Effects of conjugation length, electron donor and acceptor strengths on two-photon absorption cross sections of asymmetric zinc-porphyrin derivatives

Exceptionally large two-photon absorption cross sections at the infrared region have been revealed by time-dependent density functional theory calculations for asymmetric charge-transfer conjugated zinc-porphyrin derivatives. The largest two-photon cross section is found to be more than one order of magnitude larger than for the conventional two-photon active organic molecules. The calculations show that the formation of strong charge-transfer states depends on the length of the conjugation bridge between the zinc-porphyrin core and the electron donor/acceptor. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor.     

取代基对二噻吩并噻吩衍生物的双光子吸收性质的影响

利用ZINDO/SOS方法, 从理论上研究了对称和不对称取代两种情况下, 取代基对二噻吩并噻吩衍生物单双光子吸收性质的影响. 结果表明, 所设计的噻吩类分子具有较大的双光子吸收截面, 且双受体取代比双给体取代更有利于增大分子的双光子吸收截面. 同时发现, 此类分子受体取代可以显著增加波长较短的双光子峰附近的双光子吸收, 而给体取代则可以改善波长较长吸收峰附近的双光子吸收.     

Plasmonic enhancement of the two photon absorption cross section of an organic chromophore using polyelectrolyte-coated gold nanorods

The effect of plasmonic enhancement on the two-photon absorption cross section of organic chromophores attached to polyelectrolyte-coated gold nanorods was investigated. The magnitudes of such enhancements were confirmed using single and two photon excitations of the chromophore molecules bound to polyelectrolyte-coated gold nanorods. By synthesizing two-, four-, six-, and eight-polyelectrolyte layer coated nanorods of a particular aspect ratio, the distance dependence of the evanescent electromagnetic field on molecular two-photon absorption was observed. Enhancements of 40-fold were observed for the chromophores nearest to the surface.     

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.     

Hybrid density functional theory/molecular mechanics calculations of two-photon absorption of dimethylamino nitro stilbene in solution

The dimethylamino nitro stilbene (DANS) molecule is studied for exploring solvent effects on two-photon absorption using the quantum mechanical/molecular mechanical (QM/MM) response theory approach, where the quantum part is represented by density functional theory. We have explored the role of geometrical change of the chromophore in solution, the importance of taking a dynamical average over the sampled structures and the role of a granular representation of the polarization and electrostatic interactions with the classically described medium. The line shape function was simulated by the QM/MM technique thereby allowing for non-empirical prediction of the absolute two-photon cross section. We report a maximum in the TPA cross section for a medium of intermediate solvent polarity (i.e. in chloroform) and provide the grounds for an explanation of this effect which recently has been experimentally observed for a series of charge transfer species in solvents of different polarity. The calculations of absorption energies reproduce well the positive solvatochromic behavior of DANS and are in good agreement with experimental spectra available for the chloroform and DMSO solvents. In line with recent development of the QM/MM response technique for color modeling, we find this methodology to offer a versatile tool to predict and analyze two-photon absorption phenomena taking place within a medium.     

基于咔唑的双光子荧光次氯酸根探针光学性质及响应机理

采用含时密度泛函理论与响应函数理论相结合的方法,研究了两种实验室新合成的水溶性双光子荧光次氯酸根（ClO^-）探针分子HCH和HCM的单光子吸收、双光子吸收和荧光发射性质。计算结果表明,HCH和HCM分子与ClO^-反应后,生成物的光吸收和光发射性质均发生明显变化,相应的吸收和发射峰位都发生了明显蓝移,荧光强度展现出明显的增强。另外,两探针分子都具有较大的双光子吸收截面,且与ClO^-反应后,生成物的双光子吸收截面值显著增加,因此两分子均可作为性能优良的双光子荧光探针分子。此外,通过分析HCH和HCM分子与ClO^-反应前后的Mulliken电荷布居情况,从理论上证实了该系列荧光探针分子的识别机理是C＝N异构化。     

Enhancement of a two-photon absorption cross section (TPACS)--design and synthesis of a novel class of photochromic molecules with large TPACS

Two novel photochromic compounds were designed and synthesized; the two-photon absorption (TPA) cross section of both were measured with a nanosecond laser pulse; their TPACS (delta) values are around 25 x 10(-46) cm4 s photon-1 molecule-1 in acetonitrile; the molecular structure of the target compounds have a 3-methyl-1-benzothiophen-2-yl moiety, which can greatly enhance the two-photon absorption cross section.     

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.     

Two-photon excited fluorescence of nitrogen-vacancy centers in proton-irradiated type Ib diamond

Two-photon fluorescence spectroscopy of negatively charged nitrogen-vacancy [(N-V)-] centers in type Ib diamond single crystals have been studied with a picosecond (7.5 ps) mode-locked Nd:YVO(4) laser operating at 1064 nm. The (N-V)- centers were produced by radiation damage of diamond using a 3 MeV proton beam, followed by thermal annealing at 800 degrees C. Prior to the irradiation treatment, infrared spectroscopy of the C-N vibrational modes at 1344 cm(-1) suggested a nitrogen content of 109 +/- 10 ppm. Irradiation and annealing of the specimen led to the emergence of a new absorption band peaking at approximately 560 nm. From a measurement of the integrated absorption intensity of the sharp zero-phonon line (637 nm) at liquid nitrogen temperature, we determined a (N-V)- density of (4.5 +/- 1.1) x 10(18) centers/cm3 (or 25 +/- 6 ppm) for the substrate irradiated at a dose of 1 x 1016) H(+)/cm(2). Such a high defect density allowed us to observe two-photon excited fluorescence and measure the corresponding fluorescence decay time. No significant difference in the spectral feature and fluorescence lifetime was observed between one-photon and two-photon excitations. Assuming that the fluorescence quantum yields are the same for both processes, a two-photon absorption cross section of sigma(TPA) = (0.45 +/- 0.23) x 10(-50) cm(4).s/photon at 1064 nm was determined for the (N-V)- center based on its one-photon absorption cross section of sigma(OPA) = (3.1 +/- 0.8) x 10(-17) cm2 at 532 nm. The material is highly photostable and shows no sign of photobleaching even under continuous two-photon excitation at a peak power density of 3 GW/cm(2) for 5 min.     

