Enhanced diradical nature in oxyallyl derivatives leads to near infra red absorption: a comparative study of the squaraine and croconate dyes using computational techniques

We apply many criteria to estimate the diradical character of the ground state singlets of several oxyallyl derivatives. This is carried out as the oxyallyl derivatives like squaraine and croconate dyes can be represented by both mesoionic and diradical formulas, the domination of which would characterize its lowest energy transition. One criterion applied is the singlet-triplet gap, which is known to be inversely proportional to the diradical character. Another criterion is the occupation number; this is determined for the symmetry broken state of the molecules in the unrestricted formalism, and the difference of occupation in the HOMO and LUMO is related to the diradical character. The diradical character of all of the croconates and few squaraines is estimated to be large. All of these have absorption above 750 nm and can be classified as near infrared (NIR) dyes, leading to the inference that NIR absorptions in these molecules are largely due to the dominance of the diradical character. To understand the reliability of the DFT methods for the absorption property predictions of these molecules, TD-DFT studies to calculate the vertical excitation energies have been carried out, using the B3LYP/ BLYP exchange correlation functionals and the LB94 asymptotic functional with and without the inclusion of solvent. The deviations, in both the squaraine series (average lower diradical character), are found to be systematic, and with the inclusion of the solvent in the calculation, the deviations decrease. The best least-squares fit with the experimentally observed values using B3LYP /6-311G(d, p) for the symmetric squaraines yields an R value of 0.92 and, for the unsymmetric squaraines, an R value of 0.936. With inclusion of the solvent, the R value is 0.96 for the symmetric squaraines and 0.961 for the unsymmetric squaraines, indicating that these DFT functionals with linear scaling may be used to study these systems. The croconate dyes, however, have larger deviation from the experimentally observed values in all of the functionals studied even after inclusion of the solvent effects. The deviations are also not systematic. The deviation with respect to the experiment in this case is attributed to the average larger diradical character in this series.     

Role of the oxyallyl substructure in the near infrared (NIR) absorption in symmetrical dye derivatives: A computational study

It is well-known from experimental studies that the oxyallyl-substructure-based squarylium and croconium dyes absorb in the NIR region of the spectrum. Recently, another dye has been reported (J. Am. Chem. Soc. 2003, 125, 348) which contains the same basic chromophore, but the absorption is red-shifted by at least 300 nm compared to the former dyes and is observed near 1100 nm. To analyze the reasons behind the large red shift, in this work we have carried out symmetry-adapted cluster-configuration interaction (SAC-CI) studies on some of these NIR dyes which contain the oxyallyl substructure. From this study, contrary to the earlier reports, it is seen that the donor groups do not seem to play a major role in the red-shift of the absorption. On the other hand, on the basis of the results of the high-level calculations carried out here and using qualitative molecular orbital theory, it is observed that the orbital interactions play a key role in the red shift. Finally, design principles for the oxyallyl-substructure-based NIR dyes are suggested.     

Near-infrared absorption in symmetric squarylium and croconate dyes: a comparative study using symmetry-adapted cluster-configuration interaction methods

Symmetric croconate (CR) and squarylium dyes (SQ) are well-known near-infrared (NIR) dyes and, in general, are considered to be donor-acceptor-donor type molecules. It is established in the literature that CR dyes absorb in a longer wavelength region than the corresponding SQ dyes. This has been attributed to the CR ring being a better acceptor than the SQ ring. Thus increasing the donor capacity should lead to a bathochromic shift in both SQ and CR. On the other hand, some experiments reported in the literature have revealed that increasing the conjugation in the donor part of the SQ molecule leads first to red shift, which upon a further increase of the conjugation changes to a blue shift. Hence, to understand the role of the central ring and the substitutions in the absorption of these dyes, we carried out high-level symmetry-adapted cluster-configuration interaction (SAC-CI) calculations of some substituted SQ and CR dyes and compare the absorption energy with the existing experimental data. We found that there is very good agreement. We also carried out SAC-CI calculations of some smaller model molecules, which contain the main oxyallyl substructure. We varied the geometry (angle) of the oxyallyl subgroup and the substitution in these model molecules to establish a correlation with the bathochromic shift. We found that the charge transfer is very small and does not play the key role in the red shift, but on the other hand, the perturbation of the HOMO-LUMO gap (HLG) from both the geometry and substitution seems to be responsible for this shift. We suggest as a design principle that increasing the donor capacity of the groups may not help in the red shift, but introducing groups which perturb the HLG and decrease it without changing the MO character should lead to a larger bathochromic shift.     

Density functional theory study on the first hyperpolarizabilities of organoimido derivatives of hexamolybdates

The first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of arylimido molybdate derivatives have been investigated by density functional theory (DFT). The molecular orbital character analysis reveals that organoimido-to-polyanion charge transfer may be responsible for the NLO properties of this kind of molybdate derivatives. The NLO study shows intra-ion charge transfer is helpful to increase the first hyperpolarizability of arylimido molybdate derivatives. The lengthening of organoimido pi-conjugation enhances the betavec value. System 4 has the largest betavec value at the static electronic field, 1.238 x 10(-27)esu. Orbital analysis shows that the degree of charge transfer between polyanion cluster and organic segment was increased when the second organoimido polyanion was introduced. The present investigation provides important insight into NLO origin and properties of polyanion arylimido molybdate derivatives.     

Theoretical investigation on redox-switchable second-order nonlinear optical responses of push-pull Cp*CoEt2C2B4H3-expanded (metallo)porphyrins

The second-order nonlinear optical (NLO) properties of the Cp*Co(C(2)H(5))(2)C(2)B(4)H(3)-expanded (metallo)porphyrins (Cp* = C(5)Me(5)) have been investigated by using ab inito RHF and density functional theory (DFT) methods. The investigation shows that the compound with expand porphyrin possesses remarkable large molecular hyperpolarizability β(tot) value, ~414.1 × 10(-30) esu (at LC-ωPBE level), and might be an excellent second-order NLO material. From the character of charge transfer (CT) transition, it indicates that the -Cp*Co(C(2)H(5))(2)C(2)B(4)H(3) acts as an electron donor in this kind of systems. As a result of the redox behavior on expanded (metallo)porphyrin, the redox switching character of the NLO responses for the systems 2a-4a has also been studied. The results show that the β(tot) values of reduced forms are larger than that of neutral ones. Furthermore, the time-dependent DFT calculation illustrates that reduced forms have a significant difference on the CT patterns versus neutral ones. The present investigation provides insight into the comparison with DFT results on estimating first hyperpolarizability and the NLO properties of the series of push-pull compounds.     

Theoretical study on a novel series of fullerene-containing organometallics Fe(eta5-C55X5)2 (X = CH, N, B) and their large third-order nonlinear optical properties

Geometry structures, electronic spectra, and third-order nonlinear optical (NLO) properties of Fe(eta (5)-C 55X 5) 2 (X = CH, N, B) have first been investigated by time-dependent density functional theory. We analyzed the intramolecular interactions between ferrocene and the C 50 moiety. The calculated electronic absorption spectrum indicates that the short wavelength transitions are ascribed to the C 50 moiety mixed charge transfer transition of ferrocene itself, while the low energy excitation transitions are ascribed to the unique charge transfer transition from ferrocene to C 50 moiety in these systems. The third-order polarizability gamma values based on sum of states (SOS) method show that this class of ferrocene/fullerene hybrid molecule possesses a remarkably large third-order NLO response, especially for Fe(eta (5)-C 55B 5) 2 with the static third-order polarizability (gamma av) computed to be -10410 x 10 (-36) esu and the intrinsic second hypepolarizability to be 0.250. Thus, these complexes have the potential to be used for excellent third-order nonlinear optical materials. Analysis of the major contributions to the gamma av value suggest that the charge transfer from ferrocene to C 50 moiety along the z-axis (through Fe atom and the centers of two hybrid fullerenes) play the key role in the NLO response. Furthermore, boron substitution is an effective way of enhancing the optical nonlinearity compared to CH and N substitution, owing to smaller energy gap and better conjugation through the whole molecule.     

