Twisted intramolecular charge transfer (TICT) is an electron transfer process that occurs upon photoexcitation in molecules that usually consist of a donor and acceptor part linked by a single bond. Following intramolecular twisting, the TICT state returns to the ground state either through red-shifted emission or by nonradiative relaxation. The emission properties are potentially environment-dependent, which makes TICT-based fluorophores ideal sensors for solvents, (micro)viscosity, and chemical species. Recently, several TICT-based materials have been discovered to become fluorescent upon aggregation. Furthermore, various recent studies in organic optoelectronics, non-linear optics and solar energy conversions utilised the concept of TICT to modulate the electronic-state mixing and coupling on charge transfer states. This review presents a compact overview of the latest developments in TICT research, from a materials chemistry point of view.

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Recent advances in twisted intramolecular charge transfer (TICT) fluorescence and related phenomena in materials chemistry

The natural extracellular matrix (ECM) represents a complex and dynamic environment. It provides numerous spatio-temporal signals mediating many cellular functions including morphogenesis, adhesion, proliferation and differentiation. The cell–ECM interaction is bidirectional. Cells dynamically receive and process information from the ECM and remodel it at the same time. Theses complex interactions are still not fully understood. For better understanding, it is indispensable to deconstruct the ECM up to the point of investigating isolated characteristics and cell responses to physical, chemical and topographical cues. Two-photon polymerization (2PP) allows the exact reconstruction of cell specifi c sites in 3D at micro- and nanometer precision. Processing biocompatible synthetic and naturally-derived hydrogels, the microenvironment of cells can be designed to specifi cally investigate their behavior in respect to key chemical, mechanical and topographical attributes. Moreover, 3D manipulation can be performed in the presence of cells, guiding biological tissue formation in all stages of its development. Here, advances in 2PP microfabrication of synthetic and naturally based hydrogels are reviewed. Key components of photopolymerizable hydrogel precursors, their structure–property relationships and their polymerization mechanisms are presented. Furthermore, it is shown how biocompatible 2PP fabricated constructs can act as biologically relevant matrices to study cell functions and tissue development.

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Hydrogels for Two-Photon Polymerization: A Toolbox for Mimicking the Extracellular Matrix

Hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of reference molecules, namely, carbon tetrachloride, chloroform, trichloroacetonitrile, acetonitrile, and dichloromethane. The multipolar decomposition of the first hyperpolarizability tensor through the use of the spherical harmonics formalism is employed to highlight the impact of the symmetry of the molecular scatterers on their nonlinear optical responses. It is demonstrated that HRS is a technique of choice to probe the molecular symmetry of the compounds. Coupled-cluster calculations performed at the coupled-cluster level with singles, doubles, and perturbative triples in combination with highly extended basis sets and including environment effects by using the polarizable continuum model qualitatively reproduce the molecular first hyperpolarizabilities and depolarization ratios of the molecular scatterers.

Reference molecules for nonlinear optics: A joint experimental and theoretical investigation

With our new home-built circularly polarized luminescence (CPL) instrument, we measured fluorescence and CPL spectra of the enantiomeric pairs of two quasi-isomeric BODIPY DYEmers 1 and 2, endowed with axial chirality. The electronic circular dichroism (ECD) and CPL spectra of these atropisomeric dimers are dominated by the exciton coupling between the main π–π* transitions (550–560 nm) of the two BODIPY rings. Compound 1 has strong ECD and CPL spectra (glum=4×10−3) well reproduced by TD-DFT and SCS-CC2 (spin-component scaled second-order approximate coupled-cluster) calculations using DFT-optimized ground- and excited-state structures. Compound 2 has weaker ECD and CPL spectra (glum=4×10−4), partly due to the mutual cancellation of electric–electric and electric–magnetic exciton couplings, and partly to its conformational freedom. This compound is computationally very challenging. Starting from the optimized excited-state geometries, we predicted the wrong sign for the CPL band of 2 using TD-DFT with the most recommended hybrid and range-separated functionals, whereas SCS-CC2 or a DFT functional with full exact exchange provided the correct sign.

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Circularly Polarized Luminescence from Axially Chiral BODIPY DYEmers: An Experimental and Computational Study

