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This Perspective describes some of the recent successes in modeling properties of chromophore containing domains of proteins capable to respond to visible light by using quantum chemistry methods. Applications to two distinctively different classes of such proteins are considered: those of the green fluorescent protein family with the chromophores derived from the hydroxybenzylidene-imidazolinone moiety and those containing the flavin-based chromophore. The major emphasis is on applications of economic computational strategies to estimate geometry configurations and electronic excitation energies. We show that using the molecular cluster approach and the “black-box” methods ZINDO and SOS-CIS(D) proves to be efficient for characterization of protein spectra. © 2013 Wiley Periodicals, Inc.

In the present study, the electronic energy transfer pathways in trimeric and hexameric aggregation state of cyanobacteria C-phycocyanin (C-PC) were investigated in term of the Forster theory. The corresponding excited states and transition dipole moments of phycocyanobilins (PCBs) located into C-PC were examined by model chemistry in gas phase at time-dependent density functional theory (TDDFT), configuration interaction-singles (CIS), and Zerner's intermediate neglect of differential overlap (ZINDO) levels, respectively. Then, the long-range pigment-protein interactions were approximately taken into account by using polarizable continuum model (PCM) at TDDFT level to estimate the influence of protein environment on the preceding calculated physical quantities. The influence of the short-range interaction caused by aspartate residue nearby PCBs was examined as well. Only when the protonation of PCBs and its long- and short-range interactions were properly taken into account, the calculated energy transfer rates (1/K) in the framework of Forster model at TDDFT/B3LYP/6-31+G* level were in good agreement with the experimental results of C-PC monomer and trimer. Furthermore, the present calculated results suggested that the energy transfer pathway in C-PC monomer is predominant from -155 to -84 (1/K = 13.4 ps), however, from -84 of one monomer to -84 (1/K = 0.30.4 ps) in a neighbor monomer in C-PC trimer. In C-PC hexamer, an additional energy flow was predicted to be from -155 (or -84) in top trimer to adjacent -155 (or -84) (1/K = 0.52.7 ps) in bottom trimer. (c) 2013 Wiley Periodicals, Inc.

Kindling fluorescent protein (KFP) is an important member of the colored proteins family widely used as biomarkers in living cells. We apply quantum chemistry modeling of KFP properties by computing structure and spectra of a large molecular cluster mimicking the chromophore-containing pocket, assuming the protonated (neutral) form of the chromophore in the trans configuration. We provide evidence that this protein conformation accounts for the previously observed but unassigned absorption band at 445 nm. We demonstrate that the structure and charge of the amino acid residues nearest to the chromophore play a role in the shifts in the absorption wave length, thus underlying a strong role of intermolecular interactions when considering properties of the fluorescent proteins. © 2012 Wiley Periodicals, Inc.

Optical properties of conjugated copolymer incorporating thienylene-dioctyloxyphenyle-thienylene and bipyridine units prepared by Stille coupling reactions in the ground and lowest singlet excited states were calculated with Time Dependent Density Functional Theory (TD-DFT), CIS/3-21G*, and semiempirical (ZINDO) methods. The complementarities and the good agreement between theoretical and experimental results have permitted us to describe electronic structure and to predict a correlation between structure and properties of this new copolymer. Then, on the basis of these results, we show that this copolymer could be exploited as an active layer in optoelectronic devices. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Spurred by the pioneering modeling studies of Sylvio Canuto on absorption spectra and solvatochromic shifts of organic molecules in polar and nonpolar solvents, we report in this work a computational study for the common optical dye probe Nile red (NR) to elucidate the origin of its absorption shift between gas phase and aqueous solution. The Car-Parrinello molecular dynamics (CPMD) technique is used for gas phase NR, whereas for NR in water solvent, a hybrid quantum mechanics-molecular dynamics (CPMD-QM/MM) approach has been utilized. For the configurations obtained from CPMD and CPMD-QM/MM, the absorption spectrum has been calculated using the INDO/CIS method as implemented in the ZINDO program. Different solvation shells for NR in water have been defined based on solute-all-atoms and solvent center of mass radial distribution function (g(rX–O)rdf). The electronic excitation energies for these solvation shells were calculated in a systematic way to evaluate their individual contributions. In addition, calculations of absorption spectra were performed for NR (excluding solvent molecules) obtained from CPMD-QM/MM calculations to isolate the contribution to the solvatochromic shift just due to solvent-induced geometrical change. Interestingly, this geometrical change in NR itself contributes as much as 50 nm to the solvatochromic shift. The calculated λmax for gas phase is around 488 nm and is comparable to the values reported for NR in nonpolar solvents, whereas the inclusion of solvent molecules in the hydration shell yields a λmax of 565 nm which contributes to almost 77 nm of the solvatochromic shift. The inclusion of solvent molecules up to the fourth solvation shell in the g(rX–O) rdf yields λmax of 596 nm which is in good agreement with the experimentally reported value 593 nm. The change in λmax due to inclusion of the fourth solvation shell is only 1 nm, indicating that the spectrum has converged with respect to the solvent effect. Overall, this study suggests that the combined use of CPMD-QM/MM and ZINDO can be successfully used to model and to interpret solvatochromic and thermochromic behavior of NR and other organic dye molecules. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

The photosynthetic light-harvesting system II (LH2) of Rhodospirillum molischianum is investigated using a time-dependent combination of molecular dynamics simulations and semiempirical ZINDO/S electronic structure calculations. The classical simulations are performed on the available crystal structure of the LH2 complex. Snapshots of the atomic fluctuations along this 12 ps long trajectory serve as input for the calculation of the excitation energies of the individual bacteriochlorophylls embedded in the LH2 complex. Furthermore, the couplings between the bacteriochlorophylls are computed using the method of transition charges from electrostatic potentials and for comparison also using the point-dipole approximation. With these quantities the excitonic energies of the complete system as well as the linear absorption spectra are calculated and compared to experimental findings.

