Hong-Qiang Wang

Spirooxazine molecular switches with nonlinear optical responses as selective cation sensors

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. The switching properties are particularly large when various metal ions (Li+, Na+, K+, Mg2+, Ca2+, Fe2+, Zn2+, and Ag+) are introduced, as evidenced by density functional theory calculations, which show that the spirooxazine undergoes a pronounced change in geometry accompanied by formation of a larger π-conjugated system. The resultant merocyanine derivatives have 10–21-fold higher static second-order NLO responses. Spirooxazine can therefore be used as a powerful and multi-use detection tool. The large first hyperpolarizability (βtot) is shown to rely on the alkaline earth metal, causing βtot values to increase nearly 21-fold, as evidenced by the larger charge distribution, lower transition energy, and separate distribution of first hyperpolarizability density. In contrast, variation of βtot in the Fe2+ derivative is not obvious, owing to stronger complexation, a larger amount of charge transferred from the napthoxazine moiety to the metal, and the reduction in N⋯O distance between the ligand heads. Therefore, spiropyran-to-merocyanine molecular switching can be used to distinguish alkaline earth metals and determine the efficiency of cation detection.

Second-order nonlinear optical responses of carboranyl-substituted indole/indoline derivatives: impact of different substituents.

Carborane has been the subject of great interest over the last decades due to its high structural, chemical, biological stability and diverse applications. In the present work, carboranyl-substituted indole/indoline compounds and their functionalized derivatives have been systematically investigated by density functional theory (DFT) method with the view of assessing their electronic structures and first hyperpolarizabilities. Significantly, the first hyperpolarizabilities can be obviously enhanced by the introduction of a strong electron-withdrawing group for closed-ring forms, while the strong electron-donating group is beneficial for large first hyperpolarizabilities for open-ring forms. It indicates that the NLO properties of these compounds can be enhanced by controlling their relative substituent groups. Furthermore, the time-dependent DFT calculation illustrates that the enhancement of the first hyperpolarizabilities are found due to the obvious charge transfer (CT) transition, and closed-ring forms have a significant difference on the CT patterns versus open-ring ones. Investigation of the structure-property relationship and substituent effects at the molecular level can benefit for further exploration of carboranyl-substituted indole/indoline derivatives with versatile and fascinating NLO properties.

The novel link between planar möbius aromatic and third order nonlinear optical properties of metal–bridged polycyclic complexes

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.

A cation-selective and anion-controlled benzothiazolyl-attached macrocycle for NLO-based cation detection: variational first hyperpolarizabilities

Recently, a benzothiazolyl group bearing the NO2S2-macrocycle L has been the subject of great interest due to its unique advantages in binding metal cations. Here, we systematically investigate the second-order nonlinear optical (NLO) properties of L, its metal cation derivatives L*M (M = Na+, K+, Mg2+, Ca2+, Zn2+, Cd2+, and Hg2+), and some anion-controlled complexes [Hg(L)(ClO4)2 and Hg(L)(Cl)2] by density functional theory (DFT). The results show that the addition of the transition metal cation Hg2+ leads to the increasing transition energy of the S0 → S1 transition, which induces a significant decrement in the NLO response (βtot = 8.2 × 10−30 esu) with respect to the corresponding macrocycle L (βtot = 125.8 × 10−30 esu). The depolarization ratio (DR) and anisotropy parameter (ρ) render it possible to control the NLO contribution. Further analysis of the charge transfer (CT) parameters, transferred electrons (qCT), CT distance (dCT), and t index indicates that L*Hg2+ reaches the minimum CT and spatial charge separation. Thus, the transition metal cation Hg2+ is distinguishable and easier to detect by utilizing the variations in the NLO response. Furthermore, the addition of ClO4− to the complex L*Hg2+ forms a distorted pentagonal bipyramid configuration, which drives the significant reduction of the first hyperpolarizability. Therefore, the obvious NLO contrasts can be switched by different metal cations and anions, which will broaden the scope of optical molecular detectors and further inspire us to investigate it.