compared to the diradical ground state where Grob fragmentation proved energetically more favorable.     

Low-Temperature Photochemistry of Previtamin D: A Hula-Twist Isomerization of a Triene

What is the standard course of the cis – trans isomerization of nonpolar polyenes? Many results support a pathway in which only a central CH group rotates out of the plane, while the remaining parts of the molecule reorient within the plane (“hula-twist” mechanism). The intermediate structure through which the molecule passes corresponds to the geometry predicted for the conical intersection of the S₁ and S₀ potential energy surfaces (see picture).     

Laser induced optoacoustic spectroscopy of benzil: Evaluation of structural volume change upon photoisomerization

The cis-skew to trans-planar photoisomerization of benzil in the photoexcited state was studied by laser-induced optoacoustic spectroscopy (LIOAS) in a series of normal alkane solvents at room temperature. The structural volume change due to the photoisomerization process has been estimated and compared with the same calculated from the optimized skew and trans structures. The magnitude of the structural volume change is estimated to be 22.9 ± 1.2 mL/mol. This study reveals that an expansion in volume occurs during the photoinduced isomerization process. In non-polar alkane solvents, the estimated volume change has been argued to be free from electrostrictional contribution.     

Cis–trans isomerization and spin multiplicity dependences on the static first hyperpolarizability for the two-alkali-metal-doped saddle[4]pyrrole compounds

Doping two alkali-metal atoms (Li and Na) into the saddle-shaped saddle[4]pyrrole forms four new two-alkali-metal-doped compounds with alkalide or electride characteristic. They are cis-LiNa(saddle[4]pyrrole) isomers 1(singlet) and 2(triplet), and trans-Li(saddle[4]pyrrole)Na isomers 3(singlet) and 4(triplet). The four structures with all-real frequencies are obtained at the density functional theory (DFT) B3LYP/6-311+G(d) level. All calculations of electric properties have been carried out at the second order Møller–Plesset perturbation theory (MP2) level. The order of the β ₀ values is 3.54 × 10³ for trans-4(triplet) <1.51 × 10⁴ for cis-1(singlet) <3.57 × 10⁴ for cis-2(triplet) <2.34 × 10⁵ a.u. for trans-3(singlet). The static first hyperpolarizability (β ₀) depends on the cis–trans isomerization and spin multiplicity. The result demonstrates that the cis–trans isomerization and spin multiplicity controls of the second-order NLO response are possible.     

Theoretical Study of Solvent Effects on the Cis-to-Trans Isomerization of [Pd(C6Cl2F3)I(PH3)2]

In this work, the effect of different solvents on the mechanism of the cis-to-trans isomerization of [Pd(C₆Cl₂F₃)(I)(PH₃)₂] has been investigated theoretically in detail. Using a quantum mechanical approach, different pathways were investigated. A three-pathway mechanism has already been proposed which consists of two PH₃-sensitive steps (k ₃, k ₄) and one PH₃-insensitive (k _diss) step. Since in the k ₃ pathway the solvent has two types of explicit and implicit effects, this path was investigated for both the gas phase and solvent systems (using the PCM model). In this path, solvents with larger donor numbers (necessary condition) and smaller dielectric constant and dipole moment values (sufficient condition) are more appropriate. In the k ₄ pathway, the solvent has an implicit effect only, and the smaller are the dielectric constant and dipole moment of a solvent, the more appropriate it is. To find the best solvent, a parameter, called the average activation energy, was defined, which considers the contribution of each path in the mechanism.     

Chemicals from Synthesis Gas : By Robert A. Sheldon (Series: Catalysis by Metal Complexes) D. Reidel Publishing Company,Dordrecht, The Netherlands, 1983, xx + 216 pages,Dfl. 110.00, US$ 48.00. ISBN 90-277-1489-4.

Photolysis of cis-HMn(CO)₄PPh₃ in the presence of H₂ and 1-alkene results in catalytic hydrogenation and isomerization of the alkene. The isomerization leads to cis- and trans-2-alkene in the presence or absence of H₂. Catalytic hydrogenation also occurs when cis-CH₃Mn(CO)₄PPh₃ is irradiated in the presence of H₂; use of D₂ leads exclusively to CH₃D. The possible mechanism of the hydrogenation is discussed.     

