Fabio Momicchioli

Theoretical study of trans-cis photoisomerism in polymethine cyanines

Abstract The trans-cis photoisomerism of polymethine dyes has been interpreted so far using different and rather inconsistent models of the potential energy surfaces. In order to search for a unified electronic model, we tackled the problem again from an intramolecular point of view. Our study consisted in qualitative MO considerations followed by explicit (CS INDO) calculations of the S 0 , T 1 and S 1 potential energy curves for a proper model system: pentamethine cyanine isomerizing around the 2–3 and 3–4 bonds. Torsional energy levels for the calculated potential curves were also obtained. The main conclusions are: (i) the photoreaction proceeds through a “non-spectroscopic” (perp) S 1 minimum which exists also in the isolated molecule, (ii) this twisted excited species has charge transfer character (TICT), as opposed to the biradicaloid character of the “non-spectroscopic” S 1 minimum involved in the trans→cis photoisomerization of olefines (e.g., stilbene). The possible consequences on the dynamics of the excited state relaxation in non-polar solvents are envisaged.

Trans-cis photoisomerization mechanism of carbocyanines: experimental check of theoretical models

Abstract Two different theoretical models for the cyanine photoisomerization, emphasizing, respectively, the role of the solvent effects (model a) and that of the intramolecular forces (model b), have been experimentally tested by investigating the photophysical and photochemical behaviour of two carbocyanines (DOC and DTC). The decay kinetics of the excited-state (S1) stable isomer and of the ground-state (S0) unstable photoisomer have been measured in solvents of very different polarities, but similar and low viscosities (methanol, methylene chloride and chlorobenzene were extensively investigated, but some test measurements were also carried out in p-dioxane and in 1:1 toluene-chlorobenzene). The activation energy for the non radiative decay of the spectroscopic S1 minimum, corresponding to a twisting around one of the polymethine bonds, is independent of solvent polarity, whereas a small but significant dependence is displayed by the activation energy and preexponential factor of the S0 back isomerization. These results can be easily accounted for within model b, while they sharply contrast with the predictions of model a. We have also measured the quantum yields of photoisomer formation for DOC and DTC at various temperatures and have shown that, in both cases, a radiationless decay occurs from the perp S1 minimum to the ground state, with a branching ratio favouring reformation of the stable (trans) isomer. The parallel temperature dependence of the fluorescence decay and photoisomer formation has provided further support to model b. Finally, the temperature dependence of the quantum yield for the cis→trans photoisomerization of DOC has been analyzed. A very low activation energy has been found, suggesting that this process is probably intrinsically barrierless.

A new INDO-type procedure for conjugated non-rogid molecules. II. Extension to electronic spectra and excited-state potential curves

Abstract Starting from the modified INDO scheme suggested in part 1 of this work, a new INDO-based procedure is formulated capable of handling both ground and lowest-lying excited states of conjugated non-rogid molecules with a satisfactory degree of accuracy. A “spectroscopic” parameterization [involving scaling down of both resonance integrals (β__) and electron repulsion integrals (γ AB )] is combined with a newly designed core-core repulsion formula which ensures that the correct balance between attractive and repulsive interactions is preserved. Extensive calculations on several conjugated hydrocarbons demonstrate that use of our INDO-based scheme with Ohno-Klopman γ integrals and polyexcited CI treatments of the excited states, leads to a good description of the electronic energy spectrum (for both triplet and lowest-lying singlet states), and at the same time produces excellent potential functions for the rotation about essential single bonds (while reasonable bond lengths are predicted as well). Considering its essential characteristics, we have termed this method CS INDO (where C and S stand for conformations and spectra, respectively). Preliminary studies, however, have indicated that the real field of application may well be wider.

A new indo-type procedure for conjugated non-rigid molecules. I. Ground-state conformations and barriers to internal rotation

Abstract As a first step of a theoretical study devoted to formulating a semi-empirical all-valence-electron method capabl of correctly describing both ground- and excited-state properties of large conjugated molecules we present a new INDO SCF technique especially devised fo conformational prediction. The procedure is founded on the use of a basis set of hybrid AOs which conserve their σ or π (local) symmetry in non planar geometries. In this representation the non-diagonal elements of the core matrix correspond to chemically well-defined interactions (σ—σ, π—π, σ—π) which can be differently screened by introducing specific factors ( k σσ , k ππ , k σπ ). Internal rotation potential curves were caculated for five significant test molecules (butadiene, biphenyl, styrene, cis- and trans-stilbene) by using two different sets of screening parameters. It was found that the use of the set k σσ = k ππ = 1, k σπ = 0.94 (which means reducing hyperconjugative stabilization in non-planar forms) is sufficient to describe correctly internal rotation around single bonds, while the use of the set k σσ = 1, k ππ = 0.75, k σπ = 0.80 allows cis—trans energy differences also to be correctly reproduced. In general, conformational predictions obtained by our modified INDO SCF procedure have proved as good as those derived by minimal basis set ab initio SCF calculations.

