A chemically labeled cytotoxic agent: two-photon fluorophore for optical tracking of cellular pathway in chemotherapy

Near-infrared two-photon excited photoluminescence from Yb3+-doped CsPbClxBr3-x perovskite nanocrystals embedded into amphiphilic silica microspheres

Nonlinear absorption of metal-halide perovskite nanocrystals (NCs) makes them an ideal candidate for applications which require multiphoton-excited photoluminescence. By doping perovskite NCs with lanthanides, their emission can be extended into the near-infrared (NIR) spectral region. We demonstrate how the combination of Yb3+ doping and bandgap engineering of cesium lead halide perovskite NCs performed by anion exchange (from Cl- to Br-) leads to efficient and tunable emitters that operate under two-photon excitation in the NIR spectral region. By optimizing the anion composition, Yb3+-doped CsPbClxBr3-x NCs exhibited high values of two-photon absorption cross-section reaching 2.3 × 105 GM, and displayed dual-band emission located both in the visible (407-493 nm) and NIR (985 nm). With a view of practical applications of bio-visualisation in the NIR spectral range, these NCs were embedded into silica microspheres which were further wrapped with amphiphilic polymer shells to ensure their water-compatibility. The resulting microspheres with embedded NCs could be easily dispersed in both toluene and water, while still exhibiting a dual-band emission in visible and NIR under both one- and two-photon excitation conditions.

Physical Mechanisms of Intermolecular Interactions and Cross-Space Charge Transfer in Two-Photon BDBT-TCNB Co-Crystals

Co-crystal materials formed by stacking different molecules with weak interactions are a hot research topic. In this work, we theoretically investigate the intermolecular interactions and charge transfer properties of the supramolecular BDBT-TCNB co-crystal (BTC). The π-π bonds, hydrogen bonds, and S-N bonds in the BTC bind the BDBT and TCNB molecules together to form a highly ordered co-crystal and lead to the co-crystal's excellent two-photon absorption (TPA) properties. The intermolecular interactions of the BTC are discussed in detail by the independent gradient model based on Hirshfeld partition (IGMH), atoms in molecules (AIM), electrostatic overlay diagram, and symmetry-adapted perturbation theory (SAPT) energy decomposition; it is found that there is a strong interaction force along the stacking direction. The charge transfer properties of the one-photon absorption (OPA) and TPA of the BTC were investigated by charge density difference (CDD) and transition density matrix (TDM). It is found that the dominant charge transfer mode is the cross-space charge transfer along the stacking direction. Therefore, strong intermolecular interactions will promote intermolecular cross-space charge transfer. This work is of great significance for the design of organic optoelectronic supramolecular materials.

Upconversion Fluorescence in Naturally Occurring Calcium Fluoride

Fluorine can negatively interfere with leach and smelting processes during mineral processing. Real-time knowledge of the concentration and mineral hosts of fluorine in a mineral processing ore stream is important to protect process line equipment and product. Currently only offline methods of detection are available. Online sensors that determine specific fluorine-bearing mineral concentration in real-time would enable improved efficiency in processing decisions during mine production. Common excitation wavelengths used for fluorescence studies in minerals frequently provide signals that are not clearly host-specific, and hence of limited utility for mineral identification. We show that upconversion fluorescence, a process in which two or more photons are absorbed and one higher-energy photon is emitted, provides a more host-specific fluorescence output, minimizing spurious signals in complex environments and therefore greatly improving detection thresholds. Natural samples of fluorite (CaF₂), a major fluorine host at many mine sites, have been analyzed by near-infrared excitation and have revealed upconversion fluorescence from rare earth inclusions. Upconversion fluorescence was detected in samples with rare earth concentrations as low as one part per million and is therefore considered a potential new sensing modality for real-time fluorite monitoring.

Experimental therapy of doxorubicin resistant human uveal melanoma with targeted cytotoxic luteinizing hormone-releasing hormone analog (AN-152)

BACKGROUND

Cytotoxic analogs of LHRH (luteinizing hormone-releasing hormone) can be successfully used for the treatment of hormone-dependent cancers such as prostatic, ovarian, endometrial, but our knowledge about their effect on hormone-independent cancers such as human uveal melanoma (UM) is limited. Previously, we have demonstrated that 46% of UM express full-length LHRH receptors. This finding has led us to further examine the mechanism of action of LHRH receptor based targeted therapies in this malignancy.

