Theoretical prediction of one- and two-photon absorption properties of N-annulated quaterrylenes as near-infrared chromophores

Graphene nanoribbons (GNRs) have attracted increasing attention due to high potentiality in nanoelectronics. In the present study, quantum-chemical calculations of structural and nonlinear optical properties have been first carried out for the nanoelectronical materials, a new series of ladder-type N-annulated quaterrylenes and their imide chromophores. The effects of the solvent, terminal groups, the number of N-annulated bridges, and π-conjugated length are discussed in detail. The solvent effect is significant on the one-photon absorption (OPA). Moreover, the OPA and two-photon absorption (TPA) properties of the two series of DI and N-MI molecules show a clear solvent dependence, which is attributed to the carboximide substitution featuring larger polarization. Introducing electron-donating groups and dicarboximides and increasing the conjugated length lead to red-shifts of the OPA, emission, and TPA spectra, lower emission lifetimes, and enhanced TPA cross sections (δ(max)), but further extension of the conjugated framework does not always promote an increase of δ(max). The changing trends of δ(max) can be explained by the transition moment and the intramolecular charge transfer. All N-annulated quaterrylene and their imide derivatives possess small energy gaps, intense near-infrared absorption and emission, and large δ(max), which are important for use as two-photon fluorescent labeling materials.     

Attachment of different donor groups to a cryptand for modulation of two-photon absorption cross-section

Two-photon absorption (TPA) properties of a laterally nonsymmetric aza cryptand with attached side arms have been investigated. This series of Schiff base derivatives supports the mechanistic approach for enhancing the TPA process, which is usually dictated by molecular geometry, pi-bridging, delocalization length, and corresponding charge-transfer possibilities. The results described here suggest that on increasing the branching units, the TPA cross-section, sigma((2)), can be tuned to a larger value. The TPA activity is "switched on" when a metal atom enters the cavity and serves as a conduit of electronic delocalization. The sigma((2)) value increases as the donor strength increases. The maximum value is obtained on moving from the single-branched system to the nearly threefold symmetry. This serves as a useful synthetic strategy for designing novel octupolar molecules with high sigma((2)) values. Theoretical calculations at the B3LYP functional with the 6-31G* basis set under DFT formalism provide supporting evidence that the communication between the side arms through the metal d orbital and more ordered geometry of chromophores leads to a smaller HOMO-LUMO gap, which has a great influence upon the electronic properties of the molecules.     

Theoretical study of one- and two-photon absorption properties of pyrene derivatives

The geometrical structure, electronic structure, one-photon absorption (OPA) properties of pyrene and its derivatives have been theoretically studied by using density functional theory (DFT) method and Zerner’s intermediate neglect of differential overlap (ZINDO) methods, and their two-photon absorption (TPA) properties are calculated by the ZINDO/sum-over-states method. The results show that introducing donor groups to pyrene molecule, increasing the number of donor groups, extending the conjugated length, or forming circular conjugated dimer can increase the oscillator strength (f) in the TPA process and ultimately result in extremely large TPA cross-sections and strong OPA around 400 nm of pyrene derivatives. All these results give us some basic principles to design pyrene derivatives with large TPA cross-sections. This shed light into the significance of the pyrene derivatives as promising fluorescent probes in biochemistry when they were linked to some special recognizing groups.     

Two-photon absorption and nonlinear optical properties of octupolar molecules.

Two-photon absorption (TPA) cross sections of four representative series of octupolar molecules are theoretically investigated. The general structure--TPA-property relationship is described by using the effective four-state valence-bond three-charge-transfer model. As the charge-transfer character of the ground electronic state increases due to the strong donors or acceptors, (i) the transition dipole matrix elements between the ground and 2-fold degenerate excited states increase, (ii) the energy gap decreases, and consequently (iii) the TPA transition amplitude monotonically increases. Thus, the design strategy to maximize the TPA cross section of the octupolar molecule is established. On the basis of the four-state model, the first hyperpolarizability of the octupolar molecule is found to be linearly proportional to the TPA cross section. This theoretical relationship is confirmed by using the ab initio calculation results. The Hammett correlation analysis of the TPA cross section and first hyperpolarizability is also presented.     

