Tunable luminescence of bithiophene-based flexible Lewis pairs.

Bithiophene-based flexible Lewis pairs with P(O)R2 (R = phenyl, isopropyl) and BMes2 (Mes = 2,4,6-trimethylphenyl) functionalities are able to toggle between closed, Lewis adduct and open, unbound Lewis pair structures. The open structure is favored in strong hydrogen bond donating solvents or at higher temperatures giving rise to an intense charge-transfer (CT) luminescence, while the closed structure without this emission dominates in non-hydrogen bond donating solvents or at lower temperatures. Intermediate solvents result in an equilibrium mixture of both structures, which shows unusual mixed emission that is dependent on excitation wavelength.

Tuning the extended structure and electronic properties of gold(I) thienyl pyrazolates.

A series of thienyl pyrazole proligands and gold(I) thienyl pyrazolate cyclic trinuclear complexes (CTCs) have been synthesized. The relationship between the structure and emission properties of bridging thienyl pyrazolates within gold(I) cyclic trinuclear complexes suggests that the nature of dual emission is sensitive to ligand conjugation length. Density functional theory has been used to support the assignment of metal-sensitized, ligand-localized phosphorescence from monothienyl complexes, while low-lying, ligand-localized LUMOs present in bithienyl systems prohibit metal-sensitized phosphorescence. Soluble n-hexyl derivatives have been synthesized to explore the electrochemical properties of gold(I) thienyl pyrazolates CTCs, and conductive electropolymerized thin films were realized.

Photophysical behaviour of cyclometalated iridium(III) complexes with phosphino(terthiophene) ligands.

Six new Ir(III) complexes containing the 3'-phosphino-2,2':5',2''-terthiophene (PT3) ligand in three different coordination modes are reported. The electronic properties of the complexes are characterized by cyclic voltammetry, absorption, emission and time-resolved transient absorption spectroscopies and DFT/TDDFT calculations. The electrochemical and photophysical behaviour of the complexes was found to be dominated by the PT3 ligand. For the complexes in which the PT3 ligand is coordinated in a bidentate P,S or P,C mode, the lowest energy absorption band is attributed to π-π* PT3 localized transitions consistent with observations from DFT calculations. Emission quantum yields are low in all cases (<0.07) and emission lifetimes are short (<50 ns). Intersystem crossing leads to a long-lived triplet state ((3)L) also localized on the PT3 group. In the complex where the PT3 ligand is coordinated only via the phosphine, TDDFT calculations suggest that there is some MLCT (and Cl-PT3 CT) character in the lowest energy transition.

Enhanced photoluminescence of sulfur-bridged organic chromophores.

A general approach to enhancing the emission quantum yield of several widely studied organic chromophores is presented. The luminescence properties of a series of symmetrical sulfur-bridged chromophores are reported as a function of the oxidation state of the bridging sulfur atom. The photoluminescence quantum yield is significantly enhanced by successively oxidizing the sulfur bridge from sulfide (S), to sulfoxide (SO), to sulfone (SO2).

Coordination mode dependent excited state behavior in group 8 phosphino(terthiophene) complexes.

The ground and excited state behavior of four Ru(II) and Os(II) bipyridyl complexes containing the 3'-(diphenylphosphino)-2,2':5',2''-terthiophene (PT(3)) ligand in two different coordination modes (P,S and P,C) is reported. The complexes are generally stable under extended photoirradiation, except for [Ru(bpy)(2)PT(3)-P,S](PF(6))(2) which decomposes. Emission lifetimes and transient absorption spectra and lifetimes have been obtained for all the complexes. These data support a PT(3) ligand based lowest excited state in the case of both P,S bound complexes, and a charge separated lowest excited state in both P,C bound complexes, conclusions supported by Density Functional Theory (DFT) calculations.

Charge transfer and intraligand excited state interactions in platinum-sensitized dithienylethenes.

The photophysical behavior for two photochromic Pt-terpyridine acetylide complexes containing pendant dithienylethenes (DTEs) bound to the metal through the alkynyl linkage is presented. Selective excitation of the Pt complex with visible light resulted in the metal-sensitized ring closing of the DTE unit. The central purpose of this study was to understand how excited state interactions govern the photophysics by correlating differences in the linkage of the two components with differences in the intramolecular energy transfer processes that occur between the Pt complex and the DTE. A series of model complexes without photochromic ligands were prepared and studied to elucidate the contributions of the triplet metal-to-ligand charge transfer and triplet intraligand states. It is demonstrated that reducing the orbital overlap of the metal-based and intraligand states by lengthening the linkage and eliminating a conjugated pathway is effective at dramatically decreasing the efficiency of intramolecular energy transfer. This is evidenced by the appearance of Pt-terpyridine based phosphorescence and a significant decrease in the observed rate of metal-sensitized ring closing of the DTE.

Successful bifunctional photoswitching and electronic communication of two platinum(II) acetylide bridged dithienylethenes.

Coordinating two dithienylethenes to a platinum center results in the reversible ring closure of both photochromic units in a model for a photoresponsive pi-conjugated polymer. This system demonstrates how metal-sensitized photochemistry, from a triplet excited state, circumvents the problems associated with other multicomponent photochromic systems, where significant electronic interactions in the ground state and singlet excited state prevent full photoswitching. Changes in charge-transfer behavior based upon conversion of both dithienylethenes to their ring-closed forms illustrate how photomodulation of conductivity through a conjugated polymer might be achieved using Pt-bis(acetylide)s.