AIMS

In the present study we investigated the cellular uptake of doxorubicin (DOX) and cytotoxic LHRH analog AN-152 (AEZS-108, zoptarelin doxorubicin) on human UM cell lines (OCM3) and its DOX resistant form OCM3_DOX320 by confocal laser scanning microscopy. The LHRH receptor expression was characterized by RT-PCR and immunocytochemistry.

RESULTS

We were able to establish a new, stable and DOX resistant human UM cell line OCM3_DOX320. Our results demonstrated the expression of splice variants and isoforms of receptor for LHRH in OCM3 UM cell line and its doxorubicin resistant form OCM3_DOX320. It has been revealed by MTT assay that AN-152 inhibited cell proliferation in a dose dependent manner in OCM3_DOX320 cells. Furthermore, receptor-mediated uptake of AN-152 was demonstrated using confocal laser scanning microscopy in both cell line.

CONCLUSIONS

Our results suggest that the antiproliferative effect of AN-152 can be detected even if only LHRH receptor isoforms are expressed. Our study also demonstrates the LHRH receptor-mediated uptake of AN-152 in DOX resistant OCM3_DOX320 cells_. Our experiments provide new insights into a potential targeted therapy of UM and give further details about the accumulation of AN-152 in hormone-independent DOX-resistant cells expressing splice variants of the LHRH receptors.

Discovery of LHRH and development of LHRH analogs for prostate cancer treatment

The discovery, isolation, elucidation of structure, synthesis, and initial testing of the neuropeptide hypothalamic luteinizing hormone-releasing hormone (LHRH), which regulates reproduction, is briefly described. The design, synthesis, and experimental and clinical testing of agonistic analogs of LHRH is extensively reviewed focusing on the development of new methods for the treatment of prostate cancer. Subsequent development of antagonistic analogs of LHRH is then faithfully recounted with special emphasis on therapy of prostate cancer and BPH. The concepts of targeted therapy to peptide receptors on tumors are re-examined and the development of the cytotoxic analogs of LHRH and their status is reviewed. The endeavor to develop better therapies for prostate cancer, based on LHRH analogs, guided much of our work.

Targeted cytotoxic analog of luteinizing hormone-releasing hormone (LHRH), AEZS-108 (AN-152), inhibits the growth of DU-145 human castration-resistant prostate cancer in vivo and in vitro through elevating p21 and ROS levels

Management of castration-resistant prostate cancer (CRPC) is challenging due to lack of efficacious therapy. Luteinizing hormone-releasing hormone (LHRH) analogs appear to act directly on cells based on the LHRH receptors on human prostate adenocarcinoma cells. We explored anticancer activity of a cytotoxic analog of LHRH, AEZS-108, consisting of LHRH agonist linked to doxorubicin. Nude mice bearing DU-145 tumors were used to compare antitumor effects of AEZS-108 with its individual constituents or their unconjugated combination. The tumor growth inhibition of conjugate was greatest among treatment groups (90.5% inhibition vs. 41% by [D-Lys(6)]LHRH+DOX). The presence of LHRH receptors on DU-145 cells was confirmed by immunocytochemistry. In vitro, AEZS-108 significantly inhibited cell proliferation (61.2% inhibition) and elevated apoptosis rates (by 46%). By the detection of the inherent doxorubicin fluorescence, unconjugated doxorubicin was seen in the nucleus; the conjugate was perinuclear and at cell membrane. Autophagy, visualized by GFP-tagged p62 reporter, was increased by AEZS-108 (7.9-fold vs. 5.3-fold by DOX+[D-Lys(6)]LHRH. AEZS-108 more effectively increased reactive oxygen species (ROS, 2-fold vs. 1.4-fold by DOX+[D-Lys(6)]LHRH) and levels of the apoptotic regulator p21 in vivo and in vitro. We demonstrate robust inhibitory effects of the targeted cytotoxic LHRH analog, AEZS-108, on LHRHR positive castration-resistant prostate cancer cells.

