Lowest-lying Absorption Research Articles

Valence and Rydberg excitations of 3-fluorotoluene in the 4.4–10.8 eV photoabsorption energy region

The absolute photoabsorption cross sections for 3-fluorotoluene in the 4.4–10.8 eV energy-range were obtained for the first-time using a synchrotron radiation light source. New theoretical calculations, providing vertical excitation energies and oscillator strengths, were performed at the time-dependent density functional theory and the equation-of-motion coupled cluster method restricted to single and double excitations to qualitatively support the experimental data. The electronic transitions discernible in the photoabsorption spectrum have been assigned to valence, mixed valence-Rydberg and Rydberg transitions. Additionally, a comprehensive assignment of the vibronic structure was performed with the main contribution of C–H in plane bending mode progressions dictating the spectroscopy of the lowest-lying absorption band. From the absolute photoabsorption cross sections, the photolysis lifetimes of 3-fluorotoluene in the Earth's atmosphere were also obtained.

Theoretical insight on the electronic structure and photophysical properties of three blue cyclometalated Ir(III) complexes based on the benchmark complex FIrpic

The electronic and photophysical properties of three blue cyclometalated Ir(III) complexes have been investigated by using the density functional theory (DFT) and time dependent density functional theory (TDDFT) methods. Both HOMO and LUMO values of complexes 1, 2 and 3 are in the order of 1 > 2 > 3. The lowest-lying singlet absorption for 1, 2 and three attributed to the configurations of the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) with the d(Ir)+π(A)→π*(A) [MLCT/ILCT] character. Complexes 1, 2 and three possess almost the same phosphorescence wavelength and transition characters.

Computational Design of Rhenium(I) Carbonyl Complexes for Anticancer Photodynamic Therapy.

New Re(I) carbonyl complexes are proposed as candidates for photodynamic therapy after investigating the effects of the pyridocarbazole-type ligand conjugation, addition of substituents to this ligand, and replacement of one CO by phosphines in [Re(pyridocarbazole)(CO)3(pyridine)] complexes by means of the density functional theory (DFT) and time-dependent DFT. We have found, first, that increasing the conjugation in the bidentate ligand reduces the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gap of the complex, so its absorption wavelength red-shifts. When the enlargement of this ligand is carried out by merging the electron-withdrawing 1H-pyrrole-2,5-dione heterocycle, it enhances even more the stabilization of the LUMO due to its electron-acceptor character. Second, the analysis of the shape and composition of the orbitals involved in the band of interest indicates which substituents of the bidentate ligand and which positions are optimal for reducing the HOMO–LUMO energy gap. The introduction of electron-withdrawing substituents into the pyridine ring of the pyridocarbazole ligand mainly stabilizes the LUMO, whereas the HOMO energy increases primarily when electron-donating substituents are introduced into its indole moiety. Each type of substituents results in a bathochromic shift of the lowest-lying absorption band, which is even larger if they are combined in the same complex. Finally, the removal of the π-backbonding interaction between Re and the CO trans to the monodentate pyridine when it is replaced by phosphines PMe3, 1,4-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA), and 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP) causes another extra bathochromic shift due to the destabilization of the HOMO, which is low with DAPTA, moderate with PMe3, but especially large with CAP. Through the combination of the PMe3 or CAP ligands with adequate electron-withdrawing and/or electron-donating substituents at the pyridocarbazole ligand, we have found several complexes with significant absorption at the therapeutic window. In addition, according to our results on the singlet–triplet energy gap, all of them should be able to produce cytotoxic singlet oxygen.

