Recently, three 9,9-dimethylxanthene-based donor (D)/ acceptor (A) U-shaped space-through architectures, containing π−π intramolecular interactions between the D and A, exhibit unique advantage (i.e., a small singlet (S 1 ) − triplet (T 1 ) energy splitting (ΔE ST )) in thermally activated delayed fluorescence (TADF). To explore the TADF and second-order nonlinear optical (NLO) properties of U-shape compounds with through-space charge transfer (TSCT) between aligned D and A units compared with that of conventional conjugated D−A (L shape) ones, we theoretically investigated the geometric and electronic structures, through space D−A π−π interactions, CT properties, ΔE ST , and first hyperpolarizabilities (β tot ) of compounds 1-L∼5-U. The calculated ΔE ST values of the U-shaped molecules are relatively smaller than that of Lshaped compounds in gas phase, indicating that the U-shaped derivatives are excellent thermally activated delayed fluorescent candidates. Furthermore, a noteworthy finding was that the conjugated D−A unit of L-shaped compounds was suggested to promote the performance in NLO due to the lower excited energy and stronger oscillator strength for the crucial excited state. Especially, for compound 2-L, the β tot value is 8 times larger than that of 2-U in gas phase. In addition, we have quantitatively studied ΔE ST and β tot values in the solid-state polarization for all studied molecules using the polarizable continuum model. Importantly, the results of polarization effects (ε from 1.0 to 3.0) show that the marked reduction in the ΔE ST values of Ushaped derivatives are due to the simultaneous presence of dominant 1 TSCT and 3 TSCT excited states in the solid-state polarization, which are favorable for TADF materials. In addition, the increment in the β tot values of L-shaped compounds are preferable for NLO applications. We hope this work may provide a theoretical understanding on the influence of the heteroatom and the π−π conjugation between D and A units and polarization effects on the ΔE ST and β tot and novel design mentality of the efficiency-enhancing TADF and NLO materials.

The nonlinear optical (NLO) and thermally activated delayed fluorescence (TADF) properties of organic push–pull materials consisting of π-conjugated electron-donating (D) and electron-accepting (A) subunits are dominated by the interaction of D and A moieties via intramolecular charge transfer (ICT). Understanding the structure–property relationship, at the microscopic level, is the prerequisite for further performance optimization or improvement. In this work, we theoretically investigated the geometric and electronic structures, CT properties, polarizabilities (α), first hyperpolarizabilities (βtot), and singlet–triplet energy gap (ΔE ST) of the homoconjugation (as type I) and the conventional conjugation D–A (as type II) compounds. A noteworthy finding was that the type II molecule was suggested to promote the performance in NLO due to the lower excited energy and larger dipole moment variations for the crucial excited state, as well as the larger separate distributions of first hyperpolarizability density. In addition, the electron transition properties, second-order NLO responses, and ΔE ST values strongly depend on the nature of different electron acceptors (pyrazine → dicyanopyrazine → dicyanoquinoxaline). Further, based on the polarizable continuum model analysis, the increment in the βtot of all studied compounds is preferable for NLO applications. Moreover, the ΔE ST values of the molecules in which the acceptor are replaced by dicyanopyrazine/dicyanoquinoxaline (2, 3, and 6) in vacuum are reduced by an order of magnitude when embedded in a polarizable environment, indicating they are potentially efficient TADF materials. Overall, we envision that the various architectures and the polarization effect introduced in the present work will offer a route toward the rational design of such kind of D–A system for novel functional second-order NLO and TADF materials.

The high modularity of multicomponent photoactive BODIPY/carborane/diketopyrrolopyrrole hybrid chromophores was explored for linear/nonlinear optical applications.

Metal–bridged polcyclic aromatic complexes, exhibiting unusual optical effects such as near-infrared photoluminescence with particularly large Stokes shifts, long lifetimes and aggregation enhancement, have been established as unique “carbonloong chemistry”. Herein, the electronic structures, aromaticities, absorption spectra and third order nonlinear optical (NLO) responses of metal–bridged polcyclic aromatic complexes (M = Fe, Re, Os and Ir) are investigated using the density functional theory computations. It is found that the bridge–head metal can stabilize and influence rings, thus creating π–, σ– and metalla–aromaticity in an extended, π–conjugated framework. Interestingly, metal radius greatly influence the bond, aromaticity, liner and third order NLO properties, which reveals useful information to develop new applications of metal regulatory mechanism in NLO materials field. Significantly, the novel relationship between the aromaticity and third order NLO response has firstly been proposed, that the metal-bridged polycyclic complex with larger aromaticity will exhibit larger third order nonlinear optical response. It is our expectation that the novel link between aromaticity and NLO response could provide valuable information for scientists to develop the potential NLO materials on the basis of metal–bridged polycyclic complexes.

Spirooxazine, a photochromic material, can transform into metallic open-form merocyanine by molecular switching, giving rise to large contrasts in its second-order nonlinear optical (NLO) properties.