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A multivariate statistical analysis is used to extract a single control variable from results of 137 parameter adaptive laser pulse shaping optimization of multiphoton electronic excitation in Rh(II) coordination complex in solution. It is shown that this single variable can be sued to linearly manipulate the observed fitness which is determined by the ratio of molecular emission to second harmonic generation of the laser pulse.

The development of adaptive control algorithm with physiochemical control pathways in arbitrary complex molecular system which could be used to measure second-harmonic spectra of shaped laser fields is reported. The adaptive control experiment can be used to understand spectral control pathways of the molecule in solution.

A generally applicable method is presented for the dimension reduction of adaptive control experiments based on partial least squares regression analysis (PLS) of the normalized covariance matrix of the total data set. Experimental results shows that only seven fundamental dimensions from an original 208-dimension search space are needed to account for approximately 90% of the variance in the observed fitness of 11700 laser-pulse shapes explored during the optimization experiment.

A study of the transition metal complexes has been conducted by means of the femtosecond visible electronic absorption spectroscopy. Results include identification of the optical marker for the thermalized excited state as a unique absorptive feature.

Research has been conducted on the variable-temperature emission data for bipyridyl complexes of Ru (super)III. Results indicate the increase of the spectra through the glass-to-fluid transition.

Proton-transfer reactions involving the silaformyl anion, HSi[O.sup.-], and its conjugate acids, HSiOH and [H.sub.2]SiO, have been investigated with ab initio methods. Calculations through fourth-order perturbation theory suggest possible routes for proton transfer. Accurate estimates for the acidity of [H.sub.2]SiO and HSiOH are presented and discussed in light of earlier experimental estimates.

A series of aryl-substituted 2,2'-bipyridyl complexes of RuII were synthesized and photophysically characterized. The unusually large radiative quantum yield for (Ru(dpb)3)(PF6)2 in CH3CN solution was found to be due to the reduction of geometric distortion along the ring-stretch acceptor mode coordinates. The 3MLCT excited stae of (Ru(dpb)3) was thought to be characterized by a ligand conformation where the 4,4'-phenyl substituents are coplanar with the bipyridyl fragment.

Researchers experimentally and computationally analyzed the gas-phase reactions of trimethylaluminates with several acids. Methyl cleavage is preferred in the reaction of nine aluminates and HCl. Related calculations for ((CH3)3AlX)- (for X = F and OH) with HCl and intrinsic reaction coordinate calculations for ((CH3)3AlF)- + HCl from the transition state to products outline the transition states and forecast the cleavage results for X and methyl cleavage.

The various mechanisms and dynamics involved in the formation, as well as the trapping of the electron-transfer photoproducts as exhibited by different metal complexes are analyzed. New method for the formation and storage of the different charge-separated excited states are also explained.

The synthesis and spectroelectrochemical characterization of two new ruthenium(II) polypyridyl donor-bridge-acceptor (D-B-A) systems with an excited-state conformationally active bridge along with two new donor-bridge (D-B) model complexes without acceptor is described. The extent of electron delocalization that is acceptable following metal-to-ligand charge transfer excitation provides insight into structurally control of the key factors governing photoinduced electron transfer rates such as reorganization energy and electronic coupling.

Proton-coupled electron transfer (PCET) kinetics of a zinc (II) porphyrin donor noncovalently bound to a naphthalene-diimide acceptor through an amidinium-carboxylate interface is investigated. The conclusion states that the importance of a proton network in controlling ET rates is becomiing more apparent as the strauctural details of the biological systems are revealed.

The steric effects on ligand geometry were examined using computational methods. Neutral aryl-pyridines were studied as models for ground-state ligand structure by performing ab initio calculations of HF and MP2, reduced aryl-pyridines were investigated as models for metal-to-ligand charge transfer excited states through the use of ab initio calculations of UHF, UMP2, ROHF and ROMP2 as well density functional calculations of UB-LYP and USVWN. The findings are discussed.

Energy wasting charge recombination is an efficiency limiting process in efforts to achieve solar energy storage. Here, density functional theory is used to explore the thermodynamics of photochemical energy storage reactions in several ruthenium polypyridyl complexes where heterolytic halogen~carbon bond scission occurs after light-induced formation of the triplet metal to ligand charge transfer (3MIXT) state, as seen in the following reaction: [Ru11(A)n(L-X)]2+ + hv → [Ru111(A)n(L-X) •--]2+*→ [Ru111(A)n(L• )]3+ +X-- (L = polypyridine ligand; X = Cl, Br, and I; A = ancillary ligand). A thermochemical cycle is employed to determine structural and electronic factors influencing δΕrxn•. Significant energetic penalties in the oxidation of the metal center are mitigated through methylation of ancillary ligands or introduction of amine ancillary ligands. Methylation of the halogenated ligand maintains energy stored in the MLCT state. Reduction in δΕrxn• is obtained by exploiting strain in the coordination geometry or in sterically encumbered ligands that is released upon bond breaking. Formation of a contact ion pair is significantly more favorable than complete separation of charged products, and shows negative δΕ with respect to the 3MLCT state in certain cases. Future tunability in stored energy may be achieved through careful manipulation of ligand structure and charge on ancillary ligands. [ABSTRACT FROM AUTHOR]

