Andong Xia

Micro lens fabrication by means of femtosecond two photon photopolymerization

We report the fabrication of micro lens using an alternative annular scanning mode with continuous variable layer thickness by two-photon polymerization after multi-parameter optimization. Laser scanning mode and scanning pace parameter are optimized to achieve good appearance. As examples of the results, a 2 x 2 micro spherical lens array with diameter of 15 microm and a micro Fresnel lens with diameter of 17 microm are fabricated. Their optical performances are also tested. Compared to the conventional femtosecond two-photon fabrication, this work provides an alternative, effective and cheap processing method for the fabrication of micro optic device that requires arbitrary shape with high surface quality and small scale.

Manipulating Aggregation and Molecular Orientation in All‐Polymer Photovoltaic Cells

Manipulating molecular orientation at the donor/acceptor interface is the key to boosting charge separation properties and efficiencies of anisotropic-materials-based organic photovoltaics (OPVs). By replacing the polymeric donor PBDTBDD with its 2D-conjugated polymer PBDTBDD-T, the power conversion efficiency of OPVs featuring the anisotropic polymer acceptor PNDI is drastically boosted from 2.4% up to 5.8%.

Synergetic Integration of Cu1.94S-ZnxCd1-xS Heteronanorods for Enhanced Visible-Light-Driven Photocatalytic Hydrogen Production.

In this Communication, we present the integration of synergetic designs into high-quality, well-defined Cu1.94S-ZnxCd1-xS heteronanorods (0 ≤ x ≤ 1) for enhanced photocatalytic hydrogen evolution. These heteronanorods possess two light absorbers, intimate heterointerfaces, tunable band gaps over a wide range, and uniform one-dimensional morphology. As verified by experimental and density functional theory studies, these heteronanorods with continuous composition adjustment fully exploit the benefits of both interfacial charge separation and optimized band alignments. Even without any cocatalysts, Cu1.94S-Zn0.23Cd0.77S heteronanorods exhibit efficient hydrogen production activity (7735 μmol h(-1) g(-1)) under visible-light irradiation (λ > 420 nm), representing a 59-fold enhancement compared with the pristine CdS catalyst. Meanwhile, deposition of a Pt cocatalyst on the Cu1.94S-ZnxCd1-xS surface substantially enhances the hydrogen production performance (13 533 μmol h(-1) g(-1)) with an apparent quantum efficiency of 26.4% at 420 nm, opening up opportunities to promote the overall photocatalytic performance using rationally designed nanostructures.

Photophysical properties of rhodamine isomers: A two-photon excited fluorescent sensor for trivalent chromium cation (Cr3+)

We introduce a new rhodamine-based fluorescent chemosensor, FD8 which exhibits a distinct two-photon excited fluorescence (TPEF) on/off characteristic upon binding Cr(3+) ions. By coordination with metal cation, conformation of FD8 changes from spirocyclic to open-ring, resulting in remarkable enhancement of absorption and fluorescence both in one- and two-photon excitations. As a result, a 29-fold enhancement of two-photon excited fluorescent intensity was observed when 10 eq. Cr(3+) was added to the FD8 solution. The detection limit of Cr(3+) cation concentration down to 1 microM (0.01 eq. of FD8) was achieved under our experimental condition. Besides the excitation within ultraviolet regime by fluorescence resonance energy transfer (FRET) mechanism, the TPEF on/off behavior further extends the excitation to near infrared regime (the biological optimal window of 700-1200 nm), and shows more effective sensitivity. The broad excitation wavelength, on/off fluorescence and high selectivity to Cr(3+) enable FD8 to be a powerful Cr(3+) cation sensor with potential application, especially in biological detection. To the best of our knowledge, this is the first report about two-photon fluorescent sensor for Cr(3+) ions.

