Time-resolved absorption spectroscopy on the femtosecond time scale has been used to monitor the earliest events associated with excited-state relaxation in tris-(2,2'-bipyridine)ruthenium(II). The data reveal dynamics associated with the temporal evolution of the Franck-Condon state to the lowest energy excited state of this molecule. The process is essentially complete in approximately 300 femtoseconds after the initial excitation. This result is discussed with regard to reformulating long-held notions about excited-state relaxation, as well as its implication for the importance of non-equilibrium excited-state processes in understanding and designing molecular-based electron transfer, artificial photosynthetic, and photovoltaic assemblies in which compounds of this class are currently playing a key role.

The synthesis and photophysical characterization of a series of aryl-substituted 2,2‘-bipyridyl complexes of RuII are reported. The static and time-resolved emission properties of [Ru(dpb)3](PF6)2, where dpb is 4,4‘-diphenyl-2,2‘-bipyridine, have been examined and are contrasted with those of [Ru(dmb)3](PF6)2 (dmb = 4,4‘-dimethyl-2,2‘-bipyridine). It is shown through analysis of electrochemical data and detailed fitting of the emission spectrum that the unusually large radiative quantum yield for [Ru(dpb)3](PF6)2 in CH3CN solution at room temperature is due to reduction of the degree of geometric distortion along primarily ring-stretch acceptor mode coordinates relative to other molecules in this class. It is proposed that the 3MLCT excited state of [Ru(dpb)3]2+ is characterized by a ligand conformation in which the 4,4‘-phenyl substituents are coplanar with the bipyridyl fragment, leading to extended intraligand electron delocalization and a smaller average change in the C−C bond length upon formation of the excited state as compared to [Ru(dmb)3]2+. These conclusions are further supported by photophysical data on several new molecules, [Ru(dptb)3](PF6)2 (dptb = 4,4‘-di-p-tolyl-2,2‘-bipyridine), [Ru(dotb)3](PF6)2 (dotb = 4,4‘-di-o-tolyl-2,2‘-bipyridine), and [Ru(dmesb)3](PF6)2 (dmesb = 4,4‘-dimesityl-2,2‘-bipyridine). The systematic increase in steric bulk provided by this ligand series results in clear trends in k r, k nr, and S M (the Huang−Rhys factor), consistent with the delocalization model. In addition, time-resolved resonance Raman data reveal frequency shifts in ring-stretch modes across the series supporting the notion that, as the steric bulk of the ligand increases, the ability for the peripheral phenyl rings to become coplanar with the bipyridyl fragment is hindered. Ab initio calculations employing Hartree−Fock and second-order perturbation theory on neutral and anionic 4-phenylpyridine, put forth as a model for the ground and excited states of [Ru(dpb)3]2+, are also reported. These calculations suggest a canted geometry for the ground state, but a considerable thermodynamic driving force for achieving planarity upon reduction of the ligand. The canted ground-state geometry is also observed in the single-crystal X-ray structure of the mixed-ligand complex [Ru(dmb)2(dpb)](PF6)2. Finally, consideration of how this system evolves from the Franck−Condon state to the planar thermalized 3MLCT state is discussed with regard to the possibility of time-resolving the onset of extended electron delocalization in the excited state by using ultrafast spectroscopy.

Coherent light sources can be used to manipulate the outcome of light-matter interactions by exploiting interference phenomena in the time and frequency domain. A powerful tool in this emerging field of 'quantum control' is the adaptive shaping of femtosecond laser pulses, resulting, for instance, in selective molecular excitation. The basis of this method is that the quantum system under investigation itself guides an automated search, via iteration loops, for coherent light fields best suited for achieving a control task designed by the experimenter. The method is therefore ideal for the control of complex experiments. To date, all demonstrations of this technique on molecular systems have focused on controlling the outcome of photo-induced reactions in identical molecules, and little attention has been paid to selectively controlling mixtures of different molecules. Here we report simultaneous but selective multi-photon excitation of two distinct electronically and structurally complex dye molecules in solution. Despite the failure of single parameter variations (wavelength, intensity, or linear chirp control), adaptive femtosecond pulse shaping can reveal complex laser fields to achieve chemically selective molecular excitation. Furthermore, our results prove that phase coherences of the solute molecule persist for more than 100 fs in the solvent environment.

