Selective light emission in nonbonding electron transitions in poly(vinyl pyrrolidone) molecules on spin-coating in...

It is shown that polymer molecules of poly(vinyl pyrrolidone) (PVP) rearranged in thin layers present surface-enhanced light emission in selective bands over a wide 260-860 nm range of spectrum. Four bands occur in spin-coated films by a dilute solution in water at 288, 395, 560, and 760 nm upon irradiating with an ultraviolet 200-250 nm light. The second and third bands are strong by contributing 82% intensity of the spectrum. Randomly dispersed PVP molecules in solution exhibit a single band (broad) at 425 nm {pi(0) <-- n(1)pi(2)* transition in the nonbonding electrons n(1) in the C=O (2s(2)p(4)) group of pyrrolidone ring} and a harmonic band 650-860 nm (weak) of roughly twice its wavelength. In films, this band is split up into two well-separated pi(0) <-- n(1)pi(2)* (395 nm) and pi(0) <-- n(2)pi(1)* (560 nm) bands. Localized nonbonding electrons n(2) of the C-N (2s(2)p(3)) moiety of pyrrolidone ring excite and emit part of the energy in the pi(0) <-- n(2)pi(1)* transition with as much intensity as in the pi(0) <-- n(1)pi(2)* band. Localization of n(1) and n(2) electrons on molecular layers of films favors the resonance >N-C=O structure with three C=O stretching bands 1615, 1635, and 1665 cm(-1) against a single band 1638 cm(-1) in randomly dispersed molecules (solution). The C-N stretching frequency is decreased by 30 cm(-1). Results are useful for molecular designing of optical films for down-energy conversion, optical switching, and biological sensors.