A unified description of linear and nonlinear polarization in organic polymethine dyes

Push-Pull Heptamethines Near the Cyanine Limit Exhibiting Large Quadratic Electro-Optic Effect

Harnessing the quadratic electro-optic (QEO) of near-infrared polymethine chromophores over broad telecom wavelength bands is a subject of immense potential but remains largely under-investigated. Herein a series of push-pull heptamethines containing the tricyanofuran (TCF) acceptors and indoline or benzo[e]indoline donors are reported. These dipolar chromophores can attain a highly delocalized "cyanine-like" electronic ground state in solvents spanning a wide range of polarities, in some cases even closer to the ideal polymethine state than symmetrical cyanines. A transmission-mode electromodulation spectroscopy is used to study the electric-field-induced changes in optical absorption and refraction of polymer films doped with heptamethine chromophores, and large and thermally stable QEO effect with high efficiency-loss figure-of-merits that compare favorably to those from dipolar polyenes in poled or unpoled polymers and III-V semiconductors is obtained. The study opens a path for developing organic materials based on cyanine-like merocyanines for complementary metal oxide semiconductor -compatible, fast, efficient, and low-loss electro-optic modulation.

Tuning optical properties of π-conjugated double nanohoops under external electric field stimuli-responsiveness

Carbon nanorings have attracted substantial interest from synthetic chemists due to their unique topological structures and distinct physical properties. An intriguing π-conjugated double-nanoring structure, denoted as [8]CPP-[10]cyclacene, was constructed via the integration of [8]cycloparaphenylene ([8]CPP) into [10]cyclacene. Using the external electric field stimuli-responsiveness of [8]CPP-[10]cyclacene, directional charge transfer can be induced, resulting in the emergence of intriguing properties. The effects of the external electric field in three specific directions were explored, vertically in the [8]CPP unit (Fy), vertically in the [10]cyclacene unit (Fz), and horizontally along the double nanorings diameter (Fx). Interestingly, the external electric field vertically to the [10]cyclacene unit significantly enhanced the first hyperpolarizability (βtot) compared to that vertically to the [8]CPP unit. Notably, [8]CPP-[10]cyclacene under Fx exhibited significantly larger the βtot values (1.48 × 105 a.u.) than those of vertical Fy and Fz. This work opens up a wide range of nonlinear optics, making it a compelling area to explore in the field of carbon nanomaterials.

Theoretical Study on the Optoelectronic Properties of Merocyanine-Dyes

Merocyanines, as prototypes of highly polar π-conjugated molecules, have been intensively investigated for their self-assembly and optoelectronic properties, both experimentally and theoretically. However, an accurate description of their structural and electronic properties remains challenging for quantum-chemical methods. We assessed several theoretical approaches, TD-DFT, GW-BSE, STEOM-DLPNO-CCSD, and CASSCF/NEVPT2-FIC for their reliability in reproducing optoelectronic properties of a series of donor/acceptor (D/A) merocyanines, focusing on the first excitation energy. Additionally, we tested an all-electron perturbative method based on time-dependent coupled-perturbed density functional theory, denoted as TDCP-DFT. Particular focus was set on direct and indirect solvent effects, which affect excited-state energies by electrostatic interaction and molecular geometry. The molecular configuration space was sampled at the semiempirical tight-binding level. Our results corroborate previous investigations, showing that the S0 - S1 excitation energy strongly depends on the merocyanine molecular structure and the dielectric constant of the solvent. We found significant effects of the polar solution environment on the geometry of the merocyanines, which strongly affect the calculated excitation energies. Taking these effects into account, the best agreement between calculated and measured excitation energies was obtained with TDCP-DFT and GW-BSE. We also calculated excitation energies of molecular crystals at the TDCP-DFT level and compared the results to the corresponding monomers.

First hyperpolarizability of the di-8-ANEPPS and DR1 nonlinear optical chromophores in solution. An experimental and multi-scale theoretical chemistry study