Synthesis, crystal structures, optical properties, DFT and TD-DFT studies of Ni (II) complexes with imine-based ligands

Two new square-planar nickel (II) complexes were synthesized from (2-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)phenol) and (1-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)naphthalen-2-ol) ligands, for complex I and complex II, respectively. They were characterized by IR, UV-Vis spectroscopy and X-ray diffraction. Further measurements were also conducted as cyclic voltammetry and conductivity measurements. Both complexes crystallize in the monoclinic system with the space groups P21/n and P21/c for complex I and complex II, respectively. The electronic absorption spectrum of complex I exhibits the lowest-lying absorption band (LEB) at 381 nm and the highest energy absorption band (HEB) at 269 nm. For complex II, the LEB was located at 431 nm while the HEB was found to be at 260 nm. Theoretical structure geometries, vibrational frequencies and electronic properties of the two complexes were calculated, by means of the density functional theory (DFT). The basis set 6-31g+(d) was chosen, for non-metal atoms (C, H, N and O) and the Stuttgart/Dresden effective core potentials basis set (SDD), for Ni atoms. UV-Vis transitions nature was investigated via a time-dependent density functional theory (TD-DFT) calculation. Overall, theoretical calculations were in accordance with the experimental results.

Accurate Description of Low-Lying Excited States in a Series of Photoreactive Clusters [Os3(CO)10(α-diimine)] by DFT Calculations

Density functional theory (DFT) calculations were performed on clusters [Os3(CO)10(α-diimine)], for α-diimine = 2,2'-bipyridine (BPY), N-isopropyl 2-iminomethylpyridine (IMP), and N, N'-diisopropyl-l,4-diaza-1,3-butadiene (DAB), together with their spectroscopic study. This important family of clusters is known to convert upon irradiation with visible light into short-lived biradicals and long-lived zwitterions from a σπ* (SBLCT) excited state that has not been described accurately thus far by quantum mechanical calculations. On the basis of the combined DFT, UV-vis absorption, and resonance Raman data, the lowest-lying visible absorption band is assigned to a σ(Os1-Os3)-to-π*(α-diimine) CT transition, for α-diimine = bpy and IMP, and to a strongly delocalized σ(Os1-Os3)π*-to-σ*(Os1-Os3)π* transition for conjugated nonaromatic α-diimine = DAB. The DFT calculations rationalize the experimentally determined characteristics of this electronic transition in the studied series: (i) The corresponding absorption band is the dominant feature in the visible spectral region. (ii) The CT character of the electronic excitation declines from α-diimine = bpy to IMP and vanishes for DAB. (iii) The excitation energies decrease in the order α-diimine = DAB > BPY > IMP. (iv) The oscillator strength shrinks in the order α-diimine = DAB > IMP > BPY. Reference photoreaction quantum yields measured accurately for the formation of a cluster zwitterion from [Os3(CO)10(IMP)] in strongly coordinating pyridine demonstrate that the optical population of the lowest-energy 1σπ* and relaxed 3σπ* excited states in the DFT model scheme is still capable of inducing the initial homolytic Os1-Os3 σ-bond splitting, although less efficiently than the optical excitation into neighbor higher-lying electronic transitions due to a higher potential barrier for the reaction from a dissociative (σσ*) state.

TUNING THE ELECTRONIC, PHOTOPHYSICAL AND CHARGE TRANSFER PROPERTIES OF SMALL D-A MOLECULES BASED ON THIENOPYRAZINE-TERTHIENYLS BY CHANGING THE DONOR FRAGMENT: A DFT STUDY

Four acceptor-donor organic conjugated molecules based on thieno[3,4-b]pyrazine-terthienyls were analyzed in order to explore the effect of the donor substituent on their molecular structures, electronic and optical properties. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD/DFT) calculations were carried out employing the B3LYP hybrid functional in combination with the 6-31G(d,p) basis set. The results suggests that the addition of electron-donating substituents to the conjugated molecules can diminish their energy gap value, which is beneficial to the photon harvesting. The lowest-lying absorption spectra of compounds substituted with electron donor groups exhibited a red-shift and a high oscillation factor compared with the unsubstituted molecule. Additionally, the ionization potential ( IP ), electron affinity ( EA ), reorganization energy (λ) and open-circuit voltage ( V oc) of the molecules were evaluated. According to these values, the molecules show good photovoltaic properties, and efficient charge transfer for hole and electron and balanced charges.

