title: The nature of the light absorption and emission transitions of 4-hydroxybenzophenone in different solvents. A combined computational and experimental study.
abstract: The photophysics and photochemistry of 4-hydroxybenzophenone (4HOBP) are interesting because they can give some insight into the behavior of humic material. Here we show that 4HOBP has a number of fluorescence peaks: (i) an intense one at excitation/emission wavelengths Ex/Em ∼ 200-230/280-370 nm, likely due to an excitation transition from S0to S5or S6, followed by S2→ S0in emission (Sndenotes the singlet states of 4HOBP); (ii) a minor peak at Ex/Em ∼ 270-300/320-360 nm (S0→ S2in absorption and S2→ S0in emission), and (iii) very interesting signals in the typical emission region of humic substances, most notably at Ex/Em ∼ 200-220/400-500 nm and Ex/Em ∼ 260-280/400-470 nm (in both cases the emission corresponded to an S1→ S0transition). The peak (i) (Ex/Em ∼ 200-230/280-370 nm) is quite intense at low 4HOBP concentration values, but it undergoes an effective inner-filter phenomenon. Remarkably, 4HOBP shows fluorescence peaks that arise from S2→ S0transitions and that do not follow Kasha's rule. Fluorescence is observed in aprotic or poorly protic solvents, and to a lesser extent in aqueous solution. The excited states of 4HOBP, and most notably 4HOBP-S1, are much stronger acids than 4HOBP-S0. Therefore, excited 4HOBP is quickly deprotonated to 4OBP--S0in ∼neutral solution, with a considerable loss of the fluorescence properties. Higher fluorescence intensity can be observed under acidic conditions, where excited-state deprotonation is less effective, and in basic solution where the dissociated 4OBP--S0form prevails as the ground state. The excited states of 4OBP-are formed directly upon radiation absorption, and being weak bases they do not undergo important acid-base equilibria. Therefore, they can undergo radiational deactivation to produce significant fluorescence emission.
authors.affiliation: Dipartimento di Chimica, Università di Torino, Via Giuria 5, 10125 Torino, Italy. davide.vione@unito.it.
authors.forename: Francesco
authors.initials: F
authors.lastname: Barsotti
authors.forename: Giovanni
authors.initials: G
authors.lastname: Ghigo
authors.forename: Silvia
authors.initials: S
authors.lastname: Berto
authors.forename: Davide
authors.initials: D
authors.lastname: Vione