Lu-Yi Zou

Theoretical study on photophysical properties of ambipolar spirobifluorene derivatives as efficient blue-light-emitting materials.

The aim of this work is to provide an in-depth interpretation of the optical and electronic properties of a series of spirobifluorene derivatives. These materials show great potential for application in organic light-emitting diodes as efficient blue-light-emitting materials due to the tuning of the optical and electronic properties by the use of different electron donors (D) and electron acceptors (A). The geometric and electronic structures of the molecules in the ground state are studied with density functional theory (DFT) and ab initio HF, whereas the lowest singlet excited states are optimized by ab initio CIS. The energies of the lowest singlet excited states are calculated by employing time-dependent density functional theory (TD-DFT). The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, electron affinities, reorganization energies, and exciton binding energies for these complexes are affected by different D and A moieties. Also, it has obtained that these blue-light-emitting materials have improved charge transport rate and charge transfer balance performance and can be used as efficient ambipolar-transporting materials in organic light-emitting diodes.

Theoretical investigation on the origin of yellow-green firefly bioluminescence by time-dependent density functional theory.

The question whether the emitter of yellow-green firefly bioluminescence is the enol or keto-constrained form of oxyluciferin (OxyLH(2)) still has no definitive answer from experiment or theory. In this study, Arg220, His247, adenosine monophosphate (AMP), Water324, Phe249, Gly343, and Ser349, which make the dominant contributions to color tuning of the fluorescence, are selected to simulate the luciferase (Luc) environment and thus elucidate the origin of firefly bioluminescence. Their respective and compositive effects on OxyLH(2) are considered and the electronic absorption and emission spectra are investigated with B3LYP, B3PW91, and PBE1KCIS methods. Comparing the respective effects in the gas and aqueous phases revealed that the emission transition is prohibited in the gas phase but allowed in the aqueous phase. For the compositive effects, the optimized geometry shows that OxyLH(2) exists in the keto(-1) form when Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 are all included in the model. Furthermore, the emission maximum wavelength of keto(-1)+Arg+His+AMP+H(2)O+Phe+Gly+Ser is close to the experimental value (560 nm). We conclude that the keto(-1) form of OxyLH(2) is a possible emitter which can produce yellow-green bioluminescence because of the compositive effects of Arg220, His247, AMP, Water324, Phe249, Gly343, and Ser349 in the luciferase environment. Moreover, AMP may be involved in enolization of the keto(-1) form of OxyLH(2). Water324 is indispensable with respect to the environmental factors around luciferin (LH(2)).

An efficient strategy for designing n-type organic semiconductor materials—introducing a six-membered imide ring into aromatic diimides

The aromatic diimides are among the most promising and versatile candidates for organic optoelectronic materials due to their commercial availability, low cost, excellent optical and electric performance, such as naphthalene, anthracene and perylene diimides. But, so far, the problem is not clarified—is a five- or six-membered imide ring more helpful for n-type organic semiconductor materials? The work investigated in detail and compared various properties for molecules with a five-/six-membered imide ring from the following aspects: (1) molecular stability, reaction activity, geometries, frontier molecular orbitals as well as oxidation and reduction abilities at the single-molecule level; (2) the variation of transfer integrals at the various molecular stacking motifs; (3) the estimate of carrier mobility and its anisotropy for the actual molecule crystals. The results indicate that molecules with a six-membered imide ring should be more suitable for n-type organic semiconductor materials.

Design, Synthesis, and Optoelectronic Properties of Dendrimeric Pt(II) Complexes and Their Ability to Inhibit Intermolecular Interaction