Photophysical properties of copper(I) complexes containing pyrazine-fused phenanthroline ligands: a joint experimental and theoretical investigation.

Two copper(I) complexes [Cu(Pyz-Phen)2]PF6 (1) and [Cu(POP)(Pyz-Phen)]PF6 (2) (Pyz-Phen = pyrazino[2,3-f] [1, 10]phenanthroline, POP = bis[2-diphenylphosphino]-phenyl]ether) have been synthesized and characterized. The photophysical properties of these complexes in solution have been studied. The electronic absorption spectrum of complexes 1 exhibit the lowest-lying MLCT absorption band at 459 nm and high-energy ligand-based transitions at 275 nm, while that of complex 2 exhibits the MLCT/LLCT band at 400 nm and ligand π-π* band at 262 nm. In addition, both 1 and 2 show similar phosphorescence (3)MLCT/(3)LLCT emissions with maximum emission wavelengths of 569 and 572 nm, respectively. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to rationalize the photophysical properties of the complexes studied. The theoretical data confirm the assignment of the experimental absorption spectra and the nature of the emitting states. Graphical Abstract Photophysical properties of copper(I) complexes containing pyrazine-fused phenanthroline ligands.

An Extended Chain and Trinuclear Complexes Based on Pt(II)-M (M = Tl(I), Pb(II)) Bonds: Contrasting Photophysical Behavior.

The syntheses and structural characterizations of a Pt-Tl chain [{Pt(bzq)(C6F5)2}Tl(Me2CO)]n 1 and two trinuclear Pt2M clusters (NBu4)[{Pt(bzq)(C6F5)2}2Tl] 2 and [{Pt(bzq)(C6F5)2}2Pb] 3 (bzq = 7,8-benzoquinolinyl), stabilized by donor-acceptor Pt → M bonds, are reported. The one-dimensional heterometallic chain 1 is formed by alternate "Pt(bzq)(C6F5)2" and "Tl(Me2CO)" fragments, with Pt-Tl bond separations in the range of 2.961(1)-3.067(1) Å. The isoelectronic trinuclear complexes 2 (which crystallizes in three forms, namely, 2a, 2b, and 2c) and 3 present a sandwich structure in which the Tl(I) or Pb(II) is located between two "Pt(bzq)(C6F5)2" subunits. NMR studies suggest equilibria in solution implying cleavage and reformation of Pt-M bonds. The lowest-lying absorption band in the UV-vis spectra in CH2Cl2 and tetrahydrofuran (THF) of 1, associated with (1)MLCT/(1)L'LCT (1)[5dπ(Pt) → π*(bzq)]/(1)[(C6F5) → bzq], displays a blue shift in relation to the precursor, suggesting the cleavage of the chain maintaining bimetallic Pt-Tl fragments in solution, also supported by NMR spectroscopy. In 2 and 3, it shows a blue shift in THF and a red shift in CH2Cl2, supporting a more extensive cleavage of the Pt-M bonds in THF solutions than in CH2Cl2, where the trinuclear entities are predominant. The Pt-Tl chain 1 displays in solid state a bright orange-red emission ascribed to (3)MM'CT (M' = Tl). It exhibits remarkable and fast reversible vapochromic and vapoluminescent response to donor vapors (THF and Et2O), related to the coordination/decoordination of the guest molecule to the Tl(I) ion, and mechanochromic behavior, associated with the shortening of the intermetallic Pt-Tl separations in the chain induced by grinding. In frozen solutions (THF, acetone, and CH2Cl2) 1 shows interesting luminescence thermochromism with emissions strongly dependent on the solvent, concentration, and excitation wavelengths. The Pt2Tl complex 2 shows an emission close to 1, ascribed to charge transfer from the platinum fragment to the thallium [(3)(L+L')MM'CT]. 2 also shows vapoluminescent behavior in the presence of vapors of Me2CO, THF, and Et2O, although smaller and slower than those of 1. The trinuclear neutral complex Pt2Pb 3 displays a blue-shift emission band, tentatively assigned to admixture of (3)MM'CT (3)[Pt(d) → Pb(sp)] with some metal-mediated intraligand ((3)ππ/(3)ILCT) contribution. In contrast to 1 and 2, 3 does not show vapoluminescent behavior.