Geometry optimizations of monomers were carried out with the use of Becke's three-parameter hybrid method with Lee, Yang, and Parr (B3LYP) gradient- corrected correlation functional and the standard 6-31G(d) basis set. UV-vis spectra of different naphthalene-POPOP complexes using ZINDO/S method were calculated. In these complexes, poly(methyl methacrylate) effect on chromophores was neglected and chromophores were staked. Analyzing available experimental data and obtained theoretical results, the most probable complex structure is determined. Molecular model of POPOP and naphthalene dimer corresponds to this structure. V C 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 2540-2544, 2011

On the basis of ZINDO methods, according to the sum-overstates (SOS) expression, the program for the calculation of the second-order nonlinear optical susceptibilities βijk and βμ of molecules was devised, and the structures and nonlinear optical properties of unsymmetric bis(phenylethynyl) benzene series derivatives were studied. The influence of the molecular conjugated chain lengths, the donor and the acceptor on βμ was examined.

The title polymers PNIPzS4 and PCuPzS4 were synthesized by reaction of phthalonitrile and 2,3,5,6-tetracyano-1,4-dithiin with corresponding metal salt. Their structures and properties were characterized by associating the experimental results with MO and Cl calculations of the dimer molecule as model polymer in the ZINDO method. It has been found that the PNiPzS4 (or PCuPzS4) shows the semiconductivity at T 278 K (or 286 K) and the conductivity increase with a hoist of temperature at 253 K (or 260 K) < T < 278 K (or 286 K). The conductivity σ of the PNiPzS4 and PCuPzS4 under pressure 13.73 MPa is 1.56×10−4 and 9.33×10−5 S/cm, respectively.

On the basis of ZINDO program, we have designed a program to calculate the nonlinear second-order polarizability βyk and βμ according to the SOS expression. The second-order nonlinear optical properties of 4-nitro-4′-dimethylamino-stilbene and a series of its thiazole derivatives were studied. The calculated results were that: When replacing a benzene ring in 4-nitro-4′-dimethylamino-stilbene by a thiazole ring, the influence on β values depends on the position of thiazole ring. When the thiazole ring connects with nitro group (acceptor), the β values increase significantly compared with corresponding stilbene derivatives. The β values of 2-(p-donor-β-styryl)-5-nitro-thiazole derivatives (2–7) are larger than those of 2-(p-nitro-β-styryl)-5-donor-thiazole derivatives (8–13) and 2-(p-donor-phenyl)-azo-5-nitro-thiazole derivatives (14–19). The 2-(p-donor-β-styryl)-5-nitro-thiazole derivatives (2–7) are good candidates as chromophores duo to their high nonlinearities and potential good thermal stability.

We have theoretically investigated the one- and two-photon absorption properties of C60 and C70 using the ZINDO method. From the results it is suggested that the one-photon absorption spectra are in agreement with the experimental observations. It is found that the maximum TPA cross section of C70 is more than twice that of C60 which is consistent with the experimental results. A notable point is that the TPA process of C60 is different from that of C70 as well as other ordinary conjugated molecules.

Star-shaped rigid molecules that comprise a 1,3,5-trisubstitued benzene core and three oligoaryleneethynylene arms have great potential application in organic light-emitting devices (OLEDs). Their optical and electronic properties are tuned by the star-shaped molecular size. To reveal the relationship between the properties and structures, we perform a systemic investigation for these organic molecules. The ground and excited state molecules are studied using density functional theory (DFT), the ab initio HF, and the single excitation configuration interaction (CIS), respectively. And the electronic absorption and emission spectra are investigated with time-dependent density functional theory (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials (IP), electron affinities (EA), absorption and emission spectra are controlled by the star-shaped molecular size, which favor the hole and electron injection into OLEDs. With increasing the molecular conjugated length, the absorption and emission spectra exhibit red shifts to some extent and are in good agreement with the experimental ones. Also, the calculated emission spectra range from 330 to 440 nm. All the calculated show that the star-shaped molecules are promising as blue light emitting materials

Two types of donor(D)–acceptor(A) calix[4]arenes have been theoretically studied using DFT//B3LYP/6-31G(d) method and ZINDO/CISD method. The calculations show that the substitution of CC by the conjugation bridge CC and NN plays an important part in altering one-photon absorption (OPA) and two-photon absorption (TPA) properties. The maximum OPA wavelengths of all studied compounds are less than 400 nm, which means high transparency. The geometry of the calixarenes strongly influences the TPA properties of the studied compounds. In addition, the nitro derivatives have a wider TPA response range than other non-nitro derivatives. The tetrasubstituted calix[4]arenes (type B calixarenes) have a larger TPA cross-section values than the bisubstituted calix[4]arenes (type A calixarenes). Copyright © 2009 John Wiley & Sons, Ltd.

The Fourier transform Raman and Fourier transform infrared (FT-IR) spectra of thioguanine have been recorded. Ab initio and density functional computations of the vibrational (IR) spectrum, the molecular geometry, Highest Occupied Molecular Orbital (HOMO)–Lowest Unoccupied Molecular Orbital (LUMO) energy gaps and polarizabilities were studied. On the basis of the comparison between calculated and experimental results and the comparison with related molecules, assignments of fundamental vibrational modes are examined. The observed and simulated spectra were found to be well comparable. The electronic transition energies and intensities of spectral lines were carried out using TDDFT and ZINDO methods. Copyright © 2009 John Wiley & Sons, Ltd.

Two series of porphyrin-thiophene chromophores were theoretically studied that exhibit large two-photon absorption cross-section in the visible region. The density functional theory (DFT) associated with ZINDO/SDCI methods has been used to calculate the molecular geometries, electronic structures, one-photon and two-photon absorption properties. The results show that the number of thiophene units affects the properties of one-photon absorption (OPA) and two-photon absorption (TPA). Porphyrin-thiophene chromophores featuring two or three thiophene units have wide TPA response ranges; they can be applied to many nonlinear optical areas, such as optical limiting. Intervening ethynyl unit is beneficial to extend the conjugated pathway, and increase the two-photon absorption cross-section. At the same time, the OPA and TPA wavelengths are bathochromically shifted. From viewpoint of the high transparency and large nonlinear optical response, porphyrin-thiophene chromophores will be promising TPA materials.

