Branched charge-transfer chromophores featuring a 4,5-dicyanoimidazole unit

Synthesis, photophysics and two-photon absorption of imidazole-centred tripodal chromophores

Tripodal push-pull chromophores with D-(π-A)3 arrangement were synthesized using 1-methyl-2,4,5-triphenyl-1H-imidazole as a central electron donor, and their thermal, electrochemical, photophysical and non-linear optical properties were studied and corroborated with quantum-chemical calculations. Their facile synthesis involved Suzuki-Miyaura and Knoevenagel reactions, allowing the installation of various peripheral electron acceptors such as formyl, cyano, ester, trifluoromethyl and more complex moieties such as malonic/acetic acid derivatives, indan-1,3-dione and rhodanine. All phenyl rings appended at the central imidazole core were more or less twisted depending on the peripheral substitution. Although imidazole undergoes reversible one-electron oxidation, peripheral acceptors are reduced irreversibly in a multi-electron process. This behaviour is further seen as a variation of the LUMO, while the HOMO remained almost unaltered across the whole series. TD-DFT calculations revealed centrifugal charge transfer from the central imidazole to all C2, C4 and C5 branches occupied by the LUMO, LUMO+1 and LUMO+2. The HOMO-LUMO gap is tuneable within the range of 3.55-2.31 eV, while the longest-wavelength absorption/emission maxima were found within the broad range of 304-448/393-612 nm. Although the absorption spectra are solvent-independent, the emission depends strongly on the solvent polarity and the electron-withdrawing power of the peripheral acceptors. Extended chromophores with complex electron acceptors were investigated as two-photon absorbers, revealing relatively good cross-section values of up to 521 GM and a figure-of-merit (ΦF × δ2PA) of around 190 GM.

NLO-active Y-shaped ferrocene conjugated imidazole chromophores as precursors for SHG polymeric films

New Y-shaped ferrocene conjugated imidazole chromophores were prepared and fully characterized. The Y-shaped structure was confirmed by the single crystal X-ray diffraction technique. The chromophores show interesting second-order nonlinear optical (NLO) properties in solution, as determined by the Electric-Field Induced Second Harmonic generation (EFISH) technique. Remarkably, the trifluoro substituted compound 3 is characterized by a high μβEFISH value and has good potential as a molecular building block for composite films with Second Harmonic Generation (SHG) properties. For all compounds, the dipole moments and frontier orbital energies were calculated by the Density Functional Theoretical method.

Alphabet-Inspired Design of (Hetero)Aromatic Push-Pull Chromophores

Push-pull molecules represent a unique and fascinating class of organic π-conjugated materials. Herein, we provide a summary of their recent extraordinary design inspired by letters of the alphabet, especially focusing on H-, L-, T-, V-, X-, and Y-shaped molecules. Representative structures from each class were presented and their fundamental properties and prospective applications were discussed. In particular, emphasis is given to molecules recently prepared in our laboratory with T-, X-, and Y-shaped arrangements based on indan-1,3-dione, benzene, pyridine, pyrazine, imidazole, and triphenylamine. These push-pull molecules turned out to be very efficient charge-transfer chromophores with tunable properties suitable for second-order nonlinear optics, two-photon absorption, reversible pH-induced and photochromic switching, photocatalysis, and intercalation.

Imidazole as a parent π-conjugated backbone in charge-transfer chromophores

Research activities in the field of imidazole-derived push-pull systems featuring intramolecular charge transfer (ICT) are reviewed. Design, synthetic pathways, linear and nonlinear optical properties, electrochemistry, structure-property relationships, and the prospective application of such D-π-A organic materials are described. This review focuses on Y-shaped imidazoles, bi- and diimidazoles, benzimidazoles, bis(benzimidazoles), imidazole-4,5-dicarbonitriles, and imidazole-derived chromophores chemically bound to a polymer chain.

Effects of the nature and length of the π-conjugated bridge on the second-order nonlinear optical responses of push-pull molecules including 4,5-dicyanoimidazole and their protonated forms

Theoretical and experimental methods of characterization have been employed to investigate a series of six push-pull molecules containing a 4,5-dicyanoimidazole acceptor (A) unit, a N,N-dimethylamino donor (D) group, and systematically enlarged π-conjugated linkers (π) with the view of assessing their first hyperpolarizabilities (β) as well as their variations upon protonation. The results demonstrated that protonation occurred at the N,N-dimethylamino function, which led to disruption of the electronic charge-transfer delocalization: the A-π-D pattern of the neutral species with large β values was transformed into an A-π-A' pattern, which displayed much smaller β values. This feature makes these systems applicable as pH-triggered nonlinear optics (NLO) switches. In particular, protonation led to a decrease of the hyper-Rayleigh scattering first hyperpolarizabilities by at least one order of magnitude as well as to a decrease of the depolarization ratio, from about five for neutral species, which also supported the 1D-like NLO-phore nature, to about two for the protonated ones.