Theoretical research on the effect of regulated π-conjugation on the photophysical properties of Ir(III) complexes.

In this work, the effect of regulated host and auxiliary ligand π-conjugation on the photophysical properties of a series of Ir(III) carbene complexes is examined by using the start-of-the-art theoretical methods. According to our results, all of the lowest-lying and strongest absorption peaks can be assigned as having a mixed ligand-to-ligand/metal-to-ligand charge transfer (LLCT/MLCT) character, but the different ways of introducing phenyl have a great effect on the absorption wavelength variation. In addition, the charge transfer characteristics of lowest-lying emission have some minute differences. In addition, when the extended π-conjugation is broken, the emission wavelength can be effectively retained due to the similar emission charge transfer related electronic density distribution of occupied molecular orbitals and unoccupied molecular orbitals. However, the larger π-conjugation can give rise to remarkably blue-shifted emission. This blue-shifted emission can be attributed to the alteration in the transition character due to intense interactions between nearly degenerate unoccupied molecular orbitals. Through the evaluation of the spin-orbit coupling (SOC) effect, we can gain a deeper understanding of the radiative decay rate processes. These results reveal that the larger π-conjugation can also lead to higher quantum efficiency due to the larger radiative decay and the smaller nonradiative decay rate. Our theoretical studies highlight the role of π-conjugation of the host and auxiliary ligand, and thus, can pave the way for the design of novel and efficient blue phosphorescent materials.

Design of an ultrashort optical transmission cell for vacuum ultraviolet spectroscopy of supercritical fluids.

We present the design and characteristics of an ultrathin flow cell optimized for vacuum ultraviolet transmission spectroscopy experiments on supercritical fluids. The cell operates satisfactorily at pressures up to 300 bar and temperatures up to 390 °C. The variable path length concept of the cell allows for optical transmission studies of analytes ranging from dense condensed-phase systems to gas-phase systems. The path length of the cell can be adjusted from hundreds of nanometers to hundreds of micrometers by an exchange of a variable thickness spacer sandwiched between two sapphire windows. In the path length range from nanometers to single micrometers, metal vapor deposited on one or both of the two sandwiched optical windows constitute the spacer. Spacers with thicknesses of 2 μm and greater can be constructed from simple commercially available metal foils. The cell has been used to measure the lowest-lying absorption band of water in both the vapor and condensed phases from room temperature up to and above the critical point. It has also found application in the studies of aqueous ions and nonaqueous liquids including various common organic solvents and carbon dioxide.

Theoretical studies of ground and excited states in a series of Zn(II) complexes, derived from thiourea and thiosemicarbazide

