Fluorescence studies of porphycene in various cryogenic environments
Michał Gil1, Alexandr Gorski1, Alexander Starukhin2, and Jacek Waluk1,3
1Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 Kasprzaka, Warsaw 01-224, Poland
2B.I. Stepanov Institute of Physics of National Academy of Sciences, Мinsk 220072, Belarus
3Faculty of Mathematics and Science, Cardinal Stefan Wyszyński University, 5 Dewajtis, Warsaw 01-815, Poland
Received January 18, 2019, published online April 26, 2019
Low temperature fluorescence of porphycene, a structural isomer of porphyrin, has been studied using polymer samples, matrix isolation, and fluorescence line narrowing (FLN) techniques. Contrary to the case of the chromophore embedded in a nitrogen matrix, the emission from polymer samples at temperatures above 10 K exhibits strong dependence on the wavelength of excitation: increasing the excitation energy leads to gradual broadening and, finally, loss of vibrational structure. A rather unusual observation is the similarity of the structured fluorescence spectra obtained for excitations into S1 and S2 states. This finding indicates a correlation between the site distributions in S1 and S2. A similar idea has been put forward earlier for tetraphenylporphyrin (I. Lee, G.J. Small, and J.M. Hayes, J. Phys. Chem.94, 3376 (1990)). We propose that the correlation is due to isotropic polarizability in the molecular plane; calculations confirm such hypothesis. For porphycene, an additional factor that can contribute to the effect is a rapid transtrans tautomerization that leads to the rotation of x and y axes in-plane of the molecule. FLN spectra reveal significant band broadening for excitation into S2. This suggests that the site correlation is not of 1:1 type or that at 4.2 K the site exchange dynamics is frozen in com-parison with the situation at higher temperatures.