Laser photolysis study of trans→cis photoisomerization of trans-1-phenyl-2-(2-naphthyl) ethylene

The direct trans→cis photoisomerization of trans-1-phenyl-2-(2-naphthyl) ethylene (trans-PNE) in liquid solution at room temperature was studied by the nanosecond laser photolysis technique. The time-resolved S_n←S₁ and T_n←T₁ absorption spectra were observed with trans-PNE at 300 K and 77 K. The lifetime of the triplet state of trans-PNE was found to be much shorter in liquid solution at room temperature than in rigid solution at 77 K. This fact and the effect of a triplet quencher shows that the photoisomerization of trans-PNE occurs mainly via the triplet state.     

Light-induced cis/trans isomerization of cis-[Pd(L-S,O)2] and cis-[Pt(L-S,O)2] complexes of chelating N,N-dialkyl-N′-acylthioureas: key to the formation and isolation of trans isomers

Irradiation cis-[M(Lⁿ-S,O)₂] complexes (M = Pt^II, Pd^II) derived from N,N-dialkyl-N′-benzoylthioureas (HLⁿ) with various sources of intense visible polychromatic or monochromatic light with λ < 500 nm leads to light-induced cis?→?trans isomerization in organic solvents. In all cases, white light derived from several sources or monochromatic blue-violet laser 405 nm light, efficiently results in substantial amounts of the trans isomer appearing in solution, as shown by ¹H NMR and/or reversed-phase HPLC separation in dilute solutions at room temperature. The extent and relative rates of cis/trans isomerization induced by in situ laser light (λ = 405 nm) of cis-[Pd(L²-S,O)₂] was directly monitored by ¹H NMR and ¹⁹⁵Pt NMR spectroscopy of selected cis-[Pt(L-S,O)₂] compounds in chloroform-d; both with and without light irradiation allows the δ(¹⁹⁵Pt) chemical shifts cis/trans isomer pairs to be recorded. The cis/trans isomers appear to be in a photo-thermal equilibrium between the thermodynamically favored cis isomer and its trans counterpart. In the dark, the trans isomer reverts back to the cis complex in what is probably a thermal process. The light-induced cis/trans process is the key to preparing and isolating the rare trans complexes which cannot be prepared by conventional synthesis as confirmed by the first example of trans-[Pd(L-S,O)₂] characterized by single-crystal X-ray diffraction, deliberately prepared after photo-induced isomerization in acetonitrile solution.     

Photoisomerization of trans‐2‐[4′‐(Dimethylamino)styryl]benzothiazole

Photoreaction of trans‐2‐[4′‐(dimethylamino)styryl]benzothiazole (t‐DMASBT) under direct irradiation has been investigated in dioxane, chloroform, methanol and glycerol to understand the mechanism of photoisomerization. Contrary to an earlier report, isomerization takes place in all these solvents including glycerol. The results show that restriction on photoisomerization leads to the increase in fluorescence quantum yield in glycerol. The results are consistent with the theoretically simulated potential energy surface reported earlier using time‐dependent density functional theory (TDDFT) calculations. DFT calculations on cis isomers under isolated condition have suggested that cis‐B conformer is more stable than cis‐A conformer due to hydrogen‐bonding interaction. In the ground state, cis‐DMASBT is predominantly present as cis‐B. The fluorescence spectra of the irradiated t‐DMASBT suggested that photoisomerization follows not the adiabatic path as proposed by Saha et al., but the nonadiabatic path.     

Derivatographic studies on transition metal complexes. XII. Thermal isomerization of trans-[CrBr2en2]Br·H2O and cis-[CrBr2tn2]Br·2 H2O in the solid phase

The thermal trans-to-cis isomerization of trans-[CrBr₂en₂]Br·H₂O and cis-to-trans isomerization of cis-[CrBr₂tn₂]Br·2 H₂O were studied by means of derivatographic and isothermal measurements. In both cases isomerization took place in anhydrous state after the complete dehydration. The activation energies for dehydration of the above two complexes were evaluated isothermally to be 61 and 38 kJ mole^?1, respectively, and for their isomerization to be 420 and 275 kJ mole^?1, respectively. These data were compared with those for the corresponding chloro complexes and the ease of isomerization between them was discussed.     

Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC → TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S₁→S₀ internal conversion (major), intersystem crossing S₁→T₁, pyran ring formation, photoenolization and TC → TT isomerization. Computations support the S₁→S₀ and T₁→S₀ channels as responsible for photostabilization of the TC form.