Tuning Emission Responses of a Triphenylamine Derivative in Host–Guest Complexes and an Unusual Dynamic Inclusion Phenomenon

A newly synthesized triphenylamine derivative (1Cl₃) shows significant differences in inclusion complex formation with two different macrocyclic hosts, cucurbit[7]­uril (CB[7]) and β-cyclodextrin (β-CD). Detailed investigations by NMR spectroscopy reveal that CB[7] forms a 1:3 host–guest complex ([1·3­{CB­[7]}]­Cl₃) in which three arms of 1Cl₃ are bound to three host molecules. On the other hand, β-CD forms a dynamic 1:1 inclusion complex ([1·{β-CD}]­Cl₃) by binding to only one of the three arms of 1Cl₃ at a given time. The formation of a 1:1 host–guest complex with β-CD and 1:3 host–guest complex with CB[7] was also confirmed from the results of the isothermal titration calorimetric studies. Interestingly, 1Cl₃ exhibits a rare dual emission property in solution at room temperature with the lower and higher energy bands arising from a locally excited state and an intramolecular charge-transfer transition, respectively. The difference in inclusion complex formation behavior of 1Cl₃ with the two macrocyclic hosts results in the stabilization of different emission states in the two inclusion complexes. The fundamental difference in the electrostatic surface potentials, cavity polarities, and shapes of the two macrocyclic hosts could account for the formation of the different inclusion complexes with distinct luminescence responses.