Flavylium compounds are versatile molecules that comprise anthocyanins, the ubiquitous colorants used by Nature to confer colour to most flowers and fruits. They have found a wide range of applications in human technology, from the millenary colour paints described by the Roman architect Vitruvius, to their use as food additives, combining colour and antioxidant effects, and even as light absorbers in solar cells aiming at a greener solar energy conversion. Their rich complexity derives in part from their ability to switch between a variety of species (flavylium cations, neutral quinoidal bases, hemiketals and chalcones, and negatively charged phenolates) by means of external stimuli, such as pH, temperature and light. This critical review describes (i) the historical advancements in the understanding of the equilibria of their chemical reaction networks; (ii) their thermodynamics and kinetics; (iii) the mechanisms underlying their colour development, such as co-pigmentation and host–guest interactions; (iv) the photophysics and photochemistry that lead to photochromism; and (v) applications in solar cells, models for optical memories, photochromic soft materials such as ionic liquids and gels, and their properties in solid state materials (274 references).

Chemistry and applications of flavylium compounds: a handful of colours

An effective and widely tunable means of producing terahertz (THz) radiation using difference-frequency generation (DFG) in an organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was demonstrated by optimizing the pump wavelengths. We calculated the (wideband) refractive index and coherence length mapping of BNA to establish the optimum phase-matching condition of the DFG configuration. To satisfy the phase-matching conditions over the range 0.1-20 THz, both pump wavelengths were varied from 780 to 950 nm, and the optical wavelength dependency of the THz-wave output was clearly observed. We expanded the range of THz-wave generation and increased the power output to ten times that obtained using previous methods.

Optimized terahertz-wave generation using BNA-DFG

Organic crystals with second-order optical nonlinearity feature very high and ultra-fast optical nonlinearities and are therefore attractive for various photonics applications. During the last decade, they have been found particularly attractive for terahertz (THz) photonics. This is mainly due to the very intense and ultra-broadband THz-wave generation possible with these crystals. We review recent progress and challenges in the development of organic crystalline materials for THz-wave generation and detection applications. We discuss their structure, intrinsic properties, and advantages compared to inorganic alternatives. The characteristic properties of the most widely employed organic crystals at present, such as DAST, DSTMS, OH1, HMQ-TMS, and BNA are analyzed and compared. We summarize the most important principles for THz-wave generation and detection, as well as organic THz-system configurations based on either difference-frequency generation or optical rectification. In addition, we give state-of-the-art examples of very intense and ultra-broadband THz systems that rely on organic crystals. Finally, we present some recent breakthrough demonstrations in nonlinear THz photonics enabled by very intense organic crystalline THz sources, as well as examples of THz spectroscopy and THz imaging using organic crystals as THz sources for various scientific and technological applications.

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Organic crystals for THz photonics

A single crystal of N-benzyl 2-methyl-4-nitroaniline (BNA) whose dimensions were 8×10 mm (diameter × length) was grown using a vertical Bridgman method with sufficient optical transparency for spectroscopic use. Polarized reflection spectra of the crystal were measured in order to determine its linear optical parameters (refractive indices and extinction coefficients). The relative magnitudes of the d-tensor components of the crystal were determined to be d333=(17±9)×d322, d322>d311, d311~0 from polarization dependencies of the second harmonic generation. These results are in disagreement with previously reported theoretical studies, in which the effects of intermolecular interactions were completely neglected. Here, we have included these intermolecular interactions in the calculations assuming the presence of supra-molecular clusters. We conclude that the anomalous enhancement of d333 is due to the interactions along the direction of the hydrogen bonds in the crystal and the resonance effect.

Second Order Nonlinear Optical Properties of the Single Crystal of N-Benzyl 2-methyl-4-nitroaniline: Anomalous Enhancement of the d333 Component and Its Possible Origin

Linear and second-order nonlinear optical (NLO) properties of a single crystal of N-benzyl-2-methyl-4-nitroaniline (BNA) have been determined. The refractive indices of the crystal were measured in the visible to near infra-red region. The Maker-fringe and phase-matching methods were used to determine the magnitudes of the d-tensor components for the second harmonic generation when excited the crystals at 1064 nm. They were d333=234±31, d322=15.6±0.9, and d311~0 pm/V. The value of d333 is the largest value reported for any yellow-colored NLO materials and suggests that crystals of BNA will be very effective for terahertz wave generation.

