Cross-exchange of donor units in donor-acceptor-donor type conjugated molecules: Effect of symmetrical and unsymmetrical linkage on the electrochemical and optical properties

Solvatochromic and Proton-Responsive characteristics of Bi-1,3,4-Oxadiazole derivatives with symmetric dimethylamino substitution

D-A molecules find extensive use in intelligent stimulus-response systems due to their exceptional attributes, including high sensitivity, rapid response, wide compatibility, and structural adaptability. The strength of Intramolecular Charge Transfer (ICT) plays a pivotal role in determining the performance of these devices. To enhance the ICT strength and explore new applications for D-A molecules, we meticulously designed a pair of symmetric dimethylamino-substituted bi-1,3,4-oxadiazole derivatives (DMAOXD and DMAOXDBEN). These symmetric D-A-A-D molecules, with strong electron donor terminals, displayed a modest redshift of less than 25 nm in the UV-vis absorption spectra. However, there was a significant redshift in the emission spectra (140 nm for DMAOXD and 170 nm for DMAOXDBEN) when transitioning from cyclohexane to dimethyl sulfoxide, indicating a pronounced ICT characteristic. Theoretical calculations support the idea that the dimethylaminophenyl unit serves as an electron donor in both DMAOXD and DMAOXDBEN, while the 1,3,4-oxadiazole and central benzene ring act as acceptors. The pronounced ICT characteristic observed in DMAOXD and DMAOXDBEN can be attributed to long-distance electron transfer. Additionally, it's noteworthy that the emission of DMAOXD and DMAOXDBEN solution samples can be quenched by adding trifluoroacetic acid (TFA) and restored by the addition of triethylamine (TEA). Inspired by this, a pattern created with ink samples containing DMAOXD and DMAOXDBEN can be concealed through fumigation with TFA and subsequently revealed by treating them with TEA, suggesting their potential use in data encryption.

Ultrafast excited state relaxation dynamics of pyran-based D-π-A systems: solvent polarity controls the triplet state

The excited state relaxation dynamics of V-shaped D-π-A systems having 4H-pyranylidene appended barbituric acid as an acceptor and diphenylamine (TPAPBA) and diethyl amine (EAPBA) as donors were investigated using steady-state and time-resolved spectroscopy along with theoretical optimization. The steady-state photophysical characterization exhibited the bathochromic shift of the emission maximum (∼6400 cm-1) and large change in the dipole moment (∼24D) with an increase of solvent polarity, reflecting the occurrence of the intramolecular charge transfer state (ICT) in the excited state. The nanosecond and femtosecond transient absorption spectra of these derivatives in a non-polar solvent, toluene, reveal that the excited state relaxation pathway involving a local excited state (LE) decayed to ICT followed by the formation of a twisted ICT state by conformational relaxation, finally leading to the triplet state. The lack of observation of a triplet state in the polar solvent, acetonitrile, signifies that the relaxation dynamics of V-shaped triads in the excited state are influenced by the polarity of the solvent.

Toward High-Performance Electrochromic Conjugated Polymers: Influence of Local Chemical Environment and Side-Chain Engineering

Three homologous electrochromic conjugated polymers, each containing an asymmetric building block but decorated with distinct alkyl chains, were designed and synthesized using electrochemical polymerization in this study. The corresponding monomers, namely T610FBTT810, DT6FBT, and DT48FBT, comprise the same backbone structure, i.e., an asymmetric 5-fluorobenzo[c][1,2,5]thiadiazole unit substituted by two thiophene terminals, but were decorated with different types of alkyl chain (hexyl, 2-butyloctyl, 2-hexyldecyl, or 2-octyldecyl). The effects of the side-chain structure and asymmetric repeating unit on the optical absorption, electrochemistry, morphology, and electrochromic properties were investigated comparatively. It was found that the electrochromism conjugated polymer, originating from DT6FBT with the shortest and linear alkyl chain, exhibits the best electrochromic performance with a 25% optical contrast ratio and a 0.3 s response time. The flexible electrochromic device of PDT6FBT achieved reversible colors of navy and cyan between the neutral and oxidized states, consistent with the non-device phenomenon. These results demonstrate that subtle modification of the side chain is able to change the electrochromic properties of conjugated polymers.

D-A-D Compounds Combining Dithienopyrrole Donors and Acceptors of Increasing Electron-Withdrawing Capability: Synthesis, Spectroscopy, Electropolymerization, and Electrochromism

Five D-π-A-π-D compounds consisting of the same donor unit (dithieno[3,2-b:2′,3′-d]pyrrole, DTP), the same π-linker (2,5-thienylene), and different acceptors of increasing electron-withdrawing ability (1,3,4-thiadiazole (TD), benzo[c][1,2,5]thiadiazole (BTD), 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP), 1,2,4,5-tetrazine (TZ), and benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NDI)) were synthesized. DTP-TD, DTP-BTD, and DTP-DPP turned out to be interesting luminophores emitting either yellow (DTP-TD) or near-infrared (DTP-BTD and DTP-DPP) radiation in dichloromethane solutions. The emission bands were increasingly bathochromically shifted with increasing solvent polarity. Electrochemically determined electron affinities (|EA|s) were found to be strongly dependent on the nature of the acceptor changing from 2.86 to 3.84 eV for DTP-TD and DTP-NDI, respectively, while the ionization potential (IP) values varied only weakly. Experimental findings were strongly supported by theoretical calculations, which correctly predicted the observed solvent dependence of the emission spectra. Similarly, the calculated IP and EA values were in excellent agreement with the experiment. DTP-TD, DTP-BTD, DTP-TZ, and DTP-NDI could be electropolymerized to yield polymers of very narrow electrochemical band gap and characterized by redox states differing in color coordinates and lightness. Poly(DTP-NDI) and poly(DTP-TD) showed promising electrochromic behavior, not only providing a rich color palette in the visible but also exhibiting near-infrared (NIR) electrochromism.

4,7-Bis(5-(9-hexyl-9H-carbazol-3-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-d]pyridazine

Donor molecules of the D-π-A-π-D type structure are often used for applications in organic photovoltaics. In this communication, bromination of 4,7-di(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-d]pyridazine followed by Suzuki cross-coupling with carbazoleboronic acid gave 4,7-bis( 5-(9-hexyl-9H-carbazol-3-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-d]pyridazine. The structure of the newly synthesized compounds was established by high resolution mass-spectrometry, 1H, 13C NMR, IR, and UV spectroscopy and mass-spectrometry. A study of the luminescent properties of the dye showed that it exhibits fluorescence in the near infrared region of the spectrum, which makes it a promising compound for use as an active emitting layer in NIR OLED as well as for other possible applications as an IR luminophore.