Controlling Singlet Fission by Molecular ContortionFelisa S. Conrad-Burton,†,# Taifeng Liu,*,†,‡,# Florian Geyer,† Roberto Costantini,†,∥,⊥

Andrew P. Schlaus,† Michael S. Spencer,† Jue Wang,† Raul Hernandez Sanchez,† Boyuan Zhang,†

Qizhi Xu,†,§ Michael L. Steigerwald,† Shengxiong Xiao,‡ Hexing Li,‡ Colin P. Nuckolls,*,†,‡

and Xiaoyang Zhu*,†

†Department of Chemistry, Columbia University, New York, New York 10027, United States‡Department of Chemistry, Shanghai Normal University, Shanghai, China§Department of Chemistry, Wuhan University of Science and Technology, Wuhan, China∥CNR-IOM, AREA Science Park, Basovizza, 34149 Trieste, Italy⊥Physics Department, University of Trieste, Via Valerio 2, 34127 Trieste, Italy

*S Supporting Information

ABSTRACT: Singlet fission, the generation of two triplet excited states from theabsorption of a single photon, may potentially increase solar energy conversionefficiency. A major roadblock in realizing this potential is the limited number ofmolecules 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 astable molecular platform and use strain to tune the singlet and triplet energies. Usingperylene diimide as a model system, we tune the singlet fission energetics from endoergicto exoergic or iso-energetic by straining the molecular backbone. The result is anincrease in the singlet fission rate by 2 orders of magnitude. This demonstration opens adoor to greatly expanding the molecular toolbox for singlet fission.

Many studies on singlet fission1,2 utilize acenes and oligo-acenes due to their high singlet fission yields,3−11 but acenesare not sufficiently stable for practical applications. Pastattempts at expanding the molecular library for singlet fissionhas been limited by the very small number of knownchromophores12−17 that satisfy the energetic requirement ofa first excited singlet energy greater than or equal to twice thetriplet energy, E(S1) ≥ 2xE(T1). This limitation has confinedmuch of the chemical design for singlet fission to controllingthe linkage chemistry between chromophores or controllingthe molecular packing in solids. Here, we present a newstrategy, to create practically applicable chromophores forsinglet fission by contorting aromatic structures throughintramolecular strain to tune their excited state energies.Singlet and triplet energies are determined not only by themolecular structure but also by the degree of contortion, suchas bowing and twisting, present in that molecular structure.18

Here we explore this strategy for tuning the energetics ofchromophores from unfavorable to favorable for singlet fission.To demonstrate the strategy of using molecular contortion

to control singlet fission we employed perylene diimide (PDI,Figure 1a) as our scaffold. PDI and its derivatives are useful inthis context: it is exceedingly stable to harsh environmentalconditions, it is the basis for highly efficient organic solar cells,and it is known to undergo singlet fission in the solid state withlow rates due to unfavorable energetics leading to endoergicsinglet fission.16,19,20 Here we introduce contortion to createbowing of the PDI by adding two terphenyl groups (see Figure

1a for molecular structure and Figure 1b for crystallinepacking). Importantly, this contortion results in a lowering ofthe singlet and the triplet energies, and a larger singlet−tripletgap due to an increase in exchange energy. DFT calculations(SI8) on this structure indicate the energies are similarlylowered by ∼100−200 meV (Figure 1c). Thus, in thelongitudinally bowed structure, PDI-B, the S1 → 2T1 processgoes from endoergic in planar PDI to approximately isoergic.To test this strategy, we study crystalline films of PDI-B andfind that singlet fission occurs in 2.5 ps, which is 2−3 orders ofmagnitude faster than corresponding processes in films formedfrom planar PDI systems.16,19,20

■ RESULTS AND DISCUSSIONFigure 1a displays the molecule we designed to test whetherthe longitudinal contortion induces efficient singlet fission.Details for its synthesis and characterization are contained inthe Supporting Information. The bottom part of Figure 1adisplays the molecular structure from single crystal X-raydiffraction. Repulsion between the middle phenyl of theterphenyl bridges and the PDI bay position, as well as strongrepulsion between the outer phenyl of the terphenyl and thecarbonyl of the imide, together bend the PDI along its longaxis. Figure 1b illustrates the crystal packing structure of PDI-B. In this system, there is only pi-pi interaction along the b-axis

