Structure-dependent and environment-responsive optical properties of the trisheterocyclic systems with electron donating amino groups

Abstract A series of 2,2′:6′,2″-terpyridine (terpy), 2,6-di(thiazol-2-yl)pyridine (dtpy), and 2,6-di(pyrazin-2-yl)pyridine (dppy) derivatives with electron donating amino groups (dimethylamine, piperidine, morpholine and diphenylamine) connected to the terpy/dtpy/dppy skeleton via phenylene linkage have been thoroughly investigated in order to get an insight into the impact of amine donors and nitrogen-based π-deficient heterocycles on the thermal, redox, UV–Vis absorption and emission properties. All the compounds melted without thermal decomposition and the presence of di(pyrazin-2-yl)pyridine skeleton increased the melting temperature and prevented formation of amorphous material. The obtained compounds undergo electrochemical reduction and oxidation processes and show electrochemically estimated E g in the range of 2.30–2.83 eV. The lowest values of E g were confirmed for the compounds bearing 2,6-di(pyrazin-2-yl)pyridine core and dimethylamine or piperidine substituents. Structure-dependent fluorescence properties of the synthesized compounds were evidenced in both solution and solid state. The use of 2,2′:6′,2″-terpyridine as acceptor unit gives opportunity to obtain more efficient emitters, while 2,6-di(pyrazin-2-yl)pyridine derivatives display the largest Stokes shifts. The dramatic decrease in the fluorescence quantum yield with the increasing polarity was attributed to ICT→TICT, while dual emission behavior was assigned to LE to ICT emissions. The effect of protonation and viscosity on the emission properties was also examined and it was found to be dependent on the nature of the acceptor moiety. Under external voltage the 2,6-di(pyrazin-2-yl)pyridine derivatives, except for dtpy with diphenylamine, did not exhibit light emission, contrary to the functionalized terpy and dppy.