Switching of Two-Photon Optical Properties by Anion Binding of Pyrrole-Based Boron Diketonates through Conformation Change

Two-photon absorption (TPA) dyes with intense fluorescence can be used to detect small chemical species and as sensors and bioimaging probes for specific analytes. Various TPA dyes responding to a number of external stimuli have been reported. Among them, biologically important anionic species have not been used as agents to control TPA properties because their direct electronic influences on the transition dipole moments of dyes are typically small. In this study, dipyrrolyldiketone BF₂ complexes substituted with π-extended units exhibited efficient TPA properties that could be regulated by conformation changes induced by anion binding. The TPA intensity decreased to 1/5 of the original intensity upon anion binding, which was much larger than that observed for one-photon absorption. Anion detection was achieved by a change in the emission intensity of spatially resolved spots of two-photon-excited fluorescence (TPEF) in the sample. Experimental and theoretical studies were performed to understand the mechanism of the TPA property control and showed that the drastic changes in the transition dipole moments upon conformation changes between the straight and bending forms of the π-electronic systems caused the TPA and TPEF intensities drop.     

Blue two-photon excited fluorescence of several D-π-D, A-π-A, and D-π-A compounds featuring dimesitylboryl acceptor

Several donor-π-donor (D-π-D), acceptor-π-acceptor (A-π-A), and donor-π-acceptor (D-π-A) types of organic compounds with fluorene as π bridge and dimesitylboryl group as electron acceptor, which show strong two-photon excited blue fluorescence, have been synthesized and structurally investigated. The symmetric A-π-A type of compound exhibits the shortest wavelength of two-photon excited fluorescence (TPEF) at λ_em=405 nm under the excitation of λ_ex=730 nm; the unsymmetric D-π-A type of compound with diphenylamino as donor exhibits the most intense TPEF at blue region (λ_em=484 nm) with a two-photon absorption cross-section of 425 GM under λ_ex=800 nm.     

一个新的具有大的吸收截面的双光子引发剂的合成和非线性光学性质

E,E-1,4-Bis（4＇-N,N-diphenylaminostyryl）-2,5-dimethoxybenzene （DPAMOB） has been synthesized by a simple and effective solid phase Wittig reaction and characterized by 1^H NMR spectra and elemental analysis, Linear absorption, single-photon induced fluorescence and two-photon induced fluorescence spectra were experimentally studied. The new dye has a large two-photon absorption （TPA） cross-section of σr= 1007,2 GM [1 GM= 1 × 10^-50 cm^4·s/（photon molecule）] at 800 nm measured by the two-photon induced fluorescence method. The experimental results confirm that DPAMOB is a good TPA chromophore and can successfully initiate two-photon photopolymerization of ethoxylated trimethylolpropane triacrylate esters （SR454）. Finally, a microstructure has been fabricated by use of DPAMOB as initiator.     

Theoretical Study on One- and Two-Photon Absorption Properties of PPV Derivative with Electron-Donor Phenylanthracene as Pendent Group

Based on the equilibrium structures from quantum mechanics AM1 method, employing INDO/CI method and the sum-over-state （SOS） formula, the one-photon absorption （OPA） and two-photon absorption （TPA） properties as well as the second hyperpolarizabilities were discussed in detail for a kind of PPV derivative poly 2-（9-phenylanthracen-10-yl）-1,4-phenylenevinylene （P1）. The results indicate that the two-photon cross section （3） increases with the increasing of the number of repeating segment （n）, however only a slight increase corresponds to the increasing of molecular weight when the repeating unit number arrives at a certain number. From this point of view, the TPA cross section values of P1 were extrapolated through the linear fit of 3 value vs. 1/n. The δ value of P1 is as high as 181 GM （1 GM=10^-50 cm^4·s·photon^-1）. Concerning the influence of pendent group and extension of π-conjugation on 3 values, another four oligomers P2, PPV, P1-1, and P2-1 were investigated for comparison. The calculation results reveal that the position and property of pendent group have great influence on the OPA and TPA properties. A most crucial role for relatively larger 3 value was played by the extension of π-conjugation.     

