Synthesis and Electronic Properties of Novel Donor-π-Acceptor-Type Functional Dyes with a Carbonyl-Bridged Bithiophene π-Spacer

Abstract

In this study, we synthesized novel donor-π-acceptor (D-π-A) functional dyes bearing a carbonyl-bridged bithiophene as a π-conjugated spacer and evaluated the absorption and fluorescence properties as well as the photostability. The developed dyes 1-CO-3-CO possess an N,N-diphenylaminophenyl electron donor unit and an electron acceptor unit such as a formyl group (1-CO), an (N,N-diethylthiobarbituryl)methylene moiety (2-CO), or a (3-dicyanomethylidene-1-indanon-2-yl)methylene moiety (3-CO). The absorption spectra of 1-CO-3-CO in dichloromethane at room temperature showed absorption maxima at 569 nm, 631 nm, and 667 nm, respectively, and the stronger acceptors in 2-CO and 3-CO led to enhancement of the ICT character. In addition, 2-CO and 3-CO had a second absorption band in the visible region, showing panchromatic absorption properties. Electrochemical analyses of the developed dyes revealed that the carbonyl bridging group in the π-spacer contributes to stabilization of the frontier orbitals such as the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively), in comparison with the referential dyes bearing a dibutylmethylene-bridged bithiophene spacer, 1-CBu₂-3-CBu₂. The HOMO/LUMO stabilization brought about high photostability in the doped poly(methyl methacrylate) film.

Keywords: donor–π–acceptor-type dye; frontier orbital; intramolecular charge transfer; panchromatic absorption; photostability.

Figure 1

Molecular design of D–π–A-type functional dyes in this work.

Scheme 1

Synthesis of 1-CO–3-CO and 1-CBu₂–3-CBu₂. DME represents 1,2-dimethoxyethane.

Figure 2

UV–vis–NIR absorption spectra of (a) 1-CO–3-CO and (b) 1-CBu₂–3-CBu₂ in dichloromethane at room temperature.

Figure 3

Photoluminescence spectra of (a) 2-CO and 3-CO and (b) 1-CBu₂–3-CBu₂ in dichloromethane at room temperature.

Figure 4

Distributions of frontier orbitals of 1-CO–3-CO and 1-CBu₂–3-CBu₂. obtained by DFT and TD-DFT calculations at the M062X/6-31G(d,p) level of theory with the SMD solvation model in dichloromethane.

Figure 5

Calculated UV–vis–NIR absorption spectra (solid lines) of 1-CO–3-CO and 1-CBu₂–3-CBu₂. The corresponding experimental spectra are also shown together by dashed lines.

Figure 6

Cyclic voltammograms of 1-CO–3-CO and 1-CBu₂–3-CBu₂ in dichloromethane. Tetrabutylammonium perchlorate (0.1 M) was used as a supporting electrolyte, and the potentials were recorded relative to an Ag/AgNO₃ (0.1 M) reference electrode.

Figure 7

Time-course changes in (a) UV–vis–NIR absorption spectra and (b) relative absorbances (A/A₀s) of 2-CO-, 2-CBu₂-, 3-CO-, and 3-CBu₂-doped PMMA films upon continuous irradiation of AM-1.5G light (Xe lamp: 100 mW cm⁻²). The doping level of the dye-doped PMMA film; methyl methacrylate monomer unit/dye = 99/1 (mol/mol). The values of A/A₀ were monitored at 612, 584, 667, and 641 nm for 2-CO, 2-CBu₂, 3-CO, and 3-CBu₂, respectively.