Fluorescence resonance energy transfer in a novel two-photon absorbing system

A novel fluorescence resonance energy transfer (FRET) system containing a two-photon absorbing dye and a nile red chromophore has been synthesized. Upon two-photon excitation by laser at 815 nm this molecule displays efficient energy transfer from the two-photon absorbing dye to the nile red moiety, with an 8-fold increase in emission compared to the model compound. Similarly, single-photon excitation of the two-photon absorbing moiety at 405 nm results in >99% energy-transfer efficiency, along with a 3.4-fold increase in nile red emission compared to direct excitation of the nile red chromophore at 540 nm. This system provides an effective way to use IR radiation to excite molecules that, by themselves, have little or no two-photon absorption.     

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.     

Solvent effects on two-photon absorption of dialkylamino substituted distyrylbenzene chromophore

Solvent effects on the two-photon absorption of a symmetrical diamino substituted distyrylbenzene chromophore have been studied using the density functional response theory in combination with the polarizable continuum model. It is shown that the dielectric medium has a rather small effect both on the bond length alternation and on the one-photon absorption spectrum, but it affects significantly the two-photon absorption cross section. It is found that both one- and two-photon absorptions are extremely sensitive to the planarity of the molecule, and the absorption intensity can be dramatically reduced by the conformation distortion. It has led to the conclusion that the experimentally observed anomalous solvent effect on the two-photon absorption of dialkylamino substituted distyrylbenzene chromophores cannot be attributed to the intrinsic properties of a single molecule and its interaction with solvents.     

Two-photon state selection and angular momentum polarization probed by velocity map imaging: application to H atom photofragment angular distributions from the photodissociation of two-photon state selected HCl and HBr

A formalism for calculating the angular momentum polarization of an atom or a molecule following two-photon excitation of a J-selected state is presented. This formalism is used to interpret the H atom photofragment angular distributions from single-photon dissociation of two-photon rovibronically state selected HCl and HBr prepared via a Q-branch transition. By comparison of the angular distributions measured using the velocity map imaging technique with the theoretical model it is shown that single-photon dissociation of two-photon prepared states can be used for pathway identification, allowing for the identification of the virtual state symmetry in the two-photon absorption and/or the symmetry of the dissociative state. It is also shown that under conditions of excitation with circularly polarized light, or for excitation via non-Q-branch transitions with linearly polarized light the angular momentum polarization is independent of the dynamics of the two-photon transition and analytically computable.     

Two-photon absorption strength: a new tool for the quantification of two-photon absorption

In this paper, we define the two-photon absorption strength, a new characterization tool, similar to the oscillator strength, but for two-photon absorption. It allows the quantification of the two-photon absorption properties of molecular systems which are one-photon transparent. Its definition is such that the corresponding numerical values are around 100 for small molecules. We also show that this new theoretical tool allows the direct comparison of experimental and theoretical data without requiring the introduction of any arbitrary band width. As an example, the experimental and theoretical (AM1+CNDOS and HF+CIS3-21G) two-photon absorption properties of the 2,2'-bi(9,9-dihexylfluorene) molecule are compared.     

Laser two-photon ionization spectroscopy of molecules in liquids

A two-photon lonization technique has been developed and applied to determine the photoionization threshold of a molecule in liquid solution. The photoionization of pyrene in n-pentane was studied by monitoring the photocurrent of an≈ 10^?6 M solution as a function of the laser wavelength in the region 360–530 nm, corresponding to two-photon, transition in the region 180–265 nm. The photoionization threshold thus determined is 4.80 ± 0.02 eV. Resonances were observed in the two-photon ionization spectrum which are tentatively ascribed to preionization of one-photon forbidden transitions to states energetically degenerate with the continuum, reached via two-photon absorption.     