Multipolar symmetric squaraines with large two-photon absorption cross-sections in the NIR region

The two-photon absorption (TPA) properties of two extended symmetric squaraine dyes are thoroughly characterized from the experimental and quantum-chemical point of view. The two molecules are specially engineered to have a multipolar structure with increasing complexity, D-π-A-π-D and A'-π-D-π-A-π-D-π-A', respectively. The experimental TPA spectra, measured by means of the Z-scan technique in the femtoseconds regime, display considerably high values of TPA cross sections (σ(TPA)) for both molecules. In particular, the squaraine with the more extended structure shows the highest value of σ(TPA) ever reported for this class of molecules. CIS and TDDFT calculations of the one and two-photon absorption properties are carried out to clarify the origin of the observed TPA properties and fully characterize the electronic properties of these compounds. The calculations, in good agreement with the experimental data, suggest that the origin of this exceptionally high σ(TPA) can be ascribed to the presence of a peripheral A' group, that increases the density of excited states involved in the TPA process.     

卟啉多枝分子合成与分子内能量转移、双光子吸收性能研究

以三苯胺或硝基苯为端基, 合成了三个卟啉多枝分子: 5-(4-硝基苯甲酰氧基)苯基-10,15,20-三-(4-溴苯基)卟啉(TPP-NO₂)、5-(4-硝基苯甲酰氧基)苯基-10,15,20-三-(4-二苯胺基-1-苯乙烯基)苯基卟啉(TPP-X₃)和5,10,15,20-四-(4-二苯胺基-1-苯乙烯基)苯基卟啉(TPP-X₄), 进行了红外光谱、核磁共振光谱和质谱表征. 比较研究了分子“枝”、“核”不同键合方式与不同对称结构对分子的线性光谱、非线性光谱以及分子内能量转移行为的影响. 在钛宝石激光器(800 nm)和Nd∶YAG倍频光(532 nm)泵浦下, 样品溶液均发出卟啉环特有的红色荧光——前者系双光子吸收机制“上转换”荧光, 后者则为双光子吸收与分子内能量转移机制“下转换”荧光. 飞秒Z-scan技术测得样品双光子吸收截面最大可达130 GM, 与四苯基卟啉(TPP)同等测试条件下的双光子吸收截面相比增大了两个数量级.     

Extended Alkenyl and Alkynyl Benzotriazoles with Enhanced Two-Photon Absorption Properties as a Promising Alternative to Benzothiadiazoles

A series of donor–π–acceptor–π–donor (D -π-A-π-D) benzoazole dyes with 2H-benzo[d][1,2,3]triazole or BTD cores have been prepared and their photophysical properties characterized. The properties of these compounds display remarkable differences, mainly as a result of the electron-donor substituent. Dyes with the best properties have visible-light absorption over λ=400 nm, large Stokes shifts in the range of about 3500–6400 cm⁻¹, and good fluorescence emission with quantum yields of up to 0.78. The two-photon absorption properties were also studied to establish the relationship between structure and properties in the different compounds synthesized. These results provided cross sections of up to 1500 GM, with a predominance of S₂←S₀ transitions and a high charge-transfer character. Time-dependent DFT calculations supported the experimental results.     