Computational studies using density functional theory (DFT) are reported for a series of donor-acceptor (DA) transition metal complexes and related excited-stale and electron transfer (ET) photoproduct models. Three hybrid Hartree-Fock/DFT (HF/DFT) functionals, B3LYP, PW91,and PBE1PBE, are employed to characterize structural features implicated in the dynamical control of productive forward and energy wasting back Ef events. Energies and optimized geometries are reported for the lowest energy singlet state in [Ru(dmb)2(bpy-ø-MV)]4+ (DA1), [Ru(dmb)2(bpy-o-tolyI-MV)]4+ (DA2), [Ru(dmb)2(bpy-2,6-Me2-ø-MV)]4+ (DA3), and (Ru(trnb)2(bpy-2,6-Me2-ø-MV)]4+ (DA3'), where dmb is 4,4'-dimethyt-2,2'-bipyridine, tmb is 4,4',5,5'-tetramethyl-2,2'-bipyridine, MV is methyl viologen, and is ø phenylene spacer. These indicate that the dihedral angle θ1 between the aryl substituent and the bipyridine fragment to which it is bound, systematically increases with the addition of steric bulk. Energies, optimized geometries, and unpaired electron spin densities are also reported for the lowest enery triplet state of [Ru(dmb)2(4-p- tolyf-2,2'-bipyridine)]2+ (D1*), [Ru(dmb)2(4-(2,6-dimethylphenyl)-2,2'-bipyridine)]2+ (D2*), [Ru(dmb)2(4-mesityl-2,2'-bipyridine)]2+ (D3*), and [Ru(tmb)2(4-mesityl-2,2'-bipyridine)]2+ (D3'*). Each of these serves as a model of a reactant excited stale in the forward electron-transfer photochemistry allowing us to qualify and quantify the role of excited-state intraligand electron delocalization in driving substantial geometry changes (especially with respect to θ1) relative to its respective DA counterpart. Next, energies, optimized geometries, and spin densities are reported for the lowest energy triplet of each DA species; 3DA1, 3DA2, 3DA3, and 3DA3'. These are used to model the ET photoproduct and they indicate that θ1 increases following ET, thus, verifying switch-like properties. Finally, we report data for geometry optimized DA1 and 3DA1 in a continuum model of room temperature acetonitrile. This study shows a complete recovery θ1 to its ground state value which has implications in efforts to trap electrons in charge-separated states. [ABSTRACT FROM AUTHOR]

The synthesis, characterization, and photophysical properties of 4- and 6-coordinate Bi 3+ coordination complexes are reported. Bi(bzq) 3 ( 1 ) and [Bi(bzq) 2 ]Br ( 2 ) (bzq = benzo[ h ]quinoline) are synthesized by reaction of 9-Li-bzq with BiCl 3 and BiBr 3 , respectively. Absorption spectroscopy, electrochemistry, and DFT studies suggest that 1 has 42% Bi 6s character in its highest-occupied molecular orbital (HOMO) as a result of six σ* interactions with the bzq ligands. Excitation of 1 at 450 nm results in a broad emission feature at 520 nm, which is rationalized as a metal-to-ligand charge transfer (MLCT) and phosphorescent emission resulting from bismuth-mediated intersystem crossing (ISC) to a triplet excited state. This excited state revealed a 35 μs lifetime and was quenched in the presence of oxygen. These results demonstrate that useful optoelectronic properties of Bi 3+ can be accessed through hypercoordination with covalent organobismuth interactions that mimic the electronic structure of lead perovskites.

Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron-transfer (DET) activation of alkyl-halide initiators following absorption of light. Characterizing DET rate coefficients ( k act ) and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs, where synthetic handles of core substitution and N -aryl substitution enable tunability of the electronic and spin characters of the catalyst excited state as well as DET reaction driving force (Δ G ET 0 ). Using Stern-Volmer quenching experiments through variation of the diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of k act are reported as a function of Δ G ET 0 for four PCs: four triplet reactants and four singlets with k act values ranging from 1.1 × 10 8 M -1 s -1 , where DET itself controls the rate, to 4.8 × 10 9 M -1 s -1 , where diffusion is rate-limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of the adiabatic Marcus electron-transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where k act can be estimated if Δ G ET 0 is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on the driving force. Next, on the basis of measured k act values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In N -naphthyl-containing PCs characterized by near-unity intersystem crossing, the T 1 is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S 1 is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in N -phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S 1 under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S 1 and T 1 reactant states produce the PC •+ radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The subunity PC •+ yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.

