PHOTOPHYSICS OF CARBAZOLE-CONTAINING SYSTEMS - 2 - FLUORESCENCE BEHAVIOR OF POLY(N-VINYL CARBAZOLE) AND N-VINYL CARBAZOLE METHYL ACRYLATE COPOLYMER FILMS

The fluorescence behaviour of thin films of poly(N-vinyl carbazole), P NVCz, and a series of N-vinyl carbazole/methyl acrylate, NVCz/MA, copo lymers has been investigated using both steady-state and time-resolved emission techniques. The fluorescence of PNVCz at 298 K is dominated by emission from two excimeric traps. Trap I has the conventional 'san dwich' structure formed from two fully overlapped aromatic rings. The other (trap II) is a species involving two partially overlapped carbaz ole substituents. The fluorescences of the NVCz/MA copolymers contain contributions from unassociated or monomeric carbazoyl excited states which increase in prominence as the aromatic content of the system is reduced. The influence of intermolecular interactions in creating exci mer traps is apparent: even at a carbazole content of 8.3 mol%, excime r emission is evident. In PNVCz and copolymers of higher aromatic cont ents, the microtacticity of the macromolecule exerts a dominant influe nce upon the photophysical behaviour of the bulk polymer despite the m oderating effects of intermolecular interactions between chromophores. As the temperature of the system is reduced, the contribution from tr ap II to the overall fluorescence from the polymer films increases rel ative to that made by emission from trap I. At 77 K, the fluorescence of PNVCz contains a significant contribution from trap II emission. At 298 K, fluorescence from the conventional excimer, trap I, dominates the steady-state emission spectrum. On the basis of observations upon time-resolved fluorescence data, it is proposed that restrictions to t he reorientation of carbazole substituents which occur at low temperat ure serve to inhibit the conversion of a proportion of the trap II sit es into fully overlapped excimers, thereby reducing depletion of the t rap II population. Implicit to this explanation of the photophysical c haracteristics of PNVCz and the higher content NVCz/MA copolymers, is the belief that the high concentration of excimer-forming trap sites m itigates against significant energy migration between carbazole substi tuents which might otherwise populate the excimer traps. These observa tions are of importance to considerations of the photoconductivity dis played by polymers derived from NVCz.