Photophysical Studies on 1-(o-Aminophenyl)pyrene. Characterization of an Intramolecular Charge-Transfer State with Application to Proton-Transfer Dynamics

Article abstract of DOI:10.1021/ja00293a012

A photophysical study on 1-(p-aminophenyl)pyrene (I) reveals two principal fluorescences arising from ?,?^* (locally excited in pyrene ring) and charge-transfer (CT) (aniline as donor, pyrene as acceptor) states.The latter dominates in organic solvents and aqueous media at pH >/= 7.The estimated energy of the CT state in highly polar solvents (near 500 nm, ca. 2.5 eV) agrees well with the measured redox energetics (2.7 eV) while the dependence of the transition energy on solvent dielectric constant indicates an excited state dipole moment of ca. 14 D.The photophysics of I is pH dependent in aqueous media because of a conjugate acid/base equilibrium with ground- and excited-state constants of pK_a = 4.05, pK_a^* = 3.3 (Forster cycle), respectively, in ethanol-water (50:50).The rather similar acidities of the ground and excited states of I indicate little or no CT contribution to the deprotonation step in the excited state.Deprotonation of the locally excited ?,?^* state of I (conjugate acid form) is thought to initially yield the locally excited free base form of I which rapidly relaxes to the CT state.The latter is subject to proton-transfer quenching (most likely involving the pyrene radical anion moiety of the CT state), k_q = (7.38 +/- 1.5) * 10⁸ M^-1 s^-1 (corrected for changing proton activity coefficients in ethanol-water).A study of the rate of deprotonation of the conjugate acid of I^* from room temperature through the liquid/solid phase transition (near ca. 50 deg C) to -196 deg C allows an estimate of the enthalpy and entropy of activation in several different regions.As the temperature is lowered from 295 to 160 K, the enthalpy term decreases from 5360 to 780 cal/mol while the entropy term changes from -6.1 to -24.3 cal/(K<*>mol).