Theoretical studies for a series of mono- and binuclear zinc (II) complexes Zn(CH3COO)(2)(H2L)(2) [H2L = N-2-propenyl-N'-2-pyridinylthiourea] (A), Zn-2(CH3COO)(2)(H3L-a)(2) [H3L-a=2-[(2-hydroxy-phenyl)methylene]hydrazine-N-phenylcarbothioamide] (B), and Zn(H3L-b)(2) [H3L-b = 2-[(2-hydroxy-phenyl) methylene]hydrazine-N-(2-propenyl)carbothioamide] (C) have been performed on their structures and excited-state absorption spectra. The singlet ground-state geometries are fully optimised at three DFT levels, i.e., B3LYP, B3PW91, and M06. Different geometries, i.e., strongly distorted tetrahedral coordination environment in complex A, distorted square-pyramidal environment in complex B, and irregular octahedral mode in complex C are identified. Consequently, the spectroscopic properties are calculated by means of time-dependent density functional theory (TDDFT) with the Polarisable Continuum Model (PCM) based on the optimised gas-phase geometries. Three absorption peaks are identified for every complex, which are in good agreement with the experimental ones. For complex A, all three absorption peaks centered at 280.33 nm, 268.09 nm, and 250.87 nm, respectively, are ascribed to the (p, pi) -> pi* transition with a mixed intraligand charge-transfer (ILCT)/ligand-ligand charge-transfer (LLCT) character. The composition of frontier orbitals involved in major absorption bands for the three complexes shows similarities, which results in the almost homologous transition attributions and characteristics. A remarkable bathochromic shift in the lowest-lying absorption band is observed for complexes B and C as compared with complex A, which is attributed to the decreased H (HOMO)-L (LUMO) energy gap (Delta E vertical bar HOMO-LUMO vertical bar) by the formation of conjugate metallocycles in complexes B and C.

Theoretical Studies on the Electronic Structures and Spectral Properties of Three Iridium(III) Complexes With Different NˆN Ligands

We report a quantum chemical study of the electronic structures and spectral properties of three iridium(III) biscarbene complexes with different heteroleptic N^N ligands. The theoretical calculation reveals that the lowest-lying singlet absorptions at 434 nm for 1 and 487 nm for 3 are attributed to the mixed transition characters of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT). However, for 2, the lowest-lying singlet absorption at 509 nm is attributed to the MLCT. For 1–3, the phosphorescence at 739, 913, and 737 nm are mainly attributed to 3MLCT and 3LLCT characters. For 1 and 3, the emission energies are nearly the same, which is larger than that of 2. Ionization potentials (IP) and electron affinities (EA) calculations show that the assumed complex 3 has large EA value and enhanced electron injection ability as compared to complexes 1 and 2. Moreover, the reasons for different transition characters and phosphorescence quantum yield for the three complexes have been discussed in this paper. This theoretical contribution allows the factors determining the efficiency of radiative and nonradiative decay pathways in the three complexes.

Theoretical studies on the electronic structures and optical properties of the thiophene oligomer containing 2-(trifluoromethyl) thieno [3, 4-b] thiophene moiety and the CF3 end-caps

Structural, electronic, and optical properties of a series of π-conjugated thiophene oligomers P1-P3 and CF3P1-CF3P3 have been theoretically investigated. P1-P3 contain the 2- (trifluoromethyl) thieno [3, 4-b] thiophene moiety as the centre and 1–3 repeating thiophene units adjacent to its two sides respectively, while their corresponding derivatives CF3P1-CF3P3 with the CF3 as end-caps. The geometric structures of the oligomers in the ground and excited state were optimized by PBE1PBE and CIS methods with 6–31G (d) basis sets, respectively. All the oligomers exhibit zigzag arrangements. The absorptions and emissions were calculated by the time-dependent density functional theory method (TD-PBE1PBE). The lowest-lying absorptions of all the oligomers can be characterized as π-π* electron transition. For each series of oligomers, there is a progressive lowering in HOMO-LUMO gap with the increase of the repeating unit, being consistent with the red-shifted trend in the lowest-lying absorption and fluorescence from P1 to P3 and CF3P1 to CF3P3. To compare the P- and corresponding CF3P-oligomers, the end-cap CF3 group causes the slight blue shifts in absorption and emission spectra. The ionization potentials (IPS), electron affinities (EAs), and reorganization energies (λ) as well as the hole/electron extraction energies (HEP/EEP) of the oligomers were explored and those of the corresponding polymer were obtained by extrapolation method. The IP and HEP of P-polymer are lower than those of CF3P-polymer, indicating that the P-polymer is more suitable for hole transport than CF3P-polymer, while the higher EA and EEP for CF3P-polymer suggest the better electron transfer property. For CF3 end-caps, the CF3P-polymer exhibits the equal reorganization energy between electron and hole, which is a precondition for the charge transfer balance.

