Fluorescence Properties of An Electron Acceptor Substituted Bis-Pyrazolo-Pyridine Derivative: NO2-DMPP

K. Rechthalera, R. Schamschulea, A.B.J. Parusela, K. Rotkiewiczb,c, D. Piorunb and G. Köhlera a Institut für Theoretische Chemie und Strahlenchemie der Universitat Wien, Althanstrasse 14, 1090 Vienna, Austria b Pedagogical University of Kielce, Institute of Chemistry, Chęcińska 5, 25-020 Kielce, Poland c Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland

Received: August 22, 1998, revised version December 14, 1998

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The fluorescence properties of 3,5-dimethyl-1,7-diphenyl-4 -(4'-nitro-phenyl)-bis-pyrazolo-[3,4-b;4',3'-e]-pyridine (NO2-DMPP) and its parent compound 3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4',3'-e]-pyridine (BPP, without the nitrophenyl substituent) were investigated. BPP is a highly fluorescent blue emitter and the fluorescence properties, the emission wavelength, and the fluorescence quantum yield, depend only slightly on solvent. On the contrary, acceptor substituted NO2-DMPP shows dual fluorescence: A long-wavelength component experiences a red-shift with increasing solvent polarity but is efficiently quenched when the polarity exceeds that of solvents like 1,2-dichloroethane or 1-bromopropane. A weak short-wavelength component changes only slightly its position and intensity upon variation of the solvent but its yield increases strongly at low temperatures. The experimental results are discussed in the context of the results of semiempirical calculations which show that fluorescence originates from two closely lying fluorescent states which change their sequence and properties when the polarity of the solvent is varied. A twisted intramolecular charge transfer (TICT) state does most likely not contribute to the emission properties, because of its high energy.

DOI: 10.12693/APhysPolA.95.321 PACS numbers: 33.50.Dq, 34.70.+e