http://iopscience.iop.org/article/10.1143/JJAP.46.1528/pdf

Determination of the $d$-Tensor Components of a Single Crystal of N-Benzyl-2-methyl-4-nitroaniline

Six N-substituted derivatives of 2-methyl-4-nitroaniline (MNA) and two N-substituted derivatives of 4-nitroaniline were synthesized and their physical and optical properties were examined in the search for nonlinear optical materials with a sufficiently large second harmonic (SH) activity and applicability to practical use. Some derivatives produced single crystals and their structures were determined by X-ray crystallography. The trends of the SH intensity of these materials could be qualitatively explained based on the b tensor (the effective molecular hyperpolarizability in the crystal). Amongst the derivatives examined in this investigation, N-benzyl-MNA showed the highest SH activity (up to twice that of MNA), thermostability and phase-matching ability.

https://iopscience.iop.org/article/10.1143/JJAP.36.6754/pdf

Second-Harmonic Generation from Single Crystals of N-Substituted 4-Nitroanilines

The quantum yield of photochemical conversion of 2-hydroxy-4′-dimethylaminochalcone into 4′-dimethylaminoflavylium ion is approximately 0.23 in tetrahydrofuran solution in the presence of 5 mM p -toluenesulphonic acid on irradiation with UV light. The quantum yield is essentially constant as a function of the reaction conditions, such as the reaction time, temperature, substrate concentration and light intensity, but the absorption spectra of the reactant and product are well separated. These features are favourable for use as a chemical actinometer.

Chemical actinometer with 2-hydroxy-4'-dimethylaminochalcone

Parent and substituted flavanones undergo photochemical opening of the pyrone ring to give corresponding 2′-hydroxychalcones in benzene, pyridine, acetonitrile etc., the yields varying from 0 to 70% with substituents. The reaction is depressed by the addition of 1,3-pentadiene, nitrosobenzene, or acrylonitrile. On the other hand, flavanone and 4-chromanone undergo photoreduction in 2-propanol to give reductive coupling products and solvent adducts as isomeric mixtures. 7,8-Benzoflavanone does not undergo photoreduction in 2-propanol but the ring opening reaction instead. Mechanisms have been discussed in terms of the relative contribution and π,π* character in the lowest triplet states.

Photoreactions of Flavanones

Irradiation of an oxygen-free benzene solution of cis-4-flavanol 1a by a mercury lamp in the presence of benzophenone gave 4-flavanone (32%), benzopinacol (85%), cis- and trans-2-(diphenylhydroxymethyl)-4-flavanols (7.6%), and 2-(diphenylhydroxymethyl)-4-flavanone (7.7%), whereas photolysis of cis-4-acetoxyflavane under similar conditions gave cis-2-(diphenylhydroxymethyl)-4-acetoxyflavane(24%), 2,2′-bi-4-acetoxyflavane (6%), and benzopinacol. Photolysis of 1a in acetone gave 4-flavanone (3.6%), trans-4-(1-hydroxy-1-methylethyl)-4-flavanols (9.9%). The cis isomers of the parent and substituted 4-flavanols showed higher reactivities than the corresponding trans isomers.

Photochemical Reactions of 4-Flavanols in the Presence of Ketone Sensitizers

Polarographic behavior of o-hydroxyphenylmercury chloride was examined in aqueous nitric acid medium. An abnormal first wave and a peculiar angular-shaped second wave were observed. Equations of the current-potential curves of the first and second waves are derived on the following assumptions: the first wave is inhibited by a reduction product adsorbed on the surface of the dropping mercury electrode as solid particles, the adsorbed precipitate being reduced at the second wave. The equations agree well with the observed results.

Motonobu Morioka

Papers

Second-Harmonic Generation from Single Crystals of N-Substituted 4-Nitroanilines

Chemical actinometer with 2-hydroxy-4'-dimethylaminochalcone

Photoreactions of Flavanones

Photochemical Reactions of 4-Flavanols in the Presence of Ketone Sensitizers

Cathodic Waves of o-Hydroxyphenylmercury Chloride in Aqueous Nitric Acid Medium. Inhibition by Reaction Product and Reduction of Precipitate