Alec C. Durrell

Singlet and Triplet Excitation Management in a Bichromophoric Near-Infrared-Phosphorescent BODIPY-Benzoporphyrin Platinum Complex

Multichromophoric arrays provide one strategy for assembling molecules with intense absorptions across the visible spectrum but are generally focused on systems that efficiently produce and manipulate singlet excitations and therefore are burdened by the restrictions of (a) unidirectional energy transfer and (b) limited tunability of the lowest molecular excited state. In contrast, we present here a multichromophoric array based on four boron dipyrrins (BODIPY) bound to a platinum benzoporphyrin scaffold that exhibits intense panchromatic absorption and efficiently generates triplets. The spectral complementarity of the BODIPY and porphryin units allows the direct observation of fast bidirectional singlet and triplet energy transfer processes (k(ST)((1)BDP→(1)Por) = 7.8 × 10(11) s(-1), k(TT)((3)Por→(3)BDP) = 1.0 × 10(10) s(-1), k(TT)((3)BDP→(3)Por) = 1.6 × 10(10) s(-1)), leading to a long-lived equilibrated [(3)BDP][Por]⇌[BDP][(3)Por] state. This equilibrated state contains approximately isoenergetic porphyrin and BODIPY triplets and exhibits efficient near-infrared phosphorescence (λ(em) = 772 nm, Φ = 0.26). Taken together, these studies show that appropriately designed triplet-utilizing arrays may overcome fundamental limitations typically associated with core-shell chromophores by tunable redistribution of energy from the core back onto the antennae.

Near-IR Phosphorescence of Iridium(III) Corroles at Ambient Temperature

The photophysical properties of Ir(III) corroles differ from those of phosphorescent porphyrin complexes, cyclometalated and polyimine Ir(III) compounds, and other luminescent metallocorroles. Ir(III) corrole phosphorescence is observed at ambient temperature at wavelengths much longer (>800 nm) than those of most Ir(III) phosphors. The solvatochromic behavior of Ir(III)-corrole Soret and Q absorption bands suggests that the lowest singlet excited states (S(2) and S(1)) are substantially more polar than the ground state.

Iodinated aluminum(III) corroles with long-lived triplet excited states.

The first reported iodination of a corrole leads to selective functionalization of the four C-H bonds on one pole of the macrocycle. An aluminum(III) complex of the tetraiodinated corrole, which exhibits red fluorescence, possesses a long-lived triplet excited state.

Nitrogen Insertion into a Corrole Ring: Iridium Monoazaporphyrins†

A new route to rare porphyrinoids: The non-innocence of the corrole ring allows the oxidative ring insertion of a nitrogen atom under mild conditions (see scheme; NBS=N-bromosuccinimide). The resulting meso-substituted azaporphyrins exhibit high-energy Soret absorption bands and red luminescence. This new synthetic route will allow for the development of novel azaporphyrin complexes with relevance to the study of biomimetic oxidations.

Structural Control of 1A2u-to-3A2u Intersystem Crossing in Diplatinum(II,II) Complexes

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per(difluoroboro)tetrakis(pyrophosphito)diplatinate(II) ([Pt(2)(μ-P(2)O(5)(BF(2))(2))(4)](4-), abbreviated Pt(pop-BF(2))), yields a radiative decay rate (k(r) = 1.7 × 10(8) s(-1)) an order of magnitude greater than that of the parent complex, Pt(pop). Its temperature-independent and activated intersystem crossing (ISC) pathways are at least 18 and 142 times slower than those of Pt(pop) [ISC activation energies: 2230 cm(-1) for Pt(pop-BF(2)); 1190 cm(-1) for Pt(pop)]. The slowdown in the temperature-independent ISC channel is attributed to two factors: (1) reduced spin-orbit coupling between the (1)A(2u) state and the mediating triplet(s), owing to increases of LMCT energies relative to the excited singlet; and (2) diminished access to solvent, which for Pt(pop) facilitates dissipation of the excess energy into solvent vibrational modes. The dramatic increase in E(a) is attributed to increased P-O-P framework rigidity, which impedes symmetry-lowering distortions, in particular asymmetric vibrations in the Pt(2)(P-O-P)(4) core that would allow direct (1)A(2u)-(3)A(2u) spin-orbit coupling.

Groups 5 and 6 Terminal Hydrazido(2−) Complexes: Nβ Substituent Effects on Ligand-to-Metal Charge-Transfer Energies and Oxidation States

Brightly colored terminal hydrazido(2-) (dme)MCl(3)(NNR(2)) (dme = 1,2-dimethoxyethane; M = Nb, Ta; R = alkyl, aryl) or (MeCN)WCl(4)(NNR(2)) complexes have been synthesized and characterized. Perturbing the electronic environment of the β (NR(2)) nitrogen affects the energy of the lowest-energy charge-transfer (CT) transition in these complexes. For group 5 complexes, increasing the energy of the N(β) lone pair decreases the ligand-to-metal CT (LMCT) energy, except for electron-rich niobium dialkylhydrazides, which pyramidalize N(β) in order to reduce the overlap between the Nb═N(α) π bond and the N(β) lone pair. For W complexes, increasing the energy of N(β) eventually leads to reduction from formally [W(VI)≡N-NR(2)] with a hydrazido(2-) ligand to [W(IV)═N═NR(2)] with a neutral 1,1-diazene ligand. The photophysical properties of these complexes highlight the potential redox noninnocence of hydrazido ligands, which could lead to ligand- and/or metal-based redox chemistry in early transition metal derivatives.

Electron transfer triggered by optical excitation of phenothiazine-tris(meta-phenylene-ethynylene)-(tricarbonyl)(bpy)(py)rhenium(I).

We have investigated excited-state electron transfer in a donor-bridge-acceptor complex containing phenothiazine (PTZ) linked via tris(meta-phenylene-ethynylene) to a tricarbonyl(bipyridine)(pyridine)Re(I) unit. Time-resolved luminescence experiments reveal two excited-state (*Re) decay regimes, a multiexponential component with a mean lifetime of 2.7 ns and a longer monoexponential component of 530 ns in dichloromethane solution. The faster decay is attributed to PTZ → *Re electron transfer in a C-shaped PTZ-bridge-Re conformer (PTZ-Re ≈ 7.5 Å). We assign the longer lifetime, which is virtually identical to that of free *Re, to an extended conformer (PTZ-Re > 20 Å). The observed biexponential *Re decay requires that interconversion of PTZ-bridge-Re conformers be slower than 10(6) s(-1).