Crystal Structure and Optical Properties of the [Ag62S12(SBut)32]2+ Nanocluster with a Complete Face-Centered Cubic Kernel

The crystal structure of the [Ag62S12(SBu(t))32](2+) nanocluster (denoted as NC-I) has been successfully determined, and it shows a complete face-centered-cubic (FCC) Ag14 core structure with a Ag48(SBu(t))32 shell configuration interconnected by 12 sulfide ions, which is similar to the [Ag62S13(SBu(t))32](4+) structure (denoted as NC-II for short) reported by Wang. Interestingly, NC-I exhibits prominent differences in the optical properties in comparison with the case of the NC-II nanocluster. We employed femtosecond transient absorption spectroscopy to further identify the differences between the two nanoclusters. The results show that the quenching of photoluminescence in NC-I in comparison to that of NC-II is caused by the free valence electrons, which dramatically change the ligand to metal charge transfer (LMCT, S 3p → Ag 5s). To get further insight into these, we carried out time-dependent density functional theory (TDDFT) calculations on the electronic structure and optical absorption spectra of NC-I and NC-II. These findings offer a new insight into the structure and property evolution of silver cluster materials.

Two-photon excitation studies of hypocrellins for photodynamic therapy

The photophysical and photochemical properties of hypocrellins (HA and HB) are examined with two-photon excitations at 800 nm using femtosecond pulses from a Ti:sapphire laser. The two-photon excited fluorescence spectra of HA and HB are very similar to those obtained by one-photon excitation, which may indicate that the two-photon induced photodynamic processes of hypocrellins are similar to one-photon induced photodynamic processes. The two-photon excitation cross sections of HA and HB are measured at 800 nm as about 34.8 x 10(-50) cm(4) s/photon and 21.3 x 10(-50) cm(4) s/photon, respectively. The large two-photon cross sections of both HA and HB, suggest that the hypocrellins can be potential two-photon phototherapeutic agents. As an example for two-photon photodynamic therapy of hypocrellins, we also further examine the cell-damaging effects of HA upon two-photon illumination. Our preliminary results of cell viability test indicate hypocrellins can effectively damage the Hela cells under two-photon illumination.

Photophysical Properties of Intramolecular Charge Transfer in Two Newly Synthesized Tribranched Donor−π−Acceptor Chromophores

The nature of optical excitation and the degree of intramolecular charge transfer (ICT) as well as the dynamics of excited ICT states of two new tribranched donor-pi-acceptor molecules with acceptor-terminated (DA(3)) and acceptor-centered (AD(3)) geometries have been investigated by steady-state and femtosecond time-resolved stimulated emission fluorescence depletion (FS TR-SEP FD) measurements in different polar solvents. The interpretation of the experimental results is based on the comparative investigation of the two D-pi-A compounds with respect to the model monomer counterpart (DA). The larger solvatochromic effects and stronger solvent dependence of spectral properties of DA(3) than that of AD(3) indicate that the excited ICT state of DA(3) possesses higher polarity and larger dipole moments compared to those of AD(3). The similarity of absorption and strong solvent-dependent fluorescence spectra of DA(3) and DA reveals that the excited-state properties of DA(3) are identical to that of the model DA, which localized on one of the branches in DA(3). In contrast to DA(3), the large red shift in the absorption and the small Stokes shift of AD(3) suggest the formation of a delocalized ICT state to a certain extent in the excited state of AD(3). The dynamic behavior of excited ICT states for all three compounds are also investigated by femtosecond time-resolved stimulated emission depletion (FS TR-SEP FD) measurements, where the excited-state relaxations are highly dependent on both solvent polarity and the polar degree of the excited ICT states. Furthermore, the steady-state fluorescence excitation anisotropy shows that the intramolecular excitation transfer among the three disorder-induced localized ICT states with nondegenerate transition dipole moments is involved within DA(3). Compared to DA(3), a substantial red shift in the absorption of AD(3) results from the formation of a delocalized ICT state, where the specific excitation anisotropy spectrum shows that the excitation energy is mainly redistributed between the localized ICT state and the delocalized ICT state.