The solvent effects on the linear and second-order nonlinear optical properties of an aminonaphtylethenylpyridinium (ANEP) dye are investigated by combining experimental and theoretical chemistry methods. On the one hand, deep near infrared (NIR) hyper-Rayleigh scattering (HRS) measurements (1840-1950 nm) are performed on solutions of di-8-ANEPPS in deuterated chloroform, dimethylformamide, and dimethylsulfoxide to determine their first hyperpolarizablity (βHRS). For the first time, these HRS experiments are carried out in the picosecond regime in the deep NIR with very moderate (≤3 mW) average input power, providing a good signal-to-noise ratio and avoiding solvent thermal effects. Moreover, the frequency dispersion of βHRS is investigated for Disperse Red 1 (DR1), a dye commonly used as HRS external reference. On the other hand, these are compared with computational chemistry results obtained by using a sequential molecular dynamics (MD) then quantum mechanics (QM) approach. The MD method allows accounting for the dynamical nature of the molecular structures. Then, the QM part is based on TDDFT/M06-2X/6-311+G* calculations using solvation models ranging from continuum to discrete ones. Measurements report a decrease of the βHRS of di-8-ANEPPS in more polar solvents and these effects are reproduced by the different solvation models. For di-8-ANEPPS and DR1, comparisons show that the use of a hybrid solvation model, combining the description of the solvent molecules around the probe by point charges with a continuum model, already achieves quasi quantitative agreement with experiment. These results are further improved by using a polarizable embedding that includes the atomic polarizabilities in the solvent description.

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Efficient Synthesis, Spectroscopic Characterization, and Nonlinear Optical Properties of Novel Salicylaldehyde-Based Thiosemicarbazones: Experimental and Theoretical Studies

Currently, we reported the synthesis of six novel salicylaldehyde-based thiosemicarbazones (BHCT1-HBCT6) via condensation of salicylaldehyde with respective thiosemicarbazide. Through various spectroscopic methods, UV-visible and NMR, the chemical structures of BHCT1-HBCT6 compounds were determined. Along with synthesis, a computational study was also performed at the M06/6-31G(d,p) functional. Various analyses such as natural bond orbital (NBO) analysis, natural population analysis, frontier molecular orbital (FMO) analysis, and molecular electrostatic potential surfaces were carried out to understand the nonlinear optical (NLO) characteristics of the synthesized compounds. Additionally, a comparative study was carried out between DFT and experimental results (UV-vis study), and a good agreement was observed in the results. The energy gap calculated through FMOs was found to be in decreasing order as 4.505 (FHCT2) > 4.499 (HBCT6) > 4.497 (BHCT1) = 4.497(HMCT5) > 4.386 (CHCT3) > 4.241(AHCT4) in eV. The global reactivity parameters (GRPs) were attained through E _HOMO and E _LUMO, which described the stability and hardness of novel compounds. The NBO approach confirmed the charge delocalization and stability of the molecules. Among all the investigated compounds, a larger value (557.085 a.u.) of first hyperpolarizability (β_tot) was possessed by CHCT3. The NLO response (β_tot) of BHCT1-HBCT6 was found to be 9.145, 9.33, 13.33, 5.43, 5.68, and 10.13 a.u. times larger than that of the standard para-nitroaniline molecule. These findings ascertained the potential of entitled ligands as best NLO materials for a variety of applications in modern technology.

Reversing the Bond Length Alternation Order in Conjugated Polyenes by Substituent Effects

We have synthesised several push-pull substituted conjugated polyenes and determined their accurate C-C bond lengths and charge-density distributions by utilising quantum crystallographic techniques. In a series of alkene, dienes, and triene bearing two (trifluoromethyl)sulfonyl (triflyl) groups on the terminal carbon atom, unique reversal of the bond-length alternation (BLA) order has been observed. This is a pronounced aberration from the molecular structure predicted by the Lewis-structure-based neutral resonance structure. Such reversal of BLA order has not been observed in push-pull compounds bearing conventional electron-withdrawing groups such as carbonyl and cyano groups instead of triflyl groups. Bonding behaviour of both normal and reversed bond length alternating systems has been revealed by complementary bonding analysis using several bond descriptors based on the experimentally fitted wavefunctions.

Effect of different end-capped donor moieties on non-fullerenes based non-covalently fused-ring derivatives for achieving high-performance NLO properties

A series of derivatives (DOCD2-DOCD6) with D-π-A configuration was designed by substituting various efficient donor moieties via the structural tailoring of o-DOC6-2F. Quantum-chemical approaches were used to analyze the optoelectronic properties of the designed chromophores. Particularly, M06/6-311G(d,p) functional was employed to investigate the non-linear optical (NLO) response (linear polarizability ⟨α⟩, first (β_tot) and second ([Formula: see text]_tot) order hyperpolarizabilities) of the designed derivatives. A variety of analyses such as frontier molecular orbital (FMO), absorption spectra, transition density matrix (TDMs), density of states (DOS), natural bond orbital (NBO) and global reactivity parameters (GRPs) were employed to explore the optoelectronic response of aforementioned chromophores. FMO investigation revealed that DOCD2 showed the least energy gap (1.657 eV) among all the compounds with an excellent transference of charge towards the acceptor from the donor. Further, DOS pictographs and TDMs heat maps also supported FMO results, corroborating the presence of charge separation states along with efficient charge transitions. NBO analysis showed that π-linker and donors possessed positive charges while acceptors retained negative charges confirming the D-π-A architecture of the studied compounds. The λ_max values of designed chromophores (659.070-717.875 nm) were found to have broader spectra. The GRPs were also examined utilizing energy band gaps of E_HOMO and E_LUMO for the entitled compounds. Among all the derivatives, DOCD2 showed the highest values of β_tot (7.184 × 10^-27 esu) and [Formula: see text]_tot (1.676 × 10^-31 esu), in coherence with the reduced band gap (1.657 eV), indicating future potentiality for NLO materials.

