We have designed molecules which release alcohols upon exposure to UV light independent of the reaction media, making it possible to liberate alcohols in a controlled manner in applications. Photolysis of 2-(2-isopropylbenzoyl)benzoate ester derivatives 4 in various solvents and in thin films results in the liberation of the alcohol moiety from the ester. The reaction mechanism for the release of the alcohol has been elucidated by time-resolved laser flash photolysis. Upon irradiation the triplet excited state of ketone, 4 is formed, and its lifetime can be estimated to be between 0.08 and 0.8 ns. The triplet excited state decays by efficient intramolecular H-atom abstraction to form a 1,4-biradical, 8, that has a lifetime of less than 17 ns and is trapped by molecular oxygen. In the absence of oxygen, biradical 8 intersystem crosses to form photoenols (Z)-9 and (E)-10 in a ratio of 5:2, respectively. Photoenol (Z)-9 has a lifetime of approximately 3000 ns in protic solvents and returns to the starting material through 1,5 intramolecular hydrogen transfer. The other isomer, (E)-10, is much longer lived (>1 ms) and releases the alcohol moiety through an intramolecular lactonization.

[reaction: see text] Photolysis of 3 in argon-saturated 2-propanol led to formation of 5 via intermolecular H-atom abstraction followed by lactonization. Irradiation of 4 in 2-propanol gave compounds 6 and 7 that also come from intermolecular H-atom abstraction. In contrast, photolysis of an oxygen-saturated solution of 3 in 2-propanol yields products 8, 9, and 10, which were all formed from intramolecular H-atom abstraction and trapping of the corresponding biradical with oxygen. Laser flash photolysis of 3 in methanol showed formation of biradical 3BR (lambda(max) 330 nm, and tau = 50 ns) via intramolecular H-atom abstraction as the main photoreactivity of 3. Biradical 3BR decayed into photoenols 3Z and 3E (lambda(max) 390 nm, tau = 6.5 micros and tau = 162 micros, respectively). In comparison, laser flash photolysis of 4 yielded photoenols 4Z and 4E (lambda(max) 390 nm, tau = 15 micros and tau = 3.6 ms, respectively). Thus photoenol 3E is unusually short-lived, and therefore it does not undergo the intramolecular lactonization as we have observed for the analogous photoenol 1E. Photoenol 3Z decays back to 3 via an intramolecular 1,5-H shift, whereas photoenol 3E reforms 3 efficiently via the solvent with the aid of the ortho ester group. The intramolecular lactonization of photoenols 1E and 3E must be a slow process, presumably because the photoenols are rigid and the hydroxyl group is inhibited, by intramolecular hydrogen bonding, from acquiring the correct geometry for lactonization. Thus only photoenols that are resistant to reformation of their ketone via the solvent are long-lived enough to undergo lactonization and release the alcohol moiety.

Photolysis of 1a yields 4a in argon-saturated methanol, whereas 1b is photostable. Laser flash photolysis of 1a in acetonitrile shows formation of biradical 2a (lambda(max) = 340 nm, tau = approximately 60 ns), which undergoes intersystem crossing to form Z-3a (lambda(max) = 380 nm, tau = 270 ns) and E-3a (lambda(max) = 380 nm, tau = 300 ms). Z-3a regenerates the starting material, whereas E-3b undergoes intramolecular lactonization to release the alcohol moiety and form 4a. Similar laser flash photolysis of 1b shows formation of biradical 2b (lambda(max) = 340 nm, tau = 1.9 micros in acetonitrile), which is longer-lived than 2a is. However, 2b only undergoes intersystem crossing to form Z-3b (lambda(max) = 380 nm, tau = 4.3 micros). Calculations demonstrate that intramolecular pseudo hydrogen bonding between the OH moiety and the radical centered on the isopropyl carbon in 2b and the bulkiness of the isopropyl group prevent the necessary rotation to form E-3b. In comparison, 2a does not form an intramolecular pseudo hydrogen bond between the methylene radical center and the OH group, and as a consequence, it undergoes intersystem crossing to form both E- and Z-3a.