Gigantic Two-Photon Absorption Cross Sections and Strong Two-Photon Excited Fluorescence in Pyrene Core Dendrimers with Fluorene/Carbazole as Dendrons and Acetylene as Linkages

We report a series of stiff dendrimers (referred to as T1, T2, T3, and T4) that have both gigantic two-photon absorption (TPA) cross sections up to 25,000 GM and strong two-photon excited fluorescence (TPEF) with fluorescence quantum yield of ∼0.5. The large TPA cross sections and high quantum yields of these dendrimers are directly related to their geometrical structures, where the polycyclic aromatic pyrene is chosen as the chromophoric core because of its planar and highly π-conjugated structure, fluorene moieties as dendrons extend the conjugation length through the planar structure, and carbazole moieties are modified at three-, six-, and nine-positions as electron donor. All of these groups are linked with acetylene linkage for effective π-electron delocalization, leading to large TPA cross section and high fluorescence quantum yield. The spectral properties of all dendrimers are investigated by one- and two-photon excitations. Furthermore, steady-state fluorescence excitation anisotropy and quantum chemical calculation are also employed to determine the structure-related mechanism of these dendrimers with gigantic TPA cross sections and high TPEF efficiency. We then show that the improvement of branched chains in the T-series dendrimers enhances the light-harvesting ability. The core emission spectra, fluorescence quantum yield, and fluorescence lifetime are almost invariable by directly exciting the dendrons. These results will provide a guideline for the design of useful two-photon materials with structural motifs that can enhance the TPA cross-section and fluorescence quantum yield of a molecule without causing a red shift of the one- and two-photon excitation wavelengths for specific applications.

Intramolecular Charge Transfer and Solvation Dynamics of Thiolate-Protected Au20(SR)16 Clusters Studied by Ultrafast Measurement.

It is accepted that the monolayer ligand shell in monolayer-protected gold nanoclusters (MPCs) plays an important role in stabilizing the metal core structure. Previous reports have shown that the core and shell do not interact chemically, and very few studies investigating the intramolecular charge transfer (ICT) between the core and ligand shell in clusters have been reported. The underlying excited state relaxation mechanisms about the influence of solvents, the optically excited vibration, and the roles of the core and shell in charge transfer remain unknown to a large extent. Here we report a femtosecond transient absorption study of a Au20(SR)16 (R = CH2CH2Ph) cluster in toluene and tetrahydrofuran. The ICT from the outside shell to the inside core upon excitation in Au20(SR)16 is identified. The observed solvation-dependent oscillations in different solvents further confirm the photoinduced ICT formation in Au20(SR)16. The results provide a fundamental understanding of the structure-property relationships about the solvation-dependent core-shell interaction in Au MPCs.

Localized Emitting State and Energy Transfer Properties of Quadrupolar Chromophores and (Multi)Branched Derivatives

In order to better understand the nature of intramolecular charge and energy transfer in multibranched molecules, we have synthesized and studied the photophysical properties of a monomer quadrupolar chromophore with donor-acceptor-donor (D-A-D) electronic push-pull structure, together with its V-shaped dimer and star-shaped trimers. The comparison of steady-state absorption spectra and fluorescence excitation anisotropy spectra of these chromophores show evidence of weak interaction (such as charge and energy transfer) among the branches. Moreover, similar fluorescence and solvation behavior of monomer and branched chromophores (dimer and trimer) implies that the interaction among the branches is not strong enough to make a significant distinction between these molecules, due to the weak interaction and intrinsic structural disorder in branched molecules. Furthermore, the interaction between the branches can be enhanced by inserting π bridge spacers (-C═C- or -C≡C-) between the core donor and the acceptor. This improvement leads to a remarkable enhancement of two-photon cross-sections, indicating that the interbranch interaction results in the amplification of transition dipole moments between ground states and excited states. The interpretations of the observed photophysical properties are further supported by theoretical investigation, which reveal that the changes of the transition dipole moments of the branched quadrupolar chromophores play a critical role in observed the two-photon absorption (2PA) cross-section for an intramolecular charge transfer (ICT) state interaction in the multibranched quadrupolar chromophores.