Ultrafast energy migration in chromophore shell-metal nanoparticle assemblies

A multifunctional ligand-coated nanoparticle system containing approximately 2000 highly two-photon absorptive chromophores has been investigated by means of steady-state and femtosecond time-resolved spectroscopy. This system with a high local concentration of chromophores showed remarkably low self-quenching and a high fluorescence quantum yield, which is important for a variety of two-photon sensing and imaging applications. We have observed evidence for ultrafast energy migration in these chromophore shell-metal nanoparticle systems. Time-resolved experiments also showed non-zero residual anisotropy after the initial fast decay, which can be interpreted as due to the formation of the specific domains on the metal surfaces. This investigation opens new avenues toward the development of multi-chromophoric efficient TPA fluorescence sensing/imaging systems with large numbers of chromophores per one metal particle nanoparticle.     

Enhanced two-photon absorption and ultrafast dynamics of a new multibranched chromophore with a dibenzothiophene core

A series of novel compounds with dibenzothiophene core branched structures have been synthesized, and their two-photon absorption (TPA) properties were investigated. Two-photon fluorescence (TPF) and z-scan techniques were carried out, and a significant enhancement in the TPA cross section was observed for ST-G2, which possesses the largest generation number among the studied samples. By using different solvents, the largest nonlinear optical (NLO) response was observed in the most polar solvent. Ultrafast pump-probe experiments were performed to probe the excited state dynamics in the branched molecules, and the obtained results further confirmed the TPA enhancement mechanism. Time-resolved fluorescence (TRFL) and TRFL anisotropy measurements reveal that there is an ultrafast charge localization to the intramolecular charge transfer (ICT) state followed by relaxation with a lifetime longer than 1 ns.     

氧芴三苯胺多枝分子的双光子吸收与电化学行为

研究了3个氧芴/三苯胺衍生物: E-2,8-双(4-二苯胺基苯乙烯基)氧芴(简称OT-G1)、E-2,8-双[4-(二苯基氨基-二苯乙烯基)(4’-溴苯基)氨基-苯乙烯基]氧芴(简称OT-G1.5)和E-2,8-双-[4’,4″-二-(二苯胺基苯乙烯基)-4-二苯胺基苯乙烯基]氧芴(简称OT-G2)的双光子吸收和电化学行为. 研究结果表明, 分子“代数”从1→1.5→2增高, 氧芴三苯胺多枝分子的HOMO能级升高、双光子荧光强度和双光子吸收截面明显增大. 由于HOMO能级的升高有利于分子的电荷转移, 因而分子表现出强的双光子吸收能力, 这表明可通过电化学行为来推断出分子的双光子吸收性能.     

Hidden electronic excited state of enhanced green fluorescent protein

Precise two-photon absorption spectra of the green fluorescent protein (GFP) and the mutants sapphire-GFP (T203I) and enhanced GFP (S65T/F64L), as well as a model compound for the chromophore, 4'-hydroxybenzylidene-2,3-dimethylimidazolinone (HBDI) were measured by multiplex two-photon absorption spectroscopy. The observed TPA bands of the anionic forms of enhanced GFP and HBDI were significantly shifted to the higher energy compared with the lowest-energy bands in one-photon absorption spectra. This result indicated the existence of a hidden electronic excited state in the vicinity of the lowest excited singlet (S1) state of the anionic form of the GFP chromophore, which is the origin of the blue shift of the two-photon absorption spectra as well as two-photon fluorescence excitation spectra.     