Substituent effects on the properties of fluorene-thieno[3,4-b]pyrazine derivatives for light-emitting applications

Five thieno[3,4-b]pyrazine-based model compounds were studied to explore the effects of the substituent groups (alkyl or aryl) on the structure, atomic charge, optical properties, ionization potential (IP), electron affinity (EA), and reorganization energy. Theoretical calculations were carried out by density functional theory (DFT) using the B3LYP hybrid function combined and CAM-B3LYP with the 6-31G(d) basis set. The lowest-lying absorption and emission spectra of 9,9'-diethylhexylfluorene-alt-5,7-dithien-2-yl-thieno[3,4-b] pyrazine (FDDTTP) with alkyl groups showed a blue-shift, while those of FDDTTP with aryl groups exhibited a red-shift. The results agree well with analytical data from reorganization energies. IPs are brought down by both alkyl and aryl groups. However, EAs are raised only by aryl units. The results indicate that aryl groups are more helpful in forming excitions for FDDTTP molecules. Consequently, FDDTTP with aryl groups are more efficient acceptor segments for designing donor-acceptor copolymers than those with alkyl groups.

Synthesis, photophysical and electrochemical properties of a carbazole dimer-based derivative with benzothiazole units

A novel A-π-D-π-D-π-A type compound, containing two benzothiazole rings as electron acceptors and two N-ethylcarbazole groups as electron donors, (E)-1,2-bis(3-(benzothiazol-2-yl)-9-ethylcarbazol-6-yl)ethene (BBECE), was synthesized and characterized by elemental analysis, NMR, MS and thermogravimetric analysis. Electrochemical property of compound BBECE was studied by cyclic voltammetry analysis. The absorption and emission spectra of BBECE was experimentally determined in several solvents and simultaneously computed using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculated absorption and emission wavelengths are coincident with the measured data. The lowest-lying absorption spectra can be mainly attributed to intramolecular charge transfer (ICT), and the fluorescence spectra can be mainly described as originating from an excited state with intramolecular charge transfer (ICT) character. The molecular orbitals (HOMO and LUMO), the ionization potential (IP), the electron affinity (EA) and reorganization energy of compound BBECE were also investigated using density functional theory (DFT). The results show that compound BBECE exhibited excellent thermal stability and electrochemical stability as well as high fluorescence quantum yield, indicating its potential applications as an excellent optoelectronic material in optical fields.

Benzoquinolateplatinum(ii) complexes as building blocks in the synthesis of Pt–Ag extended structures

