HOLE-BURNING SUBTRACTED FLUORESCENCE LINE-NARROWING SPECTROSCOPY OF SQUARAINES IN POLYMER MATRICES

We present vibrationally resolved line-narrowed fluorescence spectra o f seven centrosymmetric squaraines in polyethylene and polystyrene mat rices at 1.4 K. At a fixed excitation frequency, the intensities of th e zero-phonon lines decay with lime due to nonphotochemical hole burni ng,leaving behind a broader background attributed to emission from mol ecules excited into phonon sidebands. The hole widths are independent of the part of the emission spectrum monitored, suggesting that under these conditions all spectral features arise from a single species or a collection of closely related species. Subtracting the spectra of th e hole-burned samples from the initial. spectra leaves predominantly z ero-phonon line excited spectra exhibiting enhanced vibrational resolu tion. A detailed analysis of the vibrationally resolved emission spect ra is given, aided by semiempirical AMI force field calculations on th e ground electronic state. The dominant Franck-Condon active vibration in all seven squaraines examined is a mode near 150 cm(-1) which is a ssigned as predominantly an in-plane expansion of the whole molecule b ut which mixes with other low-frequency motions of the amino group sub stituents.