双光子技术在光生酸剂中的应用研究

自2000年以来,双光子技术开始应用于光生酸剂体系中,并取得了一定的研究进展。双光子技术在光生酸剂中的应用主要有两种情况:一是单分子体系,即光生酸剂分子本身具有较高的双光子吸收截面,可以在双光子激发下产生光酸;二是双分子体系,由具有较高吸收截面的敏化剂分子和光生酸剂分子组成,通过分子间电子转移的方式产生光酸。本文就这一类具有特殊光学性质的有机分子体系的构成及其应用进行了综述,介绍了不同类型的可以利用双光子技术的光生酸剂体系,总结了双光子技术在光生酸剂发展中面临的一些关键问题,展望了双光子技术在光生酸剂中的发展方向。     

Protonation Effect on One- and Two-photon Absorption Property of a Newly Synthesized Octupolar Chromophore

The protonation effects on one- and two-photon absorption properties of an octupolar molecule TA with 1,3,5-triazine core and pyrrole electron-donating end-groups have been studied at hybrid density functional theory level. A computational scheme is developed tosimulate a proton attached to an atom. The numerical results show that large changes in both one- and two-photon absorption properties are observed when the compound is transformed from neutral to threefold protonated states. When the compound is protonated, more charge transfer states appear and the absorption band has a red-shift. Furthermore, the two-photon absorption cross-section is largely enhanced. The theoretical calculations demonstrate the protonation effect on promoting the intramolecular charge transfer strength. The results present qualitative agreement with the experimental observations. A two-photon absorption switch with the compound TA based on the protonation effect is proposed.     

苯并噻唑衍物的合成及结构-双光子吸收光物理特性关系

报道了三个具有典型A-π-A'型共轭结构的苯并噻唑衍生物的合成及结构-双光 子吸收光物理特性关系。通过对三个化合物的共轭结构增长和拉电子基强度变化对 化合物单光子荧光光谱、双光子诱导荧光光谱和双光子吸收截面等特性的影响研究 ，我们发现，苯并噻唑杂环中的杂原子硫在化合物共轭链链短时对其光物理特性影 响很强，在共轭链较长时影响减弱甚至消失。通常情况下，共轭链长度和拉电子基 强度共同对双光子吸收截面作贡献，我们的研究表明，当共轭链较长时，共轭链的 增长对增强分子双光子吸收截面的贡献远大于拉电子基强度变化的贡献。其中，新 化合物2（2-{4-[2-（4-硝基苯基）-乙烯基]苯基}-乙烯基）苯并噻唑具有双光子 吸收截面大(181*10~(-50)cm~4s/photon)和荧光量子产率高(13.8%)的特点，是双 光子荧光显微与成像应用的一个良好的候选材料。     

Boron-containing two-photon-absorbing chromophores. 3. One- and two-photon photophysical properties of p-carborane-containing fluorescent bioprobes

Boron-containing two-photon-absorbing fluorophores have been prepared as new bifunctional molecules, potentially useful in two-photon excited microscopy (TPEM) and boron neutron capture therapy. They are based on a one-dimensional conjugated system containing a p-carborane entity at one end of the molecule and various electron-donating groups containing oxygen or nitrogen atoms at the other end. We investigated their one- and two-photon photophysical properties. They showed efficient fluorescence in an organic solvent, as well as in water for two of them, allowing microscopy on cell cultures. High two-photon absorption cross sections were determined in the 700-900 nm range. TPEM images were obtained with these new p-carborane-containing fluorophores, with laser intensities in the submilliwatt range.     

Interplay of one- and two-step channels in electrovibrational two-photon absorption

We present a theory of two-photon absorption that addresses the formation of spectral shapes taking the vibrational degrees of freedom into account. The theory is used to rationalize the observed differences between the spectral shapes of one- and two-photon absorption. We find that the main cause of these differences is that the two-step and coherent two-photon spectral bands are different even considering a single final state. Our formalism is applied to the N101 molecule (p-nitro-p'-diphenylamine stilbene), which was recently studied experimentally. Simulations show that the two-step two-photon electrovibrational absorption results in a blue shift of the absorption spectrum in agreement with the measurements.     

