QM/MM study of the absorption spectra of DsRed.M1 chromophores

We report geometries and vertical excitation energies for the red and green chromophores of the DsRed.M1 protein in the gas phase and in the solvated protein environment. Geometries are optimized using density functional theory (DFT, B3LYP functional) for the isolated chromophores and combined quantum mechanical/molecular mechanical (QM/MM) methods for the protein (B3LYP/MM). Vertical excitation energies are computed using DFT/MRCI, OM2/MRCI, and TDDFT as QM methods. In the case of the red chromophore, there is a general blue shift in the excitation energies when going from the isolated chromophore to the protein, which is caused both by structural changes and by electrostatic interactions with the environment. For the lowest ππ* transition, these two factors contribute to a similar extent to the overall DFT/MRCI shift of 0.4 eV. An enlargement of the QM region to include active‐site residues does not change the DFT/MRCI excitation energies much. The DFT/MRCI results are closest to experiment for both chromophores. OM2/MRCI and TDDFT overestimate the first vertical excitation energy by 0.3–0.5 and 0.2–0.4 eV, respectively, relative to the experimental or DFT/MRCI values. The experimental gap of 0.35 eV between the lowest ππ* excitation energies of the red (cis‐acylimine) and green (trans‐peptide) forms is well reproduced by DFT/MRCI and TDDFT (0.32 and 0.37 eV, respectively). A histogram spectrum for an equal mixture of the two forms, generated by OM2/MRCI calculations on 450 snapshots along molecular dynamics trajectories, matches the experimental spectrum quite well, with a gap of 0.23 eV and an overall blue shift of about 0.3 eV. DFT/MRCI appears as an attractive choice for calculating excitation energies in fluorescent proteins, without the shortcomings of TDDFT and computationally more affordable than CASSCF‐based approaches. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Electronic structure and molecular orbital study of the first excited state of the high‐efficiency blue OLED material bis(2‐methyl‐8‐quinolinolato)aluminum(III) hydroxide complex from ab initio and TD‐B3LYP

Bis(2‐methyl‐8‐quinolinolato)aluminum(III) hydroxide complex (AlMq₂OH) is used in organic light‐emitting diodes (OLEDs) as an electron transport material and emitting layer. By means of ab initio Hartree–Fock (HF) and density functional theory (DFT) B3LYP methods, the structure of AlMq₂OH was optimized. The frontier molecular orbital characteristics and energy levels of AlMq₂OH have been analyzed systematically to study the electronic transition mechanism in AlMq₂OH. For comparison and calibration, bis(8‐quinolinolato)aluminum(III) hydroxide complex (Alq₂OH) has also been examined with these methods using the same basis sets. The lowest singlet excited state (S₁) of AlMq₂OH has been studied by the singles configuration interaction (CIS) method and time‐dependent DFT (TD‐DFT) using a hybrid functional, B3‐LYP, and the 6‐31G* basis set. The lowest singlet electronic transition (S₀ → S₁) of AlMq₂OH is π → π* electronic transitions and primarily localized on the different quinolate ligands. The emission of AlMq₂OH is due to the electron transitions from a phenoxide donor to a pyridyl acceptor from another quinolate ligand including C → C and O → N transference. Two possible electron transfer pathways are presented, one by carbon, oxygen, and nitrogen atoms and the other via metal cation Al³⁺. The comparison between the CIS‐optimized excited‐state structure with the HF ground‐state structure indicates that the geometric shift is mainly confined to the one quinolate and these changes can be easily understood in terms of the nodal patterns of the highest occupied and lowest unoccupied molecular orbitals. On the basis of the CIS‐optimized structure of the excited state, TD‐B3‐LYP calculations predict an emission wavelength of 499.78 nm. An absorption wavelength at 380.79 nm on the optimized structure of B3LYP/6‐31G* was predicted. They are comparable to AlMq2OH 485 and 390 nm observed experimentally for photoluminescence and UV‐vis absorption spectra of AlMq₂OH solid thin film on quartz, respectively. Lending theoretical corroboration to recent experimental observations and supposition, the reasons for the blue‐shift of AlMq2OH were revealed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Intramolecular electronic communication in a dimethylaminoazobenzene–fullerene C60 dyad: An experimental and TD‐DFT study