Two-photon excitation of substituted enediynes

Electronic spectroscopy of nine benzannelated enediynes and a related fulvene was studied under one-photon and two-photon excitation conditions. We utilize measured absorbance and emission spectra and time-resolved fluorescence decays of these molecules to calculate their radiative lifetimes and fluorescence quantum yields. The fluorescence quantum yields for the other compounds were referenced to the fluorescence quantum yield of compound 3 and used to determine relative two-photon absorption cross-sections. Further insight into experimental studies has been achieved using time-dependent density functional (TD-DFT) computations. The probability of two-photon absorption (TPA) increases noticeably for excitation to the higher excited states. The photophysical properties of benzannelated enediynes are sensitive to substitutions at both the core and the periphery of the enediyne chromophore. Considerably enhanced two-photon absorption is observed in an enediyne with donor substitution in the middle and acceptor substitution at the termini. Excited states with B symmetry are not active in TPA spectra. From a practical point of view, this study extends the range of wavelengths applicable for activation of the enediyne moiety from 350 to 600 nm and provides a rational basis for future studies in this field. Our theoretical computations confirmed that lowest energy TPA in benzannelated enediynes involves different orbitals than lowest energy one-photon absorbance and provided further support to the notion that introduction of donor and acceptor substituents at different ends of a molecule increases TPA.     

Photophysical properties of octupolar chromophore based on triazine core and fluorene divinylene conjugated bridge

A novel octupolar chromophore with 1,3,5-triazine as core,2,7-divinylene-9,9-dimethylfluorene as extendedπ-conjugated bridge,triarylamine as the electron-donating end-groups was successfully synthesized and characterized.Their linear photophysical and two-photon absorption(TPA) properties were investigated by UV absorption,excited fluorescence(SPEF) spectra and nonlinear transmission method,respectively.The absorption cut-off of the chromophore is below 520 nm and it has stronger fluorescence emission in a nonpolar solvent.In addition,the chromophore exhibits larger TPA cross-section(226.0 GM) in the femtosecond regime at 800 nm.     

Enhancement of two-photon absorption cross-section in macrocyclic thiophenes with cavities in the nanometer regime

The linear and nonlinear optical properties of two thiophene-based cyclic molecules have been investigated. These molecules represent nanometer sized cavities which may be useful for novel photonic devices. By virtue of long-range interactions, these chromophores serve as novel architectures for enhanced two-photon absorption (TPA) properties. Measurements of the different size ring structures showed a 550% increase in the TPA cross-section for the larger macrocycle. Electronic structure calculations have suggested an increase in coupling of the excited states in these systems as the ring size is increased. Measurements of the ultrafast transient absorption and fluorescence were carried out with these systems in order to probe the interaction between the chromophores. The results of the transient decays as well as fluorescence anisotropy decay times gives stronger proof to the suggestion of delocalized states in the cyclic macrocycles. These results provide information regarding the optical properties of these novel systems useful for potential applications in photonics.     

以N为耦合中心多枝分子的双光子上转换荧光

采用非线性透过率法研究了以N为耦合中心的多枝化合物N-[4-{2-(3,5-二-[5-(4-叔丁基苯基)-1,3,4-噁二唑-2-]苯基}-1-乙烯基}苯基}-N,N-二苯胺(BPODPA), N,N-双[4-{2-(3,5-二-[5-(4-叔丁基苯基)-1,3,4-噁二唑-2-]苯基}-1-乙烯基}苯基}-N-苯胺(BBPOPA)和N,N,N-三[4-{2-(3,5-二-[5-(4-叔丁基苯基)-1,3,4-噁二唑-2]苯基}-1-乙烯基}苯基}胺(TBPOA)的双光子吸收性质, 测定了化合物的单光子荧光光谱和双光子上转换荧光光谱, 研究了多枝化对三苯胺分子双光子吸收和双光子激发荧光性质的影响.在800 nm波长的激光激发下,化合物BPODPA、BBPOPA和TBPOA在二氯甲烷溶液中发出很强的蓝绿色双光子上转换荧光, 荧光峰分别位于502、515 和518 nm. 这些多枝结构化合物的双光子吸收截面较大, 双光子吸收增强来源于多枝分子中扩展的π共轭体系和重复单元的协同效应.     