Modulated excited state geometry and Electron-Phonon coupling of lutein by temperature and solvent polarizability

Resonance Raman spectroscopy is one of the spectroscopic methods often chosen for studying linear polyene molecules because the Raman intensities of their υ₁ (C = C) and υ₂ (C-C) stretching vibrations are sensitive to electron-phonon coupling and the π-electron energy gap. Here, the resonance Raman and absorption spectra of lutein were studied as a function of solvent polarizabilities and of temperature in the CS₂ solvent. For lutein in CS₂, as the temperature decreased and CS solidified, the Raman scattering cross-section (RSCS) and the electron-phonon coupling constant had opposite dependence trends on temperature. The wavenumber of the lutein 0-0 electronic transition showed a marked shift to lower wavenumbers when the polarizability of the solvents decreased, and the Huang-Rhys (HR) factors and electron-phonon coupling also decreased. This work helps explore the influence of the external environment (e.g., temperature and solvent) on the excited state geometry of linear polyene molecules.

Theoretical Study of Alkaline-Earth Metal (Be, Mg, and Ca)-Substituted Aluminum Nitride Nanocages With High Stability and Large Nonlinear Optical Responses

By replacing one Al or N atom of aluminum nitride nanocage Al12N12 with an alkaline-earth metal atom, two series of compounds, namely, M@Al12N11 and M@Al11N12 (M = Be, Mg, and Ca), were constructed and investigated in theory. The substituted effect of alkaline-earth metal on the geometric structure and electronic properties of Al12N12 is studied in detail by density functional theory (DFT) methods. The calculated binding energies, HOMO–LUMO gaps, and VIE values of these compounds reveal that they possess high stability, though the NBO and HOMO analyses show that they are also excess electron compounds. Due to the existence of diffuse excess electrons, these alkaline-earth metal-substituted compounds exhibit larger first hyperpolarizabilities (β0) than pure Al12N12 nanocage. In particular, these considered compounds exhibit satisfactory infrared (IR) (>1800 nm) and ultraviolet (UV) (˂ 250 nm) transparency. Therefore, these proposed excess electron compounds with high stability may be regarded as potential candidates for new UV and IR NLO molecules.

Energetic Basis and Design of Enzyme Function Demonstrated Using GFP, an Excited-State Enzyme

The past decades have witnessed an explosion of de novo protein designs with a remarkable range of scaffolds. It remains challenging, however, to design catalytic functions that are competitive with naturally occurring counterparts as well as biomimetic or nonbiological catalysts. Although directed evolution often offers efficient solutions, the fitness landscape remains opaque. Green fluorescent protein (GFP), which has revolutionized biological imaging and assays, is one of the most redesigned proteins. While not an enzyme in the conventional sense, GFPs feature competing excited-state decay pathways with the same steric and electrostatic origins as conventional ground-state catalysts, and they exert exquisite control over multiple reaction outcomes through the same principles. Thus, GFP is an "excited-state enzyme". Herein we show that rationally designed mutants and hybrids that contain environmental mutations and substituted chromophores provide the basis for a quantitative model and prediction that describes the influence of sterics and electrostatics on excited-state catalysis of GFPs. As both perturbations can selectively bias photoisomerization pathways, GFPs with fluorescence quantum yields (FQYs) and photoswitching characteristics tailored for specific applications could be predicted and then demonstrated. The underlying energetic landscape, readily accessible via spectroscopy for GFPs, offers an important missing link in the design of protein function that is generalizable to catalyst design.

