Property Tuning in Charge‐Transfer Chromophores by Systematic Modulation of the Spacer between Donor and Acceptor

Abstract

A series of donor–acceptor chromophores was prepared in which the spacer separating 4‐dimethylanilino (DMA) donor and C(CN)2 acceptor moieties is systematically varied. All of the new push–pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X‐ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual π system, whereas the entire conjugated π system in series b is nearly planar. Support for strong donor–acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge‐transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two‐dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push–pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D–A conjugation with increasing spacer length. Degenerate four‐wave mixing experiments reveal high third‐order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.

Dicyanovinylnaphthalenes for neuroimaging of amyloids and relationships of electronic structures and geometries to binding affinities

“…For all compounds except 9c, replacement of the acetyl acceptor group by the dicyanovinyl side chain resulted in considerable bathochromic shifts of the absorption maxima from 350-380 to 400-470 nm (λ max ; Table 1). This observation is consistent with previous studies that demonstrated a propensity for the dicyanovinyl to be planar and conjugated with the donor group (19). The lack of red shift in 9c is the result of the acceptor being forced out of the plane of conjugation because of the bulky t-butyl substituent.…”

Theoretical investigation of the static (hyper)polarizabilities and reorganization energy of 4,5-dicyanoimidazole chromophore and derivatives containing benzene rings and a saturated bridge

“…It has already been demonstrated that the HOMO-LUMO gap can be modulated by strong A/D groups and large p-conjugated spacers [27]. On the other hand, intramolecular charge transfer (ICT) is an essential point in the analysis of hai and hci value, which, in the almost all organic molecules, is related to the contribution of the HOMO-LUMO transition [28].…”

Push–pull chromophores comprising benzothiazolium acceptor and thiophene auxiliary donor moieties: Synthesis, structure, linear and quadratic non-linear optical properties

“…The values of end-absorption wavelength l end are useful to consider since they provide an experimental estimation for the HOMO-LUMO gaps of conjugated molecules and show good correlation with the length of the conjugated p-system of pushpull chromophores [42]. Indeed, in our case the expansion of the p-conjugated system from [10]I to [3a]I and then to [3b,c]I resulted in systematic increase of the end-absorption wavelength in all studied solvents.…”