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Controlling Singlet Fission by Molecular Contortion Felisa S. Conrad-Burton, Taifeng Liu, Florian Geyer, Roberto Costantini, Andrew P. Schlaus, Michael S Spencer, Jue Wang, Raúl Hernández Sánchez, Boyuan Zhang, Qizhi Xu, Michael L. Steigerwald, Shengxiong Xiao, Hexing Li, Colin Nuckolls, and Xiaoyang Zhu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05357 • Publication Date (Web): 29 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019
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Journal of the American Chemical Society
Controlling Singlet Fission by Molecular Contortion Felisa S. Conrad-Burton[1]a, Taifeng Liu[1]*a,b, Florian Geyera, Roberto Costantinia,d,e, Andrew P. Schlausa, Michael S. Spencera, Jue Wanga, Raul Hernández Sáncheza, Boyuan Zhanga, Qizhi Xua,c, Michael L. Steigerwalda, Shengxiong Xiaob, Hexing Lib Colin P. Nuckolls*a,b Xiaoyang Zhu*a
[1] These authors contributed equally. *To whom correspondence should be addressed. Emails of corresponding authors:
[email protected] (TL),
[email protected] (CN),
[email protected] (XYZ).
a
Department of Chemistry, Columbia University, New York, NY 10027, USA
b c
Department of Chemistry, Wuhan University of Science and Technology, Wuhan, CHINA
d e
Department of Chemistry, Shanghai Normal University, Shanghai, CHINA CNR-IOM, AREA Science Park, Basovizza, 34149 Trieste, Italy
Physics Department, University of Trieste, Via Valerio 2, 34127 Trieste, Italy
ABSTRACT Singlet fission, the generation of two triplet excited states from the absorption of a single photon, may potentially increase solar energy conversion efficiency. A major roadblock in realizing this potential is the limited number of molecules available with high singlet fission yields and sufficient chemical stability. Here, we demonstrate a strategy for developing singlet fission materials in which we start with a stable molecular platform and use strain to tune the singlet and triplet energies. Using perylene diimide as a model system, we tune the singlet fission energetics from endoergic to exoergic or iso-energetic by straining the molecular backbone. The result is an increase in the singlet fission rate by two orders of magnitude. This demonstration opens a door to greatly expanding the molecular toolbox for singlet fission.
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Main Many studies on singlet fission1,2 utilize acenes and oligo-acenes due to their high singlet fission yields3–11, but acenes are not sufficiently stable for practical applications. Past attempts at expanding the molecular library for singlet fission has been limited by the very small number of known chromophores12–17 that satisfy the energetic requirement of a first excited singlet energy greater than or equal to twice the triplet energy, E(S1) ≥ 2xE(T1). This limitation has confined much of the chemical design for singlet fission to controlling the linkage chemistry between chromophores or controlling the molecular packing in solids. Here, we present a new strategy, to create practically applicable chromophores for singlet fission by contorting aromatic structures through intramolecular strain to tune their excited state energies. Singlet and triplet energies are determined not only by the molecular structure but also by the degree of contortion, such as bowing and twisting, present in that molecular structure.18 Here we explore this strategy for tuning the energetics of chromophores from unfavorable to favorable for singlet fission. To demonstrate the strategy of using molecular contortion to control singlet fission we employed perylene diimide (PDI, Fig. 1a) as our scaffold. PDI and its derivatives are useful in this context because: it is exceedingly stable to harsh environmental conditions; it is the basis for highly efficient organic solar cells; it is known to undergo singlet fission in the solid state with low rates due to unfavorable energetics leading to endoergic singlet fission16,19,20. Here we introduce contortion to create bowing of the PDI by adding two terphenyl groups (see Fig. 1a for molecular structure and Fig. 1b for crystalline packing). Importantly, this contortion results in a lowering of the singlet and the triplet energies, and a larger singlet-triplet gap due to an increase in exchange energy. DFT calculations (SI8) on this structure indicate the energies are similarly lowered by ~100-200meV (Fig. 1c). Thus, in the longitudinally bowed structure, PDI-B, the S1à2T1 process goes from endoergic in planar PDI to approximately isoergic. To test this strategy, we study crystalline films of PDI-B and find that singlet fission occurs in 2.5 ps, which is 2-3 orders of magnitude faster than corresponding processes in films formed from planar PDI systems16,19,20. RESULTS & DISCUSSION Fig. 1a displays the molecule we designed to test whether the longitudinal contortion induces efficient singlet fission. Details for its synthesis and characterization are contained in the
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Journal of the American Chemical Society
Fig. 1 Structure, excited state energies, and optical spectra of PDI-B and PDI. a) Chemical structure of PDI-B derivative versus PDI, and curvature of PDI-B; b) Crystal packing structure along a- and baxes of PDI-B, c) representation of DFT calculations of singlet and triplet energy levels of both PDI-B and PDI, d) Absorption (solid) and emission (dashed) spectra of PDI-B (red) and PDI (black).
