1,720,986 research outputs found
Photophysics and photochemistry of diphenylsulfone. 1. The triplet mechanism of photodissociation
No full textA laser flash photolysis study of diphenylsulfone was carried out
Chemical asymmetry and alpha and beta polarizabilities of D-A-D' chromophores: a three-state-model and TDDFT-SOS analysis of apenta-heptamethine ketocyanine
No full textThe essential-state model, here amounting to a three-state model, has been employed to account for the effects of chemical asymmetry on the electronic alpha and beta polarizabilities of a pentaheptamethine ketocyanine (KC2,3), a prototypic D-A-D' chromophore. A suitable model, based on the idea of a ‘chromophoric site’, has been set up in terms of the three-state model features previously derived for the parent symmetric pentamethine and heptamethine ketocyanines, KC2 and KC3. This approach has been found to reproduce very well the experimental transition energies and dipoles. From the resulting properties of the ground and two relevant lowest excitedstates, average alpha and beta vec have been evaluated according to the SOS approach. The performances of themodel have been tested by comparison with the results of TDDFT SOS (hyper) polarizability calculations considering up to twenty excited states. A detailed analysis of the results for the threeketocyanines has shown a rapid convergence of the SOS expansion that supports the reliability of descriptions based on a few low lying excited states (here corresponding to p - p* excitations). However, while only two excited states were necessary for the symmetric compounds, for KC2,3 avalue of beta vec comparable with the converged value, as well as with that predicted by the experimentally-based three-state model, has been obtained including at least three excited states.Both the TDDFT SOS and the three-state model descriptions have emphasized the important role played by the three-level term contributions in the determination of beta vec. Moreover, bothdescriptions agree in predicting that KC2,3 features average alpha and beta vec values in between those of KC2 and KC
Potential-energy curves of the torsional mode of 1,1′-binaphthyl in the ground and lowest excited singlet states. A CS–INDO/CI study
No full textThe application of a recently developed theoretical method, CS–INDO/CI, to the study of the sections of the potential-energy surfaces of the ground and four lowest excited singlet states of 1,1′-binaphthyl corresponding to the coordinate of twisting around the inter-ring 1,1′-bond has proved to be successful in explaining the measured spectroscopic and photophysical properties of this molecule. All the changes observed in its spectroscopy and photophysics on going from low-temperature rigid solutions to room-temperature fluid solutions (strong bathochromic shift of the fluorescence maximum, change in the Snâ†�S1 absorption spectrum, shortening of the fluorescence lifetime by an order of magnitude) are qualitatively accounted for in terms of the calculated potential-energy curves. In particular, the S1 state experiences a drastic change in its electronic nature (from Lb to La) when the interplanar angle is allowed to relax from the near-orthogonal ground-state conformation to the trans twisted S1 equilibrium conformation (φ≈ 130 °). A comparison with the results of previous theoretical approaches stresses the reliability of the CS–INDO/CI method and the wealth of information on potential-energy surfaces which can be achieved through its application
Conformations and barriers to internal rotation in trans-diarylethylenes: Theoretical investigation using a new INDO-type method (C-INDO)
No full textC-INDO, a new INDO-based technique specially devised for conformational studies of conjugated systems, is used to investigate conformational equilibria in ground state trans-diarylethylenes, as revealed by a number of emission spectroscopic observations (both steady-state and time resolved). Conformations, ΔE and barriers to internal rotation are provided for all the possible rotamers of 1-StN (1-styrilnaphthalene), 2-StN, 2,2′-DNE (2,2′-dinaphthylethylene) and 1,2′-DNE. Energetic parameters are used to predict the relative abundances of the distinct rotameric species at equilibrium as well as to estimate the rate at which equilibria are established (at different T). The results prove to be a consistent basis on which experimentally observed behaviour can be rationalized
Modelling of the cis-trans partitioning in the photoisomerizations of cyanines and stilbene derivatives