An electronically push–pull type dimethylaminoazobenzene–fullerene C₆₀ hybrid was designed and synthesized by tailoring N,N‐dimethylaniline as an electron donating auxochrome that intensified charge density on the β‐azonitrogen, and on N‐methylfulleropyrrolidine (NMFP) as an electron acceptor at the 4 and 4′ positions of the azobenzene moiety, respectively. The absorption and charge transfer behavior of the hybrid donor‐bridge‐acceptor dyad were studied experimentally and by performing TD‐DFT calculations. The TD‐DFT predicted charge transfer interactions of the dyad ranging from 747 to 601 nm were experimentally observed in the UV‐vis spectra at 721 nm in toluene and dichloromethane. A 149 mV anodic shift in the first reduction potential of the N?N group of the dyad in comparison with the model aminoazobenzene derivative further supported the phenomenon. Analysis of the charge transfer band through the orbital picture revealed charge displacement from the n_(N?N) (nonbonding) and π _(N?N) type orbitals centered on the donor part to the purely fullerene centered LUMOs and LUMO+n orbitals, delocalized over the entire molecule. The imposed electronic perturbations on the aminoazobenzene moiety upon coupling it with C₆₀ were analyzed by comparing the TD‐DFT predicted and experimentally observed electronic transition energies of the dyad with the model compounds, NMFP and (E)‐N,N‐dimethyl‐4‐(p‐tolyldiazenyl)aniline (AZNME). The n_(N?N) → π*_(N?N) and π_(N?N) → π*_(N?N) transitions of the dyad were bathochromically shifted with a significant charge transfer character. The shifting of π_(N?N) → π*_(N?N) excitation energy closer to the n → π*_(N?N) in comparison with the model aminoazobenzene emphasized the predominant existence of charge separated quinonoid‐like ground state electronic structure. Increasing solvent polarity introduced hyperchromic effect in the π_(N?N) → π*_(N?N) electronic transition at the expense of transitions involved with benzenic states, and the extent of intensity borrowing was quantified adopting the Gaussian deconvolution method. On a comparative scale, the predicted excitation energies were in reasonable agreement with the observed values, demonstrating the efficiency of TD‐DFT in predicting the localized and the charge transfer nature of transitions involved with large electronically asymmetric molecules with HOMO and LUMO centered on different parts of the molecular framework. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Rydberg or Valence? The Long‐Standing Question in the UV Absorption Spectrum of 1,1′‐Bicyclohexylidene

The electronic excited states of the olefin 1,1′‐bicylohexylidene (BCH) are investigated using multiconfigurational complete active space self‐consistent‐field second order perturbation theory in its multi‐state version (MS‐CASPT2). Our calculations undoubtedly show that the bulk of the intensity of the two unusually intense bands of the UV absorption of BCH measured with maxima at 5.95 eV and 6.82 eV in the vapor phase are due to a single ππ* valence excitation. Sharp peaks reported in the vicinity of the low‐energy feature in the gas phase correspond to the beginning of the π3s_R Rydberg series. By locating the origin of the ππ* band at 5.63 eV, the intensity and broadening of the observed bands and their presence in solid phase is explained as the vibrational structure of the valence ππ* transition, which underlies the Rydberg manifold as a quasi‐continuum.     