Anthrathiadiazole Derivatives: Synthesis,Physical Properties and Two-photon Absorption

Anthrathiadiazole is a key synthon for the construction of large azaacenes, however, the attachment of different substituents onto the skeleton of anthrathiadiazole is difficult but highly desirable because it could be easy to enrich the structures of azaacenes. Here, it is demonstrated that anthrathiadiazole derivatives with −Br, −CN, and −OCH₃ groups could be easily constructed through a simple [4+2] cycloaddition reaction between a,a,a’,a’-tetrabromo-o-xylenes derivatives and benzo[c][1,2,5]thiadiazole-4,7-dione. The structures of the as-prepared compounds with different substituents were carefully characterized. Moreover, the basic physical properties of the as-prepared anthrathiadiazole derivatives were fully investigated, where the cyano-substituted derivative (BTH - CN) has the highest stability and the methoxy-substituted derivative ( BTH - OCH₃) is easy to be oxidized. Moreover, the two-photon absorption (TPA) characteristics of different anthrathiadiazoles are also studied by using the femtosecond Z-scan technique. The results show that the fused anthrathiadiazole skeletons possess large TPA cross-section values δ₂ in the range of 3000–5000 GM, where the nature, position and strength of the substituted groups have strong effect on these values.     

The Ionization Potentials of Butadiene,Hexatriene, and their Methyl Derivatives: Evidence for through space interaction between double bond π-orbitals and non-bonded pseudo-π orbitals of methyl groups?

It is shown that the correlation of the π-ionization potentials of ethylene ( 1 ), butadiene ( 2 ) and trans-1,3,5-hexatriene ( 4 ) favours the orbital sequence π, π, σ in butadiene and π, π, σ, π in the hexatriene in excellent agreement with the results of SPINDO calculations. Within the experimental error the π-ionization potentials of cis-1,3,5-hexatriene ( 3 ) and trans-1,3,5-hexatriene ( 4 ) are the same. Methyl-substitution of 2 lowers the π-ionization potentials I₁(π) and I₂(π). For 1 and/or 4 substitution the difference I₂(π)?I₁(π) remains constant (≈ 2.5 eV). On the other hand 2 and/or 3 substitution leads to a smaller gap of I₂(π)–I₁(π) ≈ 1.6 to 2.0 eV without changing the mean π-ionization potential I (π) relative to the corresponding 1,4 substituted derivatives. This result is rationalized in terms of a through space interaction between double bond π-orbitals and non-bonded pseudo-π-orbitals of the substituting methyl groups. The reduced split I₂(π)–I₁(π) in cyclopentadiene is attributed to hyperconjugation across the methylene group.     

Selective Population of the n, π* and π, π* Triplet States of 1-Indanones

The two components of the dual phosphorescence of 1-indanone ( 1 ) and six related ketones ( 2–7 ) possess different excitation spectra exhibiting the vibrational progression characteristic of the S₀ → S₁ (n, π*) transition (shorter-lived emission) and two bands of the S₀ → S_{2 and 3} (π,π*) 0–0 transitions, respectively. The most favorable intersystem crossing routes are S₁ (n, π*) → T (n, π*) and S_2,3 (π*) → T (π, π*). Internal conversion to S₁ competes more effectively with S (π, π*) → T (π, π*) intersystem crossing only from higher vibrational levels of the S₂ and S₃ states.     

Theoretical investigation of one- and two-photon spectra of pyrazabole chromophores

Abstract  

An extensive series of pyrazabole chromophores containing pseudo-conjugated systems have been theoretically constructed and investigated on the one-photon absorption (OPA) and two-photon absorption (TPA) properties by using density functional theory and Zerner’s intermediate neglect of differential overlap methods. The results indicated that all the pyrazabole chromophores show strong OPA at around 400 nm and intense TPA properties in the range of 500–600 nm with TPA cross sections (δ _max) as large as 540–3,560 GM, which are excellent candidates for optical power limiting materials. It is noteworthy that the δ _max values of the two constructed pyrazaboles, PA3 and PAF2, are 308.8 GM at 772.0 nm and 157.8 GM at 834.4 nm, respectively, which may be particularly attractive as probes for two-photon fluorescence imaging. The influence of incorporating electron acceptors in the central core, π-conjugated bridge and terminal groups on OPA and TPA properties was analyzed in detail to derive structure–property relationships and to lay the guidelines for both spectral tuning and amplification of molecular TPA in the target region. Meanwhile, the solvent effects on these properties were taken into account within the PCM model. The solvent has a significant impact on the TPA properties for chromophore PA3 and leads to the two-photon absorption spectra (λ _max ^T ) red-shift and δ _max enhancing relative to those in gas phase. In addition, from the calculations of molecule AlA2, we can draw the conclusion that the compounds with the Al₂N₄ center behave similarly to pyrazabole chromophores in the linear optical and TPA properties and increase TPA cross sections to some extent.     

