Solvatochromism as an efficient tool to study N,N‐dimethylamino‐ and cyano‐substituted π‐conjugated molecules with an intramolecular charge‐transfer absorption

Abstract

Filip Bureš,* [a] Oldřich Pytela* [a] , Milan Kivala, [b] and François Diederich [b] Hönggerberg, HCI, 8093, Zűrich, Switzerland, Keywords: solvent effects, solvatochromism, UV/Vis spectroscopy, donor-acceptor system A representative data set has been gained by the measurement of the electronic absorption spectra of twelve systematically selected push-pull systems with an intramolecular chargetransfer (CT) absorption and the general structure D-π-A (D = donor, A = acceptor) featuring electron-withdrawing CN groups, electron-donating N(CH 3 ) 2 groups, and various π-conjugated backbones in 32 solvents with different polarity. The longest-wavelength absorption maxima λ max and the corresponding wavenumbers ν were evaluated from the UV/Vis spectra measured in 32 well-selected solvents. The D-π-A push-pull systems were further characterized by quantum-chemical quantities and simple structural parameters.Structure-solvatochromism relationships were evaluated by multidimensional statistic methods. Whereas solvent polarizability and solvent cavity size proved to be the most important factors affecting the position of λ max , the solvent polarity was less important. The most important characteristics of organic charge-transfer compounds are the energy of the LUMO, the permanent dipole moment, the COSMO (COnductor-like Screening MOdel) area, the COSMO volume, the number and ratio of N,N-dimethylamino and cyano groups, and eventually the number of triple bonds (π−linkers). A relation between the first-order polarizability α, the longest-wavelength absorption maxima λ max , and the structural features 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 F o r P e e r R e v i e w 2 has also been found. The higher-order polarizabilities β and γ are not related to the observed solvatochromism. INTRODUCTIONπ-Conjugated organic molecules attract much attention due to their prospective application as efficient materials in organic electronics and optoelectronics. [1,2] A typical organic charge-transfer chromophore (D-π-A) consists of strong electron donors D (e.g. NR 2 or OR groups), strong electron acceptors A (e.g. NO 2 or CN groups), and a π-conjugated core featuring (hetero)aromatic rings and/or double or triple bonds. [3][4][5] Optical linear and nonlinear properties of such push-pull molecules depend on the polarizability of the electrons localized in π−bonding molecular orbitals. [6] Although the polarizability of a molecule is mainly given by its chemical structure, in particular by the length of the π-conjugated spacer and the electronic nature of the donors and acceptors attached, [7][8][9] it can also be affected by external factors such as strength of radiation, state of matter, and in solution also by the solvent used.However, a general description of external factors (solvent) affecting optical and nonlinear opti...

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