Shape and core‐excited resonances of thionucleobases

Thionucleobases can be used in chemoradiation therapy of cancer. Shape resonances (SRs) and core‐excited resonances (CERs) can lead to fragmentation and eventually result in strand breaks of DNA. In particular, the more energetic CERs are believed to cause double‐strand breaks that can hardly be repaired. In this work, both the SRs and CERs of exemplary 2‐thiouracil, 4‐thiouracil, 2‐thiothymine, 4‐thiothymine, and 6‐aza‐2‐thiothymine are investigated using stabilization method in conjunction with long range corrected time‐dependent density functional theory. Results indicate that the energies of (1) π*₁ and π*₂ SRs, (2) n‐π* CERs, and (3) mixed resonances of π‐π* CERs with π* SRs can be significantly stabilized due to thionation of uracil or thymine. It is noteworthy that the resonant cases of (2) and (3) can be accessed by electrons even at energies below 4 eV. Consequently, the increased decay of temporary anions can enhance strand breaks of DNA.     

Theoretical Investigation on the Absorption and Emission Properties of the Three Isomers of Bis（thiocyanato）（2,2^1-bipyridyl）platinum（ll）

This paper presents a Density Functional or Time Dependent Density Functional （DFT/TDDFT） study of the molecular and electronic structures, optical absorption and emission spectra of three linkage isomers： bis（isothiocyanato-S）（2,2^1-bipyridyl） platinum（II） （[Pt（SCN）2（bpy）]）, （isothiocyanato-S）（thiocyanato-N）-（2,2^1-bipy- ridyl） platinum（II） （[Pt（SCN）（NCS）（bpy）]）, and bis（thiocyanato-N）（2,2^1）-bipyridyl）platinum（II） （[Pt（NCS）2（bpy）]）, in which different coordination ligands based on the N- and S-coordination of the thiocyanato ligands control the luminescent color. The electronic structures were studied using the B3LYP functional. Optimized geometries Were compared to the experimentally observed structures. TDDFT calculation was carded out to investigate the excited singlet and triplet states. Calculations have been performed both in vacuo and in solvents, using a polarized continuum model （PCM） to account for solute-solvent interactions. Inclusion of the solvent led to a significant energy change, and as a consequence, the computed spectrum calculated in the presence of the solvent was in good agree- ment with the experimental determinations. The first two absorptions were found to originate from mixed plati- num-SCN （or NSC） to bipyridyl-n＊ transitions rather than pure metal-to-ligand-charge-transfer （MLCT） transitions, whereas the higher-energy bands arose from intraligand n→π＊ transitions. The stretching frequencies of C≡N have been calculated both in the ground and excited states, which are relative to the charge transition during the excitation. In addition, different sizes of basis sets were also discussed in this paper.     

Synthesis,crystal structure,spectral characterization,and DFT studies on 4‐((1‐phenyl‐1H‐tetrazol‐5‐ylthio)methyl)benzoic acid

A new thiotetrazole compound, 4‐((1‐phenyl‐1H‐tetrazol‐5‐ylthio)methyl) benzoic acid ( 1 ), has been synthesized and characterized by elemental analysis, ¹H and ¹³C NMR, ESI‐MS, FT‐IR, UV–vis, fluorescence spectra, and single‐crystal X‐ray diffraction analysis. The structural analysis reveals that compound 1 adopts a nonplanar geometric structure and exhibits an extensive but not uniform π delocalization with all members of the tetrazolyl ring and the exocyclic sulfur atom. A density functional theory (DFT) calculation at B3LYP/6‐31G** level of theory was performed to elucidate the structure of this thiotetrazole system. And the time‐dependent DFT (TD‐DFT) calculations of absorption spectra reveal two electron‐transition bands derived from the contribution of π → π* transitions, which are in agreement with experimental results. Moreover, compound 1 exhibits a blue‐light emission (λ_em = 441 nm) in the solid state at room temperature. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 23:435–443, 2012; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21034     

