Quantum-chemical calculations of the electronic structure of 2-amino-1,3-dicyano-5,6,7,8-tetrahydronaphthalene derivatives.

The UV-Visible absorption spectra of six, newly synthesized donor-substituted 2-amino-1,3-dicyano-5,6,7,8-tetrahydronaphthalene have been measured in methylcyclohexane (MCH) and assigned with the help of quantum-chemical calculations. Our calculations have been performed to assess information regarding the electronic state energy values, corresponding oscillator strengths, x-, y-, z-components of the transition dipole moments and molecular orbitals involved in the main electronic transitions of the studied compounds. Additionally, the experimental absorption transition dipole moments were calculated, on the basis of spectroscopic data, and compared with results of our quantum-chemical calculations. On the basis of the experimental results and quantum-chemical calculations, it was shown that the long-wavelength absorption band involves an overlap of three electronic transitions of different character. For all studied donor-acceptor (D-A) compounds in vapour-phase, the long-wavelength transition (S0→S1) does not possess charge transfer character, whereas the S0→S2 transition possesses electron transfer character e.g., π-electrons of the acceptor moiety are moved to the donor part. Moreover, it is found that the electronic structure of the studied biphenyl derivatives can be approximately described within composite-model of decoupled moieties: donor and acceptor.

Excited-state dynamics of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate.

The photophysics and excited-state dynamics of ethyl 5-(4-aminophenyl)-3-amino-2,4-dicyanobenzoate (EAADCy) in solvents of varying polarity and viscosity have been studied using femtosecond transient absorption spectroscopy. Global analysis of the time-resolved spectra revealed three processes occurring in an optically excited molecule. The sequence of reactions begins with a transition from an initially excited Franck-Condon state to the nonrelaxed intramolecular charge transfer (ICT(NR)) state which is associated with a partial electron transfer. This process is followed by an additional relaxation to a more relaxed intramolecular charge transfer (ICT(R)) state with stronger charge transfer character and flattened geometry. The lifetime of the flattened charge transfer form (ICT(R)) shortens from 200 to 300 ps in medium polar solvents down to 10 ps in strongly polar solvents. On the other hand, increase of viscosity by 1 order of magnitude leads to deceleration of processes involving twist of the donor and acceptor moieties by a factor of approximately 2.5. Observation of long-lived fluorescence of EAADCy in medium polar solvent suggests that charge transfer is possible only from a hot Franck-Condon state, but not from a relaxed locally excited state which exhibits short-wavelength fluorescence on a nanosecond time scale.

Influence of prototropic reactions on the absorption and fluorescence spectra of methyl p-dimethylaminobenzoate and its two ortho derivatives.

The influence of prototropic reactions on the spectral characteristics of methyl p-dimethylaminobenzoate (I) and its o-methoxy (II) and o-hydroxy (III) derivatives has been studied using steady-state spectroscopic technique and quantum-chemical calculations. This study concerns the solvent-induced shift of the absorption, locally excited (LE) and intramolecular charge transfer (ICT) fluorescence bands in the neat tetrahydrofuran (THF) and its hydrochloric acid solutions at different HCl concentrations. On the basis of the experimental results and quantum-chemical calculations, it was shown that in a hydrochloric acid solution the studied molecules exist as a mixture of neutral, mono-, and dicationic forms. Additionally, the results of spectroscopic measurements were used to calculate, according to the Benesi-Hildebrand method, the equilibrium constants of protopropic reactions in the ground, S(0), and excited, S(1), states. Our findings predestine molecules I and II to be used as acid fluorescence probes in a region of 0-2.5 M of [H(+)] concentrations.

Influence of spectral heterogeneity of prodan and laurdan solutions on the transfer of electronic energy to octadecyl rhodamine B.

Fluorescence quenching and resonance energy transfer have been studied by steady-state fluorescence spectroscopy. The experimental and theoretical values for the rate constants of the electronic energy transfer (kET) and critical radius (R0) were determined for prodan and laurdan as donors and octadecyl rhodamine B as acceptor. The spectroscopic data show, that prodan and laurdan in solution create an inhomogeneous spectroscopic medium in which multi-channel luminescence phenomena take place. This finding indicated that the modified form of the Stern-Volmer relation should be used for analyzing fluorescence quenching data. Results of performed studies point out, that dipole-dipole interaction is responsible for the resonance energy transfer from prodan and laurdan to octadecyl rhodamine B. The relative quenching efficiencies of both dyes depend on polarity of the medium and are higher for more polar solvent (AcN).

Phase transition affects energy transfer efficiency in phospholipid vesicles.

The fluorescence quenching of 6-propionyl-2-dimethylaminonaphtalene (PRODAN) and 6-dodecanoyl-2-dimethylaminonaphtalene (LAURDAN) by octadecyl rhodamine B (ORB) in a model system of small unilamellar vesicles (SUV) of dipalmitoylphosphatidyl-choline (DPPC) was investigated. Non-linear Stern-Volmer behaviour was observed in both systems in the gel phase (25 degrees C) and in the fluid phase (50 degrees C), resulting from association processes and from static quenching. The relative quenching efficiencies of both dyes depend on the phase state of the bilayer and indicate a deeper incorporation of PRODAN and LAURDAN into the membrane in its fluid phase than in its gel phase.

Influence of conformational effects on spectroscopic properties of 2-dimethylamino-9(4'-dimethylamino) phenyl-9-fluorenol.

Absorption and fluorescence spectra of 2-dimethylamino-9(4'-dimethylamino)phenyl-9-fluorenol have been studied in different solvents. The fluorescence spectra at various temperatures show the excitation wavelength dependence. The spectroscopic data are compared with the results of the quantum chemical calculation obtained using PM3 and INDO/S methods. The fluorescence decay curves of the molecule under study can be only satisfactorily fitted by a biexponential function. The steady state and time resolved spectroscopy studies as well as the theoretical calculation confirm that in methylcyclohexane and ethyl acetate solutions always two absorbing and emitting species exist, namely rotamers alpha and beta in the ground and excited states.