Photophysics of Carbazole-Containing Systems. 2. Fluorescence Behaviour of Poly(N-Vinyl Carbazole) and N -Vinyl Carbazole/Methyl Acrylate Copolymer Films

The fluorescence behaviour of thin films of poly( N-vinyl carbazole), PNVCz, and a series of N-vinyl carbazole/methyl acrylate, NVCz/MA, copolymers 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 ‘sandwich’ structure formed from two fully overlapped aromatic rings. The other (trap II) is a species involving two partially overlapped carbazole 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 excimer traps is apparent: even at a carbazole content of 8.3 mol%, excimer emission is evident. In PNVCz and copolymers of higher aromatic contents, the microtacticity of the macromolecule exerts a dominant influence upon the photophysical behaviour of the bulk polymer despite the moderating effects of intermolecular interactions between chromophores. As the temperature of the system is reduced, the contribution from trap II to the overall fluorescence from the polymer films increases relative 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 the reorientation of carbazole substituents which occur at low temperature serve to inhibit the conversion of a proportion of the trap II sites into fully overlapped excimers, thereby reducing depletion of the trap II population. Implicit to this explanation of the photophysical characteristics of PNVCz and the higher content NVCz/MA copolymers, is the belief that the high concentration of excimer-forming trap sites mitigates against significant energy migration between carbazole substituents which might otherwise populate the excimer traps. These observations are of importance to considerations of the photoconductivity displayed by polymers derived from NVCz.