Photophysics of all- trans-retinoic acid (ATRA) chemisorbed to nanoparticulate TiO 2: Evidence for TiO 2* to ATRA energy transfer and reverse electron transfer sensitisation

Abstract

The photophysics of all- trans-retinoic acid (ATRA) in methanol solution and of the system comprising ATRA chemisorbed to nanoparticulate TiO 2 have been examined in methanol solution and in aqueous dispersion (pH = 7), respectively. We found evidence for two closely spaced singlet excited states of ATRA, tentatively assigned as n− π * and π− π *, with the former primarily responsible for ATRA fluorescence. The lifetime for this 1( n− π *) state in methanol solution is (55 ± 3) ps. The principal decay pathway of ATRA observable by picosecond transient absorption spectroscopy is photoionization. Under our conditions, photoionization is monophotonic and proceeds directly from the S 1 state of ATRA, yielding the corresponding radical cation, ATRA· +. Electrochemical characterization of ATRA by square wave voltammetry supports this interpretation. ATRA chemisorbs to TiO 2 at pH 7 in large part by deprotonation, i.e., it is present on the anatase surface as the conjugate anion of ATRA. This species is much more fluorescent than free ATRA in solution. On laser flash photolysis at 355 nm, the principal observable photoproduct was deep trapped electrons in TiO 2. Unlike the TiO 2 preparation without ATRA, these species appear only slowly after photoexcitation. A mechanism is proposed, whereby chemisorbed ATRA quenches the initially photoexcited TiO 2 by energy transfer, followed by rate-limiting injection of electrons into the conduction band of TiO 2. The photoholes remain trapped in the ATRA.