多支结构的吡嗪衍生物的合成及其双光子吸收性质

合成了一系列含有双支、三支和四支的吡嗪衍生物, 测定了它们的线性吸收和发射性质以及双光子吸收和发射性质. 随着吡嗪环上侧链数目的增加(支链数目从2到4), 吸收光谱(吸收峰位于290～390 nm)、荧光谱(发射峰位于400～510 nm)和双光子荧光谱(激发波长720 nm)都发生红移, 荧光量子产率也逐渐增强(从0.13增大到0.25). 另外, 从双支到四支结构, 双光子吸收截面σ按照1∶2.8∶3.7的比例增加, 接近于支链数目的平方之比(1∶2.25∶4), 表明多支结构的双光子吸收存在显著的增强效应, 其中四支结构的σ值为1007 GM. 实验中还发现, 对于具有相同支链数目的化合物, 邻、对位的取代模式比间位取代模式具有更强的单光子和双光子荧光性质.     

Transient Species in the Photochemistry of Tiaprofenic Acid and Its Decarboxylated Photoproduct*

Photoexcitation of tiaprofenic acid (TPA) in aqueous medium leads with almost unitary efficiency to the lowest π,π* triplet, which is detected by transient absorption. The deactivation of this state occurs in the microsecond time domain and is dominated by a thermally activated spin-allowed process with –10 kcal/mol energy barrier. The occurrence of decarboxylation from an upper state, likely the second triplet of n, π* character, is confirmed by the study of the transients toward the final keto photoproduct, i.e. the benzoylthiophene ethyl derivative (DTPA). At neutral pH, upon adiabatic release of the CO₂ fragment, long-lived triplet biradicals and ground-state intermediates with a protonated carbonyl oxygen are formed. Laser flash photolysis of DTPA leads almost quantitatively to the lowest π,π* triplet, with similar T-T absorption features as those of TPA. However the DTPA triplet appears essentially unreactive in aqueous medium. In isopropanol H-abstraction from the solvent is demonstrated by the formation of the ketyl radical.     

Fluorene as the π-spacer for new two-photon absorption chromophores

We report herein the design and synthesis of two new quadrupolar D-π-A-π-D chromophores containing diphenyl amine and dicyanobenzene or 2,1,3-benzothiadiazole as electron donor (D) and acceptor (A), respectively, which are bridged by fluorene linkage (π). The introduction of coplanar fluorenes is highly beneficial for the enhancement of two-photon absorption (TPA), where 1b displays a TPA cross section (σ₂) of up to 1975±207 GM.     

A Theranostic Agent Combining a Two‐Photon‐Absorbing Photosensitizer for Photodynamic Therapy and a Gadolinium(III) Complex for MRI Detection

The convergent synthesis and characterization of a potential theranostic agent, [DPP‐ZnP‐GdDOTA]^?, which combines a diketopyrrolopyrrole‐porphyrin component DPP‐ZnP as a two‐photon photosensitizer for photodynamic therapy (PDT) with a gadolinium(III) DOTA complex as a magnetic resonance imaging probe, is presented. [DPP‐ZnP‐GdDOTA]^? has a remarkably high longitudinal water proton relaxivity (19.94 mm ^?1 s^?1 at 20 MHz and 25 °C) for a monohydrated molecular system of this size. The Nuclear Magnetic Relaxation Dispersion (NMRD) profile is characteristic of slow rotation, related to the extended and rigid aromatic units integrated in the molecule and to self‐aggregation occurring in aqueous solution. The two‐photon properties were examined and large two‐photon absorption cross‐sections around 1000 GM were determined between 910 and 940 nm in DCM with 1 % pyridine and in DMSO. Furthermore, the new conjugate was able to generate singlet oxygen, with quantum yield of 0.42 and 0.68 in DCM with 1 % pyridine and DMSO, respectively. Cellular studies were also performed. The [DPP‐ZnP‐GdDOTA]^? conjugate demonstrated low dark toxicity and was able to induce high one‐photon and moderate two‐photon phototoxicity on cancer cells.     

