QUANTITATIVE INTERPRETATION OF THE RED EDGE EXCITATION (REE) EFFECT OF 9,9'-BIANTHRYL IN POLYISOBUTENE BY BAND SHAPE-ANALYSIS OF THE TEMPERATURE-DEPENDENT OPTICAL FLUORESCENCE-SPECTRA

Optical fluorescence spectra of 9,9'-bianthryl (BA) in polyisobutene (PIB) were measured as a function of the excitation wavelength at various temperatures between 210 and 293 K. Irradiation at the red edge of the absorption spectra selectively excites distinct conformers with respect to the torsional angle This leads to a strong dependence of the vibronic band shape of the fluorescence spectra on the excitation wavelength. The marked temperature dependence of the band shape may be attributed to the viscosity-dependent deceleration of the torsional relaxation of BA in the highly viscous polymer which prevents the excited state ensemble from reaching thermal equilibrium. The predominant broadening mechanism affecting the band shape is given by a combination of the S-1, torsional distribution and the torsional angle-dependent transition frequency. These effects are interpreted quantitatively within a torsional relaxation model based on the description of the torsional dynamics according to a Smoluchowski equation which is adapted to the steady state case. The band shape analysis of the fluorescence spectra enables The determination of the effective S-1 torsional potential as well as the friction coefficient related to the internal torsional motion of BA in the polymer matrix. A value of Delta V = 325 cm(-1) is deduced for the local barrier of the S-1 double minimum potential at phi = 90 degrees while the minimum torsional angle is found to be phi(min), = 61 degrees. Additionally, model calculations on time-resolved fluorescence spectra of BA in apolar solvents are carried our as a function of excitation wavelength and temperature showing the importance of conformational relaxation for the interpretation of transient spectra of flexible molecules.