The reaction between (NBu(4))[Pt(bzq)(C(6)F(5))(2)] (1, bzq = 7,8-benzoquinolate) and AgClO(4) in a 1 : 1 molar ratio, in acetone, gives the polymer [{Pt(bzq)(C(6)F(5))(2)}Ag](n) (2). The reaction of 2 with equimolecular amounts of PPh(3) and SC(4)H(8) (tht) produces the bimetallic complexes [{Pt(bzq)(C(6)F(5))(2)}AgL] (L = PPh(3) (3), tht (4)). For L = py, decomposition takes place and [Pt(bzq)(C(6)F(5))py] (5) is obtained. All these complexes have been characterized by X-ray diffraction. The most interesting features of complexes 2-4 is the presence of Pt-Ag bonds, with Pt-Ag distances of ca. 2.75 Å. Besides, the silver centres establish short η(1) bonding interactions with the C(ipso) of the bzq ligands, with distances Ag-C of ca. 2.45 Å. Complex 2 is a one-dimensional infinite chain in which the fragments "Pt(bzq)(C(6)F(5))(2)(-)" and Ag(+) alternate. On the other hand, complexes 1 and 3-5 show intermolecular pairing through π···π interactions between the aromatic rings of the bzq ligand, having interplanar separations of ca. 3.5 Å. Complex 2 dissolves in donor solvents (acetone, THF) as discrete bimetallic solvated fragments [{Pt(bzq)(C(6)F(5))(2)}AgS(n)] (S = solvent), similar to complexes 3 and 4. The persistence of the Pt-Ag bond in 2-4, supported by multinuclear NMR spectroscopy, causes a significant blue-shift in the lowest-lying absorption in relation to 1. This fact is attributed (TD-DFT) to a remarkable modification of the orbitals contributing to the HOMO, which changes the character of the transition from (1)LC/(1)MLCT in 1 to admixture (1)L'LCT/(1)MLCT in the bimetallic complexes. The low energy feature (490-530 nm) of 2 in solid state is attributed to CT from the Pt fragments to the Ag centers. Complexes 2-4 are only emissive in rigid media (solid and glasses). In the solid state, the metallic chain 2 exhibits a bright orange emission (560 nm, 298 K; 590 nm, 77 K), assigned to an excited state involving charge transfer from the platinum fragment with a remarkable contribution of C(6)F(5) (Ar(f)) rings to the Pt-Ag bond ((3)LMM'CT/(3)L'M'CT). However, 3 and 4 exhibit in solid state at 298 K a vibronic band, which is clearly resolved in two close non-equilibrated bands at 77 K in 3, tentatively ascribed to a mixture of (3)MLCT/(3)L'LCT transitions modified by the formation of the Pt-Ag bond. In glassy solution (77 K) 2-4 display a vibronic emission ascribed primarily to (3)LC character.

Theoretical Studies on Structures and Spectroscopic Properties of Highly Efficient Phosphorescent [Ru(terpy)(phen)X]+ Complexes

The ground and the lowest-lying triplet excited state geometries, electronic structures, and spectroscopic properties of three mixed-ligand Ru(II) complexes [Ru(terpy)(phen)X]+ (terpy=2,2′,6′,2″-terpyridine, phen=1,10-phenanthroline, and X=—C≡CH (1), X=Cl (2), X=CN (3)) were investigated theoretically using the density functional theory method. The ground and excited state geometries have been fully optimized at the B3LYP/LanL2DZ and UB3LYP/LanL2DZ levels, respectively. The absorption and emission spectra of the complexes in CH3CN solutions were calculated by time-dependent density functional theory with the PCM solvent model. The calculated bond lengths of Ru—C, Ru—N, and Ru—Cl in the ground state agree well with the corresponding experimental results. The highest occupied molecular orbital were dominantly localized on the Ru atom and monodentate X ligand for 1 and 2, Ru atom and terpy ligand for 3, while the lowest unoccupied molecular orbital were π* (terpy) type orbital. Therefore, the lowest-energy absorptions of 1 and 2 at 688 and 631 nm are attributed to a dyz (Ru)+π/p(X)→π* (terpy) transition with MLCT/XLCT (metal-to-ligand charge transfer/X ligand to terpy ligand charge transfer) character, whereas that of 3 at 529 nm is related to a dyz(Ru)+π(terpy)→π*(terpy) transition with MLCT and ILCT transition character. The calculated phosphorescence of three complexes at 1011 nm (1), 913 nm (2), and 838 nm (3) have similar transition properties to that of the lowest-lying absorption. It is shown that the lowest lying absorptions and emissions transition character of these Ru(II) complexes can be tuned by changing the electron-withdrawing ability of the monodentate ligand.