It has been proved that fullerene derivatives, in which an oligophenylenevinylene (OPV) group is attached to C(60), present an interesting photophysical phenomenon and can be incorporated into photovoltaic cells. In these systems, the OPV acts as electron donor upon excitation, and then fullerene absorbs photoexcited electrons. These new organic semiconductor materials offer the prospect of lower manufacturing costs and they present several advantages: easy fabrication, large area, flexible and light weight devices when compared with inorganic counter parts. In the present theoretical study, oligomeric chains of p-phenylenevinylene (n-PPV, n = 3-8 units) and C(60)-OPV hybrids have been studied by density functional theory (DFT). Electronic properties such as electronic absorption and emission spectra were calculated in order to determinate how the increment of spectroscopic units affects their electronic behavior. These properties were carried out with time dependent-density functional theory (TD-DFT) and ZINDO semiempirical method. The theoretical calculations of the structural properties of n-PPV and fullerene-OPV hybrids were obtained using PBE1PBE/6-31G and ONIOM two-layered version, respectively. All calculations were done with Gaussian 03W program package.

We have determined the π band-gaps of the main carotenoids present in poly(methyl)methacrylate/buriti blends, namely, trans-β-carotene, 13-cis-β-carotene, 9-cis-β-carotene, phytofluene, and zeaxanthin. Semiempirical, model Hamiltonian, and density functional calculations were carried out to study these structures. The geometries were fully optimized using AM1, PM3, and B3LYP/6-31G(d,p) methods. The TD-DFT and ZINDO/S methods were applied for the calculation of the electronic absorption spectra of the optimized B3LYP geometries. The calculated spectra using the polarizable continuum model for the solvent effects were compared with the available experimental. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

We focus this work on the theoretical investigation of the block-copolymer poly[oxyoctyleneoxy-(2,6-dimethoxy-1,4phenylene-1,2-ethinylene-phenanthrene-2,4diyl) named as LaPPS19, recently proposed for optoelectronic applications. We used for that a variety of methods, from molecular mechanics to quantum semiempirical techniques (AM1, ZINDO/S-CIS). Our results show that as expected isolated LaPPS19 chains present relevant electron localization over the phenanthrene group. We found, however, that LaPPS19 could assemble in a π-stacked form, leading to impressive interchain interaction; the stacking induces electronic delocalization between neighbor chains and introduces new states below the phenanthrene-related absorption; these results allowed us to associate the red-shift of the absorption edge, seen in the experimental results, to spontaneous π-stack aggregation of the chains. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

3,5-Disubstituted and 1,3,5-trisubstituted pyrazoles are readily synthesized from acyl chlorides, terminal alkynes, and hydrazines by a consecutive one-pot three-component Sonogashira coupling/Michael addition/cyclocondensation sequence in good to excellent yields. These pyrazoles are highly fluorescent, both in solution and in the solid state. Investigation of the electronic properties by UV/Vis and fluorescence spectroscopy and by DFT and ZINDO CI computations reveal that the excited state is highly polar and allows fine-tuning of the absorption and emission properties. X-ray structure analyses of 3,5-disubstituted pyrazoles reveal self-organization by hydrogen bonding and π-stacking. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The purpose of this work is to provide an in-depth interpretation of the optical and electronic properties of a series of aromatic oligomers and polymers, including [N-(4-(5-(3-(1,3,4-oxadiazol-2,5-ylene)phenyl)-1,3,4-oxadiazol-2-ylene)phenyl)-N-(1,4-phenylene)amine]n (NPPP)n and [N-(4-(5-(3-(1,3,4-oxadiazol-2,5-ylene)phenyl)-1,3,4-oxa-diazol-2-ylene)phenyl)-N-(1,4-phenylene)naphthalene-1-amine]n (NPPN)n (n=1–4). These polymers and oligomers show great potential for application to organic light-emitting diodes (OLEDs) as efficient blue emitters due to the tuning of the optical and electronic properties. The geometric and electronic structures of the oligomers in the ground state were investigated using density functional theory (DFT) and the ab initio HF, whereas the lowest singlet excited state of NPPP1 was optimized with ab initio configuration interaction singles (CIS). To assign the absorption and emission peaks observed in the experiment, the absorption and emission spectra of the ground and lowest singlet excited states were calculated with time-dependent DFT (TD-DFT) and Zerner's independent neglect of differential overlap (ZINDO). All DFT calculations were performed using the B3LYP functional and the 6-31G basis set. The results show that the HOMO, LUMO, energy gaps, ionization potentials, and electron affinities for these polymers are affected by increasing the conjugated chain, which favors the hole and electron injection into OLED. The trend of the variation of ΔH-L and the lowest excitation energies with 1/n, and the electronic structure and optical properties of these polymers were extrapolated and analyzed. The absorption spectra exhibit red shifts to some extent [the absorption spectra: 359.47 (NPPP1)

We report the development of an accurate computational procedure for the calculation of the n π* (λmax−1) and π π* (λmax−2) transitions of a set of thiocarbonyl derivatives. To ensure converged results, all calculations are carried out using the 6-311+G(2df,p) basis set for time-dependent calculations, and the 6-311G(2df,p) for the ground-state geometrical optimization. Starting with two hybrids, PBE0 and B3LYP, the Hartree–Fock exchange percentage (α) used is optimized in order to reach excitation energies that fit experimental data. It turns out that BLYP(α) is the more adequate functional for calibration. For the n π* excitation, the optimal α value lies in the 0.10–0.20 interval, whereas for the π π* process setting α equal to 0.10 provides the most accurate results. The corresponding mean absolute errors (MAE) are limited to 17 nm for λmax−1, and to 10 nm for λmax−2, allowing a consistent and accurate prediction of both transitions. We also assess the merits of the ZINDO//AM1 scheme and it turns out that the semi-empirical method only provides a poor prediction of the λmax of thiocarbonyl derivatives, especially for the n π* transition. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Quantum-chemical techniques were applied to investigate a series of conjugated polymers: poly(3-octylthien-2,5-ylene-co-pyrid-2,6-ylene) (p1), poly[pyrid-2,6-ylenebis(3-octylthien-2,5-ylene)] (p2) and poly[pyrid-2,5-ylene- bis(3-octylthien-2,5-ylene)] (p3) comprising alternating π-excessive 3-alkylthiophene and π-deficient meta- or para-linked pyridine moieties. Their ground state and excited state structures were optimized with density functional theory B3LYP method, and the optical properties were calculated by the time-dependent density functional theory (TD-DFT) and ZINDO/S methods. Their HOMO-LUMO gaps (ΔH-L), the lowest excitation energies (Eex), ionization potentials (IP) and electron affinities (EA) were obtained by extrapolating those of the polymers to the inverse chain length equal to zero (1/n=0). The calculated results showed that the decrease of pyridylene content increased the HOMO level and decreased the LUMO level while the para-linkage further contributed to it. The IP are in the order: p1>p2>p3 but EA are: p1

