Search

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.

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.

The present study focuses on a series of carbazole derivatives, which are being investigated as potential two-photon fluorescent probes (TFP) for DNA detection and two-photon bioimaging. The geometric structures, electronic structures, and the one-photon (OPA) and two-photon (TPA) absorption properties of 3,6-bis(1-methyl-4-vinylpyridinium) carbazole (BMVC) derivatives, as well as their dications and diiodized derivatives, were studied using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) method. The results showed that the TPA spectra of the diiodized BMVC derivatives and their dications are all found in the near-infrared region (NIR). At the same time, the diiodized BMVC derivatives presented larger TPA cross-sections than the neutral BMVC derivatives and the dications. These theoretically derived values were also in good agreement with their corresponding experimentally observed values, and they indicated that the diiodized BMVC may be the form of this TFP that combines with DNA. The diiodized BMVC derivatives and the dications have the potential to be excellent TPA materials, especially when used as TFPs in nucleic acid imaging applications.

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.

Special attention has been paid to understanding the structural effect on electronic structure and absorption spectra for an extensive series of functionalized aza-BODIPY molecules. We have employed the quadratic response theory as well as a sum-over-states approach involving few intermediate states to calculate the two-photon cross section (δmax). The results suggest that chemical modifications on the aza-BODIPY core and peripheral moieties using various substituents can finely tune their linear and nonlinear optical properties. Therefore, some new fluorophores absorbing in the near infrared region and featuring considerably high δmax at telecommunication wavelengths are proposed, which are excellent candidates for nonlinear transmission and fluorescent labeling materials. The investigation contributes a useful starting point for further design of more effective aza-BODIPY dyes and can be valuable as a foundation for future experimental research and development.

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.

Two important classes of organic molecules, perylene diimide (PDI) and pyrene derivatives have been found to possess relatively excellent photophysical and photochemical properties and especially high two-photon absorption cross sections (δ(T)(max)). Herein, one-photon absorption (OPA) and two-photon absorption (TPA) properties of some novel PDI and pyrene derivatives were comparatively investigated by the density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The calculated results indicate that with respect to PDI derivatives, the maximum TPA cross-sections for pyrene compounds increase obviously, the maximum peaks of OPA and TPA spectra are blue-shifted, the ΔE(H-L) (energy gaps between the highest occupied orbital and the lowest unoccupied orbital) increase. The different π-conjugated bridges (fluorene and pyrene) and terminal groups have slight effect on the OPA properties. Nevertheless, the molecules bearing 1,6-disubstituted pyrene as the π-conjugated bridge display the largest δ(T)(max) in both series of compounds 3 and 4. Moreover, the δ(T)(max) values of molecules with benzothiazole-substituted terminal groups are larger than those of the molecules with diphenylamine, which is attributed to benzothiazole groups stabilizing the planarity of the branch parts, extending the conjugated length and increasing the π-electron delocalized extent. Furthermore, the molecular size has marked effect on OPA and TPA properties. It is worthy to mention that cruciform 8 displays the largest δ(T)(max) among all the studied molecules in the range of 600-1100 nm. This research could provide a better understanding for the origin of the linear and nonlinear optical properties, and it would be helpful to gain more information about designing two-photon absorption materials with large δ(T)(max).

The equilibrium geometries, electronic structures, one-photon absorption (OPA) and two-photon absorption (TPA) properties of starburst 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM) derivatives have been studied by using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) program. Results showed that increasing the number of branches in DCM molecules and introducing stronger electron withdrawing groups are efficient ways to enhance the TPA cross sections. For the two- and three-branched molecules of the DCM derivatives, the solvent effect on the OPA and TPA properties was also considered by adapting the self-consistent reaction field (SCRF) theory with the polarizable continuum model (PCM) to model the interaction with the solvent in ZINDO program. The calculated molecular structure, transition dipole moment and intramolecular charge transfer (ICT) in different solvents have been analyzed to approximate how the solvent affects the TPA cross section. Results indicated that the TPA cross section increases as the polarity of the solvents increases, and it reaches a maximum when the dielectric constant epsilon equals 20.7.

The molecular equilibrium structures, electronic structures, and one- and two-photon absorption (TPA) properties of C2v (Zn(II), Fe(II) and Cu(I)) dipolar and D2d (Zn(II) and Cu(I)) and D3 (Zn(II)) octupolar metal complexes featuring different functionalized bipyridyl ligands have been studied by the ZINDO-SOS method. The calculated results show that one- and two-photon absorption properties of metal complexes are strongly influenced by the nature of the ligands (donor end groups and pi linkers) and metal ions as well as by the symmetry of the complexes. The length of the pi-conjugated backbone, the Lewis acidity of the metal ions, and the increase of ligand-to-metal ratio result in a substantial enhancement of the TPA cross sections of metal complexes. Substitution of C=N and N=N for C=C plays an important role in altering the maximum TPA wavelengths and the maximum TPA cross sections of metal complexes. Of them, the C=N substituted metal complexes have relatively large TPA cross sections. Replacing styryl with thienylvinyl makes the one-photon absorption wavelength red shift and at the same time leads to a great decrease of the maximum TPA cross sections of metal complexes. The possible reason is discussed. In the range 500-1250 nm, octupolar metal complexes exhibit intense TPAs and therefore are promising candidates for TPA materials.

N-arylpyrrole-based dyes possessing excellent opto-electronic properties are promising candidates for two-photon fluorescence labeling materials. The systematic investigation of novel N-arylpyrrole derivatives is of great importance for both fine-tuning electronic spectra and designing two-photon absorption (2PA) materials. We thoroughly studied influences of the π-conjugated center and N-substituted pyrrole moieties on the linear optical and 2PA properties. Our results show that introducing N-arylpyrrole produces bathochromic-shifts of the absorption and emission bands and an enhancement of the 2PA cross section (δ(max)), demonstrating that the electron-rich pyrrole moieties can efficiently increase intramolecular charge transfer. Substitution of fluorenyl with benzothiadiazole produces the lower energy gap, higher emission lifetime, red-shift of 2PA spectra and larger δ(max). The absorption and emission bands are highly dependent on the nature of the active building blocks. The aromatic rings attached to pyrrole can modulate the absorption peaks in the high energy and thus subtly modify the electronic properties.

The geometrical structure, electronic structure, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the perylene tetracarboxylic derivatives (PTCDs) were studied theoretically by using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The results revealed that increasing the number of naphthalene nucleus, extending the conjugated length on long axis, increasing the strength of donor group on lateral side, decreasing the DeltaE(H-L) (energy gap between the highest occupied orbital and the lowest unoccupied orbital) and keeping the conjugation effect and inductive effect along the same molecular axis are the efficient ways to enlarge TPA cross section of PTCDs compounds. The results that PTCDs compounds exhibited extremely large TPA cross section of around 800-1100 nm (near infrared region) shed light into the significance of the PTCDs compounds for applications in TPA labeling materials in vivo.