Received: May 19, 2019Published: July 29, 2019

Article

pubs.acs.org/JACSCite This: J. Am. Chem. Soc. 2019, 141, 13143−13147

© 2019 American Chemical Society 13143 DOI: 10.1021/jacs.9b05357J. Am. Chem. Soc. 2019, 141, 13143−13147

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and, due to the curvature, these close contacts only occurbetween dimer pairs. The steady-state absorption and emissionspectra have a notable red-shift (Figure 1d) corresponding to alowering of the singlet (S1) consistent with the DFTcalculations. The first singlet and triplet excited state energiesof PDI and PDI-B with the same side chain (R = CH3) werecalculated 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 isΔESF = 2ET1 − ES1 ≈ 0.3 eV in PDI and ΔESF ≈ 0.0 eV in PDI-B. Within the uncertainty in DFT results, these results indicatethat 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 pumppulse and a white-light probe pulse; the time resolution of ourmeasurement is determined by the pulse width of the probe(250 fs). The time-resolved spectra are shown in differentialtransmission ΔT/T, where ΔT (=Tp − T) is the change intransmission with (Tp) and without (T) pump. The TA spectraare shown in a 2D pseudocolor plot as functions of pump−probe delay (Δt) and probe wavelength (Figure 2a). The TAspectra consist generally of ground state bleaching (GB,positive ΔT/T) and excited state absorption (ESA, negativeΔT/T). The time-evolution in the TA spectra is particularlyobvious in ESA in the spectral region of 610−660 nm onpicosecond time scale. We carry out global analysis of the TAdata using the Glotaran software (University of Amsterdam) ina sequential model;21 this model determines the kineticevolution from one species to another, each characterized by aunique spectrum. In Figure 2b, we show the evolution-associated spectra (EAS) and the corresponding kineticprofiles of the populations. Here, after initial excitation, EAS1grows within the pump pulse duration and is naturally

attributed to the promptly formed S1 state which decays witha time constant of τ1 = 2.5 ± 0.3 ps. The second excited stateassociated with EAS2 grows in with the same time constant ofτ1 = 2.5 ± 0.2 ps and is, thus, formed kinetically at the expenseof S1. It decays with a time constant of τ2 = 160 ± 10 ps.To understand the nature of the second excited state, we

take kinetic line cuts at 600 and 622 nm, Figure 2c,d on linearand logarithmic scales, respectively. 600 nm is at the peak ofGB signal and 622 nm is the wavelength where ESA is zerofrom the S1 state and is exclusively attributed to the secondexcited state. The rise in the ESA at 622 nm is well describedby single-exponential with time constant close to τ1, asexpected from the transformation of the initially formed S1state to this second excited state. The nature of this secondexcited state is revealed in the kinetic profile of the bleachingsignal. The kinetic profile at 600 nm shows an initial rise withinthe experimental time resolution followed by a slower rise witha time constant of 2.4 ± 0.2 ps, which is the same as τ1 fromglobal analysis. This 2.4 ± 0.2 ps process results in thedoubling of the initial bleaching signal from photoexcitaiton(<0.2 ps). This provides strong evidence for singlet fission,where the transformation of the intramolecular S1 state to theintermolecular triplet pair state doubles the number ofmolecules bleached. This doubling in bleaching on the singletfission time scale would suggest nearly quantitative triplet pairformation, but actual quantitative analysis is difficult due tooverlaps in ground state bleaching and excited state absorption.Based on this finding and previous studies of singlet fission inPDI thin films,16,19,20 we assign EAS2 as the spectroscopicsignature for the triplet pair formed from singlet fission. Themain difference between these two spectral components is theblue-shift in the ESA from that of S1 to that assigned to 2T1.

Figure 1. Structure, excited state energies, and optical spectra of PDI-B and PDI. a) Chemical structure of PDI-B derivative versus PDI, andcurvature of PDI-B. (b) Crystal packing structure along a and b axes of PDI-B. (c) representation of DFT calculations of singlet and triplet energylevels of both PDI-B and PDI. (d) Absorption (solid) and emission (dashed) spectra of PDI-B (red) and PDI (black).