2-Pyrazin-2-ylethanethiol (PZT) interacts with gold nanoparticles (AuNps) and pentacyanidoferrates, providing a suitable bridging ligand for the exploitation of the coordination chemistry involved in the stabilization of the colloidal solutions and in the controlled formation of aggregates in the presence of dimethyl sulfoxide. The electronic and vibrational spectra of the molecules have been investigated and discussed based on DFT and ZINDO/S theoretical calculations. The PZT-modified AuNps exhibit strong SERS, coherent with the electromagnetic enhancement mechanism. In contrast, the charge-transfer mechanism seems to predominate in the Raman spectra of the [Fe(CN)5PZT]3–-modified gold nanoparticles.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

DNA damage caused by UV light radiation is often naturally repaired in a process initiated by excited state electron transfer from the photoactivated photolyase enzyme to the DNA cyclobutane pyrimidine dimer lesion. The active cofactor in the excited state electron transfer in the photolyase is the two-electron fully reduced form of the flavin adenine dinucleotide (FADH−). To calculate electron tunneling matrix element and model the DNA binding with photolyase, the LUMO of the FADH− calculated using extended Huckel method was previously chosen from the SCF wavefunctions. Recently, the DNA–photolyase complex was crystallized in its bound form, in good agreement with our previous model in even minute details at the active site. Here we carry out molecular dynamics simulation of the entire complex using the new experimental structure of Anacystis nidulans and identify the low-lying photoactivated states of the enzyme for the dynamical confirmations. Our results from ZINDO/S CIS calculations are compared with experimental UV spectra, and their implications for excited state electron transfer and energy transfer are discussed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

The ground and excited structures of the molecules are compared basis on the calculated by HF and CIS, respectively. The ionization potentials (IPs), electron affinities (EAs) and HOMO–LUMO gaps (ΔEHOMO–LUMO) of the oligomers are studied by the density functional theory (DFT) with B3LYP functional while the vertical excitation energies (Egs) and the maximal absorption wavelength λabs of oligomers of bifluorene and its derivatives DFE, DFA, DFBT, FDBO, and FSCHD are studied employing the time dependent density functional theory (TD-DFT) and ZINDO. Compared with BF, the derivatives DFE, DFA, and DFBT are better conjugated, easier to give an electron or a hole, as well as get an electron or a hole. Their HOMO–LUMO gaps are narrower and they have lower vertical excitation energies. The absorption and emission spectra of them are red shifting. However, FDBO and FSCHD are in the other way round. It is important that FDBO and FSCHD are good blue emitters. Copyright © 2007 John Wiley & Sons, Ltd.

The one-pot cyclisation to form the indolizinoquinoline-5,12-dione ring system has been investigated starting from 6,7-dichloroquinoline-5,8-dione by reaction with pyridine and ethyl acetoacetate. Both the N,N-syn and the N,N-anti products have been fully characterised by mass spectrometry and NMR analysis, and the regioselectivity of their formation is discussed in terms of solvent polarity and/or the nature of the metal ion. We demonstrate here that, among a wide series of polar and apolar solvents, tert-butyl alcohol is the best choice to enhance the yield of the N,N-syn regioisomer, while the N,N-anti regioisomer can be obtained with a very high selectivity when the reaction is carried out with scandium triflate. In the presence of metal chelation, semi-empirical ZINDO calculations offer an appropriate tool to explain the regioselectivity observed when different metal ions are used. The study of the regioselectivity is also supported by the data for the corresponding two-step reactions with reversed addition of the nucleophilic reagents, and it has been extended to the cases involving 4-methylpyridine and 6,7-dichloroisoquinoline-5,8-dione as reagents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Reactivity of the title P-heterocycles (1-14) in the photoinduced fragmentation-related phosphinylation of methanol was found to be influenced by the extent of ring strain and the UV absorption at 254 nm. The 7-phosphanorbornene oxides (7-14) are universal precursors due to their ring strain, no matter if they are UV-active or not at 254 nm. The easily available 2,5-dihydro-1H-phosphole oxides can be applied only in case of 1-phenyl substitution that enhances the absorption at 254 nm. The ring strain of representative P-heterocycles (5-8) was evaluated by HF/6-31G* and B3LYP/6-31+G* calculations. UV spectra of compounds 5-8 were interpreted by ZINDO/S and MNDO-d calculations. The new precursors (11-14) made possible the extension of the phosphinylations.

N-Methyl-2-(4-N,N-diphenylaminophenyl)fulleropyrrolidine and N-methyl-2-(4-di-p-tolylaminophenyl)fulleropyrro- lidine were synthesized via the 1,3-dipolar cycloaddition reactions under microwave irradiation. The molecular structures were identified and characterized by MS, UV-Vis, FT-IR, 1H NMR and fluorescence spectra. Photoinduced intramolecular electron transfer process from C60 moiety to triphenylamine moiety have been studied by nanosecond laser flash photolysis. The optimized structure and the distribution of the frontier molecular orbitals for C60-TPA were obtained by using DFT method at B3LYP/6-31G(d) level. The results indicated that the intramolecular photoinduced electron transfer could occur in these compounds, which were in excellent agreement with the nanosecond transient absorption spectra observed experimentally in polar solvent. The electronic spectrum of the compound C60-TPA was studied by ZINDO method on the basis of the optimized geometrics, which was essentially consistent with experimental values.

