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

Excitons in 2D Organic–Inorganic Halide Perovskites

Layered perovskites are hybrid 2D materials, formed through the self-assembly of inorganic lead halide networks separated by organic ammonium cation layers. In these natural quantum-well structures, quantum and dielectric confinement lead to strongly bound excitonic states that depend sensitively on the material composition. In this article, we review current understanding of exciton photophysics in layered perovskites and highlight the many ways in which their excitonic properties can be tuned. In particular, we focus on the coupling of exciton dynamics to lattice motion and local distortions of the soft and deformable hybrid lattice. These effects lead to complex excited-state dynamics, presenting new opportunities for design of optoelectronic materials and exploration of fundamental photophysics in quantum confined systems. Keywords: perovskite; hybrid material; 2D material; exciton; lead halideUnited States. Department of Energy (Award DE-SC0019345

Synthesis And Structures Of Pb3O2(CH3COO)2 0.5H2O and Pb2O(HCOO)2: Two Corrosion Products Revisited

Reactions of carboxylic acids with lead play an important role in the atmospheric corrosion of lead and lead-tin alloys. This is of particular concern for the preservation of lead-based cultural objects, including historic lead-tin alloy organ pipes. Two initial corrosion products, Pb3O2(CH3COO)(2)center dot 0.5H(2)O (1) and Pb2O(HCOO)(2) (2), had been identified through powder diffraction fingerprints in the Powder Diffraction File, but their structures had never been determined. We have crystallized both compounds using hydrothermal solution conditions, and structures were determined using laboratory and synchrotron single-crystal X-ray diffraction data. Compound 1 crystallizes in P (1) over bar, and 2 in Cccm. These compounds may be viewed as inorganic-organic networks containing single and double chains of edge-sharing Pb4O tetrahedra and have structural similarities to inorganic basic lead compounds. Bond valence sum analysis has been applied to the hemidirected lead coordination environments in each compound. Atmospheric exposure experiments contribute to understanding of the potential for conversion of these short-term corrosion products to hydrocerussite, Pb-3(CO3)(2)(OH)(2), previously identified as a long-term corrosion product on lead-rich objects. Each compound was also characterized by elemental analysis, thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), and Raman spectroscopy

Characterization of Excimer Relaxation via Femtosecond Shortwave- and Mid-Infrared Spectroscopy

Excimer formation plays a significant role in trapping excitons within organic molecular solids. Covalent dimers of perylene-3,4:9,10-bis­(dicarboximide) (PDI) are useful model systems for studying these processes as their intermolecular geometries can be precisely tuned. Using femtosecond visible-pump infrared-probe (fsIR) spectroscopy in the shortwave- and mid-infrared regions, we characterize two PDI dimers with a cofacial and a slip-stacked geometry that are coupled through a triptycene bridge. In the mid-infrared region, fsIR spectra for the strongly coupled dimers are highly blue-shifted compared to spectra for monomeric ^1*PDI. The perylene core stretching modes provide a directly observable probe of excimer relaxation, as they are particularly sensitive to this process, which is associated with a small blue shift of these modes in both dimers. The broad Frenkel-to-CT state electronic transition of the excimer, the edge of which has previously been detected in the NIR region, is now fully resolved to be much broader and to extend well into the shortwave infrared region for both dimers and is likely a generic feature of π-extended aromatic excimers

Fast Triplet Formation via Singlet Exciton Fission in a Covalent Perylenediimide-β-apocarotene Dyad Aggregate

A covalent dyad was synthesized in which perylene-3,4,:9:10-bis­(dicarboximide) (PDI) is linked to β-apocarotene (Car) using a biphenyl spacer. The dyad is monomeric in toluene and forms a solution aggregate in methylcyclohexane (MCH). Using femtosecond transient absorption (fsTA) spectroscopy, the monomeric dyad and its aggregates were studied both in solution and in thin films. In toluene, photoexcitation at 530 nm preferentially excites PDI, and the dyad undergoes charge separation in τ = 1.7 ps and recombination in τ = 1.6 ns. In MCH and in thin solid films, 530 nm excitation of the PDI-Car aggregate also results in charge transfer that competes with energy transfer from ¹*PDI to Car and with an additional process, rapid Car triplet formation in <50 ps. Car triplet formation is only observed in the aggregated PDI-Car dyad and is attributed to singlet exciton fission (SF) within the aggregated PDI, followed by rapid triplet energy transfer from ³*PDI to the carotenoid. SF from β-apocarotene aggregation is ruled out by direct excitation of Car films at 414 nm, where no triplet formation is observed. Time-resolved electron paramagnetic resonance measurements on aggregated PDI-Car show the formation of ³*Car with a spin-polarization pattern that rules out radical-pair intersystem crossing as the mechanism of triplet formation as well

Singlet Fission in Covalent Terrylenediimide Dimers: Probing the Nature of the Multiexciton State Using Femtosecond Mid-Infrared Spectroscopy

