The nature of the light absorption and emission transitions of 4-hydroxybenzophenone in different solvents. A combined computational and experimental study

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 S0 to S5 or S6, followed by S2 ® S0 in emission (Sn denote the singlet states of 4HOBP); (ii) a minor peak at Ex/Em ~270-300/320-360 nm (S0 ® S2 in absorption and S2 ® S0 in 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 a S1 ® S0 transition). 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 derive from S2 ® S0 transitions 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--S0 in ~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--S0 form 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 a significant fluorescence emission.