Triplet−Triplet Annihilation of Excited States of Polypyridyl Ru(II) Complexes Bound to Polystyrene

It Is Good to Be Flexible: Energy Transport Facilitated by Conformational Fluctuations in Light-Harvesting Polymers

We investigate the mechanism of energy transfer between ruthenium(II) (Ru) and osmium(II) (Os) polypyridyl complexes affixed to a polyfluorene backbone (PF-RuOs) using a combination of time-resolved emission spectroscopy and coarse-grained molecular dynamics (CG MD). Photoexcitation of a Ru chromophore initiates Dexter-style energy hopping along isoenergetic complexes followed by sensitization of a lower-energy Os trap. While we can determine the total energy transfer rate within an ensemble of solvated PF-RuOs from time-dependent Os* emission spectra, heterogeneity of the system and inherent polymer flexibility give rise to highly multiexponential kinetics. We developed a three-part computational kinetic model to supplement our spectroscopic results: (1) CG MD model of PF-RuOs that simulates molecular motions out to 700 ns, (2) energy transfer kinetic simulations in CG MD PF-RuOs that produce time-resolved Ru and Os excited-state populations, and (3) computational experiments that interrogate the mechanisms by which motion aids energy transfer. Good agreement between simulated and experimental emission transients reveals that our kinetic model accurately simulates the molecular motion of PF-RuOs during energy transfer. Simulated results indicate that pendant flexibility allows 81% of the excited state to sensitize an Os trap compared to a 48% occupation when we treat pendants statically. Our computational experiments show how static pendants are only able to engage in local energy transfer. The excited state equilibrates across a domain of complexes proximal to the initial excitation and becomes trapped within that unique, frozen locality. Side-chain flexibility enables pendants to swing in and out of the original domain spreading the excited state out to ±30 pendant complexes away from the initial excitation.

Trapping intermediate MLCT states in low-symmetry {Ru(bpy)} complexes

The picosecond excited state dynamics of [Ru(tpm)(bpy)(NCS)]+ (RubNCS+ ) and [Ru(tpm)(bpy)(CN)]+ (RubCN+ ) (tpm = tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine) have been analyzed by means of transient absorption measurements and spectroelectrochemistry. Emissive 3MLCTs with (GS)HOMO(h+)-(GS)LUMO(e-) configurations are the lowest triplet excited states regardless of whether 387 or 505 nm photoexcitation is used. 387 nm photoexcitation yields, after a few picoseconds, the emissive 3MLCTs. In contrast, 505 nm photoexcitation populates an intermediate excited state that we assign as a 3MLCT state, in which the hole sits in a metal-centered orbital of different symmetry, prior to its conversion to the emissive 3MLCTs. The disparities in terms of electronic configuration between the intermediate and the emissive 3MLCTs have two important consequences. On one hand, both states feature very different fingerprint absorptions in transient absorption measurements. On the other hand, the reconfiguration is impeded by a kinetic barrier. As such, the conversion is followed spectroscopically and kinetically on the 300 ps timescale.

Direct observation of light-driven, concerted electron-proton transfer

The phenols 4-methylphenol, 4-methoxyphenol, and N-acetyl-tyrosine form hydrogen-bonded adducts with N-methyl-4, 4'-bipyridinium cation (MQ⁺) in aqueous solution as evidenced by the appearance of low-energy, low-absorptivity features in UV-visible spectra. They are assigned to the known examples of optically induced, concerted electron-proton transfer, photoEPT. The results of ultrafast transient absorption measurements on the assembly MeOPhO-H---MQ⁺ are consistent with concerted EPT by the instantaneous appearance of spectral features for MeOPhO·---H-MQ⁺ in the transient spectra at the first observation time of 0.1 ps. The transient decays to MeOPhO-H---MQ⁺ in 2.5 ps, accompanied by the appearance of oscillations in the decay traces with a period of ∼1 ps, consistent with a vibrational coherence and relaxation from a higher υ(N-H) vibrational level or levels on the timescale for back EPT.

Ultrafast dynamics in multifunctional Ru(II)-loaded polymers for solar energy conversion

The use of sunlight to make chemical fuels (i.e., solar fuels) is an attractive approach in the quest to develop sustainable energy sources. Using nature as a guide, assemblies for artificial photosynthesis will need to perform multiple functions. They will need to be able to harvest light across a broad region of the solar spectrum, transport excited-state energy to charge-separation sites, and then transport and store redox equivalents for use in the catalytic reactions that produce chemical fuels. This multifunctional behavior will require the assimilation of multiple components into a single macromolecular system. A wide variety of different architectures including porphyrin arrays, peptides, dendrimers, and polymers have been explored, with each design posing unique challenges. Polymer assemblies are attractive due to their relative ease of production and facile synthetic modification. However, their disordered nature gives rise to stochastic dynamics not present in more ordered assemblies. The rational design of assemblies requires a detailed understanding of the energy and electron transfer events that follow light absorption, which can occur on time scales ranging from femtoseconds to hundreds of microseconds, necessitating the use of sophisticated techniques. We have used a combination of time-resolved absorption and emission spectroscopies with observation times that span 9 orders of magnitude to follow the excited-state evolution within polymer-based molecular assemblies. We complement experimental observations with molecular dynamics simulations to develop a microscopic view of these dynamics. This Account provides an overview of our work on polymers decorated with pendant Ru(II) chromophores, both in solution and on surfaces. We have examined site-to-site energy transport among the Ru(II) complexes, and in systems incorporating π-conjugated polymers, we have observed ultrafast formation of a long-lived charge-separated state. When attached to TiO2, these assemblies exhibit multifunctional behavior in which photon absorption is followed by energy transport to the surface and electron injection to produce an oxidized metal complex. The oxidizing equivalent is then transferred to the conjugated polymer, giving rise to a long-lived charge-separated state.