Reversible redox-switchable second-order optical nonlinearity in polyoxometalate: a quantum chemical study of [PW11O39(ReN)]n- (n = 3-7)

In this paper, the relationship between the reversible redox properties and the second-order nonlinear optical (NLO) responses for the title series of complexes has been systematically investigated by using the time-dependent density functional theory (TDDFT) method combined with the sum-over-states (SOS) formalism. The results reveal that the successive reduction processes of five PW11ReN redox states should be PW11ReVII (1) --> PW11ReVI (2) --> PW11ReV (3) --> PW11ReV1e ( 4) --> PW 11ReV2e (5). Furthermore, their electrochemical properties have been reproduced successfully. It is noteworthy that the second-order NLO behaviors can be switched by reversible redox for the present studied complexes. Full oxidation constitutes a convenient way to switch off the second-order polarizability (system 1). The incorporation of extra electrons causes significant enhancement in the second-order NLO activity, especially for the third reduced state (system 4), whose static second-order polarizability (betavec) is about 144 times larger than that of fully oxidized 1. The characteristic of the charge-transfer transition corresponding to the dominant contributions to the betavec values indicates that metal-centered redox processes influence the intramolecular donor or acceptor character. Therefore, these kinds of complexes with the facile and reversible redox states could become excellent switchable NLO materials.     

Polarizability and first-order hyperpolarizability of cyclic imides

Dipole moment, polarizability, and first-order hyperpolarizability of cyclic imides (maleimide, succinimide, phthalimide and some of their derivatives) have been investigated using ab initio and density functional theory calculations. It is found that 4,5-dichloro-, and 3,4,5,6-tetrachlorophthalimide have highest mean polarizabilities and total hyperpolarizabilities among the studied molecules. Furthermore, polarized continuum model has been employed to investigate solvent effects on the nonlinear optical (NLO) properties of succinimide; results indicate that solvent polarity has considerable influence on the NLO response of the molecules.     

Lindqvist型多酸亚胺衍生物的理论研究及设计

多金属氧酸盐的修饰化学是近年发展起来的一个热点研究领域,其中多酸的亚胺化是一种非常有效的使多酸有机官能化的方法.有机胺能够将其π电子扩展到无机框架,产生较强的d-π相互作用,从而多金属氧酸盐有机胺衍生物和远程有机官能团可以作为构筑单元构建更为复杂的多金属氧酸盐-有机杂化材料.本文综述了作者研究小组运用密度泛函理论方法研究系列Lindqvist型多酸亚胺衍生物的稳定性、成键特征和非线性光学性质,深入探讨该类有机-无机杂化衍生物非线性光学性质的起源.     

链式多Li掺杂分子H(HC==N-Li)nH(n=1~6)的NLO性质及链长依赖性

对链式多Li掺杂体系H(HCN-Li)_nH(n=1~6)的结构与性质进行了研究. 发现随着链长n的增大, 体系中有两类分子出现. 当n=1, 2时, 由于额外电子轨道是空的, 从而形成了Li 盐分子; 而当n=3~6时, 额外电子轨道是占据的, 从而形成了具有大范围额外电子云的多Li电子化物分子. 对于系列体系H(HCN-Li)_nH(n=1~6), 其非线性光学(NLO)性质的依赖性呈现阶梯式增长的规律, 即静态第一超极化率β₀的次序为2179, 2776(n=1, 2)< 5492, 5487(n=3, 4)< 15235, 15377(n=5, 6), 表明增加Li原子掺杂数是提高NLO响应的新途径.     

Substituent Effects on the Structural Features and Nonlinear Optical Properties of the Organic Alkalide Li+(calix[4]pyrrole)Li−

The effects of substituents on the structure, character, and nonlinear optical (NLO) properties of the organic alkalide Li⁺(calix[4]pyrrole)Li^? were studied by density functional theory. Natural bond orbital analysis and vertical ionization energies reveal that electron‐donating substituents strengthen the alkalide character of Li⁺(calix[4]pyrrole)Li^? and that they are beneficial for a larger first hyperpolarizability (β₀) value. However, electron‐withdrawing substituents have the opposite effect. The dependence of the NLO properties on the number of substituents and their relative position was detected in multisubstituted Li⁺(calix[4]pyrrole)Li^? compounds. For both the amino‐ and methyl‐substituted derivatives, the polarizabilities and the first hyperpolarizabilities increase as more pyrrole β‐H atoms are substituted. Moreover, distribution of the substituents so that they are as far away from each other as possible resulted in an increase in the β₀ value. The new knowledge obtained in this study may provide an effective approach to enhance the NLO responses of alkalides by employing pyrrole derivatives as complexants.     