Pyrene-based organic dyes with thiophene containing π-linkers for dye-sensitized solar cells: optical, electrochemical and theoretical investigations

A new series of metal-free organic dyes containing pyrene and α-cyanoacrylic acid end groups and thiophene, bithiophene, thienylbenzene or thienylfluorene π-linkers were synthesized and characterized by absorption, emission and electrochemical measurements. Time-dependent density functional theoretical calculations were also performed to unravel the nature of the absorption induced electronic excitations. Extension of conjugation in the π-linker by the incorporation of phenyl or fluorene was found to enhance the molar extinction coefficient while the use of thiophene red-shifted the absorption. The longer wavelength absorption peaks found for the dyes were attributed to π-π* transition with a contribution from the charge transfer transition which becomes prominent for the bithiophene bridged derivative. The bithiophene containing dye showed moderate overall light-to-electron conversion efficiency attributable to the favorable absorption and redox properties originating due to the presence of a bithiophene segment. The trends observed for the various dyes in the device performance were rationalized by electron-impedance spectroscopy measurements.     

Branched charge-transfer chromophores featuring a 4,5-dicyanoimidazole unit

Six branched and stable push-pull chromophores featuring 4,5-dicyanoimidazole as an acceptor moiety, an N,N-dimethylamino group as a donor and various π-conjugated linkers are reported. Systematic extension of the π-linker revealed that the optical and electrochemical properties of A-π-D chromophores are mainly affected by the nature of the π-conjugated backbone (length and planarity) as well as by the number of appended donors.     

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.     

Interligand transmetallic charge-transfer transitions in mixed-ligand chelates

In chelates of divalent zinc which have two different aromatic ligands, a new absorption band is observed in the visible region. This new band is interpreted as arising from a π→π* interligand transmetallic charge-transfer (ITCT) transition. The nature of this type of charge-transfer transition in a chelate molecule, using a molecular orbital model, is examined. Experimental results for the chelates (3,4-toluenedithiolato)(1,10-phenanthroline)zinc(II), (3,4-toluenedithiolato)(2,2′-dipyridyl)zinc(II),and (3,4-toluenedithiolato)(2,2′-biquinoline)zinc(II) are presented.     

Theoretical study of two-photon absorption properties of a series of ferrocene-based chromophores

The electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties of a series of ferrocene-based chromophores with TCF-type acceptors (TCF = 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran) have been studied by using the ZINDO-SOS method. The results have revealed that OPA and TPA of ferrocenyl derivatives are affected by the strength of the acceptor, especially the pi-bridge conjugation length. The TPA cross section increases with increasing acceptor strength and pi-bridge conjugation length. The TCF-type acceptor with a phenyl group can lead to a larger TPA cross section. Quadrupole molecules have the largest TPA cross sections (2000-3000 GM), which are about 4 times that of the corresponding dipolar molecules, indicating larger interactions between the top and bottom branches. Finally, the origins of the two-photon excitations for ferrocenyl derivatives are analyzed. The calculations show that ferrocenyl derivatives with TCF-type acceptors (especially quadrupole molecules) are promising candidates for TPA materials.     

Solvent Effects on Two-Photon Absorption of Alkyne and Alkene π-bridging Chromophores

The present work concerns the study of solvent effects on the geometrical structures, as well as one- and two-photon absorption (TPA) processes, for two series of alkyne and alkene π-bridging molecules, within the framework of the polarization continuum model. Particular emphasis was put on the characterization of solvent effects on the molecular geometrical structures and geometric distortion, which were measured by the bond-length-alternation parameter. The π centres in the compounds are seen to play a decisive role in increasing the TPA cross section and nonlinear optical properties. All studied molecules have relatively strong TPA characteristics, while the alkyne π-bridging ones yield larger TPA cross sections.     