One Electron Makes Differences: From Heme {FeNO}7 to {FeNO}8

The first X‐ray single‐crystal structure of a {FeNO}⁸ porphyrin complex [Co(Cp)₂][Fe(TFPPBr₈)(NO)], and the structure of the {FeNO}⁷ precursor [Fe(TFPPBr₈)(NO)] are determined at 100 K. The two complexes are also characterized by FTIR and UV/Vis spectroscopy. [Fe(TFPPBr₈)(NO)]^? shows distinct structural features in contrast to a nitrosyl iron(II) porphyrinate on the Fe? N? O^? moiety, which include a much more bent Fe? N? O^? angle (122.4(3)°), considerably longer Fe? NO^? (1.814(4)) and N? O^? (1.194(5) Å) bond distances. These and the about 180 cm^?1 downshift ν_N‐O stretch (1540 cm^?1) can be understood by the covalently bonding nature between the iron(II) and the NO^? ligand which possesses a two‐electron‐occupied π* orbital as a result of the reduction. The overall structural features of [Fe(TFPPBr₈)(NO)]^? and [Fe(TFPPBr₈)(NO)] suggest a low‐spin state of the iron(II) atom at 100 K.     

Evaluation of Non‐covalent Binding Energies and Optoelectronic Properties of New CuBr2(C6H7N)2 Complex: DFT Approaches

For the first time, the structural and optoelectronic properties of a new complex formulated as CuBr₂(C₆H₇N)₂ ( 1 ) [trans‐dibromidobis(3‐methylpyridine‐κN) copper(II)] were studied by density functional theory (DFT) calculations. They are performed using B3LYP through the Gaussian 09 program and also with full potential linearized augmented plane wave (FP‐LAPW) methods within the Generalized Gradient Approximation (GGA) and Hartree‐Fock (HF) theory by the Wien2k package. The neutral monomeric complex participates in a variety of non‐covalent interactions, including hydrogen bonding and π stacking to create a 2D coordinate plane. The binding energy value of the non‐covalent interactions responsible for the crystalline network formation of 1 were calculated using the method of dispersion corrected density functional theory (DFT‐D). In this method, the independent smallest fragment (monomer) and subsequently the related network, including seven monomers bearing all non‐covalent interactions were optimized. The results demonstrate that hydrogen bonds, especially non‐conventional C–H ··· Br interactions, govern the network formation along the a and c axes. It can be mentioned because of these directed interactions, increasing of charge transfer along x and z directions results in increasement of the absorption and refractive index along y and z directions, and vice versa. The results of band structure show indirectly and directly the nature of the bandgap within GGA and HF, respectively. The bandgap value of CuBr₂(C₆H₇N)₂ is comparable to those of binary semiconductor compounds. DOSs spectra reveal that 3d Cu, 4p Br, and 2p C states play important roles in the optical transitions of the electrons. The calculated electronic absorption of the UV/Vis spectrum shows six major electron‐transition bands derived from d → d (ligand field) n → n, n → π*, π → n, and σ → n MLCT and LMCT transitions. The calculated absorption spectrum of the titled complex through FP‐LAPW within GGA method shows good consistency with the B3LYP/def2‐TZVP/6‐311+G(d,p) method. Our calculated birefringence results show that 1 has capability of nonlinear optical, which can be used in the nonlinear optoelectronic devices.     

Comprehensive SPHYB and B3LYP‐DFT Studies of Two Types of Ferrocene

Structural and optoelectronic properties of ferrocene (FeC₁₀H₁₀) using various exchange correlation potentials including Spin Polarized Generalized Gradient Approximation (SPGGA), Hybrid Density Functional Theory (SPHYB‐DFT), and hybrid density functional Becke3LYP are investigated. Obtained bandgap by the SPHYB‐DFT and SPGGA methods show consistency with the experiment, that are indirect and direct, respectively. The cell size effects on physical properties of ferrocene studied about two types of its lattice parameters ( I and II ). The calculated results reveal that the cell size and the lattice parameters have a remarkable effect on optoelectronic and magnetic properties of ferrocene. However, there is no significant difference between I and II within molecular, structural and charge transitions in calculating UV/Vis spectrum. The calculated electronic absorption spectrum is in good agreement with experiment, in which two major electron‐transition bands derived from d–d (n → n*) and n → π* metal to ligand. NBO analyses show that there are strong donor‐acceptor interactions between central Fe atoms and cyclopentadienyl (Cp) rings that these results are in close agreement with contour plots of charge densities for prediction of the strong covalent bond between C and Fe. The optoelectronic properties of ferrocene predict that it can be efficiently used in the semiconductor devices.     

