Internal conversion with 4-(azetidinyl)benzonitriles in alkane solvents. Influence of fluoro substitution

The introduction of a fluoro-substituent in the phenyl ring of 4-(1-azetidinyl)benzonitrile (P4C) leads to smaller fluorescence quantum yields Phi (f) and shorter decay times tau in alkane solvents (cyclopentane, n-hexadecane, n-hexane and 2-methylpentane). In cyclopentane at 25 degreesC, Phi (f) and tau equal 0.02 and 0.14 ns for 2-fluoro-4-(1-azetidinyl)benzonitrile (P4CF2) and 0.11 and 0.85 ns for 3-fluoro-4-(1-azetidinyl)benzonitrile (P4CF3), as compared with 0.27 and 4.55 ns for P4C. The fluorescence originates from a locally excited (LE) state and dual fluorescence due to intramolecular charge transfer is not observed for the three aminobenzonitriles at any temperature in the alkane solvents. By measuring the yields of intersystem crossing Phi (ISC), it follows that this enhancement of the radiationless deactivation of the first excited singlet state S-1 is due to thermally activated internal conversion (IC). The IC yield Phi (IC) in cyclopentane at 25 degreesC, as an example, is considerably larger for P4CF2 (0.93) than for P4CF3 (0.35) and of minor importance for P4C (0.03). The IC activation energies E-IC of P4CF2 (12.6 kJ mol(-1)), P4CF3 (19.3 kJ mol(-1)) and P4C (38.1 kJ mol(-1)) in cyclopentane were determined by fitting tau measured as a function of temperature, together with data for Phi (f) and Phi (ISC). Similar E-IC values were obtained in n-hexane and n-hexadecane. These data show that the increase in IC efficiency from P4C via P4CF3 to P4CF2 is caused by a decrease in E-IC. The radiative rate constants k(f) in cyclopentane of P4CF2 and P4CF3 are about twice that of P4C, probably due to the mixing of the S-2(L-1(a),CT) and S-1(L-1(b)) states of P4C caused by the molecular asymmetry introduced by the F-substituents. It is assumed that the lowering of the IC barriers in P4CF2 and P4CF3 is governed by an F-substituent-dependent difference in the energies of the molecular configuration of the azetidinylbenzonitriles that can be reached in S-1 as compared with those in S-0.