EFFECTS OF POLYMER MICROENVIRONMENT ON THE THERMODYNAMICS OF THE TWISTED INTRAMOLECULAR CHARGE-TRANSFER FLUORESCENCE

The fluorescence behavior of poly(alkyl methacrylate)s (alkyl: methyl, butyl, cyclohexyl, and dodecyl) containing a trace amount of (dimethylamino)benzoate (DMAB) as fluorescence probe was investigated in different solvents (ethyl acetate, butyl chloride, and tetrahydrofuran) over the temperature range between –100 and 60 °C. For poly(methyl methacrylate) the activation energy (E1) is almost constant in good solvents and comparable to or even less than the temperature dependence of solvent viscosity, suggesting that the molecular twisting motion is barrierless and the retarding effect of polymer chain is entropic. In poor solvents, an energy barrier owing to contracted polymer chain is particularly prominent when the probe is tagged to the polymer main chain via a long alkyl spacer. With increasing bulkiness of the ester alkyl group, E1 tends to increase, reflecting the steric effect. Effects of bulky neighboring side groups (particularly in the case of poly(cyclohexyl methacrylate)) are prominent in poor solvents. The reverse reaction (i.e., regeneration of the local excited state (b*) from the a* state) was also found to be suppressed by the polymer chain. Temperature effects on TICT fluorescence were discussed in terms of solvation and excitation wavelength as well. © 1990, American Chemical Society. All rights reserved.