Introduction of D-phenylalanine enhanced the receptor binding affinities of gonadotropin-releasing hormone peptides

The purpose of this study was to examine whether the introduction of D-Phe could improve the GnRH receptor binding affinities of DOTA-conjugated D-Lys(6)-GnRH peptides. Building upon the construct of DOTA-Ahx-(D-Lys(6)-GnRH1) we previously reported, an aromatic amino acid of D-Phe was inserted either between the DOTA and Ahx or between the Ahx and D-Lys(6) to generate new DOTA-D-Phe-Ahx-(D-Lys(6)-GnRH) or DOTA-Ahx-D-Phe-(D-Lys(6)-GnRH) peptides. Compared to DOTA-Ahx-(D-Lys(6)-GnRH1) (36.1 nM), the introduction of D-Phe improved the GnRH receptor binding affinities of DOTA-D-Phe-Ahx-(D-Lys(6)-GnRH) (16.3 nM) and DOTA-Ahx-D-Phe-(D-Lys(6)-GnRH) (7.6 nM). The tumor targeting and pharmacokinetic properties of (111)In-DOTA-Ahx-D-Phe-(D-Lys(6)-GnRH) was determined in MDA-MB-231 human breast cancer-xenografted nude mice. Compared to (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1), (111)In-DOTA-Ahx-D-Phe-(D-Lys(6)-GnRH) exhibited comparable tumor uptake with faster renal and liver clearance. The MDA-MB-231 human breast cancer-xenografted tumors were clearly visualized by single photon emission computed tomography (SPECT) using (111)In-DOTA-Ahx-D-Phe-(D-Lys(6)-GnRH) as an imaging probe, providing a new insight into the design of new GnRH peptides in the future.

Recent development of two-photon fluorescent probes for bioimaging

Fluorescent probes are essential tools for studying biological systems.

Synthesis and evaluation of novel gonadotropin-releasing hormone receptor-targeting peptides

The purpose of this study was to develop novel radiolabeled gonadotropin-releasing hormone (GnRH) receptor-targeting peptides for breast cancer imaging. Three novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-conjugated GnRH peptides were designed and synthesized. The radiometal chelator DOTA was conjugated to the epsilon or alpha amino group of D-lysine, or the epsilon amino group of L-lysine via an Ahx {aminohexanoic acid} linker to generate DOTA-Ahx-(D-Lys(6)-GnRH1), DOTA-Ahx-(D-Lys(6)-GnRH2) and DOTA-Ahx-(L-Lys(6)-GnRH3), respectively. The conjugation of the DOTA to the epsilon amino group of D-lysine (rather than alpha amino group of D-lysine nor epsilon amino group of L-lysine) maintained the nanomolar GnRH receptor binding affinity. The IC(50) values of DOTA-Ahx-(D-Lys(6)-GnRH1), DOTA-Ahx-(D-Lys(6)-GnRH2) and DOTA-Ahx-(L-Lys(6)-GnRH3) were 36.1 nM, 10.6 mM and 4.3 mM, respectively. Since only DOTA-Ahx-(D-Lys(6)-GnRH1) displayed nanomolar receptor binding affinity, the specific GnRH receptor binding of (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) was determined in human GnRH receptor membrane preparations. Furthermore, the biodistribution and tumor imaging properties of (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) were examined in MDA-MB-231 human breast cancer-xenografted nude mice. (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) exhibited specific GnRH receptor binding and rapid tumor uptake (1.76 ± 0.58% ID/g at 0.5 h postinjection) coupled with fast whole-body clearance through the urinary system. The MDA-MB-231 human breast cancer-xenografted tumor lesions were clearly visualized by single photon emission computed tomography (SPECT)/CT at 1 h postinjection of (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1). The profound impact of DOTA position on the binding affinity of the GnRH peptide provided a new insight into the design of novel radiolabeled GnRH peptides. The successful imaging of MDA-MB-231 human breast cancer-xenografted tumor lesions using (111)In-DOTA-Ahx-(D-Lys(6)-GnRH1) suggested its potential as a novel imaging probe for human breast cancer imaging.

Novel fluorophore based on a multi-substituted olefin skeleton with enhanced three-photon absorption in the femtosecond regime

A new multipolar fluorophore based on a multi-substituted olefin skeleton that possesses strong three-photon absorption and optical-limiting properties in the femtosecond regime has been designed and synthesized; this archetype suggests a new strategy to further optimize molecular structures toward enhanced nonlinear absorptivities based on known materials.