First- and second-order polarizabilities of simple merocyanines. An experimental and theoretical reassessment of the two-level model.

Taking four merocyanines [(CH3)2N-(CHCH)n-C(CH 3)O; n = 1-4] (Mc1-4) as test D-A systems, we performed a close experimental and theoretical examination of the two level model with reference to its ability to provide correct predictions of both absolute values and dependence on the conjugation path length of first- and second-order molecular polarizabilities. By (1)H NMR spectroscopy merocyanines Mc1-4 were found to be approximately 1:1 mixtures of two planar conformers with cis and trans arrangements of the C(CH 3)O electron-acceptor group and all trans structure of the polyene like fragment. The degree of bond length alternancy (BLA) in the -(CHCH)n- fragment, was quantified by extensive full geometry optimizations at both semiempirical and ab initio level. DFT (6-31G**/B3LYP) optimized geometries were considered to be most reliable and were used for calculations of the excited-state properties. The applicability of the two level model, reducing the general sum-over-states (SOS) expansion to only one term involving the ground state (g) and the lowest-lying (1)(pipi*) CT state (e), was checked by analysis of fluorescence and near UV absorption spectra. Measurements of the basic two-level model quantities ( Ege, microge and Deltamicro(eg)), by which the dominant components of alpha and beta tensors are expressed (alpha XX , beta XXX , X identical with long molecular axis), were designed to give approximate free-molecule values. It is proposed, in particular, an adjustment of the solvatochromic method for the determination of Deltamicro(eg), based on accurate measurements of absorption spectral shifts in n-hexane/diethyl ether mixtures with small diethyl ether volume fractions. Such an approach led to Mc1-4 beta XXX s matching well in both value and n-dependence with EFISH data reported in the literature for similar merocyanines. For the fluorescent Mc4, the results were qualitatively well reproduced by an approach, which combines absorption and fluorescence solvent shifts. All the measured quantities were calculated for both trans and cis Mc1-4 by three semiempirical INDO-based approaches aiming at evaluating the performances of different integral parametrizations and CI extensions: ZINDO/S, CS INDO SCI, CS INDO SDCI. In all cases, alpha XX and beta XXX were found to rise proportionally to about n (1.3) and n (2), respectively, in qualitatively good agreement with the experimental values. As to the absolute values, however, experimental alpha XX s and beta XXX s were best reproduced by CS INDO SDCI combining Ohno-Klopman parametrization and CI including both single and double excitations. The validity of the two-level model was checked by comparison with converged SOS calculations for the longest chain merocyanine (Mc4) and finite field calculations of linear polarizabilities for all of the four dyes (Mc1-4).

Potential energy curves and photochemical cis-trans isomerism in styrene

Cis-trans photoisomerism of styrene is investigated from a theoretical point of view. Curves of the potential energy as a function of the rotation angle (ω) around the ethylenic bond were obtained for the ground state and a few excited states by the CIPSI PCILO method. The potential surface of the lowest excited singlet is found to have both an absolute minimum at ω = π/2 and a small relative minimum at ω = 0. These findings are consistent with the singlet mechanism for the direct cis-trans photoconversion. The origin of the two energy minima in the S1 potential surface is explained in terms of weights of the local excitations in the zero-order wavefunction.

Electronic spectra and trans—cis photoisomerism of carbocyanines. A theoretical (CS INDO CI) and experimental study

