Singlet fission is a process in which a singlet exciton converts into two triplet excitons. To investigate this phenomenon, we synthesized two covalently linked 5-ethynyl-tetracene (ET) dimers with differing degrees of intertetracene overlap: BET-X, with large, cofacial overlap of tetracene π-orbitals, and BET-B, with twisted arrangement between tetracenes exhibits less overlap between the tetracene π-orbitals. The two compounds were crystallographically characterized and studied by absorption and emission spectroscopy in solution, in PMMA and neat thin films. The results show that singlet fission occurs within 1 ps in an amorphous thin film of BET-B with high efficiency (triplet yield: 154%). In solution and the PMMA matrix the S1 of BET-B relaxes to a correlated triplet pair (1)(T1T1) on a time scale of 2 ps, which decays to the ground state without forming separated triplets, suggesting that triplet energy transfer from (1)(T1T1) to a nearby chromophore is essential for producing free triplets. In support of this hypothesis, selective excitation of BET-B doped into a thin film of diphenyltetracene (DPT) leads to formation of the (1)(T1T1) state of BET-B, followed by generation of both DPT and BET-B triplets. For the structurally cofacial BET-X, an intermediate forms in <180 fs and returns to the ground state more rapidly than BET-B. First-principles calculations predict a 2 orders of magnitude faster rate of singlet fission to the (1)(T1T1) state in BET-B relative to that of crystalline tetracene, attributing the rate increase to greater coupling between the S1 and (1)(T1T1) states and favorable energetics for formation of the separated triplets.

Separation of triplet excitons produced by singlet fission is crucial for efficient application of singlet fission materials. While earlier works explored the first step of singlet fission, the formation of the correlated triplet pair state, the focus of recent studies has been on understanding the second step of singlet fission, the formation of independent triplets from the correlated pair state. We present the synthesis and excited-state dynamics of meta- and para-bis(ethynyltetracenyl)benzene dimers that are analogues to the ortho-bis(ethynyltetracenyl)benzene dimer reported by our groups previously. A comparison of the excited-state properties of these dimers allows us to investigate the effects of electronic conjugation and coupling on singlet fission between the ethynyltetracene units within a dimer. In the para isomer, in which the two chromophores are conjugated, the singlet exciton yields the correlated triplet pair state, from which the triplet excitons can decouple via molecular rotations. In contrast, the meta isomer in which the two chromophores are cross-coupled predominantly relaxes via radiative decay. We also report the synthesis and excited-state dynamics of two para dimers with different bridging units joining the ethynyltetracenes. The rate of singlet fission is found to be faster in the dimer with the bridging unit that has orbitals closer in energy to that of the ethynyltetracene chromophores.

A combined experimental and theoretical study shows a significant barrier (ca. 100 kJ/mol) to rotation through the interchromophoric carbon−carbon single covalent (1.49 Å) bond between the naphthalenimide and perylenimide units that prevents coplanarization of the two units in the dyad NP, thereby forcing them to act as independent chromophores/redox centers. Upon photoexcitation, highly efficient energy transfer is observed from the naphthalenimide (energy donor) to the perylenimide (energy acceptor) moiety predominantly through Coulombic coupling, completely isolating the orbital overlap (Dexter-type) interaction between the chromophoric units at such short separation by virtue of their orthogonal arrangement. Because Forster's ideal-dipole approximation ignores the contribution from significant higherorder Coulombic interactions at such short distances between donor and acceptor moieties, the complete coupling was computed from the transition densities, giving an estimate of the energy-transfer rate from the naphthalenimide donor to the perylenimide acceptor of k ET = 2.2 × 10 10 s −1 , in agreement with observations. Ultrafast excitation energy (ca. 40 ps, 90%) and electron (<0.5 ps, 10%) transfer from the singlet excited state of naphthalenimide to the perylenimide moiety competes with further delayed processes in the conjugate NP. Upon excitation at 345 nm, conjugate NP exhibits near-quantitative energy transfer in conjunction with solvent-polarity-dependent (solvatochromic) perylenimide fluorescence, resulting in a remarkable Stoke's shift of ca. 175−240 nm. Favorable photophysical properties such as high fluorescence quantum yield, wide excitation range, ultrafast energy transfer, marginal electron transfer, and large Stoke's shift make this conjugate a potential candidate for biological applications.

