Jing-Fu Guo

Color-tuning mechanism in firefly luminescence: theoretical studies on fluorescence of oxyluciferin in aqueous solution using time dependent density functional theory.

The first singlet excited state geometries of various isomers and tautomers of firefly oxyluciferin (OxyLH2), as well as their fluorescence spectra in aqueous solution, were studied using time dependent density functional theory (TDDFT). With changing pH in aqueous solution, three fluorescence peaks, blue (450 nm), yellow-green(560 nm), and red (620 nm) correspond to neutral keto and enolic forms, the monoanionic enolic form,and the monocationic keto form respectively. A counterion, Na+, was predicted to cause a blue shift in the fluorescence of anionic OxyLH2. The contributions of a charge transfer (CT) state upon electronic excitation of the planar and twisted structures were predicted. CT was large for the twisted structures but small for the planar ones. The differences between pK and pK* of various oxyluciferin species were predicted using a Forster cycle. A new possible light emitter, namely, the monocation keto form (keto+1), was considered.

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.

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.

Theoretical prediction of one- and two-photon absorption properties of N-annulated quaterrylenes as near-infrared chromophores.

Graphene nanoribbons (GNRs) have attracted increasing attention due to high potentiality in nanoelectronics. In the present study, quantum-chemical calculations of structural and nonlinear optical properties have been first carried out for the nanoelectronical materials, a new series of ladder-type N-annulated quaterrylenes and their imide chromophores. The effects of the solvent, terminal groups, the number of N-annulated bridges, and π-conjugated length are discussed in detail. The solvent effect is significant on the one-photon absorption (OPA). Moreover, the OPA and two-photon absorption (TPA) properties of the two series of DI and N-MI molecules show a clear solvent dependence, which is attributed to the carboximide substitution featuring larger polarization. Introducing electron-donating groups and dicarboximides and increasing the conjugated length lead to red-shifts of the OPA, emission, and TPA spectra, lower emission lifetimes, and enhanced TPA cross sections (δ(max)), but further extension of the conjugated framework does not always promote an increase of δ(max). The changing trends of δ(max) can be explained by the transition moment and the intramolecular charge transfer. All N-annulated quaterrylene and their imide derivatives possess small energy gaps, intense near-infrared absorption and emission, and large δ(max), which are important for use as two-photon fluorescent labeling materials.

A Theoretical Investigation of Two Typical Two-Photon pH Fluorescent Probes

Intracellular pH plays an important role in many cellular events, such as cell growth, endocytosis, cell adhesion and so on. Some pH fluorescent probes have been reported, but most of them are one‐photon fluorescent probes, studies about two‐photon fluorescent probes are very rare. In this work, the geometrical structure, electronic structure and one‐photon properties of a series of two‐photon pH fluorescent probes have been theoretically studied by using density functional theory (DFT) method. Their two‐photon absorption (TPA) properties are calculated using the method of ZINDO/sum‐over‐states method. Two types of two‐photon pH fluorescent probes have been investigated by theoretical methods. The mechanisms of the Photoinduced Charge Transfer (PCT) probes and the Photoinduced Electron Transfer (PET) probes are verified specifically. Some designed strategies of good two‐photon pH fluorescent probes are suggested on the basis of the investigated results of two mechanisms. For the PCT probes, substituting a stronger electron‐donating group for the terminal methoxyl group is an advisable choice to increase the TPA cross section. For the PET probes, the TPA cross sections increase upon protonation.

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.

Computational design of two-photon fluorescent probes for intracellular free zinc ions.

Two-photon fluorescence probes used in two-photon fluorescence microscopy (TPM) can achieve intact tissue imaging without destruction. Therefore, for a long time, TPM has been an important tool in biology and medicine. In this background via a quantum chemical method, a series of zinc ion probe molecules using N,N-di(2-picolyl)ethylenediamine (DPEN) as the recognition group were studied, which are based on the photoinduced electron transfer (PET) mechanism. The fact that the one-photon absorption peak is almost unchanged and the fluorescence emission intensity increased significantly upon coordination with a zinc ion reveals that these probes can be PET fluorescent bioimaging reagents. And it is predicted that when the chemically modified probe molecule is incorporated with Zn(2+), the two-photon absorption (TPA) cross-section (δmax) will greatly increase and the TPA peak will be in the near-infrared region. The molecules after changing the fluorophore become more suitable for probing Zn(2+) in vivo, and a modification at the end of the fluorophore can fine-tune the fluorescence and TPA properties. The detailed investigations will provide a theoretical basis for synthesizing new zinc-ion-responsive two-photon fluorescent probes.

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.