Fundamental Studies on Brooker's Merocyanine

Abstract

The spectroscopic properties of solvent soluble derivatives of Brooker's simple merocyanine 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one, which can in principle exist in two distinct canonical forms, have been assessed both experimentally and theoretically using molecular orbital methods. 1 H and 13 C NMR evidence in a range of solvents suggests that the merocyanine exists as a resonance hybrid which is weighted toward the zwitterion even in solvents of low dielectric constants. In protic solvents, the large hypsochromic shift observed for the merocyanine in the visible region arises from both a dielectric effect and a hydrogen bonding effect. Theoretically, the PM3/COSMO method gives a reasonable account of the structure and spectroscopic shifts of the merocyanine in aprotic solvents. The large shifts observed arise because solvents with large dielectric constants have a much greater stabilizing effect on the more polar ground state of the merocyanine than they do on the first excited state. While the same method predicts stable hydrogen-bonded structures for a dihydrate and hexahydrate, it is unable to reproduce the known hypsochromic shift for these solvated species. In contrast, a version of the CNDO/S method does predict the correct trends on hydration though the magnitude of the effect is less than that found experimentally.Recent studies have strongly suggested that one of the main factors which contribute to the large solvatochromic shift is a

Self‐Assembly of Bis(merocyanine) Tweezers into Discrete Bimolecular π‐Stacks

“…[29] After deprotection by catalytic hydrogenation, the free amine intermediate was reacted with ethyl cyanoacetate to afford bis(cyanoacetamide) 11 in 66 % yield. Condensation of 11 with ethyl acetoacetate gave bisA C H T U N G T R E N N U N G (pyridone) 12 in 72 % yield, and the reaction of 12 with DMF and salt 13 [30] afforded the bis(merocyanine) dye 3 in 10 % yield. Bis(merocyanine) dye 2 was obtained in 28 % yield by the reaction of pyridone 16 [12b] with N,N'-diphenylformamidine (DPFA) and salt 9.…”

“…[49] Naphthalene dicarbaldehyde 6, [28] pyridinium salt 13, [30] dibromide 14, [31] hydroxypyridone 16,…”

An internal charge transfer-dependent solvent effect in V-shaped azacyanines

“…In order to reveal the reason for such a striking difference, we measured the absorption and emission spectra of 4 and 5 in four solvents of different polarities ( Table 2). We observed a strong negative solvatochromism for both dyes (analogous to classical Brooker's merocyanine 30 ), indicating larger stabilization of the ground state of azacyanine molecules combined with increased reorganization energy in more polar solvents, which thus increases the energy gap between the ground state and the (Franck-Condon) excited state and leads to a shorter wavelength of the absorbed light. This effect was particularly strong for 4, where 20 and 46 nm hypsochromic shifts of respectively absorption and emission maxima were observed in the polarity range between dichloromethane and methanol.…”

Charge Transfer Transition: Investigation of the Effect of Solvents on Electronic Spectra of N-Alkyl/ N-Alkylphenyl Derivatives of p-Nitroaniline

“…This shows a change in solvent polarity can quickly lead to change in both structure and color. This will in turn cause a change in the relative energy levels of the ground and excited states, allowing for different colorimetric observations 16 .…”