No full textIn the course of photoisomerization, polymethine cyanines as well as stilbene and its derivates decay from the S1 potential energy minimum, corresponding to the perpendicular geometry, to yield either cis or trans ground-state molecules. The fraction of cis isomers obtained, alpha, spans a larger range of values for symmetric cyanines than for stilbene derivatives. It is argued that such different behaviour for the two classes of compounds should be traceable to the electronically different nature of their S1 perp species. Making use of radiationless transition theory results, it is shown that the relative location of the S1 minimum and S0 maximum along the internal rotation coordinate is crucial to the evaluation of alpha: even small differences between these critical twisting angles, which are more reasonably expected for polymethine cyanines than for stilbene-like compounds, may cause strong deviations from equipartitioning (alpha = 0. 5 )
Linear and nonlinear optical properties of V-shaped D-π-A-π-D chromophores: Effects of the incorporation of aromatic rings in the polyenic π-bridges of open-chain ketocyanines
No full textFollowing 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
Exciton effects in the dimer and higher aggregates of a simple merocyanine dye. A CSINDO CI based theoretical study
No full textA simple streptomerocyanine with five methine carbons (Mc) has been used as a suitable model system to investigate theoretically the optical properties in organized assemblies of merocyanine dyes. The subject was treated by a methodology making use of a semiempirical quantum chemistry description of the isolated chromophore within the Frenkel exciton theory. Both an extended CS INDO CIPSI and easier CS INDO SCI (with different parametrizations of electron repulsion integrals) schemes were applied in order to establish the role played by the quality of the wave functions. The exciton states were built on the (1)(pi pi*) (HOMO-LUMO) excited-state responsible for the color band of Me. Exciton effects were first analyzed for two stable dimer structures, having H- and J-type character, located by use of a semiempirical intermolecular potential and the simulated annealing method. Then, attention was shifted from dimers to higher-order model aggregates built in such a way as to obtain typical H (columnar) and J (staircase) arrangements. Exciton-state properties (energies and transition dipole moments) were studied as a function of the aggregation number N (up to N = 50). The results are discussed with reference to the approximations used to calculate the excitonic matrix as well as to the effects of changing from CIPSI to simple SCI schemes
First- and Second-Order Polarizabilities of Simple Merocyanines. An Experimental and Theoretical Reassessment of the Two-Level Model
No full textTaking 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)
TRANS CIS PHOTOISOMERIZATION MECHANISM OF CARBOCYANINES - EXPERIMENTAL CHECK OF THEORETICAL-MODELS
No full textTwo 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
Solvent influence on absorption and fluorescence spectra of merocyanine dyes: a theoretical and experimental study
No full textThe solvaton-CS INDO model, previously successfully used to describe the solvatochromic properties of merocyanines, has been extended to the study of the solvent influence on the fluorescence spectra (fluorosolvatochromism) of these dyes. A ketocyanine (M1) and a stilbazolium betaine (M2) were chosen as representatives of positively and negatively solvatochromic behaviours, respectively. The gap of experimental knowledge concerning the emission properties of M2 was filled by a spectrofluorometric analysis in a set of solvents covering a large range of the E-T(30) scale. Solvato- and fluorosolvatochromism were described by calculating the S-0 (eq.) --> S-1 (Franck-Condon) and S-1 (eq.) --> S-0 (Franck-Condon) transition energies as a function of a polarity factor related to the static dielectric constant of the solvent, and ranging from 0 to 1. The absorbing S-0 (eq.) and emitting S-1 (eq.) units (solute molecule + solvent cage) were approximated using the S-0 and S-1 geometries of the unsolvated molecule and the respective charge distributions fitted to the current value of k(epsilon). The calculation results fully confirm that S-0 and S-1 states of merocyanines can be viewed as a mixture of a neutral and a zwitterionic structure whose composition is controlled by the solvent polarity. The plots of the calculated spectral data (absorption and emission maxima and corresponding Stokes shifts) vs k(epsilon) are in fairly good agreement with those of the experimental data over almost the entire range of the normalized E-T(N) values, thus showing that specific solvent interactions are at least partly simulated within the solvaton-CS INDO scheme. The methodological prerequisites for a correct prediction of solvatochromic shifts are recalled with reference to previous conflicting theoretical interpretations
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