Amplifying the Negative Solvatochromism of Pyridinium Phenolates via Fluorene Conjugation

Abstract

New chromophores carrying a donor phenolate group and an acceptor pyridyl moiety separated by a fluorene spacer are reported. A progressive elongation of the systems was achieved by the introduction of an additional phenyl ring and one or two vinyl linkers using Suzuki−Miyaura or Horner−Wadsworth−Emmons couplings. Zwitterionic forms FL1−FL5 of these donor−bridge−acceptor molecules were generated through consequent N-methylation and deprotonation reactions leading to large redshifts in absorbance maxima. UV−vis absorbance studies also revealed a negative solvatochromic behavior, with a smooth bathochromic shift observed upon decreasing the solvent polarity. Notably, examples FL1a and FL3 exhibited wide solvatochromic shifts of 235 and 297 nm, respectively, as the solvent polarity gradually lowered from polar water to less polar pyridine. Remarkably, the magnitude of the shift for FL3 was even greater than that of Reichardt's dye (253 nm) in the same solvent range, which is known to be one of the best performing solvatochromic dyes reported. Apart from extending the conjugation, the fluorene moiety also allows facile side chain engineering, making these solvatochromic dyes attractive candidates for medium-polarity indicators and sensors.

Extended Reichardt's Dye–Synthesis and Solvatochromic Properties

“…Our recent studies on the behaviour of the more conjugated OPP dyes with fluorene bridges showed improved solvatochromism compared to the phenylene analogues. [13] Given the solvatochromism of the πextended betaine dyes reported in this paper, it is proposed that the sensitivity to solvent polarity could be increased by replacing the biphenyl bridge with a fluorene unit.…”

“…Thin layer chromatography was performed on Merck-Millipore Silica gel 60G F 254 glass plates, and spots were revealed under 254 nm and 365 nm light from a mercury lamp. 1 H-NMR and 13 C-NMR spectra were obtained on a 500 MHz Bruker spectrometer, and shifts were referenced to the NMR solvent signals. Mass spectra were obtained on a Thermo Exactive Plus Orbitrap ESI-MS spectrometer operating in positive or negative mode.…”

“…[6] We recently expanded this series to include fluorene and ethylene π-bridges which showed increased the solvatochromic effect due to increased π-conjugation. [13] While many derivatives of 1B have been synthesised [1,3,14,15] adding solubilising groups, varying the substituents ortho to the phenolate (see 1C, Figure 2), replacing the phenolate with other electron-donating groups -there has been little exploration on effect of π-extension. Given the aforementioned results for the π-extended OPP dye series, we decided to investigate the effect an addition of a phenylene spacer would have on 1B.…”