Systematic Control of the Overlapping Energy Region for an Efficient Intramolecular Energy Transfer: Functionalized Salen-Al/Triphenylamine Guest-Host Assemblies.

A series of triphenylamine (TPA)-containing salen-Al assembly dyads, [salen(3- tBu-5-R)2Al(OC6H4- p-N(C6H5)2)] [salen = N, N'-bis(salicylidene)ethylenediamine; R = H (D1), tBu (D2), Ph (D3), OMe (D4), and NMe2 (D5)], were prepared in good yield (50-80%) and fully characterized by NMR spectroscopy and elemental analysis. Both the UV/vis absorption and photoluminescence (PL) spectra of D1-D4, except for D5, in a tetrahydrofuran solution exhibited dual patterns, which are assignable to the salen-Al-centered π-π* transition (low-energy region) and the TPA-centered π-π* transition (high-energy region). In particular, the emission spectra of the dyads displayed interesting dual-emissive patterns via a significant intramolecular energy transfer (IET) process between the salen-Al moiety and TPA group. Notably, this IET process was systematically tuned by varying the substituents and dominantly observed in the rigid state. More interestingly, compared to the salen-Al complexes (A1-A4) without the TPA group, D1-D4 exhibited enhanced quantum efficiencies. Time-dependent density functional theory calculations on the S1-optimized structures of D1-D5 further supported these experimental results by indicating the existence of independent transition states between the salen-Al moiety and TPA group in the assembly dyads. The present study reports the first example of salen-Al complexes bearing electron-rich TPA moieties.