Ruby laser-induced two-photon photolysis of potassium ferrioxalate

The ruby laser-induced photolysis of potassium ferrioxalate is shown to follow a two-photon excitation mechanism. The two-photon absorption cross section is found to be δ = (1.5 ± 1.0) × 10^?50 cm⁴ s/photon molecule.     

Synthetic micelle sensitive to IR light via a two-photon process

A micellar assembly of molecules constituted of poly(ethylene glycol) as the hydrophilic component and 2-diazo-1,2-naphthoquinone as the hydrophobic component was shown to be destroyed in a two-photon photoreaction triggered by infrared light with release of an encapsulated fluorescent probe molecule.     

CALCIUM-DEPENDENT FLUORESCENCE LIFETIMES OF INDO-1 FOR ONE- AND TWO-PHOTON EXCITATION OF FLUORESCENCE

Abstract— We characterized the fluorescence intensity decays of Indo-1, which is commonly used as an emission wavelength-ratiometric calcium probe. The apparent lifetime of the long-wavelength side of the emission of Indo-1 is dependent on Ca²⁺. This long-wavelength emission displays the characteristics of an excited-state reaction, that is, a negative preexponential component in thc multiexponential analysis. The emission spectra and lifetime of Indo-1 appear to be identical for one-photon and two-photon excitation at 351 and 702 mn, respectively, suggesting that the relative one- and two-photon cross sections are similar for the calcium-free and calcium-bound forms of Indo-1. Also, the two-photon cross section of Indo-1 is relatively high, about 4 × 10⁻⁴⁹ cm⁴ s/photon molecule at 690 nm for both the calcium-free and calcium-bound forms. Hence, Indo-1 can be used for calcium imaging based on one- or two-photon excitation, using either emission wavelength ratios or lifetime imaging methods.     

Effects of vibronic dephasings on the polarization anisotropy decay in the time-resolved two-photon ionization of an isolated molecule

Effects of vibronic dephasings on the time-dependent degree of polarization in the two-photon ionization of an isolated symmetric top molecule are theoretically studied. The so-called fast (picosecond) decays of polarization in isolated trans-stilbene are explained in terms of a Coriolis-induced vibronic dephasing mechanism.     

Theoretical Studies on One-photon and Two-photon Absorption Properties of Pyrene-core Derivatives

The analytic response theory at density functional theory level is applied to investigate one-photon and two-photon absorption properties of a series of recently synthesized pyrene-core derivatives. The theoretical results show that there are a few charge-transfer states for each compound in the lower energy region. The one-photon absorption properties of the five investigated compounds are highly consistent with those given by experimental measure-ments. The two-photon absorption intensities of the compounds are greatly enhanced with the increments of the molecular sizes, in which the two-photon absorption cross section of the four-branched compound is about 5.6 times of that of the mono-branched molecule. Further-more, it is shown that the two-photon absorption properties are sensitive to the geometrical arrangements.     

Organelle-specific detection of phosphatase activities with two-photon fluorogenic probes in cells and tissues

Two-photon fluorescence microscopy (TPFM) provides key advantages over conventional fluorescence imaging techniques, namely, increased penetration depth, lower tissue autofluorescence and self-absorption, and reduced photodamage and photobleaching and therefore is particularly useful for imaging deep tissues and animals. Enzyme-detecting, small molecule probes provide powerful alternatives over conventional fluorescent protein (FP)-based methods in bioimaging, primarily due to their favorable photophysical properties, cell permeability, and chemical tractability. In this article, we report the first fluorogenic, small molecule reporter system (Y2/Y1) capable of imaging endogenous phosphatase activities in both live mammalian cells and Drosophila brains. The one- and two-photon excited photophysical properties of the system were thoroughly investigated, thus confirming the system was indeed a suitable Turn-ON fluorescence pair for TPFM. To our knowledge, this is the first enzyme reporting two-photon fluorescence bioimaging system which was designed exclusively from a centrosymmetric dye possessing desirable two-photon properties. By conjugation of our reporter system to different cell-penetrating peptides (CPPs), we were able to achieve organelle- and tumor cell-specific imaging of phosphatase activities with good spatial and temporal resolution. The diffusion problem typically associated with most small molecule imaging probes was effectively abrogated. We further demonstrated this novel two-photon system could be used for imaging endogenous phosphatase activities in Drosophila brains with a detection depth of >100 μm.     