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.     

Metal-ion sensing fluorophores with large two-photon absorption cross sections: aza-crown ether substituted donor-acceptor-donor distyrylbenzenes

Chromophores based on a donor-acceptor-donor structure possessing a large two-photon absorption cross section and one or two mono-aza-15-crown-5 ether moieties, which can bind metal cations, have been synthesized. The influence of Mg(2+) binding on their one- and two-photon spectroscopic properties has been investigated. Upon binding, the two-photon action cross sections at 810 nm decrease by a factor of up to 50 at high Mg(2+) concentrations and this results in a large contrast in the two-photon excited fluorescence signal between the bound and unbound forms, for excitation in the range of 730 to 860 nm. Experimental and computational results indicate that there is a significant reduction of the electron donating strength of the aza-crown nitrogen atom(s) upon metal ion binding and that this leads to a blue shift in the position as well as a reduction in the strength of the lowest-energy two-photon absorption band. The molecules reported here can serve as models for the design of improved two-photon excitable metal-ion sensing fluorophores.     

Charge transfer enhances two-photon absorption in transition metal porphyrins

Two-photon absorption processes were investigated in electropolymerized Fe(III), Mn(III), and Co(II) 5,10,15,20-tetrakis-(4-hydroxytetraphenyl)porphyrin films. Degenerate four wave mixing (DFWM) spectroscopy with 100 fs pulses in the near-IR spectral region was used. Metalloporphyrins with strong charge transfer (CT) transitions in the linear absorption spectra also show enhanced two-photon absorption. (Metalloporphyrin two-photon absorption cross section, delta, increases >10 times over that for the metal free porphyrin.) This effect was attributed to a two-photon induced charge transfer between the metal ion's d orbitals and the pi-system of the porphyrin. Correlation of one- and two-photon absorption properties of transition metal porphyrins suggests a new and simple approach to improve organic materials for photonic applications.     

Two-photon absorption cross sections: an investigation of solvent effects. Theoretical studies on formaldehyde and water

The effects of a solvent on the two-photon absorption of microsolvated formaldehyde and liquid water have been studied using hybrid coupled-cluster/molecular mechanics (CC/MM) response theory. Both water and formaldehyde were considered solvated in water, where the solvent water molecules were described within the framework of molecular mechanics. Prior to the CC/MM calculations, molecular dynamics simulations were performed on the water/formaldehyde and water/water aggregates and many configurations were generated. By carrying out CC/MM response calculations on the individual configurations, it was possible to obtain statistically averaged results for both the excitation energies and two-photon absorption cross sections. For liquid water, the comparison between one- and two-photon absorption spectra is in good agreement with the experimental data available in the literature. In particular, the lowest energy transition occurring in the one-photon absorption spectrum of water only occurs with a relatively small strength in the two-photon absorption spectrum. This result is important for the interpretation of two-photon absorption data as these results show that in the absence of selection rules that determine which transitions are forbidden, the spectral profile of the two-photon absorption spectrum can be significantly different from the spectral profile of the one-photon absorption spectrum.     

Two-photon absorption properties of alkynyl-conjugated pyrene derivatives

A series of pyrene derivatives having 4-(N,N-dimethylamino)phenylethynyl groups as the substituent (1-5) have been synthesized and their two-photon absorption properties were investigated. Comparison of two-photon cross section (delta(max)) with related compounds reveals that pyrene is as efficient a pi-center as anthracene in two-photon materials. Moreover, the two-photon cross section (delta(max)) increased with the number of substituents reaching at the maximum value of 1150 GM for the tetra-substituted derivative (5). Furthermore, the two-photon action cross section (Phi delta(max)) of 5 is comparable to that the most efficient two-photon materials. This result provides a useful guideline to the design of efficient two-photon materials bearing pyrene as a pi-center.     

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。     

Suggestion for a new two-photon cross section

We suggest a new two-photon (TP) absorption cross section which is independent of the wavelength of the two laser beams. The different wavelengths may cause changes in the regular cross section by orders of magnitude in the same absorbing molecule. This effect is not due to the resonance term, but to the number of modes.     

Linear and Nonlinear Optical Properties of Novel Multi-branched Oligomers

We investigate the fluorene-vinylene unit dependent photo-physical properties of multi-branched truxene based oligomers (Tr-OFV_n, n=1-4) employing steady-state absorption and emission spectroscopy, transient absorption spectroscopy, two-photon fluorescence, and z-scan technique. The results show that the increasing of fluorene-vinylene unit leads to a red-shift in the spectra of absorption and fluorescence, and shortens the excited state lifetime. Meanwhile, two-photon fluorescence efficiency and two-photon absorption cross section of truxene based oligomers gradually enhance in company with the extension of π-conjugated length. In addition, the values of two-photon absorption cross section modeled on the sum-over-state approach agree well with the experimental ones. The results indicate multi-branched truxene based oligomers bearing oligo(fluorene-vinylene) arms are promisingorganic materials for two-photon applications.     

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.     

Theoretical study of one-photon and two-photon absorption properties of perylene tetracarboxylic derivatives

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.     

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.