Dendrimeric Pt(II) complexes [(C(∧)N)Pt(dpm)] and [Pt(C(∧)N)2] (Hdpm = dipivaloylmethane, HC(∧)N = 1,2-diphenylbenzoimidazole and its derivatives containing the carbazole dendrons) have been synthesized and characterized systematically. All of the complexes display green emission in the range of 495-535 nm that originated from the 360-440 nm absorption bands, which are assigned to dπ(Pt)→π*(L) metal-to-ligand charge transfer (MLCT) mixed with intraligand π(L)→π*(L) transition. Solution photoluminescence quantum yield (φp 0.26-0.31) of the heteroleptic complexes [(C(∧)N)Pt(dpm)] obviously increases when compared with that of complex [(C(∧)N)Pt(acac)]. Organic light-emitting diode devices based on these Pt(II) complexes with a multilayer configuration were fabricated and gave desirable electroluminescent (EL) performances, such as non- or less red-shifted EL spectra, in comparison with the photoluminescence spectra and slow efficiency roll-off with increasing brightness or current density. Complex [(t-BuCzCzPBI)Pt(dpm)] (where t-BuCzCzPBI = 1-(4-(3,6-di-(3,6-di-t-butyl-carbazol-9-yl))carbazol-9-yl)phenyl-2-phenylbenzoimidazole) showed the best performance, with a maximum current efficiency of 29.31 cd/A and a maximum external quantum efficiency (EQE) of 9.04% among the fabricated devices. Likewise, for homoleptic [Pt(t-BuCzCzPBI)2] dendrimer, the powder φp (0.14) and maximum EQE (0.74%) improve by 7 and 7.4 times, respectively, as high as they do for nondendrimeric [Pt(1,2-diphenylbenzoimidazole)2] (0.02, 0.10%), although its efficiency is still lower than that of the heteroleptic counterpart due to the severely distorted square-planar geometry of the emitting core. These results reveal that large steric hindrance from ancillary ligand (dpm) or the homoleptic conformation can effectively inhibit intermolecular interaction for these dendrimeric Pt(II) complexes.

Computational Design of Two-Photon Fluorescent Probes for a Zinc Ion Based on a Salen Ligand

A two-photon fluorescent probe has become a critical tool in biology and medicine owing to its capability of imaging intact tissue for a long period of time, such as in two-photon fluorescence microscopy (TPM). In this context, a series of Salen-based zinc-ion bioimaging reagents that were designed based on an intramolecular charge-transfer mechanism were studied through the quantum-chemical method. The increase of one-photon absorption and fluorescence emission wavelength and the reduction of the oscillator strength upon coordination with a zinc ion reveal that they are fluorescent bioimaging reagents used for ratiometric detection. When the Salen ligand is incorporated with Zn(2+), the value of the two-photon absorption (TPA) cross-section (δmax) will decrease, and most of the ligands and complexes exhibit a TPA peak in the near-infrared spectral region. That is, a substituent at the end of the ligand can influence the luminescence property, besides increasing solubility. In addition, the effect of an end-substituted position on the TPA property was considered, such as ortho and meta substitution. The detailed investigations will provide a theoretical basis to synthesize zinc-ion-responsive two-photon fluorescent bioimaging reagents as powerful tools for TPM and biological detection in vivo.

Are triphenylamine-functionalized or carbazole-functionalized iridium complexes the more effective phosphorescent materials? A theoretical perspective.

The ground and excited states, charge injection/transport, and phosphorescence properties of eleven carbazole- and triphenylamine-functionalized Ir(III) complexes were investigated by using the DFT method. By analyzing the spin-orbit coupling (SOC) matrix elements, radiative decay rate constants k(r), and the electronic structures and energies at the S₀(opt) and T₁(opt) states, it was possible to rationalize the order of the experimental phosphorescence quantum yields of a series of Ir(III) complexes and to predict that [Ir(Nph-2-Cz-tz)3] has a higher phosphorescence quantum yield than [Ir(TPA-tz)3] (TPA=triphenylamine, tz=thiazolyl, Cz=carbazole, Nph=N-phenyl). Carbazole-functionalized Ir(III) complexes were shown to be efficient phosphorescent materials that have not only fast but also balanced electron/hole-transport performance as well as high phosphorescence quantum yields. The phosphorescence emission spectra can be modulated by modifying or replacing a pyridyl substituent.

Computational design of a two-photon excited FRET-based ratiometric fluorescent Cu2+ probe for living cell imaging

Though copper ion (Cu2+) is a widely distributed pollutant in the water environment, it plays a vital role in many biological processes. Hence, rapid detection and identification of Cu2+ are important. In the past few years, fluorescence sensing has become the golden standard to detect Cu2+, due to its high sensitivity, high selectivity, and useful applications in biology, medicine, environment and chemistry. Thus, researchers have widely concerned with the design and synthesis of Cu2+ fluorescent probes. In this study, a novel probe 2a with high sensitivity and selectivity for detecting Cu2+ is designed. It is illustrated that 2a is a ratiometric fluorescent probe, which recognizes Cu2+ by a Forster resonance energy transfer (FRET) mechanism. Meanwhile, the two-photon absorption (TPA) optical properties of 2a are calculated. The calculated results demonstrate that 2a possesses a higher energy transfer efficiency upon excitation and a larger TPA peak in the near-infrared region than others. Therefore, it can be inferred that the probe 2a should be an excellent two-photon (TP) excited FRET-based ratiometric fluorescent probe for Cu2+. The detailed investigations can provide a theoretical basis to synthesize copper-ion-responsive TP FRET-based ratiometric fluorescent reagents, which are powerful tools for the two-photon microscopy (TPM) and biological imaging of Cu2+ in vivo.

