INTRA-MOLECULAR EXCIMER FORMATION IN SATURATED AMINES - PROBE OF LINEAR-CHAIN ALKANE CONFORMATIONAL DISTRIBUTION

The fluorescence properties of members of the series (CH3)2N[CH2]nN(CH3)2 are reported for n = 1-13, 16, 18, and 20 in dilute (10-4 M) n-hexane solution at 23°C. Based on comparisons with monoamines, emission from diamines n = 1 and 2 is assigned as excimer/monomer fluorescence. For n = 3 and 4, emission from both monomer and excimer is distinctly observed. While the emission spectrum of n = 5 shows no evidence of an excimer contribution, the decay curve is nonexponential and suggests the involvement of an excimer state. From fluorescence quantum yield and decay curve measurements, the rate constants pertinent to monomer/excimer equilibrium are evaluated for compounds n = 1-5. For n > 6, fluorescence spectra resemble those of monoamines and, with the exception of n = 16 and 20, decay curves show single exponential decay. For diamines n ≥ 4, intramolecular self-quenching rate constants (kisq) are determined. Between n = 4 and n = 8, kisq sharply decreases, reaching a minimum at n = 8. For n> 8, km increases, reaching a maximum at ca. n = 12; for longer chain lengths, a slight decrease in kiSq is indicated. For larger chain sizes, a comparison of kisq with a prediction based on a freely linked model suggests that appreciable chain stiffness is present. In general, the dependence of kisq on chain length (for n > 5) is interpreted as a manifestation of changes in the static conformational distribution functions of the diamines. Nonexponential fluorescence decay was observed for n = 16 and 20; this might imply that the head-to-head distribution functions are broader for these diamines than for the others. © 1979, American Chemical Society. All rights reserved.