Localized excitation effects in (E,E)-2,5-bis[2-(4-N,N-dipropylaminophenyl) ethylenyl]-3,6-dimethylpyrazine and generation of its different forms in solution

Abstract

rupolar dye in 1,4-dioxane and cyclohexane in addition to its instantaneous fluorescence spectra and fluorescence lifetimes in dichloromethane (pure and with added trifluoroacetic acid) were measured. Quantumchemical calculations show that the π−π conjugated system is localized in the flattest part of the molecule due to disorder in the ground-state geometry of the dye molecule and that this part is the one responsible for light absorption. The effect of localized excitation of the dye causes a considerable change in its dipole moment Δ a μ. Various fluorescent forms of the dye that feature considerably spaced emission spectra and different fluorescence lifetimes originate by adding trifluoroacetic acid to the solution of oligophenylenevinylene in dichloromethane. Introduction.Oligophenylenevinylenes belong to a class of complicated organic compounds with an extended conjugated electron system and electron-donor and electron-acceptor groups. They are finding more and more applications in microelectronics [1-4], nonlinear optics, and biology [5][6][7][8]. They are used in microelectronics primarily in organic thin-film light-emitting diodes. Symmetric organic compounds with electron-donor and -acceptor groups in quadrupolar and octupolar positions have large transverse cross sections for two-photon absorption and are used in photodynamic therapy and other areas. These compounds are also used to generate second harmonics in nonlinear light transformers and as sensors of several biologically important cations.It was found earlier [9-14] that increasing the efficiency of donor-acceptor interactions caused the primary hyperpolarizability of complicated organic molecules to increase. Thus, efficient intramolecular charge transfer that changed significantly the dipole moment of complicated organic molecules during the transition into an electronic excited state was required in order to enhance their nonlinear optical properties. A substantial change of dipole moment of dye molecules after excitation was also important with respect to their use as fluorescent probes for studying various biological systems.The completed studies showed that the absorption and emission spectra of quadrupolar and centrosymmetric oligophenylenevinylenes underwent a significant long-wavelength shift and the Stokes shift of their fluorescence spectra increased as the solvent polarity increased [15,16]. The shape of the emission spectra of symmetric oligophenylenevinylenes typically did not change after the long-wavelength solvatochromic shift. The same was true for the long-wavelength shift of the absorption spectrum upon increasing the polarity of the organic solvent. According to the theory [17], these effects indicated that the dipole moments of the symmetric oligophenylenevinylenes changed significantly after the transition into the excited state.The electro-optical absorption method (EOAM) [18,19] (spectroscopy of the molecular Stark effect in the previous terminology [20]) enables dipole moments and polarizability of molecules in various ...