Vinyl-Pyridinium Triphenylamines: Novel Far-Red Emitters with High Photostability and Two-Photon Absorption Properties for Staining DNA

A series of mono-, bis- and trisvinyl-pyridinium triphenylamines (TP-py) has been synthesised and evaluated for its one- and two-photon absorption (2PA) induced-fluorescence properties under biological conditions. Interestingly, these compounds are only weakly fluorescent in water, whereas their fluorescence emissions are strongly restored (exaltation factors of 20-100) upon binding to double-stranded DNA. Additional measurements in glycerol indicate that the fluorescence increases are the result of immobilisation of the dyes in the DNA matrix, which inhibits rotational de-excitation modes. This particular feature is especially remarkable in the case of the bis and tris derivatives (TP-2 py, TP-3 py), which each display a high affinity (K(d) ~ microM) for dsDNA. TPIF measurements have shown that TP-2 py and TP-3 py each have a large 2PA cross section (delta up to 700 GM) both in glycerol and in the presence of DNA, which ranks them amongst the best 2PA biological fluorophores. Finally, one- and two-photon confocal imaging in cells revealed that these compounds perform red staining (lambda(em)=660-680 nm) of nuclear DNA with excellent contrast. The remarkable optical properties of the TP-py series, combined with their high photostability and their easy synthetic access, make these compounds extremely attractive for use in confocal and 2PA microscopy.

Very Large Infrared Two-Photon Absorption Cross Section of Asymmetric Zinc Porphyrin Aggregates: Role of Intermolecular Interaction and Donor−Acceptor Strengths

Very large two-photon absorption (TPA) cross sections at the infrared region have been revealed for J-aggregates of asymmetric zinc porphyrin using quantum-chemical calculation. The TPA properties are evaluated for monomer and aggregates of a series of push-pull porphyrins, whose syntheses are known in the literature. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor. We also present a quantum-chemical analysis on porphyrin aggregates to understand the role of intermolecular interactions and the relationship between structural and collective nonlinear optical properties. It has been observed that the TPA properties change tremendously as monomers undergo J-aggregation and the magnitudes of TPA cross sections are highly dependent on the nature of aggregates. The importance of our results with respect to the design of photonic and photodynamic therapy materials has been discussed.

Organic nonlinear optical materials: where we have been and where we are going

Nonlinear optical (NLO) materials can be useful for a variety of applications varying from modulation of optical signals facilitated by the electro-optic effect-the effect whereby the refractive index of a material changes in response to an applied electric field-to microfabrication, sensing, imaging, and cancer therapy facilitated by multiphoton absorption, wherein molecules simultaneously absorb two or more photons of light. This short Focus article is a brief personal perspective of some of the key advances in second-order NLO materials and in multiphoton-absorbing materials, and of how and why these advances have led to renewed interest in organic NLO materials.

Targeting of cytotoxic luteinizing hormone-releasing hormone analogs to breast, ovarian, endometrial, and prostate cancers

Targeted chemotherapy is a modern approach aimed at increasing the efficacy of systemic chemotherapy and reducing its side effects. The peptide receptors expressed primarily on cancerous cells can serve as targets for a selective destruction of malignant tumors. Binding sites for LHRH (now known in genome and microarray databases as GNRH1), were found on 52% of human breast cancers, about 80% of human ovarian and endometrial cancers, and 86% of human prostatic carcinoma specimens. Because LHRH receptors are not expressed on most normal tissues, they represent a specific target for cancer chemotherapy with antineoplastic agents linked to an LHRH vector molecule. To test the efficacy of targeted chemotherapy based on LHRH analogs, we recently developed a cytotoxic analog of LHRH, designated AN-152, which consists of [D-Lys6]LHRH covalently linked to one of the most widely used chemotherapeutic agents, doxorubicin (DOX). In addition, we designed and synthesized a highly active derivative of DOX, 2-pyrrolino-DOX (AN-201), which is 500-1000 times more potent than DOX in vitro. AN-201 is active against tumors resistant to DOX, and noncardiotoxic. As in the case of DOX, AN-201 was coupled to carrier peptide [D-Lys6]LHRH to form a superactive targeted cytotoxic LHRH analog, AN-207. Both AN-152 and AN-207 can effectively inhibit the growth of LHRH receptor-positive human breast, ovarian, endometrial, and prostate cancers xenografted into nude mice. DOX-containing cytotoxic LHRH analog AN-152 is scheduled for clinical phase I/IIa trials in patients with advanced ovarian and breast cancers in 2005.

