Charge Transfer Interactions of N-(4-Carboxyphenyl)trimellitimide Dibutyl Ester

Abstract

Charge transfer (CT) interactions of the dibutyl ester of N-(4-carboxyphenyl)trimellitimide (NCTD) dissolved in organic solvents have been investigated by means of UV absorption and fluorescence spectroscopy and time-resolved fluorescence measurements. The spectroscopic data are interpreted in connection with quantum mechanical calculations using the AM1 and ZINDO/S method. The electronic transition lowest in energy, observed as an absorption tail in the UV spectra, is interpreted as a direct intramolecular CT excitation from the N-(4-carboxyphenyl) moiety (donor) to the trimellit moiety (acceptor). Independent of the excitation wavelength, a dominant red-shifted CT emission is observed in the fluorescence spectra. Thus, in addition to the direct CT excitation, indirect methods of CT formation exist. In concentrated solutions ground-state stable dimer formation is observed. Direct monomeric CT excitation and ground-state stable dimer excitation takes place in an overlapping energy range. The ground-state stable dimer fluorescence appears blue-shifted with respect to the monomeric CT fluorescence. Ground-state stable dimers exist that have decay times longer than those of the monomeric CT species. As a distribution of decay times is observed in the fluorescence decay curves, dimers with different decay times exist. According to the performed quantum mechanical calculations, dimers with intermolecular CT character are formed.

Fluorescence of Copoly(ester‐imide)s Containing 2,6‐Naphthoate Units

“…A similar change of the fluorescence with increasing concentration was observed for the aliphatic ester derivative of the imide 3. [8] In HFIP, ground-state stable dimer formation was already observed at 10 À2 M concentration. [8] This dimer formation can be detected by a blue shift of the fluorescence spectrum compared to the monomeric fluorescence and increasing fluorescence decay times in the time-resolved fluorescence decay curves.…”

“…As already mentioned, the exchange of the ester groups leads to slight changes of the absorption behavior, which can simply be described as a blue shift of the substances with aliphatic ester groups of about 5 nm with respect to the aromatic esters. [8,9] Thus, the aliphatic esters, models 1 and 3, also contribute to the absorption spectrum of the copoly(ester-imide). The copoly(esterimide) contains 80 mol-% of hydroquinone ester groups, therefore the pronounced maxima of the copoly(esterimide) absorption spectrum are well described by the absorbance of the model substances 2 and 4.…”

“…For the imide derivative 3 it was found by fluorescence measurements that dimer absorption and intra-molecular CT-absorption takes place in an overlapping energy range. [8] For the polymer, aggregate formation should already exist in dilute solutions according to the coil conformation of the polymer chain, but as mentioned in the above paragraph, the absorption spectrum of the copoly(ester-imide) is described by a superposition of the absorptions of the different model substances. Furthermore, it is not known how many naphthoate and trimellitimide units carry one or two aliphatic ester groups instead of aromatic ones, which also influences the absorption spectrum.…”

“…[8] Quantum mechanical calculations show that dimers with intermolecular CT character exist where the trimellit parts of the molecules act as acceptors and the N-(4-carboxyphenyl) moieties as donors. [8] The performed calculations described in ref. [8] result in the existence of stronger attractive forces acting in dimers, with head-to-tail rather than head-to-head geometry.…”

“…Model substance 3: The dibutyl ester of N-(4-carboxyphenyl)trimellitimide was synthesized by the procedure described for substance 2 and published in ref. [8] The comparatively large alkyl group was selected to enhance the solubility and because it resembles a repeating unit which occurs in certain other copoly(ester-imide)s. [3] Model substance 4: The di(4-tert-butylphenyl) ester of N-(4-carboxyphenyl)trimellitimide was prepared as described for model 2, but with slightly different amounts of substances. 0.03 mol of the dichloride of N-(4-carboxyphenyl)trimellitimide and 0.06 mol of 4-tert-butylphenol were dissolved in 200 ml of dichloromethane.…”