The very peculiar characteristics of zwitterions, as well as a clearand unambiguous definition, have been overlooked in past literature. However, these compounds are particularly important in view of the impact they have had in the recent past and will likely continue to have in the future as components of performing functional organic and hybrid materials. In this Account, we primarily aim to define critically important organic concepts of zwitterions regarding both their design and nomenclature. We will particularly focus on a specific kind of zwitterions we define as π-conjugated zwitterions. These types of zwitterions are systems pertaining to the class ofdonor-acceptor (push-pull) molecules. In the ground state, they are preferentially represented in terms of an electron donor moiety bearing a negative net charge, and electron acceptor one bearing a positive net charge connected by a conjugated bridge. As such, they are possibly the most effective example of push-pull structure, possessing relevant features for applications like nonlinear optics, photovoltaics, imaging, and high capacitance dielectrics. In addition, the interaction between these dipolar compounds and the environment is highly specific and can be exploited in the construction of well-organized nanostructures, both in solution and in the solid state. According to the Gold Book of IUPAC for nomenclature, the distinction between zwitterions and the charged molecule called a betaine is subtle. The betaine is a particular class of zwitterion possessing an onium atom not bearing a hydrogen. The two terms are often considered equivalent, thus generating confusion while retrieving literature. In this Account, we define and describe π-conjugated zwitterions systems that are dipolar in the ground state, admitting resonance limiting structures that are neutral and chargeless. For the purpose of this Account and to the benefit of researchers striving to retrieve materials-related zwitterion literature data, we suggest to use the term π-zwitterions instead of the commonly used plain term "zwitterions". We show that this definition enables the clear identification of a class of compounds having unique properties distinct from "dipolar conjugated compounds." We describe the most common donor and acceptor groups in π-zwitterions. In particular, we focus our attention on the special case of the nitrile functionality, which tends to be contiguous to a negative charge. We also address special emphasis to benzenoid components that are substituted by heteroaromatic units in π-zwitterions, because the HOMO-LUMO energetic consequences are specifically involved in these cases. We make reference to the paradigmatic case of π-zwitterions second order nonlinear optical properties. Here, the value of the first hyperpolarizability β versus the alternation in bond length turns out to be a measure of the balance of the chargeless and the dipolar contribution to the description of the zwitterion ground state. We also report literature data, collected both from our group and others, concerning π-zwitterions containing heteroaromatic and/or nitrile groups, those based on the most performing acceptors so far described, and merocyanines. With particular reference to merocyanines, we show how π-zwitterions can play a fundamental role in the fast growing field of organic photovoltaics. Finally, we present π-zwitterions made up of heteroaromatic groups that open new scenarios in heteroaromatic chemistry.

π-Conjugated zwitterions as paradigm of donor-acceptor building blocks in organic-based materials.

Molecular chromophores with twisted π-electron systems have been shown to possess unprecedented values of the quadratic hyperpolarizability, β, with very large real parts and much smaller imaginary parts. We report here an experimental and theoretical study which shows that these twisted chromophores also possess very large values of the real part of the cubic hyperpolarizability, γ, which is responsible for nonlinear refraction. Thus, for the two-ring twisted chromophore TMC-2 at 775 nm, relatively close to one-photon resonance, n2 extrapolated to neat substance is large and positive (1.87 × 10−13 cm2/W), leading to self-focusing. Furthermore, the third-order response includes a remarkably low two-photon absorption coefficient, which means minimal nonlinear optical losses: the T factor, α2λ/n2, is 0.308. These characteristics are attributed to closely spaced singlet biradical and zwitterionic states and offer promise for applications in all-optical switching.

Twisted π-System Chromophores for All-Optical Switching

The systematic synthesis, structural, optical spectroscopic, and second-order nonlinear optical (NLO) characterization of a series of donor-acceptor poly-arylene chromophores which have heretofore unachieved π-extension and substantial twisting from planarity, are reported: specifically, two-ring 2TTMC, dicyano(4-(3,5-dimethyl-1-(2-propylheptyl)pyridin-1-ium-4-yl)-3-methylphenyl)methanide; three-ring 3TTMC, dicyano(4'-(3,5-dimethyl-1-(2-propylheptyl)pyridin-1-ium-4-yl)-2,2',3',5',6'-pentamethyl[1,1'-biphenyl]-4-yl)methanide; and four-ring 4TTMC, dicyano(4″-(3,5-dimethyl-1-(2-propylheptyl)pyridin-1-ium-4-yl)-2,2',3″,6,6'-pentamethyl[1,1':4',1″-terphenyl]-4-yl)methanide. Single-crystal X-ray diffraction, DFT-optimized geometries, and B3LYP/INDO-SOS analysis identify three key features underlying the very large NLO response: (1) For ring catenation of three or greater, sterically enforced π-system twists are only essential near the chromophore donor and acceptor sites to ensure large NLO responses. (2) For synthetic efficiency, deletion of one ortho-methyl group from o,o',o″,o‴-tetramethylbiaryl junctures, only slightly relaxes the biaryl twist angle from 89.6° to ∼80°. (3) Increased arylene catenation from two to three to four rings (2TTMC→ 3TTMC → 4TTMC) greatly enhances NLO response, zwitterionic charge localization, and thus the ground-state dipole moment, consistent with the contracted antiparallel solid-state π-π stacking distances of 8.665 → 7.883 → 7.361 A, respectively. This supports zwitterionic ground states in these chromophores as do significant optical spectroscopic solvatochromic shifts, with aryl-aryl twisting turning on significant intra-subfragment absorption. Computed molecular hyperpolarizabilities (μβ) approach an unprecedented 900,000 × 10(-48) esu, while estimated chromophore figures of merit, μβ(vec)/M(w), approach 1500 × 10(-48) esu, 1.5 times larger than the highest known values for twisted chromophores and >33 times larger than that of planar donor-acceptor chromophores.

Naoki Watanabe

Papers

Twisted π-System Chromophores for All-Optical Switching

Ultra-High-Response, Multiply Twisted Electro-optic Chromophores: Influence of π-System Elongation and Interplanar Torsion on Hyperpolarizability