One- and two-photon photochemical stability of linear and branched fluorene derivatives

Photochemical decomposition of the fluorene derivatives, 4,4′-[[9,9-bis(ethyl)-9H-fluorene-2,7-diyl]di-2,1-ethenediyl]bis(N,N-diphenyl)benzeneamine (1) and 4,4′,4″-[[9,9-bis(ethyl)-9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl]tris(N,N-diphenyl)benzeneamine (2), were investigated in pTHF under one- (UV-lamp) and two-photon (femtosecond laser) excitation. The quantum yields of the photoreactions were determined by absorption and fluorescence methods in air-saturated and deoxygenated solutions. The values of quantum yields were in the range (1.5–2.1) × 10−6 for both types of irradiation, indicating that similar photobleaching processes occur under both one- and two-photon excitation. Deoxygenation of pTHF increased photostability of 1 and 2 by over an order of magnitude relative to air-saturated solutions. A cursory examination of some of the photochemical products (analyzed by HPLC and APCI-MS spectroscopy techniques) revealed an important role of ground state oxygen in the photoreactions, since no oxidation products were observed upon irradiation in the presence of the well know singlet oxygen sensitizer methylene blue.