Tuning the two-photon absorption response of quadrupolar organic molecules

The Quinoline Photoremovable Group (PPG) Platform-A Medicinal Chemist's Approach for Photocage Development and Applications

Photoremovable protecting groups (PPGs) offer a straightforward solution for the temporary inactivation of biologically active substrates and their subsequent controlled release by light irradiation. Their relatively easy design and mode of application have made them useful tools for studying dynamic biological processes in vitro and in vivo. Recently, there has been a growing body of data investigating their potential application in the development of drug delivery systems. Of the various PPG scaffolds in use, quinoline photocages have a history of about 20 years. The structure-property relationships of quinoline PPGs, as well as alternative multibranch designs based on quinoline monomers have been thoroughly studied both experimentally and theoretically. Therefore, quinoline PPGs serve as a representative study of PPG development, showing how the various applications of quinoline photocages followed the chemical optimization or how the applications drove the chemical design. Since the raison d'être of PPGs lies in their application for light-activated release of various substrates or performing light-activated structural changes in materials, it is crucial to understand how PPGs are selected and utilized by their end-users, who are often not chemists themselves. Therefore, we discuss whether the conclusions drawn from the selected quinoline PPG family could lead to more general insights for the field as whole. As PPG-related applications still rely heavily on a limited number of chemical scaffolds, it is worth considering, what could be the reasons for the slow uptake of novel chemical scaffolds.

Two-Photon Absorption Strengths of Small Molecules: Reference CC3 Values and Benchmarks

We present a large dataset of highly accurate two-photon transition strengths (δTPA) determined for standard small molecules. Our reference values have been calculated using the quadratic response implementation of the third-order coupled cluster method including iterative triples (Q-CC3). The aug-cc-pVTZ atomic basis set is used for molecules with up to five non-hydrogen atoms, while larger molecules are assessed with aug-cc-pVDZ; the differences due to the basis sets are discussed. This dataset, encompassing 82 singlet transitions of various characters (Rydberg, valence, and double excitations), enables a comprehensive benchmark of smaller basis sets and alternative wavefunction methods when Q-CC3 calculations become beyond reach as well as time-dependent density functional theory (TD-DFT) approaches. The evaluated wavefunction methods include quadratic response and equation-of-motion CCSD approximations, Q-CC2, and second-order algebraic diagrammatic construction in its intermediate state representation (I-ADC2). In the TD-DFT framework, a set of five commonly used exchange-correlation functionals are evaluted. This extensive analysis provides a quantitative assessment of these methods, revealing how different system sizes, response intensities, and types of transitions affect their performances.

Tris(4'-Nitrobiphenyl)amine─An Octupolar Chromophore with High Two-Photon Absorption Cross-Section and Its Application for Uncaging of Calcium Ions in the Near-Infrared Region

Compounds with high two-photon absorption (2PA) performance in the near-infrared region have attracted great attention because of their application in the material and biological science. In this study, we have developed a simple and novel octupolar chromophore, tris(4'-nitrobiphenyl)amine 1, with three nitro peripheral groups attached to a triphenylamine core via biphenyl linkers. A mono-branched analogue 2 has also been prepared to investigate the effects of octupolar and dipolar systems on photophysical and 2PA behaviors. Compound 1, despite having a much simpler structure than the previous three-branched scaffolds, exhibits comparable σ₂ values, reaching 1330 GM at 730 nm and 900 GM at 820 nm in toluene. Combined with an outstanding σ₂/MW ratio (2.2 GM g^-1 mol) and a high fluorescence quantum yield (0.51), 1 displays potential as a promising two-photon (2P) probe for bioimaging. Subsequently, the ethylene glycol tetraacetic acid-substituted derivatives featuring octupolar (3 and 5) or dipolar (4 and 6) character have been synthesized and their one-photon (1P) and 2P photochemical reactions have been examined. Finally, 1P- and 2P-triggered uncaging of Ca²⁺ from these calcium chelators has been confirmed.

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.

Intramolecular Cooperative and Anti-Cooperative Effect on the Two-Photon Absorption Cross Section in Triphenylamine Derivatives

The intramolecular cooperative effect in branched molecules is a consequence of the interaction and extent of electronic coupling among the different axes of charge transfer. Such an effect is the key to obtain remarkable nonlinear optical response in molecular systems. Here we show that triphenylamine derivative molecules containing only two branches present the strongest electronic interaction between them at the excited state, generating exponential enhancement of the 2PA cross section. The primary factor for such behavior was ascribed to the substantial extent and interaction of the π-electron delocalization promoted by the strong electron-donating and acceptor antisymmetrical groups present in each branch. However, for the three-branch molecules we observed an anticooperative effect, i.e., the 2PA cross section decreases as compared to the one-branch structure as we normalized the signal by the effective π-electron number in each molecule.

