An Apparatus for Photochemical Studies M. J. Winter
Edward Davies Chemical Laboratories, U.C.W., Aberystwyth, Dyfed, Wales. U.K. P. V. Wlnter
The High School, Llanidloes, Powys, Wales, U.K.
Photochemistry is one of the most active and rapidly developing fields of research, with application in the fields of photography and photochromism, dyestuffs and optical brighteners, the manufacture and degradation of polymers, and the conversion of solar radiation into a more convenient form of energy. Since excited states of molecules often react totally differently from the ground state, photoexcitation may open up entirely new synthetic routes.' Photophysics attempts to explain the observed photochemistry on the molecular scale. Figure 1 illustrates the more important photophysical processes and is known as a Jablonski diagram. In the lowest, or ground, electronic states of most organic molecules, electrons occupy molecular orbitals in pairs. Above these occupied orbitals lie others which are entirely empty. Absorption (A) of a photon of appropriate energy may lead to promotion of an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) giving the excited state S1 (see Fig. 1).For most species, the difference in energies of the S1 and So states is such that the required photon is usually of visible or ultraviolet frequencies, according to Planck's relationship as it applies here, i.e., (1) E(S1) - E(So) = hvm At normal temperatures absorption occurs from the vibrational ground state of So, but, depending on the energy of the absorbed photon, excited vibrational levels of S1 may be populated. At high pressures, the collision rate is high and vibrational relaxation occurs producing St in its vibrational ground state. Emission from S1, reforming So, is known as fluorescence (F). Alternatively, SI may form the state TI by intersystem crossing (ISC). This process occurs by reversal of spin of one of the unpaired electrons a process which is formally "forbidden" by quantum theory. The arrangement corresponding to parallel spins (a so-called triplet) is preferable to antiparallel spins (singlet state), so TI is lower in energy than &. From TI, intersystem crossing may occur, giving highly vibrationally excited So. Alternatively, So may be formed by emission of a photon, a process known as phosphorescence (P). Since phosphorescence involves an electron spin reversal, it too is forbidden and thus, compared to fluorescence, is of low ~robabilitv:fluorescence from SI to Sn" tvnicallv occurrine .. about 10,000 times more readily than phosphorescence from TI to S.,. Another ~ossihlefate for molecules in excited states is;eac