Photophysical properties of core-modified expanded porphyrins: nature of aromaticity and enhancement of ring planarity

We have investigated the excited-state dynamics and nonlinear optical properties of representative core-modified expanded porphyrins, tetrathiarubyrin, tetraselenarubyrin, pentathiaheptaphyrin, tetrathiaoctaphyrin, and tetraselenaoctaphyrin, containing 26, 30, and 34 pi electrons using steady-state and time-resolved absorption and fluorescence spectroscopic measurements along with femtosecond Z-scan method, with a particular attention to the photophysical properties related to molecular planarity and aromaticity. Core-modification of macrocycles by sulfur and selenium leads to NIR-extended steady-state absorption and fluorescence spectra and short-lived excited-state due to the heavy-atom effect in time-resolved spectroscopic experiments. Large negative nucleus-independent chemical shift values ranging from -13 to -15 ppm indicate that all molecular systems are highly aromatic. The observed enhancement of two-photon absorption cross-section values over 10 (4) GM for core-modified hepta- and octaphyrins is mainly attributable to their rigid and planar structures as well as their aromaticity. Overall, the observed spectroscopic and theoretical results consistently demonstrate the enhanced molecular planarity of core-modified expanded porphyrins compared with their corresponding all-aza expanded porphyrins.     

新型双光子吸收化合物的合成、光物理及其在固态分散体系中的性质

通过多步反应制备了三个新型的双光子吸收化合物,测试了它们的光物理性质,发现该系列化合物具有较大的双光子吸收截面,并且随温度降低其双光子吸收截面增大.尤其是化合物TMVDB,其量子效率为0.83,常温时的双光子吸收截面达到1164GM,其双光子吸收截面除以分子量为2.08,表现出了优良的双光子吸收发光性能.进一步将TMVDB掺杂到固体材料中,得到强双光子诱导发光的固体材料.     

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.     

Strong two photon absorption and photophysical properties of symmetrical chromophores with electron accepting edge substituents

Two photon absorption (TPA) and photophysical properties of three new symmetrical chromophores with electron accepting phthalimide edge substituents have been studied. The three chromophores contain fluorene, alcoxy-substituted divinyl benzene, and carbazole moieties as central cores, respectively. The femtosecond time-resolved fluorescence upconversion spectroscopy and two photon excited fluorescence technique have been carried out. The effect of solvent polarity on TPA and on photophysics has also been determined. Ultrafast fluorescence dynamics, with decay times ranging from 1 to 13 ps, are revealed in polar solvents. This is attributed to the relaxation of the chromophores to the intramolecular charge transfer state. The chromophore bearing fluorene central core, being of the type A-pi-A, is the most efficient concerning TPA. Strong TPA, with a cross section value as high as 2100 GM at an excitation wavelength of 770 nm is found in acetophenone which is a solvent of intermediate polarity. The TPA spectra were also reproduced using a sum over states three-state model. A study of the TPA induced photobleaching of the fluorene molecule, doped in a solid poly(methyl-methacrylate) film, has shown that this material is very promising for efficient TPA optical data storage.     

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.     

Early aggregation in prion peptide nanostructures investigated by nonlinear and ultrafast time-resolved fluorescence spectroscopy

We report the characterization of early aggregates in the self-assembly of prion peptides using nonlinear and ultrafast time-resolved fluorescence spectroscopy. The dye-labeled peptide and dye/peptide guest-host systems were used to demonstrate the feasibility of the new approach. By measuring the two-photon absorption cross-section, small aggregates of the dye labeled peptide were characterized. Ultrafast time-resolved fluorescence anisotropy spectroscopy reveals the packing state (microenvironment) of the probes to be tightly associated with aggregates and associated with aggregation progression of the peptides. Fluorescence intensity decay shows a correlation with growth of aggregates having a high level of structured beta-sheet content. A new binding ligand Cascade Yellow shows promise for beta-sheet recognition of prion peptide nanostructures. These findings may have implications for in vivo studies of neurotoxic aggregates targeting with fluorescence markers. Also, these results may provide insight into molecular design of peptide-based nanomaterials.     