Singlet fission (SF) is a spin-allowed process that involves absorption of a photon by two electronically interacting chromophores to produce a singlet exciton state, ¹(S₁S₀), followed by rapid formation of two triplet excitons if the singlet exciton energy is about twice that of the triplet exciton. The initial formation of the multiexciton correlated triplet pair state, ¹(T₁T₁), is thought to involve the agency of charge transfer (CT) states. The dynamics of these electronic states were studied in a covalent slip-stacked terrylene-3,4:11,12-bis­(dicarboximide) (TDI) dimer in which the conformation of two TDI molecules is determined by a xanthene spacer (<b>XanTDI</b>_<b>2</b>). Femtosecond mid-infrared (fsIR) spectroscopy shows that the multiexciton ¹(T₁T₁) state has absorptions characteristic of the T₁ state in the carbonyl stretch region of the IR spectrum, in addition to IR absorptions specific to the CT state in the CC stretch region. The simultaneous presence of CT and triplet state features in both high dielectric constant CH₂Cl₂ and low dielectric constant 1,4-dioxane throughout the multiexciton state lifetime suggests that this state has both CT and triplet character

Singlet Fission within Diketopyrrolopyrrole Nanoparticles in Water

Nanoparticles (NPs) of the singlet fission chromophore 3,6-bis­(5-phenylthiophen-2-yl)­pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4­(2<i>H</i>,5<i>H</i>)-dione (PhTDPP) having average hydrodynamic diameters of 63–193 nm were prepared by rapidly injecting variable concentrations of PhTDPP solutions in tetrahydrofuran into water. These PhTDPP NPs are stable over months in water and exhibit fluorescence quantum yields ≪1%. Femtosecond transient absorption spectroscopy shows that singlet fission is more rapid in smaller NPs, likely reflecting their greater surface area-to-volume ratio and consequent exposure of more molecules to the high dielectric aqueous environment. These observations suggest that charge transfer states, whose energy is sensitive to the dielectric constant of the surrounding medium, serve as virtual intermediates in PhTDPP NP singlet fission. However, the lifetime of the triplet excitons produced by singlet fission is longest in the larger NPs having greater long-range order, which allows the triplet excitons to diffuse further from one another thus slowing triplet–triplet annihilation

Benchmark job – Watch out!

ISSN:1932-7455ISSN:1932-744

Influence of Anion Delocalization on Electron Transfer in a Covalent Porphyrin Donor–Perylenediimide Dimer Acceptor System

Photodriven electron transfer from a donor excited state to an assembly of electronically coupled acceptors has been proposed to enhance charge transfer efficiency in functional organic electronic materials. However, the circumstances under which this may occur are difficult to investigate in a controlled manner in disordered donor–acceptor materials. Here we investigate the effects of anion delocalization on electron transfer using zinc <i>meso</i>-tetraphenylporphyrin (ZnTPP) as a donor and a perylene-3,4:9,10-bis­(dicarboximide) dimer as the acceptor (PDI₂). The PDI units of the dimer are positioned in a cofacial orientation relative to one another by attachment of the imide group of each PDI to the 4- and 5-positions of a xanthene spacer. Furthermore, the distal imide group of one PDI is linked to the <i>para</i>-position of one ZnTPP phenyl group to yield ZnTPP-PDI₂. The data for the dimer are compared to two different ZnTPP-PDI monomer reference systems designed to probe electron transfer to each of the individual PDI molecules comprising PDI₂. The electron transfer rate from the ZnTPP lowest excited singlet state to PDI₂ is increased by 50% relative to that in ZnTPP-PDI, when the data are corrected for the statistics of having two electron acceptors. Femtosecond transient IR absorption spectroscopy provides evidence that the observed enhancement in charge separation results from electron transfer producing a delocalized PDI₂ anion

Probing Distance Dependent Charge-Transfer Character in Excimers of Extended Viologen Cyclophanes Using Femtosecond Vibrational Spectroscopy

Facile exciton transport within ordered assemblies of π-stacked chromophores is essential for developing molecular photonic and electronic materials. Excimer states having variable charge transfer (CT) character are frequently implicated as promoting or inhibiting exciton mobility in such systems. However, determining the degree of CT character in excimers as a function of their structure has proven challenging. Herein, we report on a series of cyclophanes in which the interplanar distance between two phenyl-extended viologen (<b>ExV</b>^<b>2+</b>) chromophores is varied systematically using a pair of <i>o</i>-, <i>m</i>-, or <i>p</i>-xylylene (<i><b>o</b></i>-, <i><b>m</b></i>-, or <i><b>p</b></i><b>-Xy</b>) covalent linkers to produce <i><b>o</b></i><b>-ExBox</b>^<b>4+</b> (3.5 Å), <i><b>m</b></i><b>-ExBox</b>^<b>4+</b> (5.6 Å), and <i><b>p</b></i><b>-ExBox</b>^<b>4+</b> (7.0 Å), respectively. The cyclophane structures are characterized using NMR spectroscopy in solution and single-crystal X-ray diffraction in the solid state. Femtosecond transient mid-IR and stimulated Raman spectroscopies show that the CT contribution to the excimer states formed in <i><b>o</b></i><b>-ExBox</b>^<b>4+</b> and <i><b>m</b></i><b>-ExBox</b>^<b>4+</b> depends on the distance between the chromophores within the cyclophanes, while in the weak interaction limit, as represented by <i><b>p</b></i><b>-ExBox</b>^<b>4+</b> (7.0 Å), the lowest excited singlet state of <b>ExV</b>^<b>2+</b> exclusively photo-oxidizes the <i><b>p</b></i><b>-Xy</b> spacer to give the <i><b>p</b></i><b>-Xy</b>^<b>+•</b>-<b>ExV</b>^<b>+•</b> ion pair. Moreover, the vibrational spectra of the excimer state show that it assumes a geometry that is intermediate between that of the locally excited and CT states, approximately reflecting the degree of CT character