Computational design of zinc-ion-responsive two-photon fluorescent probes with conjugated multi-structures

A series of conjugated multi-structured fluorescent probe molecules based on a salen ligand were designed and investigated in dimethyl sulfoxide solvent using a quantum-chemical method. The results indicate that the one-photon absorption and fluorescence emission spectra (λ (O) and λ (EM)) of these molecules generally show redshifts (of 23.1-74.5 and 22.7-116.6 nm, respectively) upon the coordination of the molecules to Zn(2+). Large Stokes shifts (1511.2-11744.1 cm(-1)) were found for the molecules, meaning that interference between λ (O) and λ (EM) can be avoided for these molecules. The two-photon absorption spectra of the molecules usually present blueshifts, but the two-photon absorption cross-section (δ) greatly increases (by 221.5-868.0 GM) upon the coordination of the molecules with Zn(2+). Most of the molecules show strong two-photon absorption peaks in the range 678.2-824.4 nm, i.e., in the near-infrared region. In a word, the expanded π-conjugated frameworks of these molecules lead to redshifted λ (O) and λ (EM) and enhanced δ values. Moreover, (L-phenyl)​2 and (L-phenyl-ethynyl)2 are the most suitable of the multi-structured molecules examined in this work for use as two-photon fluorescent probes for zinc ion detection in vivo. Graphical Abstract Scheme of the calculated transition energies (E0k and E0n) and the transition dipole moments (M0k and Mkn). NTO 109, NTO 197 and NTO 228 of Zn(L-phenyl-ethynyl), Zn2(L-phenyl-ethynyl)2 and Zn3(L-phenyl)3 for one-photon absorption, respectively.

Distinguishing the Effects of Bond-Length Alternation versus Bond-Order Alternation on the Nonlinear Optical Properties of π-Conjugated Chromophores

Understanding the relationships between the molecular nonlinear optical (NLO) properties and the bond-length alternation (BLA) or π-bond-order alternation (BOA) along the molecular backbone of linear π-conjugated systems has proven widely useful in the development of NLO organic chromophores and materials. Here, we examine model polymethines to elucidate the reliability of these relationships. While BLA is solely a measure of molecular geometric structure, BOA includes information pertaining to the electronic structure. As a result, BLA is found to be a good predictor of NLO properties only when optimized geometries are considered, whereas BOA is more broadly applicable. Proper understanding of the distinction between BLA and BOA is critical when designing computational studies of NLO properties, especially for molecules in complex environments or in nonequilibrium geometries.

Molecular structure-optical property relationships for a series of non-centrosymmetric two-photon absorbing push-pull triarylamine molecules

This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.

25th anniversary article: Design of polymethine dyes for all-optical switching applications: guidance from theoretical and computational studies

All-optical switching--controlling light with light--has the potential to meet the ever-increasing demand for data transmission bandwidth. The development of organic π-conjugated molecular materials with the requisite properties for all-optical switching applications has long proven to be a significant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π-conjugated materials for all-optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.

Holstein-Peirls-Hubbard trimer as a model for quadrupolar two-photon absorbing dyes

The linear and nonlinear optical properties of a Donor-Acceptor-Donor system have been investigated by using a two-electron three-point-site model system. Some basic features of electron correlations are included in the model by means of a bi-electronic density matrix. The polarizabilities and second hyperpolarizabilities have been computed with a modified version of the Collective Electronic Oscillators (CEO) method which allowed us to include the electron-phonon coupling. Both singly- and doubly-excited states are taken into account in the computation of (hyper-)polarizabilities. The effects of electron-phonon coupling on the two-photon absorption and on the third harmonic generation in the infrared region are discussed.

A comprehensive investigation of the interrelationships between spectroscopy and photochemistry and substituents of some azulenic derivatives

Several azulenic dyes, including six azulene hydrocarbons, two azulene aldehydes, and two olefinic azulenes, have been synthesized to survey their photophysics and photochemistry. These azulenes display S(2)-->S(0) emission, but with several differences. This is the most remarkable characteristic of the effect of orbital control on color and excited state properties of the azulenic compounds. This paper emphasizes how emission spectra and photochemistry of azulenic compounds are influenced by their chemical structure and solvent. The emission spectra of the azulene hydrocarbons suggest that their excited state properties can be controlled by their molecular structure and size. It was confirmed by emission and (1)H NMR spectroscopy that azulene monoaldehyde is protonated in a strong acid, such as trifluoroacetic acid (TFA). Photochemistry of styrylazulenes was observed during irradiation. Azulenic compounds are thermally stable and color tunable, and hence they are good candidates as non-linear optical materials. Based on their unique photochemical and photophysical characteristics, novel azulenic dyes can be constructed for different uses.