Chelating Complexes of Diethylzinc and ZnCl2 with 2,2‐Bipyridine and 1,6,7,12,13,18‐Hexaazatrinaphthylene (HATN) as Ligands

The 1,6,7,12,13,18‐hexaazatrinaphthylene (HATN) complex [(Et₂Zn)₃(μ₃‐HATN)] was synthesized and characterized by IR spectroscopy, UV/Vis spectroscopy, elemental analysis and ESI‐MS spectrometry. Attempts to prepare ZnCl₂ complexes of HATN leads only to the mononuclear [(Cl₂Zn)(HATN)] derivative, characterized by X‐ray diffraction, IR‐ and UV/Vis‐spectroscopy as well as ESI‐MS spectrometry. The bright red 2,2′‐bipyridine (bipy) complex [(Et₂Zn)(bipy)] ( 1 ) was synthesized and characterized by X‐ray diffraction and NMR spectroscopy. The UV/Vis‐spectra of the HATN‐complexes show absorptions in regions of far longer wavelengths than the corresponding 2,2′‐bipyridine or 1,10‐phenantroline complexes. Consequently the π*‐LUMO of HATN ( 5 ) is lower in energy than the π*‐LUMO of 2,2′‐bipyridine ( 2 ) or 1,10‐phenanthroline (phen).     

A DFT study on the structure‐property relationship of aminonitropyrazole‐2‐oxides

Density functional theory (DFT) calculations at the B3LYP/aug‐cc‐pVDZ level have been carried out to study the geometry and electronic structures, stability, sensitivity and band gap of the possible isomers of aminonitropyrazole‐2‐oxides. Kamlet‐Jacob equations were used to determine the performance properties of model compounds. The performance properties of model compounds P5, P18, P20, P21, P22, and P23 are higher compared with 2,4,6,8,10,12‐hexanitro‐2,4,6,8,10,12‐hexaazaisowurtzitane (CL‐20) and octanitrocubane (ONC). The heat of explosion, density, detonation velocity and detonation pressure are related to the number and positions of NO₂ and NH₂ groups in pyrazole‐2‐oxide. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Spectroscopic Study of the E/Z Photoisomerization of a New Cyrhetrenyl Acylhydrazone: A Potential Photoswitch and Photosensitizer†

The new cyrhetrenyl acylhydrazone [(CO)₃Re(η⁵‐C₅H₄)‐C(O)‐NH‐N = C(CH₃)‐(2‐C₄H₂S‐5‐NO₂)] ( E‐CyAH ) has been designed, synthesized and fully characterized to study the effect of having a cyrhetrenyl fragment (sensitizer) covalently bonded to an acylhydrazone moiety (switch), on its photophysical and photochemical properties. The crystal structure reveals that E‐CyAH adopts an E‐configuration around the iminic moiety [‐N = C(CH₃)]. The absorption spectrum of E‐CyAH displays two bands at 270 and 380 nm, which are mainly ascribed to π → π* intraligand (IL) and d_π → π* metal‐to‐ligand charge transfer (MLCT) transitions, being consistent with DFT/TD‐DFT calculations. Upon 365 nm irradiation, E‐CyAH photoisomerizes to Z‐CyAH , as evidenced by UV‐Vis and ¹H‐NMR spectral changes, with a quantum yield value Φ_{E‐CyAH →Z‐CyAH} of 0.30. Z‐CyAH undergoes a first‐order thermal back‐isomerization process, with a relatively short half‐life τ_1/2 of 277 min. Consequently, E‐CyAH was quantitatively recovered after 24 h, making it a fully reversible T‐type molecular photoswitch. This remarkable behavior allows us to measure the individual photophysical properties for both isomers. In addition, E‐CyAH and Z‐CyAH efficiently photosensitize the generation of singlet oxygen (O₂ (¹Δ_g)) with good yield (Φ_Δ = 0.342).     

