Phosphorescence of adenosine and poly(riboadenylic acid). Evidence for dual emissions and n pi states

Phosphorescence from the 9-adenylyl group in the form of microcrystalline powders of adenosine films of poly(riboadenylic acid) (poly(rA)) in hyaluronic acid has been studied at 77 K. For adenosine, clearly resolved vibronic structure consists of two progressions, A and B, with delta vA = 1363 cm-1 and delta vB = 1575 cm-1, correlated with in-plane C5-N7 and in-plane C4-C5 stretch, respectively. The relative strength of the progressions varies with excitation wavelength and this, together with the absence of a common origin, indicates the existence of two independent emitting states with 0-0' levels separated by either 300 or 1000 cm-1. Two different excitation spectra are observed lying below the normal (pi pi*) adsorption and one is assigned as a previously undetected 1(n pi*) transition. For poly(rA) films the emission band envelope is identical with that of adenosine but the vibronic structure is lost. Only one excitation peak is observed at 32.9 X 10(3) cm-1, identical with one of the adenosine spectra. The second adenosine excitation spectrum probably represents an intermolecular charge transfer transition. Comparison is made with the predictions of six semi-empirical MO calculations.