No abstract

Radiative dynamics of the hemicyanine dye N- (3-sulfopropyl-4-(p-dioctylaminostyryl)pyridinium (I) have been studied in primary alcohols and in Langmuir-Blodgett (LB) monolayer films by using time-resolved laser-induced fluorescence spectroscopy. In solution, the deactivation of the optically excited state is dominated by a radiationless transition via an activated barrier crossing to the twisted intramolecular charge transfer (TICT) state, and the decay rate decreases substantially with increasing n in the series CH 3 (CH 2 ) n OH. Observed biexponential decays are consistent with the three-level scheme describing the horizontal transition on the excited-state potential surface. In monolayer films the decay dynamics are similar whether the initial excitation occurs into the monomer or aggregate bands. The kinetics are interpreted in terms of diffusion of excitons in the aggregate with eventual energy transfer to the monomer. A Monte Carlo simulation was used to characterize the motion of the excitons within aggregates and to correlate the aggregate size with exciton lifetime. The finite size of the aggregate in monolayer films was estimated under two limiting cases: one in which exciton transport on the aggregate is the rate-limiting step in emission and another in which aggregatemonomer excitation transfer limits the radiative rate. Comparison of experimental and simulated results yielded an estimate of the size of an H-aggregate in monolayers of I to be ca. 45 molecules in the first case and 50 molecules in the second. A comprehensive kinetic scheme is presented that unifies both the solution and monolayer film behavior observed in these studies.

We synthesized a series of donor-substituted tridurylboranes containing different types and number of chromophores including 1-pyrene (PB1-3), 3-carbazole (CBC1-3), or substituted p-carbazol-N-phenyl (CBN3a-c) as various donor-acceptor (D-A) molecules. The photophysical and electrochemical properties of these twisted D-A molecules were investigated by means of UV/Vis absorption and fluorescence spectroscopy as well as cyclic voltammetry (CV). Solvent polarity, viscosity, and temperature effects on the fluorescence emission reveal the existence of three types of excited states, and their equilibria and interconversions between three excited states. In increasing order of the charge-separated extent and the conformational change, three excited states are the locally excited (LE) state, the more planar intramolecular charge-transfer (ICT) state, and the more twisted ICT (TICT) state as compared to the ground state. The TICT state undergoes a conformational change with a higher energy barrier over the ICT state. The solvent polarity effect on the state conversion is opposite to the viscosity effect, and temperature effects derive from its resulting changes of polarity and viscosity. For example, the increase of the polarity of the solvent results in excited-state conversions from the LE state to the ICT state, and/or from the ICT to the TICT state, and an increased viscosity leads to the opposite conversions. On the basis of electrochemical and spectral data, thermodynamics of a possible ICT process were estimated, and correlated with the excited-state character. Finally, three excited states have been characterized by the conformation, the photophysical properties, and the thermodynamics of the ICT processes.

Within a monomolecular layer, molecules may be present as isolated species that behave independently or in an aggregated form that acts collectively. Theoretical derivations of the energy shift expected for the aggregate transition relative to the isolated monomer are based on transition moment coupling and predict a significantly different orientational dependence for point-and extended-dipole approximations. In contrast to the commonly invoked transformation from H-to J-aggregate behavior as the molecular orientation increases from surface normal, purely H-aggregate behavior is predicted at small intermolecular separation distances using the extendeddipole approximation, regardless of axial orientation. This extended-dipole model is more accurate at the short separation distances commonly encountered in organic monolayers and shows excellent agreement with the experimentally measured energy shift of +9000 cm-' for an intermolecular distance between hemicyanine dye molecules of 3.96 f 0.04 A. As a further development to this model, orientational contributions to the energy shift and the resultant spectral band profile are derived. A Gaussian axial angular distribution for the range O S 0 I ug I 5 O is found to have a considerable effect on the predicted band profile, resulting in a variation in the transition wavelength from 3 to 10 nm and an increase in the spectroscopic line width contribution by more than 1 order of magnitude. Thus, small variations in the angular distribution have a significant effect on the spectral properties of aggregates and must be considered in correlating molecular structure with electronic transitions.