Very high temperatures of fullerene synthesis allow for a significant population of excited electronic states and thus for non-negligible electronic partition functions. The issue is studied on the C80 isomeric system, as relatively low-lying electronic excited states are known in this case. The set consists of seven isolated pentagon-rule (IPR) cages. Computations suggest a species of D5d symmetry as the lowest-energy structure in the set, although in observations a cage of D2 symmetry is more populated. The electronic partition functions are evaluated by means of the ZINDO method. The computed effects of the electronic excited states are still larger than in the previously tested examples. It is shown, however, that for the special conditions of the fullerene synthesis/isolation, the electronic partition function based on the singlet excited states only should better reflect the experimental population findings. The study is complemented with the MP2 =FC/6-31G* evaluation of the separation energy between the D5d and D2 isomers that places the D2 species ∼2.8 kcal/mol below the D5d cage, i.e., the reversed order than in several previous calculations. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem 2006

Electronic and vibrational properties of phenylene-based oligomers from biphenyl P2P to para-sexiphenil P6P, and their dependence on torsion angle are calculated using semiempirical quantum mechanic (AM1, ZINDO/S) and ab initio linear combination of atomic orbitals methods. The systematic relations between molecular size and geometry, and numerous molecular properties have been established, providing the basis both for the spectroscopic identification of different structures that could appear during material processing and for tailoring of devices with desired properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1783–1794, 2006

The structures, ionization potentials (IP), electron affinities (EA) and HOMO-LUMO gaps (ΔEH-L) of the terfluorene oligomers were studied by the density functional theory with B3LYP functional. The characters of the front orbitals were analyzed on the basis of the ground structure. The vertical excitation energies EV and the maximal absorption wavelengths λ abs of a series of ter(9,9-diarylfluorene) compounds were studied employing the time dependent density functional theory (TD-DFT) and ZINDO. The calculated maximal absorption wavelengths by both methods are in good agreement with the experimental data. The results show that the differences between terfluorene hh and ter(9,9-diarylfluorene) derivatives are slight in the structures and the electronic states except that there is the spiroconjugation in the latter. The spiroconjugation made these derivatives far from optimization in terms of stability. Excited structure of hh was calculated to be compared with the ground structure, which indicats that it has strong coplanar tendency of aromatic ring with the neighbour in the excited state. Consequently, they are good blue emitting materials with promising thermal stability.

Novel sandwich metallacarboranes commo-[3,3'-Ni(8-SMe2-1,2-C2B9H10)2] (1), commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2]+ (2+), commo-[3,3'-Ru(8-SMe2-1,2-C2B9H10)2] (4) and commo-[3,3'-Fe(8-SMe2-1,2-C2B9H10)2] (5) have been prepared by reaction of [10-SMe2-7,8-nido-C2B9H10]- with NiCl2 x 6 H2O, CoCl2, [RuCl2(dmso)4] and [FeCl2(dppe)], respectively. Reduction of 2+ with metallic Zn leads to the neutral and isolable complex commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2] (3). Theoretical calculations using the ZINDO/1 semiempirical method show three energy minima for complexes 1-3 and 5 that agree with the presence of three different rotamers in solution at low temperature, while four relative energy minima have been found for 4. The calculated rotational energy barriers for complexes 1-5 have been found in the range 5.2+/-0.2 and 11.5+/-0.2 kcal mol(-1). These values are in agreement with the experimental data calculated for complexes 2+ and 5. Only one rotamer is found in the X-ray crystal structure of complexes 1-3, while two are observed for 4. Neutral complexes 1, 3 and 4 exhibit a gauche conformation, whereas a cisoid conformation is found for the 2+ ion. Rotamers evident from X-ray diffraction studies are in agreement with the global energy minimum calculated by the ZINDO/1 method. The electrochemical studies conducted on 1, 3, 4 and 5 support the proposal that the charge-compensated ligand [10-SMe2-7,8-nido-C2B9H10]- stabilises lower oxidation states in metals than the dianionic [7,8-nido-C2B9H11]2- and even the [C5H5]- ligands.

The electronic and geometrical properties of distyrylbenzene (DSB) are investigated by using chemistry theoretical calculation methods. Specifically, the excited state properties are studied by performing ab initio correlation interaction singlet (CIS) and time-dependent density functional theory; the ground state and Raman activities are computed by density functional theory with the B3LYP method. Eight conformers of distyrylbenzene are found and they are derived from three isomers which are cis, cis-, cis, trans-, and trans, trans-, respectively. The relative energy shows that each isomer of three types is separated with a large energy barrier, but a small energy difference of each conformer is found if they are in the same type. The transition state also shows the barrier between conformers is lower than isomers. The computed excited transition energies using ZINDO/S based on the optimized geometries at a DFT/B3LYP level with 6–31+G show an excellent agreement with experimental absorption spectra.

The synthesis via the Stille coupling of a new family of oligomers derived from benzo[b]thiophene is reported. Owing to their branched molecular structure lacking any symmetry element, these compounds display a low tendency to crystallization and better film-forming properties than their linear counterparts. Spin-coated films show photoluminescence efficiencies up to 50 %. Light-emitting diodes with spin-coated films as the active layers display markedly improved performance with respect to similar devices based on linear oligothiophenes, with luminance values up to more than 10000 cd m - 2 . Semiempirical PM3 and ZINDO/S calculations provide insight into the molecular geometries and electron distribution of the frontier orbitals of the new compounds. Cyclic voltammetry data indicates that the transformation of the thienyl sulfur of benzo[b]thiophene to the corresponding thienyl-S,S-dioxide leads an increase in electron affinity by 0.5-0.7 V, analogous to that of the corresponding linear oligomers.