Harnessing Intramolecular Chalcogen-Chalcogen Bonding in Merocyanines for Utilization in High-Efficiency Photon-to-Current Conversion Optoelectronics

A novel series of donor (D)-π-acceptor (A) merocyanine molecules harnessed with intramolecular chalcogen bonding (ChaB) is designed, synthesized, and characterized. ChaB comprises periodic chalcogen atoms, S, Se, and Te, and a neighboring oxygen atom of a carbonyl moiety. Compared to the D-π-A merocyanine dye with nontraditional intramolecular hydrogen bonding, the novel molecules with an intramolecular ChaB exhibit remarkably smaller absorption spectral widths and higher absorption coefficients attributed to their cyanine-like characteristics approaching the resonance parameter (c²) ∼0.5; furthermore, they exhibit better thermal stabilities and electrical charge-carrier transport properties in films. These novel D-π-A merocyanines harnessed with intramolecular ChaB networks are successfully utilized in high-performance color-selective organic photon-to-current conversion optoelectronic devices with excellent thermal stabilities. This study reports that the unique intramolecular ChaB plays an essential role in locking the molecular conformation of merocyanine molecules and enhancing the optical, thermal, and optoelectronic properties of high-performance and high-efficiency organic photon-to-current conversion devices.

A Peierls Transition in Long Polymethine Molecular Wires: Evolution of Molecular Geometry and Single-Molecule Conductance

Molecules capable of mediating charge transport over several nanometers with minimal decay in conductance have fundamental and technological implications. Polymethine cyanine dyes are fascinating molecular wires because up to a critical length, they have no bond-length alternation (BLA) and their electronic structure resembles a one-dimensional free-electron gas. Beyond this threshold, they undergo a symmetry-breaking Peierls transition, which increases the HOMO-LUMO gap. We have investigated cationic cyanines with central polymethine chains of 5-13 carbon atoms (Cy3⁺-Cy11⁺). The absorption spectra and crystal structures show that symmetry breaking is sensitive to the polarity of the medium and the size of the counterion. X-ray crystallography reveals that Cy9·PF₆ and Cy11·B(C₆F₅)₄ are Peierls distorted, with high BLA at one end of the π-system, away from the partially delocalized positive charge. This pattern of BLA distribution resembles that of solitons in polyacetylene. The single-molecule conductance is essentially independent of molecular length for the polymethine salts of Cy3⁺-Cy11⁺ with the large B(C₆F₅)₄^- counterion, but with the PF₆^- counterion, the conductance decreases for the longer molecules, Cy7⁺-Cy11⁺, because this smaller anion polarizes the π-system, inducing a symmetry-breaking transition. At higher bias (0.9 V), the conductance of the shorter chains, Cy3⁺-Cy7⁺, increases with length (negative attenuation factor, β = -1.6 nm^-1), but the conductance still drops in Cy9⁺ and Cy11⁺ with the small polarizing PF₆^- counteranion.

The appeal of small molecules for practical nonlinear optics

Small organic molecules with a π-conjugated system that consists of only a few double or triple bonds can have significantly smaller optical excitation energies when equipped with donor- and acceptor groups, which raises the quantum limits to the molecular polarizabilities. As a consequence, third-order nonlinear optical polarizabilities become orders of magnitude larger than those of molecules of similar size without donor-acceptor substitution. This  enables strong third-order nonlinear optical effects (as high as 1000 times those of silica glass) in dense, amorphous monolithic assemblies. These properties, accompanied by the possibility of deposition from the vapor phase and of electric-field poling at higher temperatures, make the resulting materials competitive towards adding an active nonlinear optical or electro-optic functionality to state-of-the-art integrated photonics platforms.

Inorganic electrides of alkali metal doped Zn12O12 nanocage with excellent nonlinear optical response

Finding new materials with exceptionally large nonlinear optical response is an interesting and challenging avenue for scientific research. Here, we report the alkali metal doped Zn₁₂O₁₂ nanocages as inorganic electrides with excellent nonlinear optical response. Density functional theory calculations have been performed for geometric, electronic and nonlinear optical response of exo- and endohedrally alkali metal doped Zn₁₂O₁₂ nanoclusters. For exohedral doping, all different possible doping sites are considered for decoration of alkali metal on the nanocage. The electride nature of the complexes is highly dependent on the position of alkali metal doping. All exohedral complexes except for alkali metal doping on six membered ring (r₆) are electride in nature, as revealed from frontier molecular orbital analysis. Interaction energies reveal that all doped nanoclusters except endo-K@Zn₁₂O₁₂ are thermodynamically stable. The exothermic encapsulation of alkali metals in Zn₁₂O₁₂ nanocages is in marked contradiction with other inorganic fullerenes where encapsulation is an endothermic process. The barriers for boundary crossing are also evaluated in order study the interconversion of exo- and endohedral complexes. Doping of alkali metal significantly influences the properties of nanocages. HOMO-LUMO (H-L) gap is reduced significantly whereas hyperpolarizability is increased several orders of magnitude. The NLO response of exohedrally doped complexes is higher than the corresponding endohedral complexes, which is in mark contradiction with the behavior of phosphide or nitride nanocages. The highest first hyperpolarizability of 1.0 × 10⁵ au is calculated for K@r₆-Zn₁₂O₁₂ complex. Third order NLO response of these complexes is calculated and compared with the best systems reported in the literature at the same level of theory.

