EXCITED TRIPLET-STATE INTERACTIONS WITH MOLECULAR-OXYGEN - INFLUENCE OF CHARGE-TRANSFER ON THE BIMOLECULAR QUENCHING RATE CONSTANTS AND THEYIELDS OF SINGLET OXYGEN (O-2(ASTERISK),(1)DELTA(G)) FOR SUBSTITUTED NAPHTHALENES IN VARIOUS SOLVENTS

The bimolecular rate constants k(O2)(T) for oxygen (O-2((3) Sigma(g)(- ))) quenching and the efficiencies f(Delta)(T) with which singlet oxyg en (O-2()((1) Delta(g))) is thereby produced are reported for a range of substituted naphthalene triplet states in acetonitrile, benzene, a nd cyclohexane. The magnitudes of k(O2)(T) and f(Delta)(T) are inverse ly correlated, and both parameters exhibit pronounced sensitivity to t he oxidation potential (E(M)(OX)) of the naphthalene derivative and so me dependence an the solvent. Since, within the range of naphthalenes studied, the triplet state energy (ET) remains largely constant and th e molecules are structurally similar, the dominant variable is the fre e energy change (Delta G(CT)) for charge transfer to molecular oxygen. It is demonstrated that the large variations observed in k(O2)(T) and f(Delta)(T) depend on the energy of the substituted naphthalene/molec ular oxygen charge-transfer (CT) states, (1,3)(M(.+)...O-2(.-)). In ac etonitrile, for example, the respective magnitudes of k(O2)(T) and f(D elta)(T) are 7.2 X 10(9) dm(3) mol(-1) s(-1) and 0.33 for 1-methoxynap hthalene compared with 1.4 X 10(9) dm(3) mol(-1) s(-1) and 0.74 for 1- cyanonaphthalene. In the nonpolar solvent cyclohexane, the CT state en ergy levels are raised (by similar to 14 kJ mol(-1)) relative to the e nergy levels in acetonitrile and benzene and this is reflected in decr eased oxygen quenching rate constants ((1-3) x 10(9) dm(3) mol(-1) s(- 1)) and increased efficiencies of singlet oxygen production (0.56-1.0) , particularly for those naphthalenes which contain electron-donating substituents. In all three solvents the k(O2)(T) and f(Delta)(T) value s for naphthalenes containing strong electron-withdrawing substituents (e.g. -CN, -NO2) remain largely constant. In order to account for the observed data, it is necessary to invoke a potential barrier (Delta G double dagger) to charge-transfer formation or the formation of excip lexes with significant CT character in the quenching step.