Electronic states of a photochromic 2-(2,4-dinitrobenzyl) pyridine single crystal

Polarized reflection and absorption spectra, FT-IR and Raman spectra of a 2-(2,4-dinitrobenzyl)pyridine (alpha-DNBP) single crystal were measured in order to elucidate the electronic states and vibrational modes relevant to the photoinduced intramolecular proton transfer (PIPT). The molecular orbital and electronic absorption spectra of three isomers (CH2, NH and OH forms) of alpha-DNBP were also calculated. From the polarized reflection spectra, we found that the Davydov splitting for the lowest allowed transition is considerably large, suggesting that the intermolecular interaction is large. The calculation shows that the HOMO-->LUMO transition of the CH2 form is very weak and the lowest allowed absorption band located at an ultraviolet region mainly comes from both the 2nd HOMO-->LUMO and HOMO-->2nd LUMO transitions. On the other hand, the lowest absorption bands of the NH and OH forms located at a visible region are due to the HOMO-->LUMO transition. The results from the calculation are qualitatively good agreement with the observed absorption spectra. We found that new strong vibrational peaks at 488, 1293 and 3386 cm(-1) appear after UV irradiation. The low frequency torsional modes below 500 cm(-1) are most likely relevant to the PIPT reaction of alpha-DNBP.