Current state-of-the-art hybrid polymer:lead chalcogenide nanocrystal solar cells require postdeposition, thin film chemical treatments to remove insulating organic ligands from the nanocrystal surface, which is a kinetically hindered process. This is compounded by the fact that it can be especially difficult to obtain colloidally stable suspensions of PbS nanocrystals ligand exchanged with small ligands, and many atomic ligands require dispersion in solvents that are incompatible with polymer solubility. Herein, we report a novel one-step colloidal ligand exchange process for PbS nanocrystals employing lead iodide (PbI2) or ammonium iodide (NH4I) as surface ligands along with n-butylamine that allow the ligand-exchanged nanocrystals to be suspended in solvents compatible with polymer dissolution. While ligand exchange is shown to be near quantitative for both iodide sources, when compared to NH4I-exchanged PbS nanocrystals, the PbI2-exchanged PbS nanocrystals not only exhibit better colloidal stability but also display superior photovoltaic performance. When the PbI2-passivated PbS nanocrystals are combined with the donor polymer poly­[2,6-(4,4′-bis(2-ethylhexyl)­dithieno­[3,2-b:2′3′-d]­silole)-alt-4,7-(2,1,3-benzo­thiadiazole)] (Si-PCPDTBT), the optimized hybrid solar cells give a broad spectral response into the NIR, leading to a power conversion efficiency (PCE) of 4.8% under AM 1.5G illumination. Time-resolved photoluminescence and transient absorption spectroscopies suggest that excitonic processes between the PbS nanocrystals and Si-PCPDTBT are more favorable when PbS nanocrystals are ligand exchanged with PbI2, leading to superior device performance.

Circular dichroism spectra for a series of structurally analogous hairpin oligonucleotides, tethered at the 5′-end with an axially chiral naphthalenimide-perylenimide dyad (NP), is dependent on the nature (AT vs GC) and the orientation (5′-C vs 5′-G) of the adjacent base pair that stacks with the dyad. Charge transfer (CT) interaction between the naphthalenimide unit of the dyad NP and the adjacent guanine–cytosine (5′-C) base pair has been characterized by UV–vis absorption and fluorescence measurements. Molecular dynamics simulations of the dyad end-capped hairpin DNA ODN6i and TD-DFT calculations of the naphthalenimide and perylenimide units confirm that the CT transition dipole orients perpendicular to the perylenimide transition dipole. The orthogonality of the cross product of the CT and the perylenimide transition dipoles with the displacement vector connecting the two dipoles in space results in the zeroing out of the rotational strength of the perylenimide transition dipole, subsequently leading to the DNA-induced nonexciton coupled circular dichroism corresponding to perylenimide, in concurrence with experimental CD spectrum. Singular value decomposition of thermal denaturation of the hairpin DNA having CT interaction (ODN6i) revealed the denaturation proceeded through an additional intermediary stage compared to the hairpin DNA without the CT interaction (ODN6). Dissimilar CD (induced CD for ODN6i vs exciton-coupled CD for ODN6) spectra corresponding to perylenimide unit obtained for similar NP end-capped hairpin DNA sequences cautions against the indiscriminate use of the exciton chirality method, particularly in systems like DNA and proteins containing polarizable chromophores that can interact with reporter transition dipoles.

We report the conformational restriction of a freely rotating biaryl derivative resulting in strong exciton coupled circular dichroism between the two nondegenerate fragments of the dyad NP only in the presence of DNA. The napthalenimide−perylenimide dyad NP, an extended 1,1′-binaphthalene derivative, has a rotational barrier of 100 kJ/mol, leading to fast racemization. While end-stacking with a DNA hairpin, NP exhibits a significantly high rotational barrier, an additional 91 kJ/mol, resulting in atroposelective preference. Such reversible conformational constraints using biotemplates show huge potential toward conformational analysis, dynamic kinetic resolution, and asymmetric synthesis. The present work focuses on conformational analysis of a freely rotating biaryl derivative using a combination of techniques such as temperature-dependent UV−vis, circular dichroism, steady-state and time-resolved fluorescence spectroscopy, and molecular dynamics simulation.

A series of CdSe quantum dot acceptors possessing six different ligand frameworks (i.e., pivalic acid, pyridine, butylamine, tert-butylthiol, thiophenol, and tetrahydrothiophene) were used as platforms for investigating the influence of quantum dot surface chemistry on the performance of hybrid poly(3-hexythiophene-2,5-diyl) (P3HT):CdSe quantum dot bulk heterojunction (BHJ) solar cells. We confirm that the device parameters used to evaluate solar cell performance are significantly influenced by the nature of the quantum dot surface ligand. The dependence of short circuit current density (JSC) on the CdSe ligand type was probed using ultrafast time-resolved photoluminescence (PL) measurements, and good correlations between the ligand-dependent trends in JSC and excited state lifetime were found, in which the P3HT:CdSe quantum dot BHJs with the shortest PL lifetimes possess the largest device current densities. The frontier energy levels of the quantum dot acceptors are significantly influenced by surface ligands, wherein the device open circuit potentials (VOC) were found to linearly correlate with the energy difference (ΔEDA) between the HOMO of the P3HT donor and the electrochemically determined LUMO of the CdSe quantum dot acceptors over a range of 220 mV. This work demonstrates the versatility of quantum dot ligand engineering for tuning the device parameters and performance of hybrid solar cells.

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