Supporting Information. The bottom part of Fig. 1a displays the molecular structure from single crystal X-ray diffraction. Repulsion between the middle phenyl of the terphenyl bridges and the PDI bay position, as well as strong repulsion between the outer phenyl of the terphenyl and the carbonyl of the imide, together bend the PDI along its long axis. Fig. 1b illustrates the crystal packing structure of PDI-B. In this system there is only pi-pi interaction along the b-axis, and due to the curvature, these close contacts only occur between dimer pairs. The steady-state absorption and emission spectra have a notable red-shift (Fig. 1d), corresponding to a lowering of the singlet (S1) consistent with the DFT calculations. The first singlet and triplet excited state energies of PDI
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and PDI-B with the same sidechain (R = CH3) were calculated to be S1 = 2.5 eV, T1 = 1.4 eV, and S1 = 2.4 eV, T1 = 1.2 eV, respectively. The energetic changes for singlet fission is DESF = 2ET1 – ES1 ~ 0.3 eV in PDI and DESF ~ 0.0 eV in PDI-B. Within the uncertainty in DFT results, these results indicate that the energy barrier for singlet fission is removed in PDI-B. To study the excited state dynamics in this system, we used transient absorption (TA) spectroscopy with a 515 nm pump pulse and a white-light probe pulse; the time resolution of our measurement is determined by the pulse width of the probe (250 fs). The time-resolved spectra are shown in differential transmission DT/T, where DT (=Tp-T) is the change in transmission with (Tp) and without (T) pump. The TA spectra are shown in a 2D pseudo-color plot as functions of pumpprobe delay (Dt) and probe wavelength (Fig. 2a). The TA spectra consist generally of ground state bleaching (GB, positive DT/T) and excited state absorption (ESA, negative DT/T). The timeevolution in the TA spectra is particularly obvious in ESA in the spectral region of 610-660 nm on picosecond time scale. We carry out global analysis of the TA data using the Glotaran software (University of Amsterdam) in a sequential model21; this model determines the kinetic evolution from one species to another, each characterized by a unique spectrum. In Fig. 2b, we show the evolution-associated spectra (EAS) and the corresponding kinetic profiles of the populations. Here, after initial excitation, EAS1 grows within the pump pulse duration and is naturally attributed to the promptly formed S1 state, which decays with a time constant of t1 = 2.5±0.3 ps. The second excited state associated with EAS2 grows in with the same time constant of t1 = 2.5±0.2 ps and is, thus, formed kinetically at the expense of S1. It decays with a time constant of t2 = 160±10 ps. To understand the nature of the second excited state, we take kinetic line cuts at 600 nm and 622 nm, Fig. 2c and Fig. 2d on linear and logarithmic scales, respectively. 600 nm is at the peak of GB signal and 622 nm is the wavelength where ESA is zero from the S1 state and is exclusively attributed to the second excited state. The rise in the ESA at 622 nm is well-described by singleexponential with time constant close to t1, as expected from the transformation of the initially formed S1 state to this second excited state. This nature of this second excited state is revealed in the kinetic profile of the bleaching signal. The kinetic profile at 600 nm shows an initial rise within the experimental time resolution, followed by a slower rise with a time constant of 2.4±0.2 ps, which is the same as t1 from global analysis. This 2.4±0.2 ps process resulting in the doubling of the initial bleaching signal from photo-excitaiton (
