Carbazole-terpyridine donor-acceptor luminophores

With the aim to combine two versatile molecular units that are widely utilized in materials and coordination chemistry, bischromophoric electron donor-acceptor carbazole-terpyridine systems (Cbz- Tpy) have been synthesized and characterized. The connecting bridge between the two moieties is constituted by phenylene (1), methylene phenylene (2) and ethynylene phenylene (3), which allow tuning of the intercomponent electronic interactions. Electrochemical studies evidence that oxidation and reduction processes occur on the Cbz and Tpy units, respectively, suggesting the possibility of internal charge transfer states located at about 3.10 eV, which is confirmed by photophysical investigations. The absorption spectra of 1 and 3 show a tail above 300 nm, indicating that these conjugated systems exhibit low-energy transitions with charge-transfer character, as confirmed by theoretical studies. At 298 K, 1-3 show a complex pattern of fluorescence profiles as a function of solvent (toluene, dichloromethane, acetonitrile), often with double emissions attributed to transitions localized on individual chromophores or associated to internal charge-transfer processes. The variability of the spectral position affords multiple colours, including white (e.g. 3 in acetonitrile). Definitive rationalization and assignment of transitions of 1-3 is obtained through singlet and triplet luminescence spectra at 77 K and by means of DFT and TD-DFT methods using the hybrid functional PBE0 and the long range corrected functional CAM-B3LYP with the polarizable continuum model. This work opens the route to versatile materials based on the Tpy-Cbz motif exhibiting luminescence all across the visible spectral region that can be controlled through electronic conjugation, solvent polarity, temperature, functionalization and cation binding.