Computational Studies of Carbodiimide Rings

Computational studies of alicyclic carbodiimides (RNCNR) (rings five through twelve) at the MP2/6-31G­(d,p)//MP2/6-31G­(d,p) level of theory were conducted to locate the transition states between carbodiimides isomers. Transition states for rings six through twelve were found. The RNCNR dihedral angle is ∼0° for even-numbered rings, but deviates from 0° for rings seven, nine, eleven, and twelve. The even- and odd-numbered ring transition states have different symmetry point groups. C_s transition states (even rings) have an imaginary frequency mode that transforms as the asymmetric irreducible representation of the group. C₂ transition states (odd rings) have a corresponding mode that transforms as the totally symmetric representation. Intrinsic reaction coordinate analyses followed by energy minimization along the antisymmetric pathways led to enantiomeric pairs. The symmetric pathways give diastereomeric isomers. The five-membered ring carbodiimide is a stable structure, possibly isolable. A twelve-membered ring transition state was found only without applying symmetry constraints (C₁). Molecular mechanics and molecular dynamics studies of the seven-, eight-, and nine-membered rings gave additional structures, which were then minimized using ab initio methods. No structures beyond those found from the IRC analyses described were found. The potential for optical resolution of the seven-membered ring is discussed

Symmetry-Directed Control of Electronic Coupling for Singlet Fission in Covalent Bis–Acene Dimers

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 <i>C</i>_<i>s</i>-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 <i>C</i>₂ 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 <i>k</i>_SF with the faster of the two dimers (<i>C</i>₂) being predicted to undergo the process on a nearly ultrafast 1.5 ps time scale

Exploiting Conformational Dynamics of Structurally Tuned Aryl-Substituted Terpyridyl Ruthenium(II) Complexes to Inhibit Charge Recombination in Dye-Sensitized Solar Cells

To explore the impact of dye structure on photoinduced interfacial electron-transfer (ET) processes, a series of systematically tuned 4′-aryl-substituted terpyridyl ruthenium­(II) complexes have been studied in TiO₂ film and dye-sensitized solar cell (DSSC) device settings. Structural tuning is achieved by the introduction of methyl substituents at the ortho positions of a ligand aryl moiety. Solar power conversion efficiencies are measured, and these values are deconstructed to better understand the fundamental processes that control light-to-current conversion. Injection yields are identified as the primary factor limiting efficiencies, due in large part to significant nonradiative decay pathways in these bis-terpyridyl Ru­(II) systems. Encouragingly, the addition of methyl steric bulk is found to inhibit charge recombination, with measured recombination lifetimes increasing by over 12-fold across the series of structurally tuned complexes. If injection yields can be improved, the structural tuning of recombination rate constants may be an important design strategy for improving solar conversion efficiency in solar cells and water-splitting devices

Synthesis, Electrochemical Characterization, and Photophysical Studies of Structurally Tuned Aryl-Substituted Terpyridyl Ruthenium(II) Complexes

