Photophysical and photochemical processes of 2-methyl, 2-ethyl, and 2-tert-butylanthracenes on silica gel.: A substituent effect study

The photophysics and photochemistry of 2-methylanthracene (2MA), 2-ethylanthracene (2EA), and 2-tert-butylanthracene (2TBA) adsorbed on silica was studied at a silica/air interface to determine the substituent effect on the above processes. In contrast to anthracene (AN), which forms ground state stable pairs at surface coverages as low as 1% of a monolayer, 2MA, 2EA, and 2TBA show no evidence of any ground state pairing even at high surface coverages. Diffuse reflectance and fluorescence data indicate that 2MA, 2EA, and 2TBA crystallize on the surface at higher surface coverages. Photolysis of 2MA, 2EA, and 2TBA at a silica/air interface proceeds more efficiently than photolysis of AN to produce the corresponding 9,10-endoperoxides formed by the addition of singlet molecular oxygen (type II) to the ground state molecules. Thermally unstable endoperoxides slowly decompose on silica surface to give the corresponding 9,10-quinones and other hydroxylated products of 2MA, 2EA, and 2TBA. Although photolyzed AN/silica samples show a significant amount of dimer formation at low surface coverages, no evidence of any dimer is observed in photolyzed samples of 2MA, 2EA, and 2TBA at low surface coverages. Small amount of dimers (isomeric), however, are observed in the photolyzed samples of 2MA, 2EA, and 2TBA at higher surface coverages suggesting that the crystal forms of these molecules may be involved in the dimerization process. The photolysis rate decreases as the surface coverage is increased. This behavior can be attributed to the involvement of crystals (formed at higher surface coverage) which act as a light sink to absorb the incident light without producing any net chemistry. The photolysis rate decreases by an order of magnitude in the presence of less than or equal to1 monolayer of physisorbed water. Time-resolved transient studies of excited triplet states of 2MA, 2EA, and 2TBA have revealed that triplet lifetimes are shortened on wet silica. Furthermore, the efficiency of singlet molecular oxygen formation drops significantly on wet silica. These results suggest that the decrease in photolysis rate is due to lowering of the singlet oxygen quantum yield in the presence of physisorbed water.