Filip Bureš

Fundamental aspects of property tuning in push-pull molecules†

Property tuning in selected examples of D–π–A molecules has been discussed and summarized in this review article. The tuning and structure–property relationships have been demonstrated on the particular A, π and D parts of the push–pull molecule. Special emphasis has been put on the tuning of the FMO levels and optical properties. Further prospective applications of the given chromophore have also been considered.

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

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 pi system, whereas the entire conjugated pi 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.

Extended conjugation and donor-acceptor substitution to improve the third-order optical nonlinearity of small molecules

The authors use donor-acceptor substitution to increase the third-order polarizability γ of small cyanoethynylethene molecules by lowering the energy corresponding to the first optical transition. They identify a weak power-law dependence for γ depending on the number of conjugated electrons that separate the donor and acceptors (N1.5), which is the result of the two competing contributions that depend on the conjugation length: the energy separation of the ground and first excited states and the strength of the transition dipole matrix elements. The molecules are highly efficient with respect to their small masses and fall within a factor of 50 of the theoretical limit for centrosymmetric molecules, putting them among the best molecules measured to date.

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

Summary 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.

Controllable Chemoselectivity in Visible-Light Photoredox Catalysis: Four Diverse Aerobic Radical Cascade Reactions†

Reported is the controllable selectivity syntheses of four distinct products from the same starting materials by visible-light photoredox catalysis. By employing a dicyanopyrazine-derived chromophore (DPZ) as photoredox catalyst, an aerobic radical mechanism has been developed, and allows the reactions of N-tetrahydroisoquinolines (THIQs) with N-itaconimides to through four different pathways, including addition-cyclization, addition-elimination, addition-coupling, and addition-protonation, with satisfactory chemoselectivity. The current strategy provide straightforward access to four different but valuable N-heterocyclic adducts in moderate to excellent yields.

Proaromaticity: Organic Charge-Transfer Chromophores with Small HOMO-LUMO Gaps

Novel donor- and/or acceptor-substituted cross-conjugated carbocycles based on quinoids or expanded quinoids, with radiaannulene perimeters, were prepared and investigated to validate proaromaticity as a concept for reducing HOMO-LUMO gaps in push-pull chromophores. Analyses of IR, (1)H NMR, and UV/Vis/NIR spectra in conjunction with molecular structures determined by X-ray diffraction show that these push-pull quinoids have significant charge-separated ground states. This feature results in small optical gaps (near IR region) and diatropic magnetic environments inside the carbocycles, as suggested by nucleus-independent chemical shift (NICS) calculations. The NICS results, together with the bond-length analysis of the quinoid spacers, provide strong support that proaromaticity, that is, aromatized zwitterionic mesomeric contributions in the ground state, is effective. A push-pull tetrakis(ethynediyl)-expanded quinoid chromophore represents the first proaromatic radiaannulene.

Ferrocene-Donor and 4,5-Dicyanoimidazole-Acceptor Moieties in Charge-Transfer Chromophores with π Linkers Tailored for Second-Order Nonlinear Optics

A series of new nonlinear optical chromophores (1-15) that were comprised of ferrocene-donor and 4,5-dicyanoimidazole-acceptor moieties and various π linkers of different length were synthesized. Support for the presence of significant DA interactions in these NLO-phores was obtained from the evaluation of the quinoid character of the 1,4-phenylene moieties and their electronic absorption spectra, which featured intense high-energy (HE) bands that were accompanied by less-intense low-energy (LE) bands. The redox behavior of these compounds was investigated by cyclic voltammetry (CV) and by rotating-disc voltammetry (RDV); their electrochemical gaps decreased steadily from 2.64 to 2.09 V. In addition to the experimentally obtained data, DFT calculations of their absorption spectra, HOMO/LUMO levels, and second-order polarizabilities (β) (-2ω,ω,ω) were performed. A structure-property relationship study that was performed by systematically altering the π linker revealed that the intramolecular charge-transfer and nonlinear optical properties of these inorganic-organic hybrid D-π-A systems (1-15) were primarily affected by: 1) The presence of olefinic/acetylenic subunits; 2) the length of the π linker; and 3) the spatial arrangement (planarity) of the π linker.

Optical operation by chromophores featuring 4,5-dicyanoimidazole embedded within poly(methyl methacrylate) matrices.

We have studied photoinduced absorption, birefringence, and optical second-harmonic generation in poly(methyl methacrylate) (PMMA) films doped by organic chromophores featuring 4,5-dicyanoimidazole in the weight content equal to 5%. The chromophores indicated as IM1-IM6 were synthesized from 2-bromo-1-methylimidazole-4,5-dicarbonitrile by either nucleophilic substitution or Suzuki-Miyaura cross-coupling reaction. The samples were obtained as films of several micrometers thickness by the spin-coating method on a quartz substrate. Measurements of the optically induced birefringence were done by the Senarmont method at wavelength 1150 nm, and photoinduced absorption was studied in the spectral range 250-700 nm under optical treatment by 300 mW cw 532 nm laser. Photoinduced optical effects were studied by bicolor 1064 and 532 nm coherent laser pulses. The maximal changes were observed for the ratio between fundamental and writing beam intensities equal to about 7:1. To interpret the observed experimental measurements, theoretical simulations of photoinduced optical properties were performed by quantum chemical computational methods.

N,N′-Dibutylbarbituric acid as an acceptor moiety in push–pull chromophores

Twelve novel D–π–A chromophores with the N,N′-dibutylbarbituric acid acceptor, the N,N-dimethylamino donor and a systematically extended π-linker were synthesized. The extent of intramolecular charge-transfer, structure–property relationships and nonlinear optical properties were further investigated by X-ray analysis, electrochemistry, UV/Vis absorption spectra, calculations and EFISH experiments.

Dicyanopyrazine-derived push–pull chromophores for highly efficient photoredox catalysis

Here, we report dicyanopyrazine (DPZ)-derived push–pull chromophores, easily prepared and tunable organic compounds, as new kinds of photoredox catalysts. In particular, the DPZ derivative H, containing 2-methoxythienyl as electron-donating moiety, exhibits a broad absorption of visible light with an absorption edge up to 500 nm and excellent redox properties, and has been demonstrated as a desirably active and efficient photoredox catalyst in four challenging kinds of photoredox reactions. The amount of catalyst in most reactions is less than 0.1 mol% and even 0.01 mol%, representing the lowest catalyst loading in the current photoredox organocatalysis.