Synthesis, electrochemical potentials, static emission, and temperature-dependent excited-state lifetimes of several 4′-aryl-substituted terpyridyl complexes of ruthenium­(II) are reported. Synthetic tuning is explored within three conceptual series of complexes. The first series explores the impact of introducing a strong σ-donating 4,4′,4″-tri-<i>tert</i>-butyl-2,2′:6′,2″-terpyridine (tbtpy) opposite to an arylated terpyridine ligand 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (ttpy). It is found that ³MLCT (triplet metal-to-ligand charge-transfer state) stabilization concomitant with ³MC (triplet metal-centered state) destabilization in the heteroleptic parent complex [Ru­(ttpy)­(tbtpy)]²⁺ leads to an extended excited-state lifetime relative to the structurally related bis-homoleptic species [Ru­(ttpy)₂]²⁺. The second series explores the impact of introducing a carboxylic acid or a methyl ester moiety at the para-position of the arylterpyridyl ligand (R₁ = R₂ = H) within heteroleptic complexes as a platform for future semiconductor attachment studies. This substitution leads to further lifetime enhancements, understood as arising from ³MLCT stabilization. Such complexes are referred to as [Ru­(<b>1</b>)­(tbtpy)]²⁺ (for the acid at R₃) and [Ru­(<b>1′</b>)­(tbtpy)]²⁺ (for the ester at R₃). In the final series, methyl substituents are sequentially added at the R₁ and R₂ positions for both the acid ([Ru­(<b>2</b>)­(tbtpy)]²⁺ and [Ru­(<b>3</b>)­(tbtpy)]²⁺) and ester ([Ru­(<b>2′</b>)­(tbtpy)]²⁺ and [Ru­(<b>3′</b>)­(tbtpy)]²⁺) analogues to eventually explore dynamical electron transfer coupling at dye/semiconductor interfaces. In these complexes, sequential addition of steric bulk decreases excited state lifetimes. This can be understood to arise primarily from the increase of the ³MLCT level, as excited-state electron delocalization is limited by inter-ring twisting in the lower-energy arylated ligand. The introduction of a dimethylated sterically encumbered ligand lead to a notable 14-fold increase in <i>k</i>_nr from [Ru­(<b>1′</b>)­(tbtpy)]²⁺ to [Ru­(<b>3′</b>)­(tbtpy)]²⁺ (or [Ru­(<b>1</b>)­(tbtpy)]²⁺ to [Ru­(<b>3</b>)­(tbtpy)]²⁺)

Dynamics of the ³MLCT in Ru(II) Terpyridyl Complexes Probed by Ultrafast Spectroscopy: Evidence of Excited-State Equilibration and Interligand Electron Transfer

Ground- and excited-state properties of [Ru­(tpy)₂]²⁺, [Ru­(tpy)­(ttpy)]²⁺, and [Ru­(ttpy)₂]²⁺ (where tpy = 2,2′:6′,2″-terpyridine and ttpy =4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine) in room temperature acetonitrile have been investigated using linear absorption, electrochemical, and ultrafast transient pump–probe techniques. Spectroelectrochemistry was used to assign features observed in the transient spectra while single wavelength kinetics collected at a variety of probe wavelengths were used to monitor temporal evolution of the MLCT excited state. From these data, the excited-state lifetime of each complex was recovered and the rate limiting decay step was identified. In the bis-heteroleptic complex [Ru­(tpy)­(ttpy)]²⁺, photoexcitation to the ¹MLCT manifold generates both tpy-localized and ttpy-localized excited states. Accordingly, interligand electron transfer (ILET) from tpy-localized to the ttpy-localized ³MLCT excited states is observable and the time scale has been measured to be 3 ps. For the homoleptic complex [Ru­(tpy)₂]²⁺, evidence for equilibration of the ³MLCT excited-state population with the ³MC has been observed and the time scale is reported at 2 ps

Exploring Non-Condon Effects in a Covalent Tetracene Dimer: How Important Are Vibrations in Determining the Electronic Coupling for Singlet Fission?

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 <i>C</i>_2v structure, the “nonhorizontal” couplings between HOMOs and LUMOs (<i>t</i>_HL and <i>t</i>_LH) vanish by symmetry. We then explore the impact of molecular vibrations through the calculation of <i>t</i>_AB coupling gradients along 183 normal modes of motion. Rules are established for the types of motions (irreducible representations in the <i>C</i>_2v point group) that turn on <i>t</i>_HL and <i>t</i>_LH 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

Inverse Kinetic Isotope Effect in the Excited-State Relaxation of a Ru(II)–Aquo Complex: Revealing the Impact of Hydrogen-Bond Dynamics on Nonradiative Decay

Photophysics of the MLCT excited-state of [Ru­(bpy)­(tpy)­(OH₂)]²⁺ (<b>1</b>) and [Ru­(bpy)­(tpy)­(OD₂)]²⁺ (<b>2</b>) (bpy = 2,2′-bipyridine and tpy = 2,2′:6′,2″-terpyridine) have been investigated in room-temperature H₂O and D₂O using ultrafast transient pump-probe spectroscopy. An inverse isotope effect is observed in the ground-state recovery for the two complexes. These data indicate control of excited-state lifetime via a pre-equilibrium between the ³MLCT state that initiates H-bond dynamics with the solvent and the ³MC state that serves as the principal pathway for nonradiative decay