Theoretical study of the mixed π-conjugated bridge effect on the nonlinear optical properties of corannulene derivative

A new series of corannulene derivatives with two mixed π-conjugated bridge have been theoretically designed and investigated by means of density functional theory. It is found that all molecules exhibit large energy gaps. The holes and electrons analysis show that charge transfer from long-chain connected with NH₂ to long-chain connected with NO₂. The small transition energy brings corannulene derivatives larger first hyperpolarizabilities. Furthermore, the polarization scan of the hyper-Rayleigh scattering (HRS) intensity indicates that all studied compounds belong to dipolar characteristic. The results indicate that employing two mixed π-conjugated bridge can significantly increase the first hyperpolarizability.

Electronic structure and bridge geometric distortion in push–pull imine-bridged triads. A theoretical study

Intramolecular electron transfer (IET) in imine-bridged triads is studied by analyzing electric charge distribution and ferrocene and bridge distortion parameters.

Enumerating Intramolecular Charge Transfer in Conjugated Organic Compounds

Charge transfer across conjugated organic molecules is the functional basis of many optoelectronic and semiconductor devices. The ability to design such molecules to suit a given device application is highly desirable; yet, realizing this prospect is impeded by the lack of an algorithm that quantifies the extent of intramolecular charge transfer (ICT) in absolute terms. In turn, an algorithm to describe ICT is held back by a poor definition of one of its key dependent terms: conjugation. Current equations assume that π-bonding operates solely across two bonds, even though conjugation extends beyond these limits, and such equations only yield relative measures of π-conjugation. This work presents a four-step algorithm that enumerates ICT on an absolute scale. The method is applied successfully to four types of optoelectronic materials; results demonstrate the need to reconsider certain fundamental chemical-bonding and ICT concepts for conjugated molecules. These findings have implications for all optoelectronic and semiconducting materials.

Green-Light-Selective Organic Photodiodes with High Detectivity for CMOS Color Image Sensors

Stacked structures employing wavelength-selective organic photodiodes (OPDs) have been studied as promising alternatives to the conventional Si-based image sensors because of their color constancy. Herein, novel donor (D)-π-acceptor (A) molecules are designed, synthesized, and characterized as green-light-selective absorbers for application in organic-on-Si hybrid complementary metal-oxide-semiconductor (CMOS) color image sensors. The p-type molecules, combined with two fused-type heterocyclic donors and an electron-accepting unit, exhibit cyanine-like properties that are characterized by intense and sharp absorption. This molecular design leads to improved absorption properties, thermal stability, and higher photoelectric conversion compared to those of a molecular design based on a nonfused ring. A maximum external quantum efficiency of 66% (λ_max = 550 nm) and high specific detectivity (D*) of 8 × 10¹³ cm Hz^1/2/W are achieved in an OPD consisting of a bulk heterojunction blend with two transparent electrodes on both sides. Finally, the green-light-detection capability of the narrow-band green-selective OPD is demonstrated by the optical simulation of an organic-on-Si hybrid, stacked-type, full-color photodetector comprising the green-light-selective OPD and a bottom Si photodiode with only blue and red color filters. Based on this molecular design, further optimization of the OPDs can allow the development of various optoelectronic sensors including 3D-stacked image sensors with enhanced sensitivities to replace the conventional Si-based CMOS image sensors.

Electrostatic Field-Induced Oscillator Strength Focusing in Molecules

The development of light-harvesting devices based on molecular materials depends critically on the ability to focus the electronic oscillator strength of molecules into the UV-vis spectral window. Typical molecular chromophores have only about 1% of their total electronic oscillator strength in this spectral region and thus perform at a small fraction of their possible effectiveness. This theoretical study finds that the electronic oscillator strength of polyenes in the UV-vis region may be enhanced by 1 order of magnitude using electrostatic fields, motivating specific experimental studies of oscillator strength focusing. We find scaling relationships between the polyene length, the intensity of the applied field, and the field-induced increase in oscillator strength that are useful for the implementation of light-harvesting strategies based on polyenes.

