Biphotonic Ionization: A Flash Photoconductivity Study of TMPD in 3-Methylpentane Solutions

Abstract

Flash photoconductivity studies are presented for a liquid organic solution. It was found that a dilute solution of N,N,N′,N′-tetramethylpharaphenylenediamine in 3-methylpentane at room temperature could be photoionized by light quanta with energies of the order of one-half those required for gas-phase ionization. This is fully reminiscent of the low-energy photoionizations seen when this and similar solutions are cooled to a very rigid state at 77°K. These ionizations are now known to be biphotonic, and the possibility of a similar mechanism holding for the liquid, room-temperature system is explored. A detailed kinetic analysis of one class of flash-biphotonic phenomena is examined. The general category treated consists of all cases in which a one-photon-produced intermediate state is subsequently ionized by a second photon. The charge-carrier density (and peak photocurrent) is proportional either to [ ∫ 0T I(t)dt]2or to ∫ 0T I2(t)dt, depending on whether the intermediate-state lifetime is either long or short compared to the flash duration T, respectively. Experimental studies verify proportionality of the peak photocurrent to both of these expressions, which, it happens, are equivalent under the conditions employed. The biphotonic character of the ionization is confirmed. A molecular-weight study of the solute molecule verifies the monomer nature of the photosensitive species, and a linear concentration dependence of the photocurrent at low concentrations eliminates a bimolecular (solute–solute) mechanism in this instance. The sublinear dependence of photocurrent on concentration at higher concentrations is analyzed as a bimolecular quenching of the intermediate state. The range of possible intermediate-state lifetimes obtained suggests (but does not prove) that, just as in the rigid solution at 77°K, the triplet state serves as the intermediate state in this fluid solution as well. The physical basis of the peak photocurrent as a meaningful parameter is discussed in an appendix.

The Biphotonic Photoionization of Chlorpromazine During Conventional Flash Photolysis: Spin Trapping Results With 5,5‐dimethyl‐1‐pyrroline‐n‐oxide

“…Our computer analysis of the excitation processes originating in the So+S1 electronic transition of CPZ also led us to conclude that the photoionization of CPZ can show monophotonic behavior during laser flash photolysis over a significant range of laser flash intensities. Similar studies have been carried out with other photoionizing chemicals (Alkaitis et al, 1975;Lachish et al, 1976;Pileni et al, 1975; hv Pillof and Albrecht, 1968).…”

Effect of laser intensity on the determination of intermolecular electron transfer rate constants—Observation of Marcus inverted region in photoinduced back electron transfer reactions

“…28,29 This cluster is suitable for quantitative study of monophotonic and biphotonic processes owing to its stability under photolysis. Though the light-intensity dependence has been studied previously, [30][31][32][33] our theoretical treatment further emphasizes both the light saturation and the inhomogeneous distribution of the species. The relationship between the observed back ET rate constant, intrinsic back ET rate constant, and the initial concentration of the excited-triplet molecule is derived by a model of diffusional encounter pairs.…”

The Stepwise Biphotonic Photoionization of Chlorpromazine as Seen by Laser Flash Photolysis

“…where n is an integer and k is a constant encompassing the specified experimental conditions (Pillof and Albrecht, 1968). The value of n can be conveniently determined by plotting the logarithms of the terms in Eq.…”

“…7: log(D0) = log k + n log(lp) (8) A plot cf log (Do) vs log(fp) then yields a straight line, the slope of which gives n. Note that the absolute values of [e;,] and light intensity are not necessary to obtain n . Lachish et al (1976) and Pillof and Albrecht (1968) have analyzed the conditions under which it is possible to use the value of n to determine mechanisms of photoionization. Using the Beer Lambert law, Lachish et al (1976) have derived a qualitative criterion for determining when the value of n obtained from equation 8 can be used to distinguish single-from multi-photonic processes.…”