1,2-Naphthalene-ring-expanded tetraazachlorins (TACs), tetraazabacteriochlorins (TABCs), and tetraazaisobacteriochlorins (TAiBCs) have been synthesized. Procedures for the synthesis of the starting materials, that is, derivatives of 1,2-naphthalenedicarboxylic acid, have been reinvestigated and improved. Nine possible derivatives, including four, two, and three structural isomers of TACs, TABCs, and TAiBCs, respectively, were separated by using thin-layer chromatography (TLC) or high-performance liquid chromatography (HPLC), and the structure of each isomer was determined by (1)H NMR spectroscopy combined with the NOE technique. The formation ratio of each isomer was rationalized in terms of the intramolecular steric repulsion effect, which was predicted by geometry optimizations at the DFT level. The derived compounds were characterized by using IR, electronic, and magnetic circular dichroism (MCD) spectroscopy, and by electrochemical methods. Frequency calculations at the DFT level correctly reproduced the experimental IR spectra and, in particular, distinguished between the three isomers of the TAiBCs. In the electronic absorption and MCD spectra of the TAC and TABC species, the Q-band splits into two intense components similarly to the 2,3-naphthalene-fused derivatives described in our preceding paper, although no significant spectral differences were observed from species to species. On the other hand, the spectra of the TAiBCs showed moderate differences depending on the structure of the isomer. The spectroscopic properties as well as the electrochemical behavior of these chlorins resemble those of the corresponding benzene-fused derivatives rather than the 2,3-naphthalene-fused derivatives. Molecular-orbital and configuration-interaction calculations within the framework of the ZINDO/S method were helpful in the discussions of the above observations.

The absorption and emission energies for diphenylboron analogs of Alq3 (Ph2Bq) and its methyl substituents (Ph2Bmq) were systematically investigated at the Zerner's intermediate neglect of differential overlap (ZINDO), configuration interaction singles (CIS), and time-dependent density functional theory (TD-DFT) levels of theory. The lowest excited-state geometries were optimized at the ab initio CIS level. The TD-DFT method provides the most reliable results for the absorption and emission transition energies, compared with other methods. Moreover, the TD-DFT calculations reliably estimate the changes of absorption and emission λmax values upon methyl substitution, with errors of 1.2% and 1.8%, respectively. The Stokes shifts are well reproduced by TD-DFT calculations. Various density functional theory methods have been tested and the B3LYP functional clearly seems to be the best choice for this class of compounds. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Equimolar amounts of hexamethylenetetramine (HMT) and trans-1,2-cyclohexanedicarboxylic acid (CDA) formed a self-assembled unit through a hydrogen bonding network. Room temperature single-crystal diffraction studies and Fourier transform infrared (FTIR) spectroscopy of the system indicated adduct formation between HMT and CDA. The crystalline adduct sample exhibited appreciable macroscopic nonlinear optical (NLO) properties. Density functional theory (DFT) calculations, using the 6-31G(d,p) basis set, have been performed to investigate the gas-phase structure and IR frequencies. The dynamic NLO properties were calculated with the ZINDO/CV method, including solvent effects. Theoretical parameters (geometry and vibrational frequencies) predicted by the DFT method for the adduct are in good agreement with the experimental data. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

We present a quantum chemical analysis of the molecular structure and first hyperpolarizabilities of a series of stilbazolium ions and compare their results with corresponding neutral molecules. The molecular geometries are obtained via B3LYP/6-31G** optimization, including the self-consistent reaction field/polarizable continuum model (SCRF/PCM) approach. The static first hyperpolarizabilities (β0) are calculated using the ab initio coupled perturbed Hartree–Fock (CPHF) method, while the dynamic first hyperpolarizability (β) is investigated using the ZINDO/correction vector (CV) method, including solvent effects. The effects of donor–acceptor properties, solvent polarity, the dispersion effect, and aggregation on the first hyperpolarizibility are methodically evaluated with an attempt toward developing the engineering guidelines for the enhancement of the molecular optical nonlinearities of charged chromophores. We compare our theoretical results with the experimental values wherever available in the literature and evaluate the results in detail about the agreements and disagreements between theoretical and experimental findings. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

In a recent study, the internal electron transfer in bovine heart cytochrome c oxidase (CcO) was examined using the tunneling currents method at the extended Huckel level of electronic structure. No internal water was present in the published structure of the enzyme, and none was taken into account in the calculations. Through computer simulation, we recently found an intriguing water hydrogen bond network in the vicinity of CcO catalytic site and in the region of electron tunneling. In this work, we reexamine the problem, with the focus on the possibility of the influence of water on electron transfer process. The calculations are carried out with the tunneling currents method implemented using more accurate ZINDO/S quantum chemical model. Results for the electron transfer from heme a to heme a3, a key step in the function of the enzyme, are presented. With molecular dynamics simulations, we identify configurations in which the water molecules interfere with the tunneling pathway. The tunneling matrix element is evaluated by the flux theorem of the tunneling currents method, and is found to be 0.34 cm−1 in the “dry” system and 1.38 cm−1 in the “wet” system. The analysis of the tunneling flux indicates that the impact of water is localized on Phe377 and spread to the methyl group and to His376, both on the heme a3 side of the ET pathway. The increase of the tunneling coupling by a factor of 4 in this case demonstrates that internal water in proteins can influence ET rate in proteins by as much as an order of magnitude. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

The electron energetic characteristics of laser dye molecules 4-dicyanomethylene-2-methyl-6-p-dimethylamino-styryl-4H-pyran (DCM), (E)-2-{2-[2-(4-dimethylaminophenyl)ethynyl]-6-threefluoromethyl-4H-4-pyranylidene}-malonitrile (DCMF3), (E)-2-{2-[2-(4-dimethylaminophenyl)ethynyl]-6-n-heptafluoropropyl-4H-4-pyranylidene}-malonitrile (DCMF7), (E)-2-{2-[2-(5-piperidine-2-thienyl)ethynyl]-6-threefluoromethyl-4H-4-pyranylidene}-malonitrile (PTFDN) were investigated thoroughly using semiempirical methods ZINDO/1 and ZINDO/S. The geometry optimization was carried out by the method of conjugated gradients. On the basis of the electronic structure and optimum geometry of the molecules, the spectra of electronic transitions to the excited states, the dipole moments, and oscillator strengths of the transitions were calculated by the method of configuration interaction. Typical features of the electronic absorption spectra of these molecules were calculated and compared with the experimental data. The influence of the solvent on the optical absorption spectra was explored. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

The interest in poly(p-phenylene) (PPP) and poly(p-phenylene vinylene) (PPV) copolymers stems from the fact that these homopolymers present interesting optical and electronic properties that allow a great variety of technological applications. Combining different numbers of PPP and PPV units it is possible, in principle, to obtain new structures presenting intermediate gap values (2.8 eV and 2.4 eV for PPP and PPV, respectively). For this study we used a Huckel Hamiltonian tight-binding coupled to the negative factor counting (NFC) technique. We carried out a systematic search to determine optimum relative concentrations for disordered binary polymeric alloys with predefined gap values. Once these structures were obtained, we used the semiempirical methods AM1/PM3 and ZINDO/S-CI for geometrical and optical studies, respectively. Our theoretical results show that it is possible to obtain copolymers of PPP and PPV with intermediate gap values of their parent structures.