Mechanism of Color and Photoacidity Tuning for the Protonated Green Fluorescent Protein Chromophore

The neutral or A state of the green fluorescent protein (GFP) chromophore is a remarkable example of a photoacid naturally embedded in the protein environment and accounts for the large Stokes shift of GFP in response to near UV excitation. Its color tuning mechanism has been largely overlooked, as it is less preferred for imaging applications than the redder anionic or B state. Past studies, based on site-directed mutagenesis or solvatochromism of the isolated chromophore, have concluded that its color tuning range is much narrower than its anionic counterpart. However, as we performed extensive investigation on more GFP mutants, we found that the color of the neutral chromophore can be more sensitive to protein electrostatics than can the anionic counterpart. Electronic Stark spectroscopy reveals a fundamentally different electrostatic color tuning mechanism for the neutral state of the chromophore that demands a three-form model as compared to that of the anionic state, which requires only two forms ( J. Am. Chem. Soc. 2019, 141, 15250-15265). Specifically, an underlying zwitterionic charge-transfer state is required to explain its sensitivity to electrostatics. As the Stokes shift is tightly linked to excited-state proton transfer (ESPT) of the protonated chromophore, we infer design principles of the GFP chromophore as a photoacid through the color tuning mechanisms of both protonation states. The three-form model could also be applied to similar biological and nonbiological dyes and complements the failure of the two-form model for donor-acceptor systems with localized ground-state electronic distributions.

Electrostatic control of photoisomerization pathways in proteins

Rotation around a specific bond after photoexcitation is central to vision and emerging opportunities in optogenetics, super-resolution microscopy, and photoactive molecular devices. Competing roles for steric and electrostatic effects that govern bond-specific photoisomerization have been widely discussed, the latter originating from chromophore charge transfer upon excitation. We systematically altered the electrostatic properties of the green fluorescent protein chromophore in a photoswitchable variant, Dronpa2, using amber suppression to introduce electron-donating and electron-withdrawing groups to the phenolate ring. Through analysis of the absorption (color), fluorescence quantum yield, and energy barriers to ground- and excited-state isomerization, we evaluate the contributions of sterics and electrostatics quantitatively and demonstrate how electrostatic effects bias the pathway of chromophore photoisomerization, leading to a generalized framework to guide protein design.

Coinage metalides: a new class of excess electron compounds with high stability and large nonlinear optical responses

The possibility of using coinage metal atoms as excess electron acceptors is examined for the first time by designing a new class of M⁺-1-M'^- (M = Li, Na, and K; M' = Cu, Ag, and Au) compounds termed "coinage metalides" on the basis of an intriguing Janus-type all-cis1,2,3,4,5,6-hexafluorocyclohexane (1) molecule. Under the large facial polarization of 1, the outermost ns¹ electrons of alkali metal atoms can be transferred to coinage metal atoms, forming diffuse excess electrons around them. Consequently, the resulting M⁺-1-Cu^- and M⁺-1-Ag^- compounds exhibit significantly large nonlinear optical (NLO) responses. In particular, these novel M⁺-1-M'^- compounds exhibit much higher stability (larger VIEs and E_c values) than that of the corresponding M⁺·1·M'^- (M, M' = Li, Na, and K) alkalides. We hope this work could open up new possibilities for NLO material design by using coinage metal atoms as excess electron acceptors and, on the other hand, attract more experimental interest and efforts to synthesize such stable compounds in the laboratory.

A Design-to-Device Pipeline for Data-Driven Materials Discovery

The world needs new materials to stimulate the chemical industry in key sectors of our economy: environment and sustainability, information storage, optical telecommunications, and catalysis. Yet, nearly all functional materials are still discovered by "trial-and-error", of which the lack of predictability affords a major materials bottleneck to technological innovation. The average "molecule-to-market" lead time for materials discovery is currently 20 years. This is far too long for industrial needs, as highlighted by the Materials Genome Initiative, which has ambitious targets of up to 4-fold reductions in average molecule-to-market lead times. Such a large step change in progress can only be realistically achieved if one adopts an entirely new approach to materials discovery. Fortunately, a fundamentally new approach to materials discovery has been emerging, whereby data science with artificial intelligence offers a prospective solution to speed up these average molecule-to-market lead times.This approach is known as data-driven materials discovery. Its broad prospects have only recently become a reality, given the timely and major advances in "big data", artificial intelligence, and high-performance computing (HPC). Access to massive data sets has been stimulated by government-regulated open-access requirements for data and literature. Natural-language processing (NLP) and machine-learning (ML) tools that can mine data and find patterns therein are becoming mainstream. Exascale HPC capabilities that can aid data mining and pattern recognition and also generate their own data from calculations are now within our grasp. These timely advances present an ideal opportunity to develop data-driven materials-discovery strategies to systematically design and predict new chemicals for a given device application.This Account shows how data science can afford materials discovery via a four-step "design-to-device" pipeline that entails (1) data extraction, (2) data enrichment, (3) material prediction, and (4) experimental validation. Massive databases of cognate chemical and property information are first forged from "chemistry-aware" natural-language-processing tools, such as ChemDataExtractor, and enriched using machine-learning methods and high-throughput quantum-chemical calculations. New materials for a bespoke application can then be predicted by mining these databases with algorithmic encodings of relationships between chemical structures and physical properties that are known to deliver functional materials. These may take the form of classification, enumeration, or machine-learning algorithms. A data-mining workflow short-lists these predictions to a handful of lead candidate materials that go forward to experimental validation. This design-to-device approach is being developed to offer a roadmap for the accelerated discovery of new chemicals for functional applications. Case studies presented demonstrate its utility for photovoltaic, optical, and catalytic applications. While this Account is focused on applications in the physical sciences, the generic pipeline discussed is readily transferable to other scientific disciplines such as biology and medicine.