Linker-dependent metal-sensitized photoswitching of dithienylethenes.

Intramolecular energy-transfer results in sensitized ring closing of a pendant dithienylethene from a platinum terpyridyl complex only when the two components are connected with a short pi-conjugated linker.

Role of s-p orbital mixing in the bonding and properties of second-period diatomic molecules.

Qualitative molecular orbital theory is widely used as a conceptual tool to understand chemical bonding. Symmetry-allowed orbital mixing between atomic or fragment orbitals of different energies can greatly complicate such qualitative interpretations of chemical bonding. We use high-level Amsterdam Density Functional calculations to examine the issue of whether orbital mixing for some familiar second-row homonuclear and heteronuclear diatomic molecules results in net bonding or antibonding character for a given molecular orbital. Our results support the use of slopes of molecular orbital energy versus bond distance plots (designated radial orbital-energy slope: ROS) as the most useful criterion for making this determination. Calculated atomic charges and frontier orbital properties of these molecules allow their acid-base chemistry, including their reactivities as ligands in coordination chemistry, to be better understood within the context of the Klopman interpretation of hard and soft acid-base theory. Such an approach can be extended to any molecular species.

Metal-metal bonding in tetracyanometalates (M=PtII, PdII, NiII) of monovalent thallium. Crystallographic and spectroscopic characterization of the new compounds Tl2Ni(CN)4 and Tl2Pd(CN)4.

The new crystalline compounds Tl2Ni(CN)4 and Tl2Pd(CN)4 were synthesized by several procedures. The structures of the compounds were determined by single-crystal X-ray diffraction. The compounds are isostructural with the previously reported platinum analogue, Tl2Pt(CN)4. A new synthetic route to the latter compound is also suggested. In contrast to the usual infinite columnar stacking of [M(CN)4]2- ions with short intrachain M-M separations, characteristic of salts of tetracyanometalates of NiII, PdII, and PtII, the structure of the thallium compounds is noncolumnar with the two TlI ions occupying axial vertices of a distorted pseudo-octahedron of the transition metal, [MTl2C4]. The Tl-M distances in the compounds are 3.0560(6), 3.1733(7), and 3.140(1) A for NiII, PdII, and PtII, respectively. The short Tl-Ni distance in Tl2Ni(CN)4 is the first example of metal-metal bonding between these two metals. The strength of the metal-metal bonds in this series of compounds was assessed by means of vibrational spectroscopy. Rigorous calculations, performed on the molecules in D4h point group symmetry, provide force constants for the Tl-M stretching vibration constants of 146.2, 139.6, and 156.2 N/m for the NiII, PdII, and PtII compounds, respectively, showing the strongest metal-metal bonding in the case of the Tl-Pt compound. Amsterdam density-functional calculations for isolated Tl2M(CN)4 molecules give Tl-M geometry-optimized distances of 2.67, 2.80, and 2.84 A for M = NiII, PdII, and PtII, respectively. These distances are all substantially shorter than the experimental values, most likely because of intermolecular Tl-N interactions in the solid compounds. Time-dependent density-functional theory calculations reveal a low-energy, allowed transition in all three compounds that involves excitation from an a1g orbital of mixed Tl 6pz-M ndz2 character to an a2u orbital of dominant Tl 6pz character.

Intraligand Charge Transfer in Pt(qol)(2). Characterization of Electronic States by High-Resolution Shpol'skii Spectroscopy.

Pt(qol)(2) (qol(-) = 8-quinolinolato-O,N) is investigated in the Shpol'skii matrices n-heptane, n-octane-h(18), n-octane-d(18), n-nonane, and n-decane, respectively. For the first time, highly resolved triplet phosphorescence as well as triplet and singlet excitation spectra are obtained at T = 1.2 K by site-selective spectroscopy. This permits the detailed characterization of the low-lying singlet and triplet states which are assigned to result mainly from intraligand charge transfer (ILCT) transitions. The electronic origin corresponding to the (3)ILCT lies at 15 426 cm(-)(1) (FWHM approximately 3 cm(-)(1)) exhibiting a zero-field splitting smaller than 1 cm(-)(1), which shows that the metal d-orbital contribution to the (3)ILCT is small. At T = 1.2 K, the three triplet sublevels emit independently due to slow spin-lattice relaxation (slr) processes. Therefore, the phosphorescence decays triexponentially with components of 4.5, 13, and 60 &mgr;s. Interestingly, two of the sublevels can be excited selectively, which leads to a distinct spin polarization manifested by a biexponential decay. At T = 20 K, the decay becomes monoexponential with tau = 10 &mgr;s due to a fast slr between the triplet sublevels. From the Zeeman splitting of the (3)ILCT the g-factor is determined to be 2.0 as expected for a relatively pure spin triplet. The (1)ILCT has its electronic origin at 18 767 cm(-)(1) and exhibits a homogeneous line width of about 12 cm(-)(1). This feature allows us to estimate a singlet-triplet intersystem crossing rate of about 2 x 10(12) s(-)(1). This relatively large rate compared to values found for closed shell metal M(qol)(n)() compounds displays the importance of spin-orbit coupling induced by the heavy metal ion. Moreover, this small admixture leads to the relatively short emission decay times. All spectra show highly resolved vibrational satellite structures. These patterns provide information about vibrational energies (which are in good accordance with Raman data) and shifts of equilibrium positions between ground and excited states. These shifts are different in the (1)ILCT and (3)ILCT states. The vibrational satellite structures support the assignment of ILCT character to the lowest excited states.