2‐(2,4‐Dinitrobenzyl)pyridine (DNBP): A Potential Light‐Activated Proton Shuttle

Abstract

The well-known photochromic tautomerism of 2-(2,4-dinitrobenzyl)pyridine (1; CH; Scheme 1) was re-investigated by flash photolysis in aqueous solution in view of its potential application as a lightactivated proton pump. Irradiation of 1 yields the enamine tautomer NH (l max ¼ 520 nm) that rapidly equilibrates with its conjugate base CNO À (l max ¼ 420 nm). The pH -rate profile for the first-order decay of NH and CNO À provides a direct determination of the acidity constant of NH, pK NH a;c ¼ 5.94 AE 0.12 (I ¼ 0.1m) and serves to clarify the mechanisms of proton transfer prevailing in aqueous solutions. The acidity constant of protonated 1 (CHNH þ ), pK CHNH a;c ¼ 4.18 AE 0.02, was determined by spectrophotometric titration.1. Introduction. -In 1925, Tschitschibabin et al. [1] reported that exposure of the pale yellow crystals of 2-(2,4-dinitrobenzyl)pyridine (1; labeled CH in Scheme 1) to sunlight developed a blue color that faded away within a day in the dark. They tentatively identified the blue product as the enamine tautomer NH. Irradiation of 2-nitrobenzyl compounds such as 1 generally yields the corresponding aci-nitro compounds by H-atom transfer from the ortho-alkyl group to the NO 2 group [2]. Thus, the tautomer OH is the expected primary photoproduct of 1 (CH) [3]. But numerous studies have confirmed the original assignment identifying the long-lived blue photoisomer as the enamine tautomer NH. Its absorption spectrum (l max % 520 nm in aqueous solution, 530 -580 nm in other media) is similar to that of the corresponding N-methylated enamine NMe [1] [4] [5]. IR-and NMR-spectral data [6], and timeresolved resonance Raman [7] and polarized optical absorption spectra of oriented molecules [8] all confirm that the blue isomer is NH, and X-ray analysis recently established this structure [9]. Studies of 1 by flash photolysis [10 -14] with sufficient time resolution [12 -14] have identified OH as the primary photoproduct (l max % 410 nm) that is subsequently converted to NH in a thermal reaction. Delayed formation of the enamine NH in MeCN was proposed to occur by proton transfer through the solvent [7b], but calculations and experiments indicate that intramolecular proton transfer OH ! NH is also feasible (see Sect. 2.1). In fact, both pathways may be operating; Kleinschmidt and Graness [5] report a quantum yield of 0.019 for formation of NH via OH, and of 0.024 for a direct formation of NH from 1. According to calculations (see Sect. 2.1), the thermochemical, direct (suprafacial, Woodward -

Synthesis and properties of di‐ and trinitrobenzyl substituted pyridine derivates

“…The structure supports theoretical calculations and spectroscopic measurements which predict that the proton is not directly shifted from carbon to nitrogen. The initial step is the proton transfer to one of the oxygen atoms of the nitro group (forming an aci ‐nitro group, OH form), which slightly rotates and hands over the proton to the pyridine nitrogen (CH → OH → NH) 19, 35. According to our X‐ray structure geometry, one of the protons of the CH 2 group points towards the nitro group.…”

“…Photoswitchable acids such as DNBP 1 could be eventually used as shuttles in artificial membranes to pump protons directly uphill across a membrane without linking it to a light driven electron transport (symport) (Scheme ). In a recent study, we determined the pK a values of all species in the photochemical and thermochemical equilibria using flash photolysis and by spectrophotometric titration 19. Based on the pK a values a hypothetical proton transport could be performed as follows.…”