Abstract Two carbocyanine dyes, DOC(3,3′-diethyloxacarbocyanine) and DTC (3,3′-diethylthiacarbocyanine), were studied in detail by a combined experimental and theoretical treatment previously adjusted on simple streptocyanines. The BPPC (bisphenylaminopentamethine cyanine) dye was also studied to better correlate the properties of DOC and DTC with those of their parent streptocyanine BMPC (bisdimethylaminopentamethine cyanine). In the experimental part of the work, carried out on methanol solutions of BPPC + , Cl − , DOC + , I − and DTC + , I − , we determined the separate spectra of the stable forms and the photoisomers produced by irradiation into the visible absorption region. The theoretical part, based on CS INDO S + D + T - CI calculations, was especially devised to provide as thorough an interpretation as possible of the electronic spectra (both S 0  S n and S 0  T n ) of BPPC, DOC and DTC in terms of molecular subunits and to try to identify each observed species as a specific geometrical isomer (all- trans or mono- cis ). In all cases the properties of the stable forms were consistent with the molecules assuming the all- trans structure. The lowest singlet excited state ( S 1 ), responsible for the colour band, retained nearly pure cyanine character ( 1 B in Platts notation), while the states falling in the second (medium UV) absorption region had prevailing aromatic ( 1 H ) or charge transfer ( 1 G ) character. This provided an explanation for the absence of a definite “ cis peak” effect in these compounds. BPPC was found to give rise to the same photoisomer as BMPC (3–4 cis ) and an additional photoisomer clearly identified as the 1–2 cis form. On the other hand, in DOC and DTC, where the spectral changes caused by the irradiation were far weaker, the single observed photoisomer could be better assigned as 2–8 cis , i.e. corresponding to the 2–3 cis form of BMPC.

Concerning interconversion of rotamers of trans-1,2-diarylethylenes in the triplet state

Abstract The applicability of the NEER (non-equilibration of excited rotamers) hypothesis to the lowest triplet state of trans -1,2-diarylethylenes is investigated theoretically in the light of recent triplet-related experimental findings. CS-INDO CI calculations indicate that in the T 1 state the barrier hindering interconversion of rotamers is rather high, 11 – 13 kcal mol −1 , for stilbene and its naphthyl analogues, but decreases to about 6 kcal mol −1 in anthryl analogues, such as 2-styrylanthracene (2-StA), in consequence of considerable localization of the triplet excitation on the anthracene moiety. This suggests that, in fluid solutions, triplet rotamers of 2-StA and related compounds may interconvert during their fairly long lifetime. In highly viscous solutions the viscosity barrier to interconversion is much higher, impeding such an interconversion. The existence of solvent viscous drag, in addition to the theoretically predicted barriers in the triplet state, explains the experimental results in a wide range of temperatures and viscosities.

The lowest excited singlet states in biphenyl and p-terphenyl. A polyexcited CI treatment

Abstract The singlet excited state description previously obtained for biphenyl and p -terphenyl by the PPP method (in its monoexcited CI approximation) was criticized by extending the calculations to include polyexcited configurations. For biphenyl all double and part of triple π-electron excitations were introduced, while calculations on p -terphenyl were limited to (most) double excitations. Correlation effects on state energies were analysed and were found to conform with the rules given by other authors for alternant hydrocarbons. Number and sequence of states within the two relevant UV absorption bands were argued by combining information from calculated energy levels and transition moments. The long-wavelength absorption region of biphenyl turns out to be composed of three states belonging to the 1 B 1g − , 1 B 3u − and 1 B 2u + species. The fluorescence characteristics reflect the known “g” character of the emitting state, while those of the absorption are well accounted for by the 1 B 2u + ( 1 L a ) state. In p -terphenyl, the gross features of both absorption and emission are determined by the 1 L a type state which is the lowest lying excited singlet. Some inconsistencies remain with regard to the electric-dipole allowed transitions falling in the second one-photon absorption region while good general agreement was obtained between calculated and observed g ← g transition energies.

Photoisomerization of simple merocyanines: a theoretical and experimental comparison with polyenes and symmetric cyanines

The structure and electronic spectra of merocyanines are intermediate between those of acyclic polyenes and cationic symmetric cyanines of comparable sizes, shifting from one to the other as a result of a solvent polarity change. In this paper we address, both theoretically and experimentally, the question of how the photochemical behaviour of simple merocyanines compares with that of polyenes and symmetric cyanines. After a brief theoretical re-appraisal of the absorption maxima dependence on the chain length in the three classes of chromogens, we analyse the calculated potential-energy curves for two photoisomerization coordinates of a polyene, a symmetric cyanine and a merocyanine of comparable sizes. The results concerning both the energy curves and the nature of the relevant electronic states, particularly the TICT (twisted intramolecular charge transfer) character of the S1 state at the perpendicular geometry, reveal the existence of a near relationship between merocyanines and symmetric ionic polymethines, traceable back to their being odd-alternant systems. The presence of a low-lying 1(nπ*) state at the trans geometry and the associated solvent-modulated vibronic coupling with the 1(πHπL*) state dominate the experimentally accessible photochemical behaviour of two simple merocyanines and make it peculiar with respect to those of both polyenes and symmetric ionic cyanines.