Acene Size-Dependent Transition of The Radical Centers From the Metal to The Acene Parts In Monocationic Dinuclear (Diethynylacene)diyl Complexes

Controlling radical localization/delocalization is important for functional materials. The present paper describes synthesis and results of electrochemical, spectroscopic, and theoretical studies of diruthenium (p-diethynylacene)diyl complexes, Me₃Si-(C≡C)₂-Ru(dppe)₂-C≡C−Ar-C≡C−Ru(dppe)₂-(C≡C)₂-SiMe₃ ( 1–6 ) (dppe: 1,2-bis(diphenylphosphino)ethane), and their monocationic radical species ([ 1 ]⁺–[ 6 ]⁺). The HOMO-LUMO energy gaps can be finely tuned by the acene rings in the bridging ligands installed, as indicated by the absorption maxima of the electronic spectra of 1–6 ranging from the UV region even to the NIR region. The cationic species [ 1 ]⁺–[ 6 ]⁺ show two characteristic NIR bands, which are ascribed to the charge resonance (CR) and π-π* transition bands, as revealed by spectroelectrochemistry. Expansion of the acene rings in [ 1 ]⁺–[ 6 ]⁺ causes (1) blue shifts of the CR bands and red shifts of the π-π* transition bands and (2) charge localization on the acene parts as evidenced by the ESR, DFT and TD-DFT analyses. Notably, the monocationic complexes of the larger acene derivatives are characterized as the non-classical acene-localized radicals.     

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。     

Hydrogen bond effect on the bonding and B2 molecular vibrations of pyrrole

The influence of the hydrogen bond on the infrared B₂ symmetry modes of pyrrole (C_2ν) has been studied. CNDO/2 calculations suggest that the most probable structure for the associated pyrrole involves the interaction NH ⋯ π. The shortest distance between the nitrogen atom and the ring of the nearest neighbor molecule is about 2.9 Å. The absorption bands of pyrrole in the gaseous state at 474, 626 and 720 and 826 cm⁻¹ have been assigned to the NH, ring and CH out-of-plane deformation modes.     

Synthesis of Anthracene Derivatives with Azaacene-Containing Iptycene Wings and the Utilization as a Dopant for Solution-Processed Organic Light-Emitting Diodes

Substituted acene derivatives are regarded as promising materials for organic electronic devices such as organic light-emitting diodes (OLEDs). In particular, anthracene derivatives are known to exhibit good fluorescence property, with the air stability and solubility in common organic solvents expected to give advantages for solution-processed device fabrication. In this study, a series of bistriisopropylsilyl(TIPS)ethynyl anthracene derivatives with azaacene-containing iptycene wings have been synthesized by using condensation reactions. Effects of size of azaacenes on optical properties and packing structures were investigated. UV/Vis absorption and fluorescence spectra indicate that the π-elongation of iptycene units has small effects on the overall π-system, which is also supported by electrochemical measurements. Secondly, single-crystal X-ray analysis implies that the molecules likely have interactions with the iptycene units of adjacent molecules, while the iptycene wings and TIPSethynyl groups can prevent the central anthracene unit from undesirable non-radiative energy loss. Finally, the most emissive derivative was used as a dopant for solution-processed OLEDs, showing obvious electroluminescence with a luminance of over 920 cd m⁻².     

具有大的双光子吸收截面的四羧酸二萘嵌苯衍生物的理论研究

用密度泛涵方法和ZINDO方法,从理论上研究了一系列四羧酸二萘嵌苯的衍生物(PTCDS)。对其平衡几何、电子结构和单双光子吸收性质进行了详细的计算研究。结果表明,此系列衍生物的双光子吸收截面主要由其主体结构(二萘嵌苯)决定。△EH-Lgap (HOMO与LUMO间的能隙)的减小,HOMO轨道能量的增大,分子主轴两端取代基给电子强度的增强,共轭长度的增大,分子刚性的增强都有利于此系列化合物双光子吸收截面的增大。并设计了比文献[22]报道的分子C有更大的双光子吸收截面的两个分子D1,D2。     

