Merocyanines based on 1,2-diphenyl-3,5-pyrazolidinedione

Abstract

The acceptor strength of the 1,2-diphenyl-3,5-pyrazolidinedione residue is re-evaluated based on the spectral-fluorescent properties of donor–acceptor dyes. Low fluorescence of the title compounds is explained by the low-lying forbidden ππ*-state.

Design and Photonics of Merocyanine Dyes

“…We conducted systematic investigations of the fluorescence properties of merocyanines for the first time, which allowed us to discover a number of specific features and make generalizations about the electronic symmetry of their chromophore in the fluorescent state S 1 . [67,[69][70][71]73,74,98] Based on the mirror symmetry (likeness) between the absorption and fluorescence (Levshin's law), it could be assumed that the regularities in the fluorescence spectra of merocyanines would be similar to those in their absorption spectra. However, it was found that even typical electronically nonsymmetric merocyanines exhibit fluorescence similar to symmetric ionic polymethines.…”

“…It was found that the FQYs of 3,5-pyrazolidinedione-based merocyanines comprising indole (62)(63)(64) and benzo[cd]indole (68-70) as donor end-groups are notably inferior to that of their analogs with other strong acceptor groups, such as barbiturate and thiobarbiturate. [74] The PCM/TD-B3LYP calculations revealed that a probable cause for this is the enhancement of internal conversion, which was attributed to the presence of a low-lying 1 π HÀ 1 π L * state associated with a dipole-forbidden transition from HOMO-1, localized on the acceptor 1,2-diphenyl-3,5-pyrazolidinedione fragment, to LUMO. This conclusion was supported by the second-order approximate coupled cluster singles and doubles model (CC2) calculation.…”

“…This conclusion was supported by the second-order approximate coupled cluster singles and doubles model (CC2) calculation. [74] The modeling revealed that this effect persists when the phenyl groups of the 3,5-pyrazolidinedione moiety are replaced with methyl groups. Hence, it can be attributed to the presence of a hydrazodicarbonyl fragment (O=C)À NRÀ NRÀ (C=O) in the acceptor end-group.…”

“…[62][63][64] Considering the existing gaps in understanding the basic structure-property relationships of merocyanines, we conducted a comprehensive and systematic investigation of a large set of dyes 6-70 (n = 0-2), where all key structural elements were varied, including the donor/acceptor end-groups and the polymethine chain length, and the medium polarity changed over a wide range from low-polarity n-hexane and toluene to high-polarity aprotic (DMF) and protic (ethanol) solvents. [65][66][67][68][69][70][71][72][73][74] In these works, the mathematical method of moments was applied for the first time to analyze bandshapes in the electronic spectra of merocyanines. [75,76] The shape and width of absorption and fluorescence bands were compared in coordinates ɛ(ν)/ν-ν and F(ν)/ν 4 -ν [where ɛ(ν) is the absorption coefficient, and F(ν) is the fluorescence intensity], as recommended by B. I. Stepanov for assessing the mirror symmetry of optical spectra.…”

“…[72] Further, the 1,2-diphenylpyrazolidine-3,5-dione residue, which was regarded one of the strongest uncharged acceptor groups in polymethines, [2] was shown to be inferior to the 1,3dialkylthiobarbituric acid residue. [74] A systematic comparison of many aspects of various malonic acid derived acceptor groups -cyanoacetic acid, malononitrile, diethyl malonate, Meldrum's acid, dimedone, 1,3-indandione, N,N'-dibutylbarbituric acid, and N,N'-dibutyl-2-thiobarbituric acid -was carried out in the paper by M. Klikar et al [80] In this study, their relative acceptor strengths were evaluated using the experimental first reduction potential E 1/2(red) (which is directly related to the LUMO position) of the derived merocyanines with 2-(N-piperidinyl)thiophene as the donor end-group. The assumption was made that LUMO in the studied molecules is localized on the acceptor moieties, which is basically true considering that the polymethine chain in the studied dyes consists of merely 1 or 3 methine groups; withal, since the donor and polymethine parts of the molecules were identical, the acceptor group variation was the only operational factor in any case.…”