Recently, π-conjugated compounds have received attention because they are known to be key building blocks for pharmaceuticals and potential candidates for electrochemical and optical materials. From this perspective, we have investigated the use of electrochemically assisted coupling reactions for the construction of π-conjugated compounds, and recently reported sequential coupling reactions including the Pd/Cu-catalyzed electro-oxidative homo-coupling of p-bromophenylacetylene (1) to give 1,4-bis(p-bromophenyl)-1,3-diyne (2) and subsequent Suzuki–Miyaura coupling with arylboronic acids leading to 1,4-bis(diaryl)-1,3-butadiynes 3 (Scheme 1).¹ Two reaction sites of p-bromophenylacetylene, the terminal alkyne and Br group, could be almost completely discriminated by the on/off application of electricity, and we called the strategy as “Electrochemical Reaction Site Switching (e-RSS)”. Several arylboronic acids can be used in these reactions, which proceed selectively to give 1,4-bis(diaryl)-1,3-butadiynes in good to excellent yields.

During the course of our investigation, we found that 1,4-bis(diaryl)-1,3-butadiynes bearing two amino groups at the p-position of the terminal benzene rings exhibited solvatochromic fluorescence. There have been many reports on amines that exhibit solvatochromic fluorescence. Most of the known amines that exhibit solvatochromic fluorescence have a strong acceptor moiety. There have been a few reports on the solvatochromic fluorescence of diamines, which have a small dipole moment. This unexpected result encouraged us to gain deeper insight into this unusual phenomenon. This time, we synthesized their analog 4 which contains an alkyne, and 5 which does not contain a triple bond (Figure 1), and compared their properties with those of 1,4-bis(diaryl)-1,3-butadiynes bearing two amino moieties. To estimate the solvent effect, we also carried out DFT and TD-DFT calculations of their compound. The results of TD-DFT calculations in different solvents indicated that the excitations and the emissions were mainly due to the HOMO–LUMO transitions of these compounds.

(1) Mitsudo, K.; Kamimoto, N.; Murakami, H.; Mandai, H.; Wakamiya, A.; Murata, Y.; Suga, S. Org Biomol Chem 2012, 10, 9562-9569.