Two-photon spectroscopy of the biphenyl chromophore. The electronic excited states of biphenyl and fluorene below 50000 cm-1

The two-photon excitation spectra of biphenyl and fluorene in dil. soln. were measured up to 50,000 cm-1. Both spectra exhibit a medium intense band system in the range 32,000-42,000 cm-1, and a strong band above 45,000 cm-1. The lowest frequency feature is assigned to a B3 symmetry transition in biphenyl and the corresponding B2 transition in fluorene. The polarization of the higher bands leads to the assignment of 2 A states at 38,000 and 47,000 cm-1. The origin of the electronically excited states of the biphenyl chromophore is discussed by simple composite mol. considerations as well as CNDO CI calcns. The latter give a semiquant. picture of transition energies and transition probabilities for 1- and 2-photon allowed excitations. A compilation of 1-photon spectra and calcns. from the literature is included in the anal. to provide a consistent picture of the electronically excited states of the biphenyl chromophore up to 50,000 cm-1

TRANS CIS PHOTOISOMERIZATION MECHANISM OF CARBOCYANINES - EXPERIMENTAL CHECK OF THEORETICAL-MODELS

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 (Sl) 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 S(l) 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 S(O) 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, radiationless decay occurs from the perp S(l) minimum to 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

First- and Second-Order Polarizabilities of Simple Merocyanines. An Experimental and Theoretical Reassessment of the Two-Level Model

Taking four merocyanines [(CH3)2N-(CHdCH)n-C(CH3)O; n ) 1-4] (Mc1-4) as test D-A systems, weperformed a close experimental and theoretical examination of the two level model with reference to itsability to provide correct predictions of both absolute values and dependence on the conjugation path lengthof first- and second-order molecular polarizabilities. By 1H NMR spectroscopy merocyanines Mc1-4 werefound to be ∼1:1 mixtures of two planar conformers with cis and trans arrangements of the sC(CH3)Oelectron-acceptor group and all trans structure of the polyene like fragment. The degree of bond lengthalternancy (BLA) in the -(CHdCH)n- fragment, was quantified by extensive full geometry optimizations atboth semiempirical and ab initio level. DFT (6-31G**/B3LYP) optimized geometries were considered to bemost reliable and were used for calculations of the excited-state properties. The applicability of the two levelmodel, reducing the general sum-over-states (SOS) expansion to only one term involving the ground state (g)and the lowest-lying 1(ππ*) CT state (e), was checked by analysis of fluorescence and near UV absorptionspectra. Measurements of the basic two-level model quantities (Ege, μge and Δμeg), by which the dominantcomponents of r and tensors are expressed (RXX, XXX, X ≡ long molecular axis), were designed to giveapproximate free-molecule values. It is proposed, in particular, an adjustment of the solvatochromic methodfor the determination of Δμeg, 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 XXX’smatching well in both value and n-dependence with EFISH data reported in the literature for similarmerocyanines. For the fluorescent Mc4, the results were qualitatively well reproduced by an approach, whichcombines absorption and fluorescence solvent shifts. All the measured quantities were calculated for bothtrans and cis Mc1-4 by three semiempirical INDO-based approaches aiming at evaluating the performancesof different integral parametrizations and CI extensions: ZINDO/S, CS INDO SCI, CS INDO SDCI. In allcases, RXX and XXX were found to rise proportionally to about n1.3 and n2, respectively, in qualitatively goodagreement with the experimental values. As to the absolute values, however, experimental RXX’s and XXX’swere best reproduced by CS INDO SDCI combining Ohno-Klopman parametrization and CI including bothsingle and double excitations. The validity of the two-level model was checked by comparison with convergedSOS calculations for the longest chain merocyanine (Mc4) and finite field calculations of linear polarizabilitiesfor all of the four dyes (Mc1-4)

Linear and nonlinear optical properties of V-shaped D–p–A–p–D chromophores: effects of the incorporation of aromatic rings in the polyenic p-bridges of open-chain ketocyanines

Following previous studies on a and b polarizabilities of ketocyanines, a subgroup of D–p–A–p–D quadrupolar chromophores with moderately V-shaped structure, the present work analyses the effects of modifying the p-bridges connecting the D (NMe2) and A (CO) groups. This aim is pursued through a detailed comparison between the previously studied ketocyanines (KC2, KC3) and a Michler’s ketone analogue (KM1) bearing styrenic (in the place of polyenic) p-bridges. First, we report a spectroscopic study, including absorption and fluorescence anisotropy spectra, aimed to probe the electronic peculiarities of KM1 as well as to derive consistent three-state model (TSM) parameters for the three compounds. The paper goes on with an extensive theoretical study, carried out in the framework of the density functional theory (DFT), encompassing the structure, the electronic spectrum, a and b polarizabilities and two-photon absorption (TPA) cross-sections (sTP). Calculations performed according to the sum-over-states (SOS) approach are discussed with reference to the performances of few-state descriptions, it is shown that such descriptions (including TSM), which have been proved to be quite reliable in the case of KC2 and KC3, lose their effectiveness with KM1 because of the electronic characteristics related to the styrenic p-bridges. As to the TPA cross-sections, the results of TSM and SOS approaches concerning the TSM g - c and g - e transitions are supplemented by those obtained using the quadratic response theory. A common qualitative conclusion, traceable to the degree of bending of the V-shaped structure, is that in the case of KM1 the allowed (g - e) and the ‘‘forbidden’’ (g - c) transitions both should be observable in the TPA spectrum, as confirmed by experiment

Conformations and internal rotation properties of molecules containing one geminal diphenyl group : diphenylethylene, diphenylketimine, benzophenone, diphenylether and diphenylmethane

The internal rotation potential-energy surfaces for a group of Ph2X type molecules (X = CCH2, CNH, CO, O, CH2) are investigated using an improved INDO-type method (C-INDO) particularly suitable for conformational studies of conjugated compounds. The quantum chemical results, presented as contour energy diagrams, are discussed in detail with regard to both the equilibrium conformations and the features determining internal rotation dynamics. Except for Ph2CH2, which is found to have a gable equilibrium form, all the other Ph2X compounds appear to prefer helical conformations. The height of internal rotation barriers and the size of the low energy regions around the minima indicate that Ph2CH2 and pH2O molecules are extremely flexible, while Ph2CCH2, Ph2CNH and Ph2CO are, comparatively speaking, rather rigid systems. On this basis a scale of conjugating ability is suggested for the series of molecules studied. In addition to the theoretical study, a Raman and infrared investigation of the conformation of all the molecules in question is also reported in order to allow a consistent comparison between theory and experiment, at least as regards the shape of the equilibrium conformations. It is shown that the experimental behaviour of the Ph2X molecules in condensed (solid and liquid) phase (as deduced from vibrational spectroscopy and other sources) can always be reconciled with the C-INDO description for the isolated molecule