A novel self-assembly based on double-layer zinc porphyrin sensitizers in supramolecular solar cell

Two new self-assemblies based on zinc porphyrin dyes ZnPx-ZnPA (x?=?1, 2) were synthesized and used for dye-sensitized solar cells. Capping layer dyes consist of zinc porphyrin (ZnP) as core unit and carbazole (P1), phenothiazine (P2) as electron-donating group. The dye ZnPA contains carboxylic group as an anchor porphyrin with upper dyes (denoted as ZnP1 and ZnP2) formed dyads by coordination bonds of Zn-to-ligand self-assembled devices. The assembly modes were also verified by transmission electron microscopy (TEM). The influence of the upper porphyrins on molecular structure as well as photovoltaic performance was investigated via photophysical and electrochemical studies and calculations. With addition of the phenothiazine unit, the ZnP2-ZnPA possesses better light-harvesting capability with a significantly red-shifted Q-band. The photoelectrochemical efficiencies for ZnP2-ZnPA are better than those of ZnP1-ZnPA ascribed to larger J_sc and V_oc.

     

Tailoring the donor moieties in TPA-based organic dyes for efficient photovoltaic,optical and nonlinear optical response properties

The current study reports tailoring the electronic donor structures of organic dyes to modify their optical and nonlinear optical (NLO) response properties. Five (5) tri-phenyl amine (TPA) based Donor-π-Acceptor (D-π-A) organic dyes with the codes ICAA1 , ICAA2 , ICAA3 , ICAA4 , and ICAA5 were designed and investigated for their optical and NLO properties using quantum chemical methods. Optical and NLO properties of these dyes were studied by CAM-B3LYP method and 6-311G* basis set. The focus has been on the impact of adding secondary donors and shifting their substitutions at ortho (o), meta (m) and para (p) positions. Among all designed compounds, ICAA4 showed the highest amplitude of average third-order NLO polarizability <γ>, which is calculated to be 1316 × 10⁻³⁶ esu. Time-dependent Density Functional Theory (TD-DFT) method was used to determine how a change in the position of the donor affected the excitation energy (E_g) and NLO response properties. The findings showed that changing the position of the secondary donor results in a red shift among absorption spectra as well as the increase in their NLO responses. Complete process of intramolecular charge transfer (ICT) has been investigated in terms of different optical parameters such as frontier molecular orbitals (FMOs), molecular electrostatic potentials (MEPs), transition density matrix (TDMs), density of states (DOS), electron density difference (EDD), and natural bond orbital (NBO) analysis. Our calculations for study of ICT process indicate that p-position of methoxy group performs better among all other positions and even it has better NLO response properties than the compound with three collective methoxy groups. The calculated V_oc values of all designed molecules range from 1.09 to 1.30, all of them are positive while their ΔG_inject is found to be in the range of −0.87 to −1.79 eV indicating their decent potential for photovoltaic applications. The studied optical, NLO and photovoltaic parameters illustrated that ICAA1 to ICAA5 are appropriate molecules not only for NLO applications but also for efficient photovoltaic purposes.     