Boron Difluoride Curcuminoid Fluorophores with Enhanced Two‐Photon Excited Fluorescence Emission and Versatile Living‐Cell Imaging Properties

The synthesis of boron difluoride complexes of a series of curcuminoid derivatives containing various donor end groups is described. Time‐dependent (TD)‐DFT calculations confirm the charge‐transfer character of the second lowest‐energy transition band and ascribe the lowest energy band to a “cyanine‐like” transition. Photophysical studies reveal that tuning the donor strength of the end groups allows covering a broad spectral range, from the visible to the NIR region, of the UV–visible absorption and fluorescence spectra. Two‐photon‐excited fluorescence and Z‐scan techniques prove that an increase in the donor strength or in the rigidity of the backbone results in a considerable increase in the two‐photon cross section, reaching 5000 GM, with predominant two‐photon absorption from the S₀–S₂ charge‐transfer transition. Direct comparisons with the hemicurcuminoid derivatives show that the two‐photon active band for the curcuminoid derivatives has the same intramolecular charge‐transfer character and therefore arises from a dipolar structure. Overall, this structure–relationship study allows the optimization of the two‐photon brightness (i.e., 400–900 GM) with one dye that emits in the NIR region of the spectrum. In addition, these dyes demonstrate high intracellular uptake efficiency in Cos7 cells with emission in the visible region, which is further improved by using porous silica nanoparticles as dye vehicles for the imaging of two mammalian carcinoma cells type based on NIR fluorescence emission.     

Physicochemical studies of bioactive heterocycles of some novel imidazole derivatives as sensitive NLO materials

Some novel imidazole derivatives were developed as highly sensitive chemisensors for transition metal ions. A prominent fluorescence enhancement was found in the presence of transition metal ions such as Hg(2+), Pb(2+), Cu(2+), Zn(2+), Co(2+) and Fe(2+) and this was suggested to result from the suppression of radiationless transitions from the n-π* state in the chemisensors. By DFT calculation HOMO-LUMO energies were calculated, the electric dipole moment (μ) and the hyperpolarizability (β) of the investigated molecules have been studied experimentally and also theoretically. These synthesized molecules were found to have microscopic non-linear optical (NLO) behaviour with non-zero tensor components.     

Stereoselectivities and regioselectivities of (4 + 3) cycloadditions between allenamide-derived chiral oxazolidinone-stabilized oxyallyls and furans: experiment and theory

A systematic investigation of the regioselectivities and stereoselectivities of (4 + 3) cycloadditions between unsymmetrical furans and a chiral oxazolidinone-substituted oxyallyl is presented. Cycloadditions were performed using an oxyallyl containing a (R)-4-phenyl-2-oxazolidinone auxiliary (2(Ph)), under either thermal or ZnCl(2)-catalyzed conditions. Reactions of 2(Ph) with 2-substituted furans gave syn cycloadducts selectively, while cycloadditions with 3-substituted furans gave selectively anti cycloadducts. The stereoselectivities were in favor of a single diastereoisomer (I) in all but one case (2-CO(2)R). Density functional theory calculations were performed to explain the selectivities. The results support a mechanism in which all cycloadducts are formed from the E isomer of the oxyallyl (in which the oxazolidinone C═O and oxyallyl oxygen are anti to each other) or the corresponding (E)-ZnCl(2) complex. The major diastereomer is derived from addition of the furan to the more crowded face of the oxyallyl. Crowded transition states are favored because they possess a stabilizing CH-π interaction between the furan and the Ph group.     

12顶点[1-R-CB11-Me11]-碳硼烷氧化-还原过程二阶NLO效应的理论研究

采用密度泛函理论（DFT）对12顶点[1-R-CB₁₁-Me₁₁]^-碳硼烷的结构和二阶非线性光学（NLO）调节效应进行计算分析. 结果表明,C位连接的取代基R供、吸电子能力的不同以及分子发生可逆氧化-还原反应对分子构型有一定影响. 由自然键轨道（NBO）电荷和电子自旋密度分析可知,分子的氧化中心是碳硼笼,分子的氧化反应可导致碳硼笼部分给、受体特性发生改变. 氧化态分子的第一超极化率总有效值（β_tot）大于相应还原态分子,当C位取代基R为供电子基团（—NH₂）的分子时,氧化态与还原态的β_tot值变化最大. 这类分子的氧化-还原反应可以有效调节二阶NLO光学效应.     