Oxidation of Guanine in Double-Stranded DNA by [Ru(bpy)2dppz]Cl2in Cationic Reverse Micelles

DNA oxidation has been investigated in the medium of cationic reverse micelles (RMs). The oxidative chemistry is photochemically initiated using the DNA intercalator bis(bipyridine)dipyridophenazine ruthenium(II) chloride ([Ru(bpy)2dppz]Cl2) bound to duplex DNA in the RMs. High-resolution polyacrylamide gel electrophoresis (PAGE) is used to reveal and quantify guanine (G) oxidation products, including 8-oxo-7,8-dihydroguanine (8OG). In buffer solution, the addition of the oxidative quenchers potassium ferricyanide or pentaamminechlorocobalt(III) dichloride leads to an increase in the amount of piperidine-labile G oxidation products generated via one-electron oxidation. In RMs, however, the yield of oxidatively generated damage is attenuated. With or without ferricyanide quencher in the RMs, the yield of oxidatively generated products is approximately the same. Inclusion of the cationic quencher [CoCl(NH3)5]2+ in the RMs increases the amount of oxidation products generated but not to the extent that it does in buffer solution. Under anaerobic conditions, all of the samples in RMs, with or without added oxidative quenchers, show decreased levels of piperidine-labile oxidation products, suggesting that the primary oxidant in RMs is singlet oxygen. G oxidation is enhanced in D2O and deuterated heptane and is diminished in the presence of sodium azide in RMs, also supporting 1O2 as the main G oxidant in RMs. Isotopic labeling experiments show that the oxygen atom in 8OG produced in RMs is not from water. The observed change in the G oxidation mechanism from a one-electron process in buffer to mostly 1O2 in RMs illustrates the importance of both DNA structure and DNA environment on the chemistry of G oxidation.

Electron injection dynamics of Ru polypyridyl complexes on SnO2 nanocrystalline thin films

Ultrafast infrared spectroscopy was utilized to investigate the electron-transfer dynamics from Ru(dcbpy)(2)(X)(2) complexes (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X(2) = SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470, respectively) to nanocrystalline SnO(2) films. For both films exposed to air (dry) and submerged in a pH 2 buffer solution, all traces show biphasic dynamics with a small ultrafast component (less than 10%) and nonexponential slow component, indicating that most injection occurs from thermalized excited state of the dye. In the dry film, the injection rate becomes slower, comparing RuN3, Ru505, and Ru470, correlating with decreasing excited-state oxidation potentials in these dyes. However, the variation of injection rate with dye potential is less noticeable at pH 2. The possible reason for the different injection dynamics in these dyes and under different environments are discussed. These injection dynamics are also compared with those on TiO(2) and ZnO.

Photexcitation of Aqueous Ruthenium(II)-tris-(2,2‘-bipyridine) with High-Intensity Femtosecond Laser Pulses

We report a femtosecond pump-probe study on the photochemistry of concentrated aqueous solutions of [RuII(bpy)3]2+, as a function of pump power (up to 2 TW/cm2) at 400 nm excitation. The transient absorption spectra in the 345-660 nm range up to 1 ns time delay enable the observation of the following photoproducts: the triplet 3MLCT (metal-to-ligand-charge-transfer) excited state, the reduced form [RuII(bpy)3]+, the oxidized species [RuIII(bpy)3]3+, and the solvated electron e(aq). The 3MLCT state is formed within the excitation pulse and undergoes vibrational relaxation in 3-5 ps, as evidenced by the shift of the ligand-centered (LC) absorption band below 400 nm. Even at the highest pump powers, the majority of e(aq) originates from multiphoton ionization of [RuII(bpy)3]2+ and not from the solvent, generating [RuIII(bpy)3]3+ as a byproduct. At 10 ps time delay, the total concentration of the three product species is balanced by the depleted concentration of [RuII(bpy)3]2+, even at the highest fluences used, indicating that no further reaction products significantly contribute to the overall photochemistry. On the 100 ps time scale, most probably diffusion-controlled reduction of ground-state [RuII(bpy)3]2+ by solvated electrons occurs, next to recombination between e(aq) and [RuIII(bpy)3]3+.