Geometry of intermolecular aromatic hydrocarbon-dialkylaniline exciplexes

Abstract

The geometry of intermolecular aromatic hydrocarbon-M.V-dimethylaniline exciplexes was investigated by comparing the singlet quenching rate constants, exciplex energies, and heats of formation of the exciplexes formed upon quenching pyrene excited singlets by M/V-dimethylaniline (1) and 3,5-di-iW-butyl-/V,/V-dimethylaniline (2).The results are in accord with a sandwich-pair geometry for the 1 exciplex and a localized-pair geometry for the 2 exciplex in which the aromatic rings are adjacent but not parallel as in the sandwich-pair case. The dramatic second-order quenching behavior exhibited by 2 also is in accord with the localized-pair geometry. It is shown that about one-half of the exciplex binding energy can be attributed to localized interactions. tudies of the fluorescence spectra of aromatic hydrocarbon crystals,1 sandwich dimers,2•3 and intramolecular excimers4•5 have shown that a perfectly overlapping sandwich geometry is preferred for excimers which are stabilized by excitation reasonance binding. Recent investigations of intramolecular aromatic hydrocarbon-amine exciplexes6•7 (both anilines and tertiary aliphatic amines) suggest that such specific geometries are not required when charge resonance provides the (1) J.

Amine quenching of fluorescence of phenylated anthracenes

“…S02, in heptane, K and -AH are 2550 M™1 and 11 kcal/ mol™1 5. The latter is very close to the value obtained for the complex of the same base with iodine (-AH = 12.1 kcal/mol™1).30 For this last complex, the charge transfer is obviously more pronounced, and so one can conclude that other effects such as the dipole-dipole interaction contribute to the stability of the complex.…”

The Photophysics of 5‐methoxyindole. A Non‐exciplex Forming Indole: Additional Evidence for Two Classes of Exciplexes*

“…The magnitude of this value is too small for a strong exciplex, and it is more likely that the slope is due to dipolar interactions. In addition, recent work by Taylor et a/. (1974) on the geometry of strong exciplexes has shown that chargetransfer is the important stabilizing force in strong exciplexes.…”

New Insights into an Old Problem. Fluorescence Quenching of Sterically-Graded Pyrenes by Tertiary Aliphatic Amines

“…Furthermore, we have used a crystalline complex between PyOMe and trans -perhydroquinoline to demonstrate that the nitrogen lone-pair must adopt an orthogonal (or near orthogonal) approach to the pyrenyl basin in order for quenching to occur . Although the latter has been an integral part of the accepted quenching model for many years, it has never been demonstrated unequivocally to the best of our knowledge . Additionally, the optimal distance and angle for approach of the nitrogen lone-pair of electrons to the pyrenyl surface in the ground-state of PyOMe have been examined in detail by DFT calculations.…”