Solid-State Photoinitiated Cycloaddition Reaction of 4,4′-(Ethene-1,2-diyl)bis(pyridinium) Dinitrate: Charge-Density Perspective on Initial Stage of the Reaction

Abstract

Solid-state photoinitiated [2 + 2] cycloaddition reaction 2(H2bpe)(NO3)2 → (H4tpcb)(NO3)4 (bpe = 1,2-bis(pyrid-4-yl)ethylene; tpcb = 1,2,3,4-tetrakis(pyrid-4-yl)cyclobutane) was carried out in a single-crystal-to-single-crystal manner. The reaction product was characterized by means of X-ray diffraction and 1H NMR spectroscopy. Only the rctt-isomer of tpcb was found as the reaction product. Intermolecular interactions in a single crystal of (H2bpe)(NO3)2 were studied within the QTAIM approach. Although sum energy of strong and weak hydrogen bonds dominates in total packing energy, contribution of π…π stacking interactions to the packing energy is also prominent. At solid (H2bpe)(NO3)2, stacking of photoreactive H2bpe2+ cations is realized via N…C, C…C and C–H…C bonding, although no four-membered cycles formed by these bond paths was found in molecular graph representation. Reduced density gradient (RDG) surfaces and molecular Voronoi surfaces clearly demonstrate accumulation of charge density between olefin groups prone to take part in photoinitiated cycloaddition reactions. Good correlation between description of hydrogen bonding in terms of QTAIM and Voronoi approaches was demonstrated. The Voronoi approach confirmed that during the photoreaction the system of hydrogen bonds remained almost unchanged.

Photo‐ and Stress‐Induced Bending of (E)‐1,2‐Bis(pyridinium‐4‐yl)ethene Dinitrate Crystals

“…However, it is evident that the [2+2] photocycloaddition reaction advances, leading to further loss of reversibility. Given that irradiation is known to result ultimately in near complete conversion to the rctt dimer, [11c] with crystal fracture only occurring at this point, these observations seem consistent with photoinduced dimerization occurring at a decreasing rate across a crystal due to absorption by the molecules closer to the light source (Figure 4c). To investigate the kinetics of photodimerization of Ebpe, 1 H NMR spectra of powdered crystals dissolved in DMSO‐ d 6 were measured before and after UV irradiation (365 nm, 250 W/m 2 ).…”

“…The integrals for the pyridyl protons at 8.87 (peak a) and 8.13 ppm (peak b) decreased significantly after UV irradiation for 10 minutes (Figure S4). The formation of a dimer via the [2+2] reaction of the alkene units of [H 2 Ebpe](NO 3 ) 2 was evident from the appearance of new peaks corresponding to pyridyl protons and cyclobutyl tertiary carbon protons at 8.58 (peak a’), 7.61 (peak b’), and 5.02 ppm (peak c’) (Figure S4) [11c] . Integration of the separate signals showed that photodimerization was 94 % complete within 10 minutes (Figures S4 and S5) and continued to the point where monomer signals were indetectable after 50 minutes.…”

“…This was first observed in quaternized derivatives formed by reactions with methylating agents, [12] however, a more elegant procedure for crystallization of this molecule was later developed based on H‐bonding in co‐crystals formed with resorcinol as a neutral H‐bond donor [10,13] . Alternative methods also suited to the ready isolation of the neutral dimer involve crystals of labile metal ion complexes of Ebpe [14] and those of salts of protonated Ebpe [11c,15] . Although they are not invariably successful, where they are, the dimer is usually by far the dominant product, and the complications of solution reactions in which isomerisation [12b,16] and intramolecular cyclization [17] can complete with dimerization are avoided.…”

“…Of particular interest to us was the salt [H 2 Ebpe](NO 3 ) 2 in that, of the three known polymorphs, [18] the monoclinic, C 2/c species can be readily obtained as laths extended by crystallization of Ebpe from dilute aqueous nitric acid (Figure 1) and irradiation of these crystals (without an additional template [15a] ) has been shown to result in dimer formation with high yield and without crystal fracture, [11c] indicating both that the crystal form would be well‐suited to evaluation of mechanical properties and that these properties should be light‐sensitive. To the best of our knowledge, pure organic ionic crystals showing both mechanical bending and photoactuation properties have rarely been explored.…”

Influence of High Pressure on the Pedal-like Motion and Photoreactivity in the Cocrystals of 1,2-Di(4-pyridyl)ethylene with trans-2-(4-Fluorophenyl)vinylboronic Acid

“…The [2 + 2] photodimerization of 1,2-di­(4-pyridyl)­ethylene, bpe (Figure a), in photostable matrices has been vastly studied, − starting from bpe and resorcinol cocrystals, , which are the flagship example showing how crystal structures can be modified in order to obtain materials of desired properties. The [2 + 2] photodimerization of bpe can lead to different isomers (Figure b), which is a consequence of crystal packing …”