Theoretical insight into linear optical and two-photon absorption properties for a series of N-arylpyrrole-based dyes

N-arylpyrrole-based dyes possessing excellent opto-electronic properties are promising candidates for two-photon fluorescence labeling materials. The systematic investigation of novel N-arylpyrrole derivatives is of great importance for both fine-tuning electronic spectra and designing two-photon absorption (2PA) materials. We thoroughly studied influences of the π-conjugated center and N-substituted pyrrole moieties on the linear optical and 2PA properties. Our results show that introducing N-arylpyrrole produces bathochromic-shifts of the absorption and emission bands and an enhancement of the 2PA cross section (δ(max)), demonstrating that the electron-rich pyrrole moieties can efficiently increase intramolecular charge transfer. Substitution of fluorenyl with benzothiadiazole produces the lower energy gap, higher emission lifetime, red-shift of 2PA spectra and larger δ(max). The absorption and emission bands are highly dependent on the nature of the active building blocks. The aromatic rings attached to pyrrole can modulate the absorption peaks in the high energy and thus subtly modify the electronic properties.     

An INDO-CI method in π-approximation for the calculation of transition metal complexes with organic ligands—application to iron(II)-trisdiimine complexes

An INDO-CI method in π-approximation extended for transition metal complexes with organic ligands is presented. The σ-polarization in the complex is estimated using a two-dimensional EHT calculation and considered in the π-calculation by means of the diagonal Fock matrix elements. The method is tested for four trisdiimine iron(II) complexes. In agreement with the experimental absorption spectra the calculations show that diimine complexes with non-heterocyclicly bound nitrogen atoms exhibit a large red-shift of the characteristic charge-transfer transition increasing the number of aromatic rings. Contrarily, for diimine complexes with heterocyclicly bound nitrogen atoms no shift of the charge-transfer transition was obtained.     

Building symmetric two-dimensional two-photon materials

Two-dimensional multi-annulenic carbon networks are important molecules with possible applications in optoelectronic devices and nonlinear optics. Investigations of two-photon absorption (TPA) cross sections have been carried out in a series of annulenes with a basic building block approach and variable symmetries. Enhancement of the TPA cross section has been observed with an increase in number of building blocks and order of symmetry. Evaluations of the ground-state transition dipole moment and chromophore density are not sufficient to explain the observed enhancement. Estimates of excited-state transition dipole moments made by femtosecond transient absorption measurements are able to successfully predict the observed trend in TPA cross section. It has been observed that the symmetry of the molecule plays a vital role in enhancing the TPA cross section by virtue of increasing the excited-state transition dipole moment.     

Molecular engineering and theoretical investigation of organic sensitizers based on indoline dyes for quasi-solid state dye-sensitized solar cells

Novel indoline dyes, I-1-I-4, with structural modification of π-linker group in the D-π-A system have been synthesized and fully characterized. Molecular engineering through expanding the π-linker segment has been performed. The ground and excited state properties of the dyes have been studied by means of density functional theory (DFT) and time-dependent DFT (TD-DFT). Larger π-conjugation linkers would lead to broader spectral response and higher molar extinction coefficient but would decrease dye-loaded amount on TiO(2) electrode and LUMO level. While applied in DSSCs, the variation trends in short-circuit current density (J(sc)) and open-circuit voltage (V(oc)) were observed to be opposite to each other. The internal reasons were studied by experimental data and theoretical calculations in detail. Notably, I-2 showed comparable photocurrent values with liquid and quasi-solid state electrolyte, which suggested through molecular engineering of organic sensitizers the dilemma between optical absorption and charge diffusion lengths can be balanced well. Through studies of photophysical, electrochemical, and theoretical calculation results, the internal relations between chemical structure and efficiency have been revealed, which serve to enhance our knowledge regarding design and optimization of new sensitizers for quasi-solid state DSSCs, providing a powerful strategy for prediction of photovoltaic performances.     

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.