Some remarks on the semiempirical one-center electron repulsion integrals

The identification of the stage of ionization for various kinds of one-center electron repulsion integrals, occurring when nonbonding or lone-pair electrons are considered explicitly as well as π-electrons, is discussed for conjugated organic molecules containing heteroatoms N. It is concluded that the value for the negative ions should be used for (π_Cπ_C | π_Cπ_C) in all the states but for (π_Nπ_N | π_Nπ_N) only in the π-π states. In the n-π states, the appropriate value of (π_Nπ_N | π_Nπ_N) is that of the neutral atom if the molecule contains only one N atom. If more than one N atom is involved in the molecule, some weighted mean of the values for the negative ion and for the neutral atom should be used. The value for the neutral atom is most adequate for one-center repulsion integrals other than the (ππ | ππ) type in both the π-π and the n-π states. The method of determining these integrals is also discussed. It is concluded that they are to be determined from the consideration of appropriate electron-transfer reactions except for exchange integrals. The exchange integrals are shown to have to be determined from the Slater–Condon parameters derived from the analysis of the experimental atomic energy levels. Illustrative calculations are given for the lower singlet levels of the formaldehyde, pyrazine, pyridine, and the p-benzoquinone molecule. It is found that the calculated energies of the n-π transitions become much too low unless the (ππ | ππ) values of the heteroatoms in the molecule are chosen differently in the n-π states and in the π-π states as pointed out theoretically in this article.     

Systematic Control of Structural and Photophysical Properties of π-Extended Mono- and Bis-BODIPY Derivatives

A series of π-extended mono- and bis-BODIPY (BODIPY=boron-dipyrromethene) derivatives, namely, benzo[a]phenanthrene-fused BODIPY (Phena-Mono-BDP), benzo[a]anthracene-fused BODIPY (Ant-Mono-BDP), and dibenz[a,h]anthracene (DBA)-bridged bis-BODIPY (Ant-Bis-BDP), were designed and synthesized to examine their structural, electrochemical, and photophysical properties. Single-crystal X-ray crystallographic analyses demonstrated the planar configuration of Ant-Mono-BDP, in contrast to that of nonplanar Phena-Mono-BDP, whereas Ant-Bis-BDP has a DBA-centered planar configuration and two terminal nonplanar units of BODIPYs. The oxidation and reduction potentials agree with the estimated energies obtained through DFT calculations. The localized HOMO and LUMO states suggested the intramolecular charge-transfer characteristics in these BODIPY derivatives. The absorption spectra of these compounds extended up to the near-IR region. Strong redshifted trends of fluorescence spectra were observed in Ant-Bis-BDP with increasing solvent polarity, as supported by the differences in dipole moments estimated from Lippert–Mataga plots. To evaluate the excited-state dynamics of these molecules, the fluorescence quantum yield (Φ_FL) of Ant-Bis-BDP dramatically increased in the range from 0.05 to 0.86, with decreasing solvent polarity. Finally, the efficient two-photon absorption cross section of Ant-Bis-BDP (ca. 1200 GM at λ=1000 nm) was also obtained by considering the large π-extended structure (acceptor–donor–acceptor type).     

Photoelectron-spectroscopic Evidence Concerning “Homo-aromaticity”

The first of the two π-bands in the photoelectron spectrum of cis-cis-cis-1, 4, 7-cyclononatriene (I, symmetry C_3v) shows a Jahn-Teller split. This is consistent with the prediction of molecular orbital theory that the top occupied orbitals of I are e (π) and a ₁(π) respectively. From the difference ?( e (π)) - ?( a ₁(π)) = 0.90 to 0.97 eV a value of β_1,3 = ?0.68 eV = 0.27 β (β = -2.5 eV) is obtained for the homoconjugative interaction of two π-orbitals in I. This value represents almost exclusively through-space interaction between the π-orbitals. Through-bond interaction (hyperconjugation) is a minor effect in I. A comparison of the photoelectron data of bicyclo [4.2.1] nonatriene with those of norbornene and cycloheptadiene shows that homoconjugation (homo-aromaticity) can only be detected by photoelectron spectroscopy if the interacting π-bonds (basis orbitals) are symmetry equivalent or have accidentally (almost) degenerate energies.