Heptametallic,Octupolar Nonlinear Optical Chromophores with Six Ferrocenyl Substituents

New complexes with six ferrocenyl (Fc) groups connected to Zn^II or Cd^II tris(2,2′‐bipyridyl) cores are described. A thorough characterisation of their BPh₄^? salts includes two single‐crystal X‐ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(Fe^II)→π* metal‐to‐ligand charge‐transfer (MLCT) bands accompany the π→π* intraligand charge‐transfer absorptions in the near UV region. Each complex shows a single, fully reversible Fe^III/II wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper‐Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities β₀ reaching as high as approximately 10^?27 esu and generally increasing with π‐conjugation extension. Z‐scan cubic NLO measurements reveal high two‐photon absorption cross‐sections σ₂ of up to 5400 GM in one case. DFT calculations reproduce the π‐conjugation dependence of β₀, and TD‐DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature.     

Low-pressure pyrolysis of tBu2SO: synthesis and IR spectroscopic detection of HSOH

Sulfenic acid (HSOH, 1 ) has been synthesized in the gas‐phase by low‐pressure high‐temperature (1150 °C) pyrolysis of di‐tert‐butyl sulfoxide (tBu₂SO, 2 ) and characterized by means of matrix isolation and gas‐phase IR spectroscopy. High‐level coupled‐cluster (CC) calculations (CCSD(T)/cc‐pVTZ and CCSD(T)/cc‐pVQZ) support the first identification of the gas‐phase IR spectrum of 1 and enable its spectral characterization. Five of the six vibrational fundamentals of matrix‐isolated 1 have been assigned, and its rotational‐resolved gas‐phase IR spectrum provides additional information on the O–H and S–H stretching fundamentals. Investigations of the pyrolysis reaction by mass spectrometry, matrix isolation, and gas‐phase FT‐IR spectroscopy reveal that, up to 500 °C, 2 decomposes selectively into tert‐butylsulfenic acid, (tBuSOH, 3 ), and 2‐methylpropene. The formation of the isomeric sulfoxide (tBu(H)SO, 3 a ) has been excluded. Transient 3 has been characterized by a comprehensive matrix and gas‐phase vibrational IR study guided by the predicted vibrational spectrum calculated at the density functional theory (DFT) level (B3LYP/6‐311+G(2d,p)). At higher temperatures, the intramolecular decomposition of 3 , monitored by matrix IR spectroscopy, yields short‐lived 1 along with 2‐methylpropene, but also H₂O, and most probably sulfur atoms. In addition, HSSOH ( 6 ), H₂, and S₂O are found among the final pyrolysis products observed at 1150 °C in the gas phase owing to competing intra‐ and intermolecular decomposition routes of 3 . The decomposition routes of the starting compound 2 and of the primary intermediate 3 are discussed on the basis of experimental results and a computational study performed at the B3LYP/6‐311G* and second‐order Møller–Plesset (MP2/6‐311G* and RI‐MP2/QZVPP) levels of theory.     

Effect of 5-methylresorcinol on spin conversion in molecular oxygen

Geometric parameters of 5-methylresorcinol (MR) molecules in different spin states (^1,3,5MR) and its complexes with oxygen (^1,3(O₂-MR)) are determined within the DFT/B3LYP method. Cross-sections of potential energy surfaces of the formation reaction of the intermolecular complex ^1,3(O₂-MR) are plotted in MCSCF functions by the MRCI method. With regard to the spin-orbit interaction the moments of spin-allowed and spin-forbidden transitions of the complexes are calculated and the possibility of spin conversion in ³O₂ under the effect of MR is analyzed.     

