An Epr Study of the Species Produced During the Uv Photolysis of Heterocyclic Compounds in Methyltetrahydrofuran at 77 K

Abstract

Abstract—The ultraviolet irradiation (290 nm ≤Λ≤ 390 nm) of indole, purine, indazole, acridine and quinoline in 2‐methyltetrahydrofuran glass at 77 K produces trapped radicals. Two electron‐paramagnetic‐resonance (EPR) signals are found at 77 K during illumination, one at high magnetic field (3–25 times 10‐1 T) assigned to the matrix radical and the other at low field (1.3 times 10‐1 to 1–5 times 10‐1 T) attributed to the lowest triplet state of the heterocyclic molecule. Quantum yields for triplet production at 77 K are 0–34 for indole, 0.51 for purine, 0.55 for indazole, 0.15 for acridine, and 0.94 for quinoline. The rate of formation of matrix radicals varies as the nRth power of the incident light intensity, I0nR, where 1.6 ≤nR=≤ 2. Solvent radical yields, which depend on the light intensity, have been determined. Under the experimental conditions, no signals attributable to trapped electrons or cations have been observed. The dependence of the reciprocal value of the rise lifetime of the low field EPR signal as a function of the intensity of exposure is in accordance with a biphotonic mechanism.

Excited States of Biomolfecules—iii

“…Neither products are found on photooxidation of tryptophan, gly-trp or trpgly, although the indole ring was reported to be severed in each dipeptide. The formation of trapped radicals of tryptophan in 2-methyltetrahydrofuran at 77 K has been reported [4] to occur from the lowest triplet state (@* = 0.34) via a biphotonic process. The induced recombination luminescence of tryptophan in low temperature polar matrices has also been discussed [87].…”