Electrolyte-Concentration and Ion-Size Dependence of Excited-State Intramolecular Charge-Transfer Reaction in (Alkylamino)benzonitriles: Steady-State Spectroscopic Studies

Abstract

Steady-state spectroscopic studies have been performed with three intramolecular charge-transfer molecules, 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C), in ethyl acetate and acetonitrile in presence of lithium perchlorate (LiClO(4)) at room temperature to investigate the effects of electrolytes on excited-state intramolecular charge-transfer reaction. Electrolyte-concentration and ion-size dependences of several spectroscopic properties such as quantum yield, absorption and emission transition moments, radiative and nonradiative rates, and changes in reaction free energies associated with LE --> CT conversion have been determined for these molecules and reported. For P4C, quantum yield decreases by a factor of approximately 7 at the highest electrolyte concentration relative to that in pure ethyl acetate whereas it is a factor of approximately 4 for both P5C and P6C. However, in acetonitrile with 1.0 M LiClO(4), quantum yield reduces to almost half of that in the pure solvent. Formation of a charge-transfer (CT) state is found to be strongly favored over the locally excited (LE) state as the electrolyte (LiClO(4)) concentration is increased, electrolyte effects being more pronounced in ethyl acetate than in acetonitrile. Relative to pure ethyl acetate, reaction free energy change (-DeltaG(r)) increases by a factor of approximately 5, approximately 4, and approximately 2 for P4C, P5C, and P6C, respectively, at 2.5 M LiClO(4) in this solvent. -DeltaG(r) for P4C exhibits a change in sign (from negative to positive) upon addition of electrolyte in ethyl acetate. In acetonitrile, however, these changes are within a few percent, except for P4C where it is about 4 times greater at 1.0 M LiClO(4) than that in pure acetonitrile. The electrolyte-induced total red shift of the CT band of these TICT molecules is 3 times higher in ethyl acetate than in acetonitrile. Although both the quantum yield and CT emission peak frequency decrease linearly with the increase in ion size, -DeltaG(r) remains largely insensitive. Further studies using a nonreactive probe (coumarin 153) in concentrated electrolyte solutions also show qualitatively similar results.