Molecular modeling of 3,4-pyridinedicarbonitrile dye sensitizer for solar cells using quantum chemical calculations

The geometries, electronic structures, polarizabilities, and hyperpolarizabilities of organic dye sensitizer 3,4-pyridinedicarbonitrile was studied based on Hartree–Fock (HF) and density functional theory (DFT) using the hybrid functional B3LYP. Ultraviolet–visible (UV–Vis) spectrum was investigated by time dependent DFT (TD-DFT). Features of the electronic absorption spectrum in the visible and near-UV regions were assigned based on TD-DFT calculations. The absorption bands are assigned to π → π1 transitions. Calculated results suggest that the three lowest energy excited states are due to photoinduced electron transfer processes. The interfacial electron transfer between semiconductor TiO₂ electrode and 3,4-pyridinedicarbonitrile is due to electron injection process from excited dye to the semiconductor’s conduction band. The role of cyanine in 3,4-pyridinedicarbonitrile in geometries, electronic structures, and spectral properties were analyzed.     

Cyclometalated IrIII Complexes with Substituted 1,10‐Phenanthrolines: A New Class of Efficient Cationic Organometallic Second‐Order NLO Chromophores

Cyclometalated cationic Ir^III complexes with substituted 1,10‐phenanthrolines (1,10‐phen), such as [Ir(ppy)₂(5‐R‐1,10‐phen)]Y (ppy=cyclometalated 2‐phenylpyridine; R=NO₂, H, Me, NMe₂; Y^?=PF₆^?, C₁₂H₂₅SO₃^?, I^?) and [Ir(ppy)₂(4‐R,7‐R‐1,10‐phen)]Y (R=Me, Ph) are characterized by a significant second‐order optical non linearity (measured by the electrical field induced second harmonic generation (EFISH) technique). This nonlinearity is controlled by MLCT processes from the cyclometalated Ir^III, acting as a donor push system, to π* orbitals of the phenanthroline, acting as an acceptor pull system. Substitution of cyclometalated 2‐phenylpyridine by the more π delocalized 2‐phenylquinoline (pq) or benzo[h]quinoline (bzq) or by the sulfur‐containing 4,5‐diphenyl‐2‐methyl‐thiazole (dpmf) does not significantly affect the μβ absolute value, which instead is affected by the nature of the R substituents on the phenanthroline, the higher value being associated with the electron‐withdrawing NO₂ group. By using a combined experimental (the EFISH technique and ¹H and ¹⁹F PGSE NMR spectroscopy) and theoretical (DFT, time‐dependent‐DFT (TDDFT), sum over states (SOS) approach) investigation, evidence is obtained that ion pairing, which is controlled by the nature of the counterion and by the concentration, may significantly affect the μβ values of these cationic NLO chromophores. In CH₂Cl₂, concentration‐dependent high absolute values of μβ are obtained for [Ir(ppy)₂(5‐NO₂‐1,10‐phen)]Y if Y is a weakly interacting anion, such as PF₆^?, whereas with a counterion, such as C₁₂H₂₅SO₃^? or I^?, which form tight ion‐pairs, the absolute value of μβ is lower and quite independent of the concentration. This μβ trend is partially due to the perturbation of the counterion on the LUMO π* levels of the phenanthroline. The correlation between the μβ value and dilution shows that the effect of concentration is a factor that must be taken into careful consideration.     

HeII(304Å) photoelectron spectrum of N2O4

The 304 Å HeII photoelectron (PE) spectrum of a mixture of NO₂ and N₂O₄ in the gaseous phase has been measured in the region up to 29 eV. By subtracting the HeII spectrum of NO₂ from the mixed spectrum, the HeII spectrum due to N₂O₄ has been deduced, indicating that there are three new PE bands in the region 20—29 eV. Relative band intensities have also been obtained from the HeII of N₂O₄ below 16 eV, corrected for electron collecting efficiency.