Intersystem Crossing Processes in Nonplanar Aromatic Heterocyclic Molecules

A comprehensive study of the photophysical properties of a series of monoaza[5]helicenes is presented on the basis of joint optical spectroscopy and quantum chemistry investigations. The molecules have been characterized by absorption and CW/time-resolved luminescence measurements. All quantities related to spin-orbit-coupling processes, such as intersystem crossing rates and radiative phosphorescence lifetimes, were found to depend strongly on the nitrogen position within the carbon backbone. Density functional theory and semiempirical quantum-chemical methods were used to evaluate the molecular geometries, the characteristics of the excited singlet and triplet states, and the spin-orbit coupling matrix elements. We demonstrate that the magnitude of spin-orbit coupling is directly correlated with the degree of deviation from planarity. The trends from the calculated photophysical quantities, namely, radiative fluorescence and phosphorescence decay rates and intersystem crossing rates, of the mono-aza-helicenes are fully consistent with experiment.

Novel organic materials through control of multichromophore interactions

The function of organic semiconducting and light-harvesting materials depends on the organization of the individual molecular components. Our group has tackled the problem of through-space delocalization via the design and synthesis of bichromphoric pairs held in close proximity by the [2.2]paracyclophane core. The linear and nonlinear optical properties of these molecules provide a challenge to theory. They are also useful in delineating the problem of intermolecular contacts in molecular conductivity measurements. Another area of research described here concerns conjugated polyelectrolytes. These macromolecules combine the properties of organic semiconductors and conventional polyelectrolytes. We have used these materials in the development of optically amplified biosensors and have also incorporated them into organic optoelectronic devices. Of particular interest to us is to derive useful structure/property relationships via molecular design that address important basic scientific problems and technological challenges.

Structure-property relationships for three-photon absorption in stilbene-based dipolar and quadrupolar chromophores

Based on essential-state models for three-photon absorption (3PA), we have investigated the structure-property relationships for stilbene-based dipolar and quadrupolar chromophores. The emphasis lies on the evolution of the 3PA cross section with the degree of ground-state polarization. For dipolar systems, we find a dominant role played by Deltamu, which expresses the change in dipole moment between the ground state and the 3PA active excited state. Thus, the strategies usually applied to maximize the second-order polarizability beta are also applicable to optimize the 3PA cross section. For quadrupolar systems, the 3PA response is dominated by contributions from channels including various low-lying two-photon allowed states, which limits the applicability of essential-state models. Optimization strategies can be proposed but vary for different ranges of ground-state polarization.

Structure to property relationships for multiphoton absorption in covalently linked porphyrin dimers: a correction vector INDO/MRDCI study

The correction vector method has been used to investigate structure to property relationships for multiphoton absorption properties in covalently linked porphyrin dimers. The electronic structure of the system is described within the multireference single and double configuration interaction (MRDCI) method coupled with the intermediate neglect of differential overlap (INDO) Hamiltonian. We find a strong increase in the two-photon absorption (2PA) and three-photon absorption (3PA) cross sections when going from an isolated porphyrin to the dimers. The nature of the 2PA and 3PA active states as well as the cross sections show a strong but not straightforward dependence on the length of the bridge between the two porphyrins. Our theoretical results are in very good agreement with experimental data for 2PA. The resulting structure to property relationships are analyzed on the basis of essential-state models, where it turns out that a three-state model considering only the Q(x) intermediate state proposed in literature does not provide a full description of the actual situation.

The correction vector method for three-photon absorption: the effects of pi conjugation in extended rylenebis(dicarboximide)s

A correction vector method within the multireference determinant single and double configuration interaction approximation coupled with the semiempirical intermediate neglect of differential overlap Hamiltonian has been developed for the computation of single and multiphoton absorption spectra of conjugated molecules. We study the effect of pi conjugation on these properties in the extended rylenebis(dicarboximide)s. The one-, two-, and three-photon absorption cross sections of the lowest-lying excited states show a power law dependence on the conjugation length, with exponents of about 1.3, 2.6, and 5.6, respectively. The maximum value of the three-photon absorption cross section in these molecules is calculated to be 1.06x10(-78) cm6 s2photon2 for photon energy at 0.57 eV.