Surface enhanced optical absorption and photoluminescence in nonbonding electrons in small poly(vinylpyrrolidone) molecules

Molecularly dispersed poly(vinylpyrrolidone) (PVP) in water (after mechanochemical stirring in hot condition) absorbs strongly in a triplet band (pi-->pi* electronic transitions) in the 200-400 nm range. Absorption maximum shifts nonlinearly from 222 nm [full width at half maximum (FWHM) approximately 22 nm] in a dilute solution 0.1 g/dl PVP to a value as large as 247 nm (FWHM approximately 55 nm) in 10 g/dl PVP. Disobeying the Beer-Lambert relation, the absorbance increased in dilute samples. Irradiating in this region induces two emission groups; (i) 300-580 nm and (ii) 600-850 nm in six distinct bands in three major PVP conformers. The first band group involves an order of larger intensity in the other group. Strong electron-phonon coupling results in the C=O group of pyrrolidone ring, which governs the conformation, in a vibronic band 1665 cm-1 as intense as the (0,0) band. The group (ii) occurs in part of the energy loss in interactions of excited photons with surrounding in the n<--pi* transition [band group (i)] in the nonbonding C=O (2sp4) electrons. Such transitions may be useful for optical switching and other devices.