Two-photon transitions in quadrupolar and branched chromophores: experiment and theory

A combined experimental and theoretical study is conducted on a series of model compounds in order to assess the combined role of branching and charge symmetry on absorption, photoluminescence, and two-photon absorption (TPA) properties. The main issue of this study is to examine how branching of quadrupolar chomophores can lead to different consequences as compared to branching of dipolar chromophores. Hence, three structurally related pi-conjugated quadrupolar chromophores symmetrically substituted with donor end groups and one branched structure built from the assembly of three quadrupolar branches via a common donor moiety are used as model compounds. Their photophysical properties are studied using UV-vis spectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments using femtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoretical calculations. The applied theoretical methodology is based on time-dependent density functional theory, the Frenkel exciton model, and analysis in terms of the natural transition orbitals of relevant electronic states. Theory reveals that a symmetrical intramolecular charge transfer from the terminal donating groups to the middle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branching via a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches. Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfer character upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited state leads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to what was earlier observed when dipolar chromophores are branched via the same common electron-donating moiety, we find only a slight enhancement of the maximum TPA response of the branched compound with respect to an additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectral shape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetry caused by branching.     

An insight into a novel class of self-assembled porphyrins: geometric structure, electronic structure, one- and two-photon absorption properties

We have theoretically investigated a series of butadiyne-linked porphyrin derivatives that exhibit large two-photon absorption (TPA) cross sections in the visible-IR range. The electronic structure, one-photon absorption (OPA), and TPA properties have been studied in detail. We found that the introduction of a butadiyne linkage and the increase of the molecular dimensionality from monomer to dimer determine the OPA intensities of Q band and Soret band, respectively. A most important role for the enhancement of the TPA cross section is played by introducing a butadiyne bridge. The complementary coordination and the combination of the terminal free base and the core zinc porphyrin are also two effective factors for the enhancement of the TPA efficiency. The dimer with two porphyrins linked at meso-positions by a butadiyne linkage results in a maximum TPA cross section (79.35 x 10(-48) cm4 s per photon). Our theoretical findings are consistent with the recent experimental observations. This series of porphyrin derivatives as promising TPA materials are the subject of further investigation.     

含1,3,5-三嗪和三苯胺的二苯乙烯类双光子材料

合成了以三苯胺为电子给体、1,3,5-三嗪为电子受体的新型二苯乙烯类化合物．用吸收光谱、荧光光谱、飞行质谱、核磁共振氢谱和碳谱进行了表征。这些化合物具有大的双光子吸收截面和强的频率上转换荧光,其中,由三个D-π-A结构的发色团形成的三枝状八偶极分子具有最大的双光子吸收截面和最强的双光子荧光。     

One- and two-photon absorption and emission properties of a Zn(II) chemosensor

This paper presents the synthesis and two photon-induced absorption (TPA) properties of a functionalized distyrylbenzene (DSB) 1 containing a tetra-azacyclododecane (cyclen) receptor for Zn(II). The influence of Zn(II) on one- and two-photon absorption characteristics of 1 has been investigated in dimethyl sulfoxide. The experiments show that the TPA action spectrum of uncomplexed 1, at 750 nm employing nanosecond-long excitation pulses, is 5 times more intense than that of the complexed form. This moderate contrast between the bound and unbound species confirms, however, the potential of this design scheme for the development of molecular structures with enhanced sensitivity and contrast to be used as Zn(II) sensors through TPA-induced fluorescence microscopy.     

Relationship between two-photon absorption and the pi-conjugation pathway in porphyrin arrays through dihedral angle control

Recently, covalently linked or self-assembled porphyrin array systems have attracted much attention for their enhanced two-photon absorption (TPA) behaviors. In this study, we have investigated the TPA properties of various dihedral angle controlled, directly linked porphyrin dimers and arrays to elucidate the relationship between the pi-conjugation pathway and TPA properties. We have demonstrated a strong correlation between pi-conjugation (aromaticity) and TPA properties in porphyrin assemblies.     

Benzothiazoles with tunable electron-withdrawing strength and reverse polarity: a route to triphenylamine-based chromophores with enhanced two-photon absorption

A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure-property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δ(TPA)) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ～500-2300 GM in the near-infrared region (740-810 nm); thereby the molecular weight normalized δ(TPA)/MW values of the best performing dyes within the series (2.0-2.4 GM·g(-1)·mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.