Influence of the Attachment of Chromophores to a Polymer Chain on Their Twisted Intramolecular Charge-Transfer State in Dilute Solution

Abstract

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 (MOR), 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 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 (E1) 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 ν0 vibrational state and less forbidden ν1 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.