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Similar small changes in the ESA spectra between singlet andtriplet have been observed previously for singlet fission in PDIthin films.16,19,20

Note that, unlike the main 0−0 vibronic absorption peak at600 nm, the doubling in ground state bleaching on the singletfission time scale of 2.4 ps is not observed for the weaker 0−1vibronic transition at 577 nm. This may result from the time-dependent increase in the 0−1/0−0 vibronic peak ratios inground state absorption. Spano has shown that an increase inthe 0−1/0−0 peak intensity ratio for an S0 − S1 transition isrelated to increased disordering in J-aggregates.22 Here, the

transient disordering may be induced by singlet fission. Thisissue deserves further investigation.We note two major differences between singlet fission in

PDI-B and PDI thin films. The first is the time constant forsinglet fission, τ1 ≈ 2.5 ps, in PDI-B films is two- to three-orders of magnitude faster than the τ1 = 180−3800 ps lifetimefound for PDI films.16,19,20 This is consistent with the removal(or substantial reduction) of the energetic barrier for singletfission in PDI-B due to strain-based energetic tuning of thesinglet and triplet states. The second major difference is thatthe decay lifetime of triplets, τ2 ≈ 160 ps in the PDI-B film, isalso 2 orders of magnitude shorter than those in PDIfilms.16,19,20 The faster decay of triplets (or the triplet pair)in the PDI-B film could result from unique molecular packingin the PDI-B crystal structure (Figure 1b): the lack of p-stacking for electronic delocalization. The packing allows forpi-pi overlap between two neighboring molecules while thenext nearest neighbor has almost no electronic contributiondue to its distance. As a result, the phase space for the tripletpair to diffuse apart via triplet energy transfer, i.e., an entropicdriving force,2,23,24 is much reduced in the PDI-B film ascompared to those in PDI films. This explains the fastertriplet−triplet recombination in the former. Consistent withthe fast triplet−triplet recombination, we are not able todirectly detect phosphorescence signal from the triplets. Theshort triplet pair lifetime also makes it difficult to harvest thetriplets in solar energy applications and further moleculardesign, e.g., using molecular dimers with flexible linkers, maybe necessary from an application point of view.Twisted conjugated aromatic molecular systems, including

PDI derivatives, have been studied before,25−28 and a commonobservation is the increased rate of intersystem crossing withincreased degree of curvature. To eliminate this possibility asan explanation for the ultrafast formation (2.5 ps) of thetriplets, we compare the PDI-B in the crystalline film with thatin the solution and a dilute film of PDI-B dispersed within apolymer matrix. Intermolecular singlet fission requires theclose contact between at least two molecules while intersystemcrossing is a unimolecular event. We carry out photo-luminescence (PL) spectroscopy and time-resolved (TR) PLmeasurements on PDI-B. Figure 3a shows PL spectra of thethree systems. The PL spectrum from PDI-B in the solution ischaracterized by a main peak at 619 nm and a weaker peak at∼650 nm which are assigned to 0−0 and 0−1 vibronictransitions, similar to the PL spectrum from solution phase

Figure 2. Transition absorption spectroscopy of PDI-B in the solidstate reveals singlet fission. Transient absorption spectroscopy andglobal analysis of PDI-B film taken at 77K in vacuum. (a) 2D color(ΔT/T) plot of TA spectra as functions of probe wavelength andpump−probe delay (Δt). (b) Evolution-associated spectra (EAS)determined from global analysis using a sequential model. EAS1 isinitially excited and decays with a time constant of 2.5 ps. EAS2 growsin with a time constant of 2.5 ps and decays with a time constant of160 ps, inset: representation of kinetics for EAS1 and EAS2. (c and d)Kinetic line cuts at 600 and 622 nm on short (linear) and long(logarithmic) time scales, respectively. In panel c, the dots areexperimental data and solid curves are fits; the red curve is singleexponential fit with time constant of 2 ps. The blue curve is abiexponential fit with time constants of 0.15 and 2.4 ps, respectively;the former represents the rise in GB signal due to initial excitation bythe pump pulse, and the latter is assigned to singlet fission.

Figure 3. Time-resolved photoluminescence supports singlet fission in PDI-B film. (a) PL profiles of PDI-Bowl in chloroform, in a polymer(PMMA) matrix, and an annealed thin film. (b) TCSPC profiles of the same systems from panel a. The solution and polymer matrix systems havemonoexponential decays with τ = 4.7 ns and 5.5 ns, respectively; the film has a biexponential decay with τ1 = 0.17 ns and τ2 = 1.7 ns.

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PDI (see Figure 1d). In the polymer matrix, the spectrumshows only a small blue-shift which can be attributed tochanges in the local dielectric environment and is otherwisenearly identical to the solution spectrum. In the crystalline film,the PL spectrum exhibits a broadening and an increase in therelative intensity of the 0−1 vibronic peak with respect to the0−0 peak. This is indicative of increased electronic couplingