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 o rder to elucidate the electronic states and vibrational modes relevant to t he photoinduced intramolecular proton transfer (PIPT). The molecular orbita l 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 c onsiderably large, suggesting that the intermolecular interaction is large. The calculation shows that the HOMO-->LUMO transition of the CH2 form is v ery weak and the lowest allowed absorption band located at an ultraviolet r egion mainly comes from both the 2nd HOMO-->LUMO and HOMO-->2nd LUMO transi tions. On the other hand, the lowest absorption bands of the NH and OH form s located at a visible region are due to the HOMO-->LUMO transition. The re sults from the calculation are qualitatively good agreement with the observ ed absorption spectra. We found that new strong vibrational peaks at 488, 1 293 and 3386 cm(-1) appear after UV irradiation. The low frequency torsiona l modes below 500 cm(-1) are most likely relevant to the PIPT reaction of a lpha-DNBP.