Luminescent host–guest complexes involving molecular clips and tweezers and tetracyanobenzene

Abstract

The molecular tweezer 1 and the clips 2 and 3, containing naphthalene or anthracene in the sidewalls, form host–guest complexes in CHCl₃ solution with the guest TCNB (1,2,4,5-tetracyanobenzene). The interaction leading to formation of the adduct is essentially of CT (charge-transfer) nature. A luminescence emission of CT origin is observed from the host–guest complexes both in fluid solution at 298 K and in rigid matrix at 77 K; to our knowledge, this is the first case of CT luminescence from a host–guest complex. At room temperature, the luminescence maxima are observed at 570, 614, and 668 nm, respectively, for the complexes based on the hosts 1, 2, and 3. Spectrophotometric and spectrofluorimetric titrations were performed to investigate the association processes. In all cases 1∶1 complexes are formed, with association constants 7.3 × 10⁵, 5.4 × 10⁶, and 1.24 × 10⁴ L mol⁻¹, respectively, for the receptors 1, 2, and 3. In the case of 2, a species with a 2∶1 host∶guest ratio is also formed. The system 1+TCNB was further investigated by cyclic and differential pulse voltammetry, giving the rate constants for adduct formation (1.9 × 10⁸ L mol⁻¹ s⁻¹) and disassembling (2.0 × 10² s⁻¹). The association/dissociation dynamics between the receptors 1 and 2 and the guest TCNB is discussed in relation to the receptor topology.