Charge-transfer character in a covalent diketopyrrolopyrrole dimer: implications for singlet fission

Diketopyrrolopyrrole (DPP) is a strongly absorbing, photostable chromophore that can undergo singlet fission (SF), a photophysical process that promises to significantly enhance solar‐cell performance. In the solid state, DPP packs in a herringbone arrangement that maximizes intermolecular donor–acceptor interactions, suggesting that charge‐transfer (CT) states play a role in DPP SF. In order to characterize intermolecular DPP CT states in molecular assemblies, we have synthesized a covalent DPP dimer bridged by a xanthene linker, which places two thiophene‐substituted DPPs (TDPPs) in a cofacial arrangement that mimics chromophore π–π stacking in the thin film. After photoexcitation in polar solvents, symmetry‐breaking charge separation forms the fully charge separated TDPP+.–TDPP−. ion‐pair state. In nonpolar solvents, charge separation is incomplete leading to the TDPPδ+–TDPPδ− CT state, which is in pseudoequilibrium with the relaxed S1S0 state observed by transient absorption and emission spectroscopy. This study highlights the importance of intramolecular coupling as well as the importance of entropy to promoting SF in chromophore dimers for which SF is endo‐ or isoergic

Electron Hopping and Charge Separation within a Naphthalene-1,4:5,8-bis(dicarboximide) Chiral Covalent Organic Cage

We present the stereoselective synthesis of a chiral covalent organic cage consisting of three redox-active naphthalene-1,4:5,8-bis­(dicarboximide) (NDI) units by dynamic imine chemistry. Single crystal X-ray diffraction analysis shows that host–guest interactions and racemic cocrystallization allow for controlling the solid state structure. Electronic interactions between the NDI units probed by absorption and circular dichroism spectroscopies, electrochemistry and theoretical calculations are shown to be weak. Photoexcitation of NDI leads to intracage charge separation with a longer lifetime than observed in the corresponding monomeric NDI and dimeric NDI cyclophane imines. The EPR spectrum of the singly reduced cage shows that the electron is localized on a single NDI unit at ambient temperatures and transitions to rapid hopping among all three NDI units upon heating to 350 K. Dynamic covalent chemistry thus promises rapid access to covalent organic cages with well-defined architectures to study charge accumulation and electron transport phenomena

Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution Scattering

Dimeric complexes composed of d8 square planar metal centers and rigid bridging ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain in order to assist the development of reliable methods to model metal dimer complexes more broadly. [Ir2 (dimen)4]2+ (dimen = para-diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteract the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the nonequilibrium depletion of the electronic ground state population─often termed the ground state hole─with ultrafast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d8-d8 metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added, and a hybrid functional, which includes exact exchange, is used.</p

Characterization of deformational isomerization potential and interconversion dynamics with ultrafast x-ray solution scattering

Dimeric complexes composed of d8 square planar metal centers and rigid bridg- ing ligands provide model systems to understand the interplay between attractive dispersion forces and steric strain, in order to assist the development reliable meth- ods to model metal dimer complexes more broadly. [Ir2(dimen)4]2+ (dimen = para- diisocyanomenthane) presents a unique case study for such phenomena, as distortions of the optimal structure of a ligand with limited conformational flexibility counteracts the attractive dispersive forces from the metal and ligand to yield a complex with two ground state deformational isomers. Here, we use ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) to reveal the nature of the equilibrium distribution and the exchange rate between the deformational isomers. The two ground state isomers have spectrally distinct electronic excitations that enable the selective excitation of one isomer or the other using a femtosecond duration pulse of visible light. We then track the dynamics of the non-equilibrium depletion of the electronic ground state population – often termed the ground state hole – with ultra- fast XSS and OTAS, revealing a restoration of the ground state equilibrium in 2.3 ps. This combined experimental and theoretical study provides a critical test of various density functional approximations in the description of bridged d8-d8 metal complexes. The results show that density functional theory calculations can reproduce the primary experimental observations if dispersion interactions are added and a hybrid functional, which includes exact exchange, is used