Push-pull thiophene chromophores for electro-optic applications: from 1D linear to β-branched structures

We report the synthesis and characterization of a novel series of push-pull chromophores bearing 1D linear and β-branched thiophenes as π-conjugated spacers between a 2,2,4,7-tetramethyl-1,2,3,4-tetrahydroquinoline electron donor unit and dicyano- and tricyanovinylene electron acceptor groups. The effect of the introduction of β-thiophenes on the linear and nonlinear (NLO) optical properties as well as electrochemical and thermal data is studied in detail by performing a comparative study between the branched and 1D linear systems. In addition, a parallel DFT computational study is used to evaluate structure-property relationships. The non-linear optical behavior of the molecules both in solution and in solid state as electro-optic (EO) films using a guest-host approach shows very promising performance for electro-optic applications with high molecular first hyperpolarizabilities (μβ) of 4840 × 10^-48 esu and electro-optic coefficients r₃₃ reaching 650 pm V^-1. One highlight is that the electro-optic films of the β-branched chromophores are superior in terms of thermal stability in device operation as measured by a transmissive modified reflective Teng-Man method. This work provides guidelines for the design of improved electro-optic materials including β-branched chromophores which could be useful for practical EO applications, where both enhanced β and r₃₃ values together with chemical and thermal stability are necessary.

Designing a new class of excess electron compounds with unique electronic structures and extremely large non-linear optical responses

New Ca⁺-1-M′⁻ (M′ = Li, Na, and K) compounds with typical alkalide features and electride-like characteristics have been obtained.

Triplet state structure-property relationships in a series of platinum acetylides: effect of chromophore length and end cap electronic properties

To develop quantitative structure-spectroscopic property relationships in platinum acetylides, we investigated the triplet state behavior of nominally centrosymmetric chromophores trans-Pt(PBu3)2(C[triple bond, length as m-dash]C-Phenyl-X)2, where X = diphenylamino, NH2, OCH3, t-Bu, CH3, H, F, benzothiazole, CF3, CN, and NO2. We measured ground state absorption, phosphorescence, excitation and triplet state absorption spectra and triplet lifetimes. By DFT we calculated the phosphorescence emission energy (ET), the spin density on the end cap (SD(X)), triplet state geometry and the distance between the triplet centroid and the central platinum atom (RS-Pt(X)). Compounds with electron-donating X have smaller triplet state lifetime, blue-shifted phosphorescence and larger triplet potential energy surface displacement associated with the C[triple bond, length as m-dash]C bond. Compounds with electron-withdrawing X have larger triplet lifetime, red-shifted phosphorescence and smaller triplet potential energy surface displacement associated with the C[triple bond, length as m-dash]C bond. The range of spin-orbit-coupling between the platinum atom and the triplet centroid was determined to be 6 Å. The quantity RS-Pt is shown to be a linear function of one-dimensional well length calculated from experimental ET. The multiple examples demonstrate RS-Pt is a useful descriptor for analyzing triplet state behavior.