Tunable Electronic Coupling and Driving Force in Structurally Well-Defined Tetracene Dimers for Molecular Singlet Fission: A Computational Exploration Using Density Functional Theory

Singlet fission (SF), a process by which two excited states are formed in a chromophoric system following the absorption of a single photon, has the potential to increase the theoretical efficiency of solar energy conversion devices beyond the single-junction Shockley–Quiesser limit. Although SF is observed with high yield in the solid state of certain molecules, linearly linked dimers based on these same constituents exhibit small yields in part due to small interchromophore electronic coupling. Previous work from our group demonstrated enhancement of SF yield in polycrystalline tetracene (Tc) via excitation of intermolecular motions, which increased direct overlap of monomer π-systems. In this current work, a series of norbornyl-bridged bistetracene (BT) dimers are investigated using DFT and the ability to control SF thermodynamics along with important interchromophore electronic coupling parameters via bridging geometry is shown. Although the electronic coupling of a series of <i>C</i>_2<i>v</i>-symmetric dimers (BT1–BT3) that differ in norbornyl bridge length is larger than in previously studied Tc dimers, a key nonhorizontal electron-transfer (ET) matrix element used in determining the SF rate is zero due to symmetry. In these systems, SF may be expected but electronic excitation will require coupling to vibrational modes that break symmetry. Singly bridged dimer isomers BT1-trans and BT1-cis, which break the <i>C</i>_2<i>v</i> symmetry of BT1 by exploiting attachment of the norbornyl bridge at the 1,2 instead of the 2,3 Tc positions, are expected to be significantly more favorable for SF due to an exoergic driving force, increased electronic coupling, a lower charge-transfer-state energy (particularly in the case of BT1-cis), and nonhorizontal ET matrix elements that are nonzero

Ultrafast Hole Transfer from CdS Quantum Dots to a Water Oxidation Catalyst

The first step in light-driven multielectron redox catalysis driven by semiconductor nanocrystals is the transfer of a photoexcited carrier to the catalyst. The efficiency of that step defines the upper limit on the efficiency of catalysis. Although progress has been made in transferring electrons from nanocrystals to catalysts for multielectron reduction reactions, there has been little success in moving photoexcited holes to water oxidation catalysts (WOCs). Here, we describe the kinetics of hole transfer from a photoexcited CdS quantum dot to a ruthenium polypyridine molecular WOC. Ultrafast transient absorption and photoluminescence (PL) upconversion experiments reveal that this hole transfer is surprisingly fast, occurring on a picosecond time scale. To determine the rate constant for hole transfer, we develop a model for PL quenching as a function of catalyst loading that takes into account both the catalyst binding equilibrium and the finite quenching efficiency. The rate constant for hole transfer is found to be 1.3 × 10¹¹ s^–1 per adsorbed catalyst, which is comparable to the fastest reported hole transfer from CdS nanocrystals to a noncatalyst hole acceptor. The catalyst binds strongly to the surface with an equilibrium constant of 7 × 10⁵ M^–1 and can remove up to 37% of the photoexcited holes. Our results suggest that valence band hole harvesting from CdS quantum dots for water oxidation can in principle be an efficient process. However, fast competing hole trapping limits this efficiency in the current system. We propose strategies for mitigating this limitation

Charge Transfer Dynamics between Photoexcited CdS Nanorods and Mononuclear Ru Water-Oxidation Catalysts

We describe the charge transfer interactions between photoexcited CdS nanorods and mononuclear water oxidation catalysts derived from the [Ru­(bpy)­(tpy)­Cl]⁺ parent structure. Upon excitation, hole transfer from CdS oxidizes the catalyst (Ru²⁺ → Ru³⁺) on a 100 ps to 1 ns timescale. This is followed by 10–100 ns electron transfer (ET) that reduces the Ru³⁺ center. The relatively slow ET dynamics may provide opportunities for the accumulation of multiple holes at the catalyst, which is necessary for water oxidation