The electronic absorption and magnetic circular dichroism (MCD) spectra, and differential pulse (DP) voltammograms of planar heterodinuclear phthalocyanine (Pc) dimers consisting of Pc and pyrazinoporphyrazine (PyZ), [Mt2PcPyZ (Mt = H2 and Zn)] and of Pc and naphthalocyanine (Nc), [Mt2PcNc (Mt = H2 and Co)] have been recorded. These dimers commonly show broad absorption bands between ca. 500 and 1000 nm, while the DP voltammograms suggest that the dimers are unstable towards oxidation, and that the two relatively independent chromophore units interact with one another. Molecular orbital (MO) calculations using the ZINDO/S Hamiltonian were performed in order to enhance the interpretation of the experimental results. Dimer orbitals were expressed by coupling the orbitals of the same symmetry, of the constituting mononucleates. It was suggested that for the HOMO of the dimers, the contribution of the more electron-rich moiety is large, while conversely for the LUMO, the contribution of the more electron-deficient moiety is large. The calculations suggested that there are three prominent transitions in the Q band region of the heterodimers. The intensity and position of these bands were suggested to be affected by the constituting mononucleates. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

There is one factor in relative stabilities of isomeric fullerenes that has rarely been studied so far—the contribution of excited electronic states. The contribution is clearly quantified by the electronic partition function, supposing the related excitation energies can be evaluated. As temperatures in fullerene synthesis are high, the term should be taken into account. In this article the problem is studied on two isomeric systems. One is the set of five isolated pentagon rule isomers of C78, relatively well known from experiments. The other is a model set of four isomers of Mg@C72 (not isolated yet). The electronic excitation energies are computed by the limited configuration interaction (CI) method. In the case of the C78 isomeric set, the ZINDO semiempirical method is used. Isomers of Mg@C72 are evaluated by means of the single-excitation CI or CI-singles (CIS) in the standard LanL2MB and LanL2DZ basis sets—CIS/LanL2MB and CIS/LanL2DZ. It is found that the electronic partition function can cause significant changes in the computed equilibrium relative concentrations of isomers at high temperatures. Metallofullerenes are more likely candidates for such enhanced effects. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

The semiempirical PM5 method has been used to calculate fully optimized structures of magnesium-bacteriochlorin, magnesium-chlorin, magnesium-porphin, mesochlorophyll a, chlorophylls a, b, c(1), c(2), c(3), and d, and bacteriochlorophylls a, b, c, d, e, f, g, and h with all homologous structures. Hartree-Fock/6-31G* ab initio and density functional B3LYP/6-31G* methods were used to optimize structures of methyl chlorophyllide a, chlorophyll c(1), and methyl bacteriochlorophyllides a and c for comparison. Spectroscopic transition energies of the chromophores and their 1:1 or 1:2 solvent complexes were calculated with the Zindo/S CIS method. The self-consistent reaction field model was used to estimate solvent shifts. The PM5 calculations predict planar structure of the porphyrin ring and central position of the four coordinated magnesium atoms in all pigments studied, in accord with the experimental, ab initio, and density functional results, a significant improvement as compared to the older semiempirical PM3 approach. Only small differences in PM5 and B3LYP/6-31G* or Hartree-Fock/6-31G* minimum energy geometries of the reference molecules were observed. Calculations show that in 1:1 solvent complexes, where the magnesium atom is five coordinated, the magnesium atom is shifted out of the plane of the porphyrin ring towards the solvent molecule, while the hexa coordinated 1:2 complexes are again planar. The PM5 method gives atomic charges that are comparable with those obtained from the Hartree-Fock/6-31G* and B3LYP/6-31G* calculations. The single point ZINDO/S CIS calculations with PM5 minimum energy structure gave excellent correlations between calculated and experimental transition energies of the chlorophylls and bacteriochlorophylls studied. Such correlations may be used for prediction of transition energies of the chromophores in protein binding sites. Calculations also predict existence of dark electronic states below the main Soret absorption band in all chromophores studied. The results suggest that the semiempirical PM5 method is a fairly reliable and computationally efficient method in predicting molecular parameters of porphyrin-like molecules.

The molecular structure of the parent bis- and tris(2-thienyl)methinium ions were studied by first-principles methods (second-order Moller–Plesset many-body perturbation theory and density functional theory). In contrast to bis(2-thienyl)methinium, tris(2-thienyl)methinium is non-planar with torsion angles between the thienyl groups and the central plane of about 30°. The calculations on tris(2-thienyl)methinium resulted in two almost isoenergetic conformers with barriers to isomerization less than 10 kcal mol−1 (1 kcal = 4.184 kJ). In order to discuss the 13C chemical shifts of Crystal Violet and of heteroanalogous compounds with thienyl-2-, with 5-amino-thienyl-2-, with 5-methylmercapto-thienyl-2- and with 5-amino-1,3-thiazolyl-2-heteroaryl groups, the structure was optimized at the DFT B3-LYP/6–31G(d) level and the chemical shifts were calculated at the DFT-GIAO/6–311 + G(2d,p) level of theory. In general, the experimental chemical shifts correlate very well with the experimental values. The lowest energy electronic transitions in the UV–VIS region were calculated by time-dependent density functional response theory (TD-DFRT) using the DFT B3-LYP/6–31 + G(d) basis set. For cationic dyes the agreement between the results of TD-DFRT calculations and experiment was less good than for neutral sulfur-containing compounds. Semiempirical calculations (PPP, ZINDO/S) are better in that case and are more efficient. Copyright © 2002 Wiley & Sons, Ltd.

