Effect of Excitation Wavelength in Single-Molecule Photochemistry of Terrylene.

The reaction of terrylene in p-terphenyl with molecular oxygen is reinvestigated by TIRF-microscopy with λ_exc=488 nm or λ_exc=561 nm and 488 nm. A similar range of fluorescent products is obtained under both experimental conditions with a reaction quantum yield Φ_r>10^-7 for those molecules which undergo the photoreaction. The majority of these oxygen-susceptible molecules reacts via an electronically relaxed, dark intermediate, presumably an endoperoxide, with a lifetime of off>~20 s. From this time constant, an activation energy E_A<0.8 eV is estimated for the transition from the intermediate to the final product, the diepoxide, which nicely agrees with values calculated for the terrylene-derivative TDI. However, ~20 % of all reacting molecules at λ_exc=561 nm and even ~40 % at λ_exc=488 nm show an immediate change of the fluorescence colour within the time resolution of the experiment, bypassing any dark intermediate. Based on this experimentally observed impact of the excitation energy and the lack of relevant excited-state absorption, we hypothesize that oxygen forms a complex with ground-state terrylene which then undergoes a quasi-unimolecular reaction in the excited-state before vibrational relaxation takes place.