Experimental and theoretical study of three new benzothiazole-fused carbazole derivatives

Three new D–π–A type compounds, each containing one benzothiazole ring as an electron acceptor and one N-ethylcarbazole group as electron donor, were synthesized and characterized by elemental analysis, NMR, MS and thermogravimetric analysis. The absorption and emission spectra of three compounds were experimentally determined in several solvents and were simultaneously computed using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculated reorganization energy for hole and electron indicates that three compounds are in favor of hole transport than electron transport. The calculated absorption and emission wavelengths are well coincident with the measured data. The calculated lowest-lying absorption spectra can be mainly attributed to intramolecular charge transfer (ICT). And the calculated fluorescence spectra can be mainly described as originating from an excited state with intramolecular charge transfer (ICT) character. The results show that three compounds exhibited excellent thermal stability and high fluorescence quantum yields, indicating their potential applications as excellent optoelectronic material in optical field.

Experimental and theoretical study of 10-methoxy-2-phenylbenzo[h]quinoline

10-Methoxy-2-phenylbenzo[h]quinoline (MPBQ) has been synthesized and characterized by NMR and X-ray single crystal diffraction. Both the ground and the lowest singlet excited-state geometries of MPBQ were optimized by B3LYP and ab initio CIS methods at 6-31G (d,p) level, respectively. The absorption and emission spectra of the compound were experimentally determined in CH 3CN solution and were simultaneously computed using density functional theory (DFT) and time-dependent density functional theory (TDDFT) in CH 3CN solution. The calculated absorption and emission wavelengths were in good agreement with the experimental ones. The calculated lowest-lying absorption spectra can be mainly attributed to intramolecular charge transfer (ICT). And the calculated fluorescence spectra can be mainly described as originating from an excited state with intramolecular charge transfer (ICT) character. These results show that MPBQ exhibited excellent thermal stability and could serve as a useful photoluminescence material.

Theoretical studies on structures and spectroscopic properties of a series of heteroleptic iridium complexes based on tridentate bis(benzimidazolyl)pyridine ligand

Electronic structures and spectroscopic properties of series of novel mixed-ligand Ir(III) complexes, [Ir(Mebip)(bpy)Cl] 2+ ( 1), [Ir(Mebip)(ppy)Cl] + ( 2), [Ir(Mebip)(ppz)Cl] + ( 3) and [Ir(Mebip)(ptz)Cl] + ( 4) [bpy = 2,2′-bipyridine, ppy = phenylpyridine, ppz = 5-(2-pyridyl)-pyrazole and ptz = 5-(2-pyridyl)-triazole, Mebip = bis(N-methylbenzimidazolyl)pyridine] have been investigated in detail by theoretical calculation. The geometries in the ground and excited state for the cationic complexes 1– 4 were optimized by density functional theory (DFT) and single excitation configuration interaction method (CIS), respectively. The absorptions and emissions of 1– 4 in CH 3CN solution were calculated using time-dependent density functional theory (TDDFT) associated with the polarized continuum model (PCM). With the assistance of the analysis of frontier molecular orbitals, HOMOs of 2– 4 mainly comprise Ir(III) ion and different chelating ligands, but that of 1 is dominated by Ir(III) ion and Mebip. LUMO and LUMO + 1 of 1– 4 predominantly reside on the Mebip ligand. The lowest-lying absorption of 1 at 460 nm (from HOMO to LOMO) is assigned to MLCT and ILCT transitions, while that of 2– 4 occurs at 457, 448 and 418 nm, respectively, originating from a mixture of MLCT, LLCT and ILCT transitions. The 542 nm phosphorescence of 1 is assigned as 3MLCT and 3ILCT characters, while the emission for 2– 4 at 591, 572 and 549 nm is described as 3MLCT, 3LLCT and 3ILCT transitions. Phosphorescent emissions of 1– 4 mirroring their lowest-lying absorption transitions, display a red shift of 2 > 3 > 4 > 1 which is in line with the σ donor ability of bidentate ligands following an order of ppy( 2) > ppz( 3) > ptz( 4) > bpy( 1).