Quenching of Benzophenone Triplets by Naphthalene
P. Nataraian University of Madras A.C. College Campus India, 600 025
I
A physical-organic chemistry experiment
Energy transfer processes appear to he an important phenomenon in photobiology and related activities in nature. A considerable amount of work has been done in the last two decades on the electronic energy transfer processes involvine oreanic molecules ( 1 1. Excitation transfer involving the triplets of a donor molecule and acceptor molecules with a sindet mound state has been intensivelv investi" gated hy the pioneering studies of Hammond and coworkers (2). An organic molecule when excited hy a spin allowed transition reaches the singlet state which has a life time in the range 10-9-10-6 s depending on the various deactivation processes of the excited state. It may return to the ground state by the fluorescence decay, it may non-radiatively return to the ground state by internal conversion, it may undergo intersystem crossing to the triplet state, or it may react photochemically. The triplet state can have a lifetime many orders of magnitude greater than that of the corresponding singlet state. Deactivation of the triplet state could occur by the emission of light, i.e. phosphorescence, non-radiative decay either to the ground state or leading to the decomposition of the molecule. Photosensitized reactions differ from direct photochemical reactions in that the photosensitizer absorbs the light hut some other species in the system reacts. In general most photosensitized reactions invoke excitation energy transfer where the photosensitizer is the donor and the reacting molecule is the acceptor. During the life time of the triplet state electronic energy transfer occurs to an acceptor molecule in the vicinity along with the other deactivation processes of the excited state. The energy transfer process occurring hetween a triplet donor and a singlet acceptor
-
=*
(triplet)
+ ,A
(smglet)
-
+ A* (singlet) (triplet)
is a spin allowed process and in a fluid medium the limiting rate for this process is controlled by the rate of diffusional encounter of the molecules involved. An experiment involving the ahow principles in a senior undereraduate rurriculum should be valuahle for the understanding of an important aspect of photochemistry. Discussion Reduction of aromatic ketones, by the action of light, in hydrogen donor solvents is a well-known reaction. The discovery that a solution of henzophenone in ethanol on exposure to sunlight gave good yields of henzpinacol was made in 1900 (3). Later researchers (4, 5) have interpreted that henzophenone triplet abstracts hydrogen atom from the solvent molecule to form ketyl radicals. Molecules like naphthalene and oxygen compete for the henzophenone triplets more efficiently than the hydrogen donor solvents and the yield for the photoreduction of henzophenone is suppressed when these molecules are present in the solution. The following mechanism was proposed when the solvent is 2-propanol(6).
The quantum yield for the photoreduction is given by the relationship
where C is the yield for the henzophenone triplet. By assuming 12-propanol] is approximately constant for deaerated solutions, eqn. (1) becomes
where k,' = h,[2-propanol]. This is known as the SternVolmer relationship and from a plot of 21% versus [Q],k, is calculated. For the calculation of h,, values of C and h, are provided from the literature. Experiment An irradiation cell (Pyrex) with two arms fitted with stoppers is filled with 0.1 M benzophenone solutions in 2-propanol containing naohthalene at various concentrations (usuallv lo-' M to M in five or six samples). The solution was deaerated far ebout 15 min with oxygen-free nitrogen and the cell was placed in a 365-nm light beam (50-nm band width) of a boa type General Electric high pressure mercury vapor lamp fitted with an interference glass filter. On the optical bench are mounted the lamp, the glass filter, and the cell holder in the given order. Change in the concentration of benzophenone during the irradation was determined speetrophotometrically by diluting the irradiated and unirrsdiated solutions 25 times using 2-propanol. Intensity of the light beam was determined either by a ferrioxalate actinometry (7) (if the experiment runs for 6 hr) or by a Kipp and Zonen calibrated thermaphile-galvanometer system.' Comments The exoeriment was eiven to the participants of the Universlty Grants Commiision Summer Institute on Rates and Eauilihria nmducted at the De~arrmenrof Phvqical ChemisGy, University of Madras in