New design strategy for the two-photon active material based on push-pull substituted bisanthene molecule

In the present work, we have critically examined the origin of strong two-photon transition probability of a donor-acceptor substituted bisanthene molecule that imitates a small piece of edge passivated (4, 4) graphene nanoribbon. In our calculations, we have considered -OMe, and -NH(2) as donors and -NO(2) as an acceptor. The one- and two-photon absorption parameters are evaluated using state-of-the-art linear and quadratic response theory, respectively, and all these calculations are carried out within the framework of time dependent density functional theory. To give a proper judgment on our findings, we have used the long-range corrected CAMB3LYP functional for all of the time dependent calculations. The present investigation reveals that the bisanthene molecule with three pairs of donor/acceptor moiety has a lower two-photon transition probability than that of a suitably designed bisanthene with only a single pair of donor/acceptor moiety. This in silico observation is consistent for all of the donor/acceptor moieties chosen in the present work. A comprehensive analysis at the two state model level of theory clearly offers us a verdict that by placing the donor/acceptor moiety in a suitable position of bisanthene, we can create a significant asymmetry in the electron density in the first excited state, which eventually leads to a significant difference in the ground and excited state dipole moment and is attributed to the higher two-photon transition probability of a particular bisanthene with a single pair of donor/acceptor moiety than bisanthene with three pairs of donor/acceptor.     

Two-photon absorption and effective broadband optical power limiting properties of a multi-branched chromophore containing 2,3-diarylquinoxalinyl moieties as the electron-pulling units

A novel multi-branched chromophore containing 2,3-diarylquinoxalinyl units as the electron-acceptors had been synthesized and its nonlinear optical properties were characterized in the femtosecond and nanosecond regime. The experimental results show that the studied fluorophore possesses strong and wide-dispersed two-photon absorption in near infrared (NIR) region. It is demonstrated that the incorporation of 2,3-disubstituted quinoxaline moieties as a part of π-conjugation in a dye molecule could be a useful approach toward large molecular two-photon absoptivities within the studied spectral region. Effective optical-power-attenuation behaviors in the nanosecond time domain of this compound were also investigated and the results indicate that such dye molecule can be a potential material as a broadband and quick-responsive optical limiter especially when against those laser lights with longer pulses.     

两个具有强双光子荧光的有机硼化合物

以二米基硼为电子受体, 苯乙烯基噻吩为共轭桥, 合成了两个新的稳定的有机硼化合物: 反式,反式-2-二米基 硼-5-{2-[4-(2-噻吩乙烯基)苯基]乙烯基}噻吩(1)和反式,反式-1,4-二-[2-(5-二米基硼噻吩)乙烯基]苯(2). 前者为不对称结构的偶极分子, 后者为对称的A-π-A型四极分子. 对称性不同的化合物表现出不同的双光子吸收性质. 对于偶极分子1, 单双光子吸收达到的激发态能级接近, 而对于四极分子2, 双光子吸收达到的激发态则比单光子吸收所达到的激发态高出0.35 eV. 在波长为710到 900 nm范围的飞秒脉冲激光激发下, 化合物1和2在THF溶液中都可以发出很强的绿色上转换荧光 (1, λ_max＝505 nm; 2, λ_max＝513 nm). 用双光子荧光法测得A-π-A型化合物2在775 nm处的双光子吸收截面达1340 GM.