Second-Order Nonlinear Optical Response of Electron Donor–Acceptor Hybrids Formed between Corannulene and Metallofullerenes

A charge transfer (CT) complex was formed between C₂₀H₁₀ and Li⁺@C₆₀ in the ground state by the concave–convex π–π CT interaction. Herein, the structures, binding interactions, electronic absorption spectra, and first hyperpolarizabilities of a series of Li⁺ and Li atom in contact with C₆₀ have been explored with density functional theory methods. It is found that independent of the doping position, doping Li atom can significantly narrow the wide gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) (<i>E</i>_gap = 2.82 eV) of the pure C₂₀H₁₀/C₆₀ in the range of 0.86–0.99 eV. Among them, the Li-doped outer isomer C₂₀H₁₀/LiC₆₀ can exhibit the intriguing n-type characteristic, where a high energy level containing the excess electron is introduced as the new HOMO orbital in the original gap of C₂₀H₁₀/C₆₀. Furthermore, the diffuse excess electron also brings C₂₀H₁₀/C₆₀ the considerable first hyperpolarizability, which is 3.53 × 10^–29 esu. When Li⁺ and Li were encapsulated into the C₆₀ cage, inner complexes C₂₀H₁₀/Li⁺@C₆₀ and C₂₀H₁₀/Li@C₆₀ of enhanced static first hyperpolarizabilities (5.39 and 2.13 × 10^–29 esu, respectively) are also delivered due to that encapsulated Li⁺@C₆₀ and Li@C₆₀ show enhanced electron acceptability as compared to pristine C₆₀, leading to more obvious intermolecular CT transitions. From a certain point of view, such systems can be considered as high-performance NLO materials that combine the basic characteristics of a classical donor–acceptor superstructure and systems with cations and easily polarizable excess electrons

Impact of Redox Stimuli on Ferrocene–Buckybowl Complexes: Switchable Optoelectronic and Nonlinear Optical Properties

A series of redox-active complexes <b>1</b>–<b>3</b> (1,1′-diquadrannulenylferrocene, 1,1′-dicorannulenylferrocene, 1,1′-disumanenylferrocene) and their corresponding conformational isomers that are composed of ferrocene donor and various acceptors of different buckybowl (open bowl-shaped polyaromatic hydrocarbon) subunits have been investigated by density functional theory. The nature of the redox property makes it possible to develop novel examples of chromophores that are amenable to molecular switches, which can therefore be used in optical devices. The complexes <b>1</b>–<b>3</b>, with high thermal and chemical stabilities, show π–π stacking interaction between two buckybowl subunits, and support for the presence of significant donor–acceptor interaction was obtained from the employment of the natural bond orbital charge and charge decomposition analysis. Both one-electron oxidation and one- and two-electron reduction have been considered. The results show some important electronic structure changes upon oxidation/reduction that are accompanied by significant differences in the corresponding absorption spectra and second-order nonlinear optical properties. These differences are due to a change in the charge transfer pattern. The redox switch ability suggests that these ferrocene–buckybowl complexes can be viewed as redox-triggered nonlinear optical switches with one of the complexes having an on/off ratio of 100.2 for the hyperpolarizability values. Thus, our work has helped to establish ferrocene–buckybowl complexes as versatile and fascinating nonlinear optical switching compounds with a promising future

Second-Order Nonlinear Optical Responses and Concave–Convex Interactions of Size-Selective Fullerenes/Corannulene Recognition Pairs: The Effect of Fullerene Size

The size-selective formation of eight molecular recognition pairs between the host corannulene and fullerene guest of various size have been studied by taking advantage of concave–convex π–π interactions. Herein, the structures, binding interactions, electronic absorption spectra, and first hyperpolarizabilities have been explored using density functional theory calculations. It is found that with the aim to maximize the concave–convex shape complementarity, the base of fullerene can be modified at certain angles with the central ring plane of C₂₀H₁₀ (0° for complexes <b>1</b>–<b>4</b> and 46°, 34°, 19°, and 5° for complexes <b>5</b>–<b>8</b>, respectively). The interaction energies depend linearly on the convex surface area and the size of the fullerene sphere. Further, the results of first hyperpolarizabilities show that the shape of the fullerene is the dominant factor for complexes <b>1</b>–<b>4</b> because of the intramolecular charge transfer (CT) within fullerene cage. Among them, complex <b>4</b> presents the largest β_tot value as 5.64 × 10^–30 esu because of the more obvious intramolecular CT from the upper part to the bottom part of the C₇₀ cage, derived from the larger height of the cage. On the other hand, low-lying CT character accounts for a large part of the first hyperpolarizability. The achieved understanding provides the prospect of size-selective strategy for enhancing the concave–convex interaction and second order nonlinear optical response in the recognition of fullerenes

Large Nonlinear Optical Responses of Dimers Bearing a Donor and Acceptor: Long, Intradimer Multicenter Bonding

Unusual long, multicenter dimers bearing a large electron donor and acceptor have been the subject of great interest over the last decades due to their better conducting, superconducting, magnetic, or other physical properties. Two-electron, multicenter bonding between two interplanar fragments has been recently recognized as a novel and important bonding interaction. Herein, the [TTF]­[TCNE], [TTF]­[TCNQ], [TTF]­[TCNP], and [TTF]­[TCNB] dimeric species have been studied by quantum mechanics methods with the view of assessing their interactions and first hyperpolarizabilities. It is found that the stabilities of the dimers primarily originate from the electrostatic bonding component. Although to a lesser extent, long, multicenter interactions due to the overlap of the molecular orbitals of the monomers are important also in the stability of these systems. Significantly, a severe hyperpolarizability decrease with changing the acceptor monomers of the dimeric species is qualitatively explained in terms of change in their charge transfer patterns. It indicates that the first hyperpolarizabilities of these dimers can be optimized by controlling their relative acceptor monomers. We believe that these results shall provide important information for further exploration of long, multicenter dimers with versatile and fascinating nonlinear optical properties

Strategy for Enhancing Second-Order Nonlinear Optical Properties of the Pt(II) Dithienylethene Complexes: Substituent Effect, π‑Conjugated Influence, and Photoisomerization Switch

The second-order nonlinear optical (NLO) properties of a series of Pt­(II) dithienylethene (DTE) complexes possessing the reversible photochromic behavior have been investigated by density functional theory (DFT) combined with the analytic derivatives method. The results show that the calculated static first hyperpolarizabilities (β_tot) of the open-ring and closed-ring systems significantly increase in the range of 2.1–4.5 times through strengthening of the electron-withdrawing ability of the substituent R (R = H, CF₃, NO₂) and an increase of the number of thiophene rings. Moreover, there is a large enhancement of the β_tot values from the open-ring systems to the corresponding closed-ring systems. This efficient enhancement is attributed to the better delocalization of the π-electron system, the more obvious degree of charge transfer, and the larger <i>f</i>_os/<i>E</i>_gm³ (<i>f</i>_os is the oscillator strength, and <i>E</i>_gm is the transition energy between the ground and the excited states) values in the closed forms according to the bond length alternation (BLA) and time-dependent density functional theory (TDDFT) calculations. In addition, the dispersion has less influence on the frequency-dependent first hyperpolarizabilities (β_tot(ω)) of the studied systems at the low-frequency area ω (0.000–0.040 au). Our present work would be beneficial for further theoretical and experimental studies on large second-order NLO responses of metal complexes