Studies in the mechanism of radiationless conversion of electronic energy. I. p-xylene in solution

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Abstract
The fluorescence quantum yields of p-xylene in isooctane excited at 184.9 nm have been determined for the transitions S3→S0 (Φ3), S2→S0 (Φ2), and S1→S0 (Φ1) as functions of the concentration of p-xylene (cx) and of the concentration of added quenchers (CCl4, CHCl3, c−C7F14), and compared with the S1→S0 fluorescence quantum yield (φ1) for excitation at 253.7 nm in the same solutions. Also vapor fluorescence quantum yields have been redetermined for the S3→S0 and S2→S0 transitions. It has been found that the total nonradiative decay rate constants of S3 (2.5×1014 sec−1) and of S2 (1.0×1013 sec−1) are essentially the same for dilute solution and vapor phases. Also it is found that in dilute solution, Φ3 is independent of concentration of quencher, whereas Φ2 is strongly quenched and to about the same extent as is the ratio β=Φ1/φ1. In more concentrated solutions (cx≳3M), Φ3 rapidly declines to unobservable levels, whereas Φ2 and β appear to increase—but only slightly. On the basis of these results the following is concluded for a dilute solution of p-xylene in isooctane excited into S3 (at 184.9 nm): (i) The S3 state either radiates (kr=3.0×109 sec−1) or makes internal conversion to S2 (k=2.5×1014 sec−1). No autoionization occurs nor do any other decay channels such as S3→S1 or S3→S0 have appreciable probability. (ii) The S2 state either radiates (kr=1.3×108 sec−1), makes internal conversion to S1 (k=1.0×1013 sec−1), or autoionizes (k≳1014 sec−1). All other decay channels are of negligible importance. (iii) In the absence of a quencher, the ejected electron returns to the parent positive ion to regenerate S2 and not a lower state. In the presence of quencher the electron is scavenged and no longer can contribute to any emission. Possible mechanisms for the disappearance of Φ3 at high cx are considered.