The transition metal complexes [Ru(dmb) 3 ] 2+ and [Ru(dpb) 3 ] 2+ , where dmb is 4,4′-dimethyl-2,2′-bipyridine and dpb is 4,4′-diphenyl-2,2′-bipyridine, have been studied by femtosecond visible electronic absorption spectroscopy. Spectroelectrochemical measurements in conjunction with nanosecond time-resolved absorption spectroscopy allow for the assignment of various features in the excited-state differential absorption spectra as both ligand-based π* r π* and ligand-to-metal charge transfer (LMCT) in nature. A unique absorptive feature centered at ∼ 530 nm in [Ru(dpb) 3 ] 2+ was identified as an optical marker for the thermalized (and hence fully intraligand delocalized) excited state. Single wavelength and full spectrum transient absorption data were obtained on both molecules in CH 3 CN solution at room temperature following metal-to-ligand charge transfer (MLCT) excitation at 400 nm. Data on [Ru(dmb) 3 ] 2+ at 532 nm, a region of net excited-state absorption, revealed biphasic decay kinetics (∼120 fs and 5 ps) attributed to a combination of 1 MLCT f 3 MLCT intersystem crossing and vibrational cooling dynamics. Dynamics for [Ru(dpb) 3 ] 2+ under identical conditions revealed biphasic rise times in the region of the ligand-based π* r π* absorption at λ probe ) 532 nm. Although the origin of the fast component (∼ 200 fs) is not yet clear, the ca. 2 ps rise is assigned to rotation of the peripheral aryl ring and thus corresponds to the time scale for intraligand electron delocalization.

Through the study of structure-property relationships using a combination of experimental and computational analyses, a number of phenoxazine derivatives have been developed as visible light absorbing, organic photoredox catalysts (PCs) with excited state reduction potentials rivaling those of highly reducing transition metal PCs. Time-dependent density functional theory (TD-DFT) computational modeling of the photoexcitation of N-aryl and core modified phenoxazines guided the design of PCs with absorption profiles in the visible regime. In accordance with our previous work with N, N-diaryl dihydrophenazines, characterization of noncore modified N-aryl phenoxazines in the excited state demonstrated that the nature of the N-aryl substituent dictates the ability of the PC to access a charge transfer excited state. However, our current analysis of core modified phenoxazines revealed that these molecules can access a different type of CT excited state which we posit involves a core substituent as the electron acceptor. Modification of the core of phenoxazine derivatives with electron-donating and electron-withdrawing substituents was used to alter triplet energies, excited state reduction potentials, and oxidation potentials of the phenoxazine derivatives. The catalytic activity of these molecules was explored using organocatalyzed atom transfer radical polymerization (O-ATRP) for the synthesis of poly(methyl methacrylate) (PMMA) using white light irradiation. All of the derivatives were determined to be suitable PCs for O-ATRP as indicated by a linear growth of polymer molecular weight as a function of monomer conversion and the ability to synthesize PMMA with moderate to low dispersity (dispersity less than or equal to 1.5) and initiator efficiencies typically greater than 70% at high conversions. However, only PCs that exhibit strong absorption of visible light and strong triplet excited state reduction potentials maintain control over the polymerization during the entire course of the reaction. The structure-property relationships established here will enable the application of these organic PCs for O-ATRP and other photoredox-catalyzed small molecule and polymer syntheses.

Photoexcited intramolecular charge transfer (CT) states in N,N-diaryl dihydrophenazine photoredox catalysts are accessed through catalyst design and investigated through combined experimental studies and density functional theory (DFT) calculations. These CT states are reminiscent of the metal to ligand charge transfer (MLCT) states of ruthenium and iridium polypyridyl complexes. For cases where the polar CT state is the lowest energy excited state, we observe its population through significant solvatochromic shifts in emission wavelength across the visible spectrum by varying solvent polarity. We propose the importance of accessing CT states for photoredox catalysis of atom transfer radical polymerization lies in their ability to minimize fluorescence while enhancing electron transfer rates between the photoexcited photoredox catalyst and the substrate. Additionally, solvent polarity influences the deactivation pathway, greatly affecting the strength of ion pairing between the oxidized photocatalyst and the bromide anion and thus the ability to realize a controlled radical polymerization. Greater understanding of these photoredox catalysts with respect to CT and ion pairing enables their application toward the polymerization of methyl methacrylate for the synthesis of polymers with precisely tunable molecular weights and dispersities typically lower than 1.10.

Photoredox catalysis is a versatile approach for the construction of challenging covalent bonds under mild reaction conditions, commonly using photoredox catalysts (PCs) derived from precious metals. As such, there is need to develop organic analogues as sustainable replacements. Although several organic PCs have been introduced, there remains a lack of strongly reducing visible light organic PCs. Herein, we establish the critical photophysical and electrochemical characteristics of both a dihydrophenazine and a phenoxazine system that enables them success as strongly reducing visible light PCs for trifluoromethylations and dual photoredox/nickel catalyzed C-N and C-S cross-couplings, reactions which have been historically exclusive to precious metal PCs.

Redox-driven proton pumps, radical initiation and propagation in biology, and small-molecule activation processes all involve the coupling of electron transfer to proton transport. A mechanistic framework in which to interpret these processes is being developed by examining proton-coupled electron transfer (PCET) in model and natural systems. Specifically, PCET investigations are underway on the following three fronts: (1) the elucidation of the PCET reaction mechanism by time-resolved laser spectroscopy of electron donors and acceptors juxtaposed by a proton transfer interface; (2) the role of amino acid radicals in biological catalysis with the radical initiation and transport processes of E. coli ribonucleotide reductase (RNR) as a focal point; and (3) the application of PCET towards small-molecule activation with emphasis on biologically relevant bond-breaking and bond-making processes involving oxygen and water. A review of recent developments in each of these areas is discussed.