The effect of neighbouring cyclopropyl groups on the luminescence of aromatic molecules

Abstract

The luminescence of benzenoid and naphthalenic compounds is quenched by neighbouring cyclopropyl groups.THE fluorescence yield of (I) (#, = 0,005) is low relative to that of (11) = 0.6).l We report the effect of neighbouring cyclopropyl groups on the luminescence of some aromatic compounds, and show that introduction of a cyclopropyl group near a chromophore which absorbs in the near-u.v. region can result in both fluorescence-and phosphorescence-quenching. The degree of quenching is shown to be dependent on temperature and on the energy and lifetime of the emitting state. The compounds studied will be discussed in pairs ; the fluorescence and phosphorescence yields (#, and #p) of the cyclopropane derivative being compared to those of its open-chain analogue within the framework of the following scheme.? ( 4, ) were observed. At room temperature, in isopentane, 4, for ( 111) is less than a factor of two lower than 4, for (IV).

Photochemical studies on deuterium-labeled 2,2-dimethylphenylcyclopropanes

“…2 or 3 to 1. For example, tosylation and reduction of (£)or (Z)-2-(hydroxymethyl-r/2)-2-methylphenylcyclopropane (25,26) afforded (£)or (Z)-2-(methyl-d2)-2-methylphenylcyclopropane (16,17) and 2-methyl-4-phenyl-1 -butene-/,/ -t/2 (31). The yields of purified cyclopropanes were typically less than 10%.…”

“…A series of equations was written for compounds 16-19 and for a 1:1 mixture of 7 and 12 (20), relating the experimentally measurable (vide infra) ratio of 1,3-hydrogen to 1,3-deuterium migration (H/D) to the theoretical value for this quantity in terms of the primary = 7r, secondary type I = a, secondary type II = ß isotope effects and the fraction of migration originating from the m-methyl group = X.…”

“…It should be noted that the isotope effects observed in [16][17][18][19] are intramolecular effects, whereas those observed in system 20 are intermolecular effects. This point will be discussed further later.…”

“…Consideration of the results from our isotopically labeled compounds allows some decisions to be made. The fact that 19 gives a nonstatistical product mix- It should also be noted that since 16 and 17 differ only in the stereochemistry of their deuterium labels, the isotope effects on product formation should be identical. Clearly the wide variation in product ratios for 16 2.49 and 5.68, respectively) can only be accounted for by a hydrogen-migration transient in which the methyl groups are not chemically equivalent.…”