Vibronic and magnetic coupling in the radiative deactivation of the lowest excited state of tris(2,2'-bipyridine)osmium(2+) doped into tris(2,2'-bipyridine)ruthenium(II) bis(hexafluorophosphate)

Thermally Tunable Dual Emission of the d(8)-d(8) Dimer [Pt2(μ-P2O5(BF2)2)4](4)

High-resolution fluorescence, phosphorescence, as well as related excitation spectra, and, in particular, the emission decay behavior of solid [Bu4N]4[Pt2(μ-P2O5(BF2)2)4], abbreviated Pt(pop-BF2), have been investigated over a wide temperature range, 1.3-310 K. We focus on the lowest excited states that result from dσ*pσ (5dz(2)-6pz) excitations, i.e., the singlet state S1 (of (1)A2u symmetry in D4h) and the lowest triplet T1, which splits into spin-orbit substates A1u((3)A2u) and Eu((3)A2u). After optical excitation, an unusually slow intersystem crossing (ISC) is observed. As a consequence, the compound shows efficient dual emission, consisting of blue fluorescence and green phosphorescence with an overall emission quantum yield of ∼ 100% over the investigated temperature range. Our investigation sheds light on this extraordinary dual emission behavior, which is unique for a heavy-atom transition metal compound. Direct ISC processes in Pt(pop-BF2) are largely forbidden due to spin-, symmetry-, and Franck-Condon overlap-restrictions and, therefore, the ISC time is as long as 29 ns for T < 100 K. With temperature increase, two different thermally activated pathways, albeit still relatively slow, are promoted by spin-vibronic and vibronic mechanisms, respectively. Thus, distinct temperature dependence of the ISC processes results and, as a consequence, also of the fluorescence/phosphorescence intensity ratio. The phosphorescence lifetime also is temperature-dependent, reflecting the relative population of the triplet T1 substates Eu and A1u. The highly resolved phosphorescence shows a ∼ 220 cm(-1) red shift below 10 K, attributable to zero-field splitting of 40 cm(-1) plus a promoting vibration of 180 cm(-1).

Chemical approaches to artificial photosynthesis. 2

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Theoretical Studies of Phosphorescence Spectra of Tris(2,2‘-bipyridine) Transition Metal Compounds

Phosphorescence spectra of tris(2,2'-bipyridine) metal compounds, [M(bpy)3]n+, where M = Zn(II), Ru(II), Os(II), Rh(III), and Ir(III), were calculated using a harmonic oscillator approximation of adiabatic potential surfaces obtained by density functional theory (DFT). Using the Huang-Rhys (S) factors calculated by theoretical Franck-Condon analysis of T1 and S0 geometries, we successfully reproduced the emission spectra observed under various conditions by nonempirical calculations. The simulations of well-structured spectra of the Zn(II), Rh(III), and Ir(III) compounds confirmed that the emission originated from localized ligand-centered excited states with considerably distorted geometries of C2 symmetry. The spectrum simulation revealed that the phosphorescence state of [Ru(bpy)3]2+ was localized 3MLCT both in a solution and a glass matrix. Furthermore, a highly resolved phosphorescence spectrum observed for [Ru(bpy)3]2+ doped in a [Zn(bpy)3](ClO4)2 crystal was reproduced well using the geometry of the localized 3MLCT by assuming mode-specific broadening of low-frequency intramolecular vibrational modes. The deuterium effects of the electronic origins of the doped crystal observed by Riesen et al. were in excellent agreement with those predicted for the localized 3MLCT. However, the calculated satellite structures of the localized 3MLCT involving bpy-h8 in [Ru(bpy-h8)(3-x)(bpy-d8)x]2+ (x = 1,2) exhibited only the bpy-h8 vibrational modes, inconsistent with the simultaneous appearance of both bpy-h8 and bpy-h8 modes in the observed spectra. A simulation on the basis of the geometry of the delocalized 3MLCT was in reasonable agreement with an unresolved spectrum observed for a neat crystal of [Ru(bpy)3](PF6)2, which is inconsistent with the assignments of localized 3MLCT on the basis of the electronic origins. The inconsistency of the assignment on the basis of the adiabatic model is discussed in terms of vibronic coupling between the localized 3MLCT states. The 3MLCT state in [Os(bpy)3]2+ seems to vary with the environment: a fully localized 3MLCT in a solution, partially localized in a glass matrix, and delocalized in PF6 salts.

Intraligand charge transfer in the Pd(II) oxinate complex Pd(qol)2. Site-selective emission, excitation, and optically detected magnetic resonance

The first spectroscopic investigation of Pd(qol)2 (qol- = 8-quinolinolato-N,O = oxinate) dissolved in an n-octane matrix (Shpol'skii matrix) is reported. Application of several spectroscopic methods at liquid helium temperatures (typically, T = 1.2 K), such as site-selective and highly resolved luminescence and excitation spectroscopy, time-resolved emission spectroscopy, optically detected magnetic resonance, microwave recovery, phosphorescence microwave double-resonance, and magnetic fields, allows us to characterize the lowest excited electronic states in detail. In accord with previous assignments for the related Pt(qol)2 it is shown that these lowest states represent-intraligand charge-transfer states, namely, 1ILCT and 3ILCT. The electronic origin of the 1ILCT state lies at 20,617 cm(-1) (site A). It exhibits a nearly homogeneous line width with a half-width of about 80 cm(-1) (fwhm), which corresponds to a lifetime of tau(1ILCT) approximately equals 2 x 10(-13) s. This value is even shorter than that found for Pt(qol)2, presumably due to intersystem crossings and relaxations to dd* states. The electronic origin of the 3ILCT state lies at 16 090 cm(-1) (site A), and its zero-field splittings (zfs) into three sublevels are 2E = 2356 MHz (0.0785 cm(-1)) and D - E = 5241 MHz (0.175 cm(-1)). The emission decay times of the three sublevels are determined as tauI = 90 +/- 30 ms, tau(II) = 180 +/- 10 mus, and tau(II) = 80 +/- 10 mus. (Slightly different values are found for a second site B at 16,167 cm(-1).) From the small values of zfs and the long emission decay times it is concluded that metal-d or MLCT admixtures to 3ILCT are very small. This result clearly reflects the ligand-centered character of the transition. The assignment as an ILCT transition is supported by the occurrence of relatively strong vibrational satellites of Pd-N and Pd-O character in highly resolved emission spectra. Although the transition is ascribed to a charge-transfer process, the geometry changes between the ground state and 3ILCT are very small. The results found for Pd(qol)2 are compared to those of companion studies of Pt(qol)2 and Pt(qtl)2 (qtl- = 8-quinolinethiolato-N,S).

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.