含Schiff碱基噻唑衍生物二阶NLO性质的理论研究

采用密度泛函理论B3LYP/6-311G**方法,对一系列含Schiff碱基噻唑衍生物的二阶非线性光学(NLO)性质和电子光谱进行了研究.结果表明,分子中推、拉电子取代基强度的提高,噻唑环数的增加以及Schiff碱基的引入有利于提高分子的βtot值.光谱数据显示,分子中HOMO轨道向LUMO,LUMO+2及LUMO+3轨道的电子跃迁对分子的电子光谱有重要贡献.该系列化合物由于具有较大的二阶NLO响应与良好的透光范围,可以作为潜在的NLO材料.     

Theoretical study on the chiroptical optical properties of chiral fullerene C60 derivative

Time-dependent density functional theory (TDDFT) calculations have been used to investigate UV/CD spectra and nonlinear optical (NLO) property of the C(60)-fullerene bisadduct (R,R,(f,s)A)-[CD(+)280] for the first time. The electron transition natures of the four main measured bands are analyzed, and their results are used to designate the excited states involved in an electron-transfer process of the studied compound. On a comparative scale, the predicted excitation energies and oscillator strengths are in reasonable agreement with the observed values, demonstrating the efficiency of TDDFT in predicting the localized and charge transfer transitions. The good agreement between the experimental and the simulated CD spectra shows that TDDFT calculations can be used to assign the absolute configurations (ACs) of chiral fullerene C(60) derivatives with high confidence. The observed large dissymmetry ratio g (g = Δε/ε) at about 700 nm results from the orbital characters of the local fullerene excited state, which leads to large transition magnetic dipole moment and small transition electronic dipole moment. The different functionals and solvent effects on UV/CD spectra were also considered. The studied compound has a possibility to be an excellent second-order NLO material from the standpoint of transparency and large second-order polarizability value.     

Heterocycle-based isomeric chromophores with substantially varying NLO properties: a new structure-property correlation study

A number of heterocycle-based aromatic and quinonoid molecular systems have been considered for the theoretical study of their electric response properties. The nonlinear optical (NLO) parameters have been calculated by using the ab initio MO and DFT methods. An approximate scheme for calculating the first hyperpolarizability (beta) and second hyperpolarizability (gamma) in the framework of the sum-over-state (SOS) method have been proposed by exploiting the generalized Thomas-Kuhn sum rule (TK-SR). The NLO properties in the present scheme can be evaluated solely from the ground-state dipole moment (mu) and linear polarizability (alpha) and have been found to correlate fairly with the ab initio calculated values. The approximate scheme can be reasonably used to explain the wider range of variation of higher-order polarizabilities in terms of the above quantities. The position of the N atom in the thiazole ring at the ortho position (versus meta position) to the acceptor increases beta and decreases gamma for aromatic compounds, while the reverse trend is found with quinonoid compounds. In the case of the pyridine ring, the shifting of the N atom toward the acceptor enhances gamma, with insignificant variation of beta predicted for both the aromatic and quinonoid molecules. The negative contribution of the cubic polarizability of the quinonoid species increases linearly with alpha(2)/mean transition energy (Delta E).     

Theoretical study on the considerable second-order nonlinear optical properties of naphthylimido-substituted hexamolybdates

The static first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of [(2-methylnaphthyl)imido]hexamolybdates derivatives have been investigated by density functional theory (DFT). The [(2-methylnaphthyl)imido]hexamolybdate has considerable large first hyperpolarizability, 6.780 x 10(-30) esu, and it is larger than that of [(2,6-dimethylphenyl)arylimido]hexamolybdate due to the double aromatic rings in the naphthylimido ligand. The naphthylimido ligand acts as an electron-donor and the polyanion acts as an electron-acceptor. The substituent position on the naphthylimido is a key factor to determine the first hyperpolarizability of (naphthylimido)hexamolybdate derivatives. The derivative, which the iodine atom locates on the para nitrogen on the naphthylimido ligand, has the largest betao(o) value among the iodine-substituted derivatives. It suggests that the iodine atom is quasi linear with nitrogen and Mo, which is bonded to thenitrogen atom, could generate a large static electronic field and give the large contribution to NLO response.The introducing of electron-donors significantly enhances the first hyperpolarizabilities of (naphthylimido)hexamolybdates comparing with the electron-acceptors as the electron-donating ability is significantly enhanced when the electron-donor is attached to the naphthylimido segment. The present investigation provides important insight into NLO properties of (arylimido)molybdate derivatives.