Bis(diarylethenyl)-thiophenes, -bithiophenes, and -terthiophenes: a new series of electrochromic systems that exhibit a fluorescence response

Abstract

2,5-Bis(diarylethenyl)thiophenes (1) and their bithiophene (2) and terthiophene (3) homologues were designed as a new series of violene/cyanine-hybrid-type electrochromic materials. Alkoxyphenyl and dialkylaminophenyl groups are used, respectively, as the aryl group in the cyanine parts of 1/2/3O and 1/2/3N, which emit fluorescence in the neutral state but not in the oxidized state. Voltammetric analyses of 18 newly prepared electron donors show that they all undergo reversible two one-electron oxidations. Redox potentials, UV/Vis spectral features, and fluorescence quantum yields are important parameters for characterizing the present electrochromic behavior with a fluorescence response, and can be finely tuned by changing aryl groups in the cyanine part, the number of thiophene rings in the violene part, and the alkyl-chain length of the alkoxyphenyl or diaklylaminophenyl groups (R = CH 3 , C 8 H 17 , C 16 H 33 ), which makes this a versatile platform for the design of novel electrochromic materials. (Scheme 1), the characteristics of which can be finely-tuned by changing the Ar 2 C end-groups (cyanine-parts). π-Extension of the violene-part is another method for modifying these properties, but simplely extended vinylogues 5 are not suitable due to their instability in the presence of oxygen and proton. Thus, there is a growing need for methods to insert cyclic π-systems 6 such as a p-phenylene unit as in 1,4-bis[2,2-(4-dimethylaminophenyl)]ethenylbenzene p-6N-a, which was prepared as a stable molecule. 7 While it showed a reversible two-electron oxidation-wave in the voltammogram, the dicationic state was not stable enough for isolation and instead transformed into oligomeric polycations (Scheme 2) despite the stability of its p-quinod structure. 8 Intermolecular C-C bond formation at the methine carbons 9 also occurs upon oxidation of the corresponding m-isomer (m-6N-a), since effective conjugation does not occur between the two cyanine-dye parts. Keywords 8During the course of our continuing studies on novel redox-based response systems, 10,11 a series of violene/cyanine-hybrids were designed, in which one (1), two (2), or three (3) thiophene-2,5-diyl units are inserted between the two Ar 2 C=CH end-groups 12 based on the above results. Compared with the p-quinodimethane skeleton, thienoquinoid-type the thiophene sulfur atom (S) and the ipso carbon (C ipso ) of the aryl group (sum of the van der Waals radii: 3.50 Å) ( Fig. 2 and Table 1). The observed short contacts are highly likely to be associated with electronic interaction between σ* (C-S) and π (C ipso ), resulting in conformational stability for the observed geometries. According to the theoretical calculation (B3LYP/6-31G*) for 2/3O-a, a geometry similar to that found in the crystal was obtained by structural optimization (Table 1 and Figs. 1d-f), 22 which supports the notion described above. In the solid-state geometry of 1O-a, slight deformation of the violene π-system is caused by steric repulsion between the two inner aryl groups, ...

Direct synthesis of aryl-annulated [c]carbazoles by gold(i)-catalysed cascade reaction of azide-diynes and arenes

“…The steady decrease in fluorescence with increasing electrochemical oxidation time could be rationalised by the non-fluorescent nature of its cation radical. Such dual electrochromism in which changes occur in both UV-Vis-NIR and FL spectra is rare, 30 but was also realised in our previous study on benzo[ g ]indolo[2,3- c ]carbazole derivatives, 9c – e which were synthesised through a different mode of the gold( i )-catalysed cascade reaction. 9 a Thus, the gold-catalysed synthesis of annulated carbazoles is a powerful tool for exploring the little developed category of advanced electrochromic systems.…”

“…Thus, the observed red shift was induced through π-extension by the fusion of the indole rings. The carbazole skeleton gives rise to fluorescence properties, 30 so the electrolysis of 11A-Me 2 also caused a change in the fluorescence (FL) spectrum [ λ em 424, 442 (sh) nm in CH 2 Cl 2 ( λ ex 354 nm)]. The steady decrease in fluorescence with increasing electrochemical oxidation time could be rationalised by the non-fluorescent nature of its cation radical.…”

Hexaarylbutadiene: A Versatile Scaffold with Tunable Redox Properties towards Organic Near‐Infrared Electrochromic Material

“…The precipitates were collected, washed with dry ether and dry hexane three times, and dried in vacuo to give 3 b 2 + (BF 4 À ) 2 (149 mg) as a dark green powder in 83% yield. (14), 508.61 (15), 508.12 (22) [M] 2 + , 507.61 (8), 507.11 (12); HR-MS (FD): m/z calcd for C 56 H 52 Br 2 F 4 N 4 : 1014.24948; found: 1014.24952.…”

“…Thus, the redox pair of 1 a / 1 a 2+ exhibits reversible electrochromism, by which not only absorption in the UV/Vis region but also that in the NIR‐region are changed. Although only a limited numbers of such NIR EM have been reported so far, they are attracting considerable recent attention since the NIR absorption can be applicable for bioimaging and phototherapy . Absence of heavy metals in organic EM is also favorable for their use in biological systems.…”