Influence of the attachment of chromophores to a polymer chain on their twisted intramolecular charge-transfer state in dilute solution

Fluorescence from the initially excited singlet state (LE) and the twisted intramolecular charge-transfer (TICT) state of 4-(N, N-dimethylamino)benzoate (DMAB), 4-(N, N-diethylamino)benzoate (DEAB), 4-(N-pyrrolidino)benzoate (PYR), 4-(N-piperidino)benzoate (PIP), 4-(N-morpholino)benzoate (MDR), and 4-(N-2, 6-dimethylmorpholino)benzoate (26DMM) were compared with the free and polystyrene-bound chromophores. The ground-state twisting of the donor with respect; to the acceptor can have a large influence on the charge distribution. The polymeric chain can force a less planar geometry and cause a bathochromic shift and a broadening of the absorption band in the electronic absorption spectra. In the area of 350 nm the emission is assigned to the locally excited state !with a planar geometry) and the emission with lambda(FI)(max) in the area of 450-520 nm to the TICT state (with a perpendicular conformation). Experimental results show (i) no polymeric chain effect in emission distribution for the DMAB molecule, (ii) a significant red edge effect (REE) for the PIP probe, (iii) a slight red edge effect for 26DMM, (iv) a specific blue edge effect (BEE) for PYR probe, and (v) a combination of REE and BEE for MOR. Adaptation of Grabowski's TICT hypothesis allows one to calculate the activation energy of TICT state formation, the backreaction TICT --> LE, and the thermally activated TICT fluorescence. The polymeric chain essentially does not change the activation energy (E-1) for the thermally activated TICT state formation, indicating that the polymeric chain does not change the microscopic viscosity in the range affecting measurable changes in the TICT-LE equilibrium. The polymeric chain increases the gap between the lowest forbidden v(0) vibrational state and less forbidden v(1) vibrational state by 195-225 cm(-1) (0.6-2.4 kJ mol(-1)). Experimental results suggest that the polymer chain affects only the vibrational relaxation of tested molecules without any significant effect on the thermodynamics of the TICT and LE states equilibrium.