Unified Model for Photophysical and Electro-Optical Properties of Green Fluorescent Proteins

Green fluorescent proteins (GFPs) have become indispensable imaging and optogenetic tools. Their absorption and emission properties can be optimized for specific applications. Currently, no unified framework exists to comprehensively describe these photophysical properties, namely the absorption maxima, emission maxima, Stokes shifts, vibronic progressions, extinction coefficients, Stark tuning rates, and spontaneous emission rates, especially one that includes the effects of the protein environment. In this work, we study the correlations among these properties from systematically tuned GFP environmental mutants and chromophore variants. Correlation plots reveal monotonic trends, suggesting that all these properties are governed by one underlying factor dependent on the chromophore's environment. By treating the anionic GFP chromophore as a mixed-valence compound existing as a superposition of two resonance forms, we argue that this underlying factor is defined as the difference in energy between the two forms, or the driving force, which is tuned by the environment. We then introduce a Marcus-Hush model with the bond length alternation vibrational mode, treating the GFP absorption band as an intervalence charge transfer band. This model explains all of the observed strong correlations among photophysical properties; related subtopics are extensively discussed in the Supporting Information. Finally, we demonstrate the model's predictive power by utilizing the additivity of the driving force. The model described here elucidates the role of the protein environment in modulating the photophysical properties of the chromophore, providing insights and limitations for designing new GFPs with desired phenotypes. We argue that this model should also be generally applicable to both biological and nonbiological polymethine dyes.

Crystal structure of tetra-methyl-ammonium 1,1,7,7-tetra-cyano-hepta-2,4,6-trienide

The title compound, C₄H₁₂N⁺·C₁₁H₅N₄ ^-, contains one tetra-methyl-ammonium cation and one 1,1,7,7-tetra-cyano-hepta-2,4,6-trienide anion in the asymmetric unit. The anion is in an all-trans conjugated C=C bonds conformation. Two terminal C(CN)₂ di-nitrile moieties are slightly twisted from the polymethine main chain to which they are attached [C(CN)₂/C₅ dihedral angles = 6.1 (2) and 7.1 (1)°]. The C-C bond distances along the hepta-dienyl chain vary in the narrow range 1.382 (2)-1.394 (2) Å, thus indicating the significant degree of conjugation. In the crystal, the anions are linked into zigzag chains along the [10] direction by C-H⋯N(nitrile) short contacts. The anti-parallel chains stack along the [110] direction with alternating separations between the neighboring anions in stacks of 3.291 and 3.504 Å. The C-H⋯N short contacts and stacking inter-actions combine to link the anions into layers parallel to the (01) plane and separated by columns of tetra-methyl-ammonium cations.

On-the-Fly Femtosecond Action Spectroscopy of Charged Cyanine Dyes: Electronic Structure versus Geometry

Understanding optical properties of molecular dyes is required to drive progress in molecular photonics. This requires a fundamental comprehension of the role of electronic structure, geometry, and interactions with the environment in order to guide molecular engineering strategies. In this context, we studied charged cyanine dye molecules in the gas phase with a controlled microenvironment to unravel the origin of the spectral tuning of this class of molecules. This was performed using a new approach combining femtosecond multiple-photon action spectroscopy of on-the-fly mass-selected molecular ions and high-level quantum calculations. While arguments based on molecular geometry are often used to design new polymethine dyes, we provide experimental evidence that electronic structure is of primary importance and hence the decisive criterion as suggested by recent theoretical investigations.

Membrane-Permeant, Environment-Sensitive Dyes Generate Biosensors within Living Cells

Dyes with environment-sensitive fluorescence have proven useful to study the spatiotemporal dynamics of protein activity in living cells. When attached to proteins, their fluorescence can reflect protein conformational changes, post-translational modifications, or protein interactions. However, the utility of such dye-protein conjugates has been limited because it is difficult to load them into cells. They usually must be introduced using techniques that perturb cell physiology, limit throughput, or generate fluorescent vesicles (e.g., electroporation, microinjection, or membrane transduction peptides). Here we circumvent these problems by modifying a proven, environment-sensitive biosensor fluorophore so that it can pass through cell membranes without staining intracellular compartments and can be attached to proteins within living cells using unnatural amino acid (UAA) mutagenesis. Reactive groups were incorporated for attachment to UAAs or small molecules (mero166, azide; mero167, alkyne; mero76, carboxylic acid). These dyes are bright and fluoresce at long wavelengths (reaching ε = 100 000 M^-1 cm^-1, ϕ = 0.24, with excitation 565 nm and emission 594 nm). The utility of mero166 was demonstrated by in-cell labeling of a UAA to generate a biosensor for the small GTPase Cdc42. In addition, conjugation of mero166 to a small molecule produced a membrane-permeable probe that reported the localization of the DNA methyltransferase G9a in cells. This approach provides a strategy to access biosensors for many targets and to more practically harness the varied environmental sensitivities of synthetic dyes.

Unlocking the action mechanisms of molecular nonlinear optical absorption for optical conjugated polymers under aggregation states

Controlling the molecular microstructure and the molecular aggregation state under different conditions to improve the MNOA performance of OCPs.