The effect of micro-environment on luminescence of aequorin: The role of amino acids and explicit water molecules on spectroscopic properties of coelenteramide

Despite the fact that the luminescence reaction mechanism of aequorin has been intensively investigated, details in luminescence such as the effect of important amino acids residues and explicit water molecules on spectroscopic properties of coelenteramide remain unclear. In this work, the effect of amino acids residues His16, Tyr82, Trp86, Phe113, Trp129, Tyr132, explicit water molecules Wat505 and Wat405 on the spectral properties of CLM(-) has been studied by CAM-B3LYP, TD M06L and TD CAM-B3LYP methods in hydrophobic environment and aqueous solution. In hydrophobic environment, the amino acids or water molecules have no significant effect on the absorption. Tyr82 and Trp86 move close to CLM(-) changes the hydrogen bond network, and thus, the spectral properties is significantly affected by the hydrogen bonds between His16H(+)+Tyr82+Trp86 and CLM(-). Tyr82, Trp86 hydrogen bonding to CLM(-) upshifts the excited energy and helps emission spectra shift to blue region. Therefore, it is concluded that His16H(+)+Tyr82+Trp86 modify the emission spectra. The molecular electrostatic potential indicated that the greater electron density is located at the oxygen atom of 6-p-hydroxyphenyl group of CLM(-), and it facilitates the formation of hydrogen bond with His16H(+)+Tyr82+Trp86. It is a critical condition for the modification of emission spectra. It is expected to help to understand the interactions between emitter and amino acids in the micro environment.

Theoretical investigation on the one- and two-photon responsive behavior of fluoride ion probes based on diketopyrrolopyrrole and its π-expanded derivatives

Because of the special physiological and chemical properties of the fluoride ion (F−) and its important roles in the environment and living organisms, a series of novel F− probes based on 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) and its π-expanded derivatives have been investigated in detail in this study. Their electronic structures and one-photon absorption (OPA) are investigated by employing the density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Moreover, their two-photon absorption (TPA) properties have also been further investigated by using quadratic response theory. Our calculated results reveal that the modifications of the compounds by both increasing the number of electron-donating groups (thiophene groups) and introducing fluorene units can effectively enhance the TPA responses, making the probes, such as DPP3, DPP4 and DPP5, show relatively higher TPA cross sections (δTPA) in the range of 5380.0–9500.0 GM in the near-infrared region (885.6–991.9 nm). Additionally, the strategy of modifying the compounds by using fluorene (DPP6), aza-BODIPY (DPP7) and BODIPY (DPP8) moieties as π-expanded central structures, respectively, is proved ideal for obtaining large δTPA values at longer wavelengths, especially for DPP7 and DPP8, whose δTPA values are about 2–3 times larger than that of the related DPP-Fs, which is superior to the previously reported two-photon F− probes because large δTPA differences between probes and the reaction products are highly desirable for practical two-photon F− detection in biological systems.

A theoretical study of a series of novel two-photon nitric oxide (NO) fluorescent probes based on BODIPY

In this work, a series of novel nitric oxide (NO) probes and the corresponding reaction products are designed based on boron dipyrromethene (BODIPY) and heteroaryl-fused BODIPY or 3,5-distyryl substituted BODIPY (KFL). Furthermore, the mechanism of recognizing NO controlled by intramolecular photoinduced electron transfer (PET) is verified by theoretical chemistry computation in this work. More importantly, the two-photon absorption properties of these novel chromophores are explored by using DALTON program. The results of our study show that the two-photon absorption cross sections of the designed molecules are as large as 1056.9–39702.5 GM with the wavelengths ranging from 700 to 850 nm, especially for KFLs based on the 3,5-distyryl substituted BODIPY core, which have more potential for applications in two-photon absorption fluorescence imaging with larger two-photon absorption cross sections in the near-infrared region, because of their better rigidity and π-conjugation that are more conducive to intramolecular charge transfer. Finally, this work presents structure modification strategies for increasing two-photon response. We hope the study can provide helpful information for further investigating two-photon NO probes.