Due to the spatial extent of the 5s and 5p filled orbitals of lanthanide ions, the shielding of the nuclear charge is not perfect at close distances to the ion. Thus, ligand atoms should experience different effective charge depending upon their distance to the central ion. Using electrostatic arguments and ab initio calculations for Eu3+, we have proposed an improved model that describes the ion as an effective charge, q(r), whose value has the following radial dependence, q(r)=3+14e. This functional form of the point charge has been implemented into the ZINDO program and INDO/S-CI calculations have been performed for the Eu(btfa)3bipy, Eu(btfa)3⋅2H2O, Eu(bzac)3bipy, and Eu(bzac)3⋅2H2O compounds. The comparison with the experimental absorption spectrum was used to optimize the exponent A parameter. Application of this model to several other Eu3+ coordination compounds yielded better results for the calculated absorption spectra than the fixed 3+ point-charge model. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

The geometries of four isomers of the trimethylsilyl substituted phthalocyanine (Pc)—I, II, III, and IV—have been optimized at the B3LYP/3-21G level of density functional theory. Normal-mode vibrational analyses have been performed and their standard thermodynamic functions, molar fractions, and electronic absorption spectra calculated. Single-point energies have been calculated at the B3LYP/6-311G* level for all isomers to evaluate the heats of formation from an isodesmic reaction. It is found that substitution has little influence on the geometry and electronic structures of the Pc framework. The corresponding geometric parameters in various isomers are close. According to the B3LYP/6-311G*//B3LYP/3-21G results, substitution at the peripheral position of the isoindole with an inner hydrogen is most favorable. The energies increase in the order of IV < II < III < I, and the energy difference between IV and I is 5.75 kJ/mol. The molar fractions of IV, II, III, and I are 0.80, 0.17, 0.02, and 0.02 and the heats of formation are 2009.96, 2010.10, 2015.85, and 2016.52 kJ/mol, respectively. This indicates that nonperipheral substituted Pcs have higher energy and little production because they are not stable under the considered conditions. The electronic spectra of the substituted Pcs calculated using the ZINDO method have two strong Q absorption bands around 700 nm and one B band around 300 nm that are slightly shifted compared with those in Pc. The ratios of the oscillator strength of the B band to the Q bands are much lowered by trimethylsilyl substitution. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

Oligotiophenes derivatives present potential applicability for the construction of electronic devices. In this work we analyzed the structure–property relationship for a class of cyanothiophene compounds using ab initio [Hartree–Fock (HF) and second-order Moller–Plesset (MP2)] and density functional theory (DFT) theoretical approaches. The energy gap between the valence and conduction bands was evaluated using distinct methodologies, being the results compared with experimental data. At the DFT level using the HF optimized geometries (DFT//HF) the average deviation was 0.22 eV (SVWN//HF) and 0.23 eV (BLYP//HF). The results from the configuration interaction implemented in the semiempirical ZINDO/S-CI program were also in good agreement with experiment, being the average deviation found to be 0.20 eV when the HF optimized geometries were used. In general, the energy gap for the cyanothiophene derivatives was lower than the values for the unsubstituted parent compounds, showing that these molecules should be considered as a good prototype to develop new organic conductor polymers. We believe this study may be useful to predict electronic properties that are related with the conductivity of organic materials. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

The spectral features of Co(II)-substituted metallo-β-lactamases were analyzed. Nuclear magnetic resonance (NMR) evidence is provided that shows no magnetic coupling between the two metal ions in Co(II),Co(II)-βLII (β-lactamase II from Bacillus cereus), suggesting that no bridging ligand connects them. Ultraviolet–visible (UV–vis) data are rationalized on the basis of semiempirical quantum chemical calculations of the intermediate neglect of differential overlap type (ZINDO/S). The binuclear active sites of the enzymes from B. cereus (βLII) and B. fragilis (CcrA) were modeled from the protein data bank (PDB) coordinates, and calculations were performed at the multireference configuration interaction level. The active site geometries of these enzymes were examined using the experimental electronic spectra as a guide. The model which best fits to the spectroscopic data lacks a bridging solvent molecule, in agreement with the NMR data. The model could also reproduce spectral changes seen in the mixed Zn(II),Co(II) adduct. The bands in the visible range are associated with ligand field transitions at the high-affinity site, whereas the UV-calculated features originate in ligand-to-metal charge transfer to the open shell Co(II) in the low-affinity site. These results indirectly support the hypothesis that the Co(II),Co(II) derivative of CcrA is also unbridged, in contrast with the structure of the native zinc enzyme. These results indicate that the existence of a bridging ligand is not necessary for metallo-β-lactamase activity, and that the second zinc ion is not essential for lowering the pKa of the bound water. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 88: 118–132, 2002

Synthesis, structure, reactivity, and theoretical studies of a new type of Ni triple-decker complexes are reported. A prominent feature of its molecular structure is the (η3:η3)-bis(enyl) coordination of two [(η5-Me4EtC5)Ni] fragments to the central arene ring of the polycondensed π-perimeter decacyclene. The central arene ring of decacyclene displays a distinct chair conformation with significant tilt angles of 25.5 and 26.1°. The three naphthalene units are twisted in a propeller-like fashion with torsion angles of 16°, 16.6°, and 19°, which is twice the twisting observed for the free decacyclene ligand. According to Extended Huckel Molecular Orbital (EHMO) calculations, a qualitative measure for arene lability in the title compound and a number of related arene-bridged metal complexes can be attributed to a subsequent filling of metal−arene antibonding orbitals. The experimentally observed reactivities of π-arene exchange in these complexes are reproduced nicely by the EHMO calculations. The title compound offers access to a number of organonickel chalcogenato complexes under mild conditions by cleavage of the S−S, Se−Se, or Te−Te bonds of diorganyl dichalcogenides R−E−E−R. The title compound is also capable of C−H activation of azobenzene and [(dimethylamino)methyl]ferrocene resulting in orthometallated complexes containing five-membered metallacycles. PES and semi-empirical ZINDO/S calculations were used to study the product of the orthometallation of the title compound with azobenzene. These studies reveal that the HOMO of the metallacycle (IE = 6.45 eV) has dominant Ni−Cp and Ni−Caryl π-antibonding character.