Two-Photon Absorption and First Nonlinear Optical Properties of Ionic Octupolar Molecules: Structure−Function Relationships and Solvent Effects

We present a quantum-chemical analysis of the two-photon absorption properties and first hyperpolarizabilities of a series of ionic octupolar molecules and a comparison of their characteristics with corresponding neutral molecules. The molecular geometries are obtained via BL3YP/6-31G (d,p) level optimization including the SCRF/PCM approach, while the dynamic NLO and two-photon absorption properties are calculated with the ZINDO/CV method including solvent effects. The effects of donor or acceptor substitution and elongation of the conjugation path length are established to demonstrate the engineering guidelines for enhancing two-photon absorption cross section and molecular optical nonlinearities. It is found that the chain length dependence of the two-photon absorption and the first nonlinearity follow the same trend, displaying a saturation limit at n = 5. The solvent induced effect on the two-photon absorption and NLO properties are studied using the ZINDO/CV/SCRF method. It has been observed that two-photon absorption and the first nonlinearity peaks at epsilon approximately = 20 and then decreases slightly, approaching saturation. We also compare our theoretical findings with the experimental results wherever available in the literature.

Two-photon microscopy of cells and tissue

Two-photon excitation fluorescence imaging provides thin optical sections from deep within thick, scattering specimens by way of restricting fluorophore excitation (and thus emission) to the focal plane of the microscope. Spatial confinement of two-photon excitation gives rise to several advantages over single-photon confocal microscopy. First, penetration depth of the excitation beam is increased. Second, because out-of-focus fluorescence is never generated, no pinhole is necessary in the detection path of the microscope, resulting in increased fluorescence collection efficiency. Third, two-photon excitation markedly reduces overall photobleaching and photodamage, resulting in extended viability of biological specimens during long-term imaging. Finally, localized excitation can be used for photolysis of caged compounds in femtoliter volumes and for diffusion measurements by two-photon fluorescence photobleaching recovery. This review aims to provide an overview of the use of two-photon excitation microscopy. Selected applications of this technique will illustrate its excellent suitability to assess cellular and subcellular events in intact, strongly scattering tissue. In particular, its capability to resolve differences in calcium dynamics between individual cardiomyocytes deep within intact, buffer-perfused hearts is demonstrated. Potential applications of two-photon laser scanning microscopy as applied to integrative cardiac physiology are pointed out.

Detection of cervical intranodal metastasis in oral cancer using elastic scattering spectroscopy

While histopathology remains the gold standard for tissue diagnosis, several new diagnostic techniques are being developed that rely on physical and biochemical changes that mirror or precede malignant change within tissue. The aim of this study was to compare findings of elastic scattering spectroscopy (ESS) with histopathology on formalin fixed neck dissection specimens to see if this technique could be used as an adjunct or alternative to histopathology in defining nodal involvement. The technique involves the use of Mie scattering and is a simple non-invasive method of tissue interrogation. One hundred and thirty lymph nodes were examined from 13 patients who underwent neck dissection. The nodes were formalin fixed, bivalved and examined by ESS using a pulsed xenon lamp. The intensity of the spectrum at 4 points was considered for comparison; at 360, 450, 630 and 690 nm. The nodes were then routinely processed and haematoxylin and eosin-stained sections examined histopathologically, and the results compared. Using this technique, a sensitivity of 98% and a specificity of 68% were obtained.

One- and two-photon absorption of three-coordinate compounds with different centers (B,Al,N) and a 2,2'-dipyridylnitrogen functional group

A series of three-coordinate octupolar compounds with varied centers (boron, aluminum, and nitrogen), which exhibit very large effective two-photon absorption cross sections have been theoretically studied. The ground state geometries and electronic structures are obtained using the density functional theory with the B3LYP functional and 6-31G(d) basis set, and the results are comparable to the available experimental determinations. Based on the correct geometrical and electronic structures, the one- and two-photon absorptions are predicted by the ZINDO-SOS method. Among these compounds, the boron (B) and aluminum (Al) centers act as acceptors, while the nitrogen center acts as donor according to the net charge changes during the excitation. It is found that (i) the compounds with boron and aluminum centers show two large two-photon absorption peaks, while the molecule with nitrogen center show only one two-photon absorption peak; (ii) the cross sections of the molecules with B or Al as centers are larger than that of the molecule with nitrogen as center; furthermore, the two-photon absorption cross section of the molecule with Al center is larger than that of the molecule with B center, from this point of view, our theoretical prediction provides for the experiment a good new candidate with large two-photon absorption cross section for further research; (iii) lengthening the conjugation bridge by inserting a benzene ring on the organoborane compounds (forming the investigated molecule B-2) enhances the two-photon absorption cross section, and keeping good transparency at the same time.

Internalization of cytotoxic analog AN-152 of luteinizing hormone-releasing hormone induces apoptosis in human endometrial and ovarian cancer cell lines independent of multidrug resistance-1 (MDR-1) system

OBJECTIVE

Eighty percent of human ovarian and endometrial cancers express receptors for luteinizing hormone-releasing hormone (LHRH-R). These receptors can be used for targeted chemotherapy with agents such as AN-152, in which doxorubicin is linked to analog [D-Lys(6)]-LHRH. Direct receptor-mediated antiproliferative effects of AN-152 have been shown in vitro and in vivo. In LHRH-R positive cell lines, AN-152 was more effective than doxorubicin at equimolar concentrations. This study was designed to investigate the mechanism of action of AN-512 in ovarian and endometrial cancer cells in vitro. Study design Three ovarian (SKOV-3, NIH:OVCAR-3, EFO-21) and 2 endometrial carcinoma cell lines (Ishikawa, HEC-1A) were evaluated for doxorubicin- or AN-152-induced apoptosis. Internalization and cytoplasmic release of AN-152 was monitored by confocal laser scanning microscopy and inhibited by chloroquine. Cleavage of doxorubicin from AN-152 was inhibited by carboxylesterase inhibitor, diisopropyl fluorophosphate (DFP). The surface expression of multidrug resistance-1 (MDR-1) gene product P-glycoprotein (Pgp) was measured by flow cytometry.

RESULTS

Induction of apoptosis by AN-152 in LHRH-R positive Ishikawa, HEC-1A, EFO-21, and NIH:OVCAR-3 cells was significantly higher than that induced by doxorubicin, whereas the percentage of apoptotic cells in LHRH-R negative SKOV-3 was higher after treatment with doxorubicin. In EFO-21 cells, apoptosis induced by AN-152 was inhibited by pretreatment with chloroquine. Pretreatment with DFP increased AN-152-induced apoptosis in LHRH-R positive cells and reduced apoptosis in LHRH-R negative SKOV-3. Both AN-152 and doxorubicin induced surface expression of MDR-1 gene product Pgp, but the effect of AN-152 was smaller than that of doxorubicin. Pgp surface expression induced by AN-152 was inhibited by pretreatment with DFP.

CONCLUSION

AN-152 is internalized through the LHRH-R and induces apoptosis in LHRH-R-positive human ovarian and endometrial cancer cell lines without activating the MDR-1 efflux pump system. The efficacy and specificity of AN-152 is inversely correlated with carboxylesterase activity.

Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn(2+) or Cu(+) as coordinate centers and 4,4'-bis(dibutylaminostyryl)-[2,2']-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn(2+) is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu(+) only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn(2+) as the center with that of Cu(+) as the center, it is found that the complex with the Cu(+) center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn(2+), both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.

Degenerate two-photon-absorption spectral studies of highly two-photon active organic chromophores

Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption spectra on the scale of absorbed photon(s) energy. Moreover, for some sample (such as AF-380), the TPA spectrum is totally different from the linear spectrum, which implies the difference of molecular transition pathways and selection rules for one- and two-photon excitation processes. At high excitation intensity levels (>or=15 GW/cm(2)), the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.

Combinatorial discovery of two-photon photoremovable protecting groups

A design principle for a two-photon photochemically removable protecting group based on sequential one-photon processes has been established. The expected performance of such groups in spatially directed photoactivation/photodeprotection has been shown by a kinetic analysis. One particular molecular class fitting into this design, the nitrobenzyl ethers of o-hydroxycinnamates, has been presented. An initial demonstration of two-photon deprotection of one such group prompted further optimization with respect to photochemical deprotection rate. This was accomplished by the preparation and screening of a 135-member indexed combinatorial library. Optimum performance for lambda >350 nm deprotection in organic solvent was found with 4,5-dialkoxy and -cyano substitution in the nitrobenzyl group and 4-methoxy substitution in the cinnamate.