Geometry of Structurally Non-Rigid Pyridinium Cations in an Excited State
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ORIGINAL ARTICLE
Geometry of Structurally Non-Rigid Pyridinium Cations in an Excited State Vladimir Kharlanov 1 & Vladislav Papper 2 Received: 18 February 2020 / Accepted: 17 July 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Quantum-chemical calculations of phenyl-substituted pyridinium cations established the formation of two non-rigid equilibrium structures in a singlet excited S1 state. These two structures are characterized by a significant torsion of the methyl group and flattened geometry of the phenyl rings relative to the plane of the heteroaromatic ring, as well as sp3 hybridization of a nitrogen atom. Structural features of the S1 equilibrium structures and their deviation from the pyridinium cation geometry in the ground state explain the experimentally detected abnormally large Stokes shift. Keywords Phenyl-substituted pyridinium cations . Fluorescence . Fluorescence of structurally non-rigid molecules . Stokes shift . Intramolecular charge transfer . Structure of molecules in excited states . Quantum-chemical calculations
Introduction Structurally non-rigid N-methyl aryl- and phenyl-substituted pyridinium cations 1 (see Fig. 1) in solutions of different polarity have unusual fluorescent properties. The Stokes shift (Δνaf), which is a shift of the fluorescence emission band maxima relative to the long-wave absorption band maxima, reaches the magnitude of 10,000 cm−1 [1–7]. A possible cause of such an abnormally large shift may be a significant change in geometry of a singlet excited state. Comparison of the fluorescence properties of the non-rigid cations under consideration with the fluorescence properties of their structurally rigid analogues allowed to assume that in the excited FrankCondon state, a structure with significantly twisted (distorted from the planar arrangement) 2- and 6-aryl-substituted rings is rearranged to more planar structure [1, 2, 6]. The non-planar geometry of cation 1 in the ground state, and, consequently, the Frank-Condon structure, is evidenced This manuscript is dedicated to a memory of a most brilliant physicist, a passionate educator, a well-respected colleague and a great friend Dr. Vladimir Kharlanov. * Vladislav Papper [email protected] 1
Institute of Chemistry, Humboldt University of Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany
2
Leo Piccard 40/7, 8471039 Be’er-Sheva, Israel
by X-ray diffraction data analysis of a crystalline compound similar to cation 1, but having a chlorine atom in the paraposition of the 4-phenyl ring. This non-planarity is confirmed by the data of the crystalline structure of cation 2 (see Fig. 1), the steric hindrance of the N-phenyl ring of which is very likely to be similar to the same of the N-phenyl ring bearing the methyl substituent in cation 1. According to these data, the rotation angles of 2- and 6-phenyl cation rings relative to the heteroaromatic nucleus are in the range of 60–70° (angles α2 and α6) (see Fig. 1), while the 4-phenyl ring is twisted significantly less (α4 = 18°) [5, 8].
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