Exploration of [V(bpy) 3 ] 2+ and [V(phen) 3 ] 2+ (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) using electronic spectroscopy reveals an ultrafast excited-state decay process and implicates a pair of low-lying doublets with mixed metal-to-ligand charge-transfer (MLCT) and metal-centered (MC) character. Transient absorption (TA) studies of the vanadium(II) species probing in the visible and near-IR, in combination with spectroelectrochemical techniques and computational chemistry, lead to the conclusion that after excitation into the intense and broad visible 4 MLCT ← 4 GS (ground-state) absorption band (ε 400-700 nm = 900-8000 M -1 cm -1 ), the 4 MLCT state rapidly (τ isc < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (τ ∼ 2.5-3 ps) to the long-lived excited state follows, which we attribute to formation of the lower mixed state. Following these initial dynamics, GS recovery ensues with τ = 430 ps and 1.6 ns for [V(bpy) 3 ] 2+ and [V(phen) 3 ] 2+ , respectively. This stands in stark contrast with isoelectronic [Cr(bpy) 3 ] 3+ , which rapidly forms a long-lived doublet metal-centered ( 2 MC) state following photoexcitation and lacks strong visible GS absorption character. 2 MLCT character in the long-lived states of the vanadium(II) species produces geometric distortion and energetic stabilization, both of which accelerate nonradiative decay to the GS compared to [Cr(bpy) 3 ] 3+ , where the GS and 2 MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in first-row transition-metal complexes and (ii) the presence of a 2 MLCT state at lower energy than the 4 MLCT state has not been previously considered. The spin assignment of charge-transfer states in open-shell transition-metal complexes is not trivial; when metal-ligand interaction is strong, low-spin states must be carefully considered when assessing reactivity and decay from electronic excited states.

Phenothiazine, owing to its ease of oxidation and modularity with respect to facile functionalization, is an attractive central chemical unit from which to construct highly reducing organic photoredox catalysts. While design improvements have been made in the community, the yield of intersystem crossing (Φ ISC ), which determines access to the long-lived triplet excited state, has yet to be systematically optimized. Herein, we explore the impacts of N -aryl substituent variation on excited-state dynamics using picosecond to millisecond transient absorption and emission spectroscopies. Design principles are uncovered that center on controlling the energy of an intermediate charge transfer (CT) state within the singlet excited-state manifold, which, in turn, dictates the yield of CT-state formation and the rate constants for its depletion. Ultimately, we find Φ ISC to be highly sensitive to the electron-withdrawing character of the N -aryl electron acceptor in the aforementioned CT state, with Φ ISC ranging from ∼0 to 0.96.

While singlet fission (SF) has developed in recent years within material settings, much less is known about its control in covalent dimers. Such platforms are of fundamental importance and may also find practical use in next-generation dye-sensitized solar cell applications or for seeding SF at interfaces following exciton transport. Here, facile theoretical tools based on Boys localization methods are used to predict diabatic coupling for SF via determination of one-electron orbital coupling matrix elements. The results expose important design rules that are rooted in point group symmetry. For Cs-symmetric dimers, pathways for SF that are mediated by virtual charge transfer excited states destructively interfere with negative impact on the magnitude of diabatic coupling for SF. When dimers have C2 symmetry, constructive interference is enabled for certain readily achievable interchromophore orientations. Three sets of dimers exploiting these ideas are explored: a bis-tetracene pair and two sets of aza-substituted tetracene dimers. Remarkable control is shown. In one aza-substituted set, symmetry has no impact on SF reaction thermodynamics but leads to a 16-fold manipulation in SF diabatic coupling. This translates to a difference of nearly 300 in kSF with the faster of the two dimers (C2) being predicted to undergo the process on a nearly ultrafast 1.5 ps time scale.

Singlet fission (SF) offers opportunities for wavelength-selective processing of solar photons with an end goal of achieving higher efficiency inexpensive photovoltaic or solar-fuels-producing devices. In order to evaluate new molecular design strategies and for theoretical exploration of dynamics, it is important to put in place tools for efficient calculation of the electronic coupling between single-exciton reactant and multiexciton product states. For maximum utility, the couplings should be calculated at multiple nuclear geometries (rather than assumed constant everywhere, i.e., the Condon approximation) and we must be able to evaluate couplings for covalently linked multichromophore systems. With these requirements in mind, here we discuss the simplest methodology possible for rapid calculation of diabatic one-electron coupling matrix elements-based on Boys localization and rediagonalization of molecular orbitals. We focus on a covalent species called BT1 that juxtaposes two tetracene units in a partially cofacial geometry via a norbornyl bridge. In BT1, at the equilibrium C2v structure, the "nonhorizontal" couplings between HOMOs and LUMOs (t(HL) and t(LH)) vanish by symmetry. We then explore the impact of molecular vibrations through the calculation of t(AB) coupling gradients along 183 normal modes of motion. Rules are established for the types of motions (irreducible representations in the C2v point group) that turn on tHL and tLH values as well as for the patterns that emerge in constructive versus destructive interference of pathways to the SF product. For the best modes, calculated electronic coupling magnitudes for SF (at root-mean-squared deviation in position at 298 K), are within a factor of 2 of that seen for noncovalent tetracene dimers relevant to the molecular crystal. An overall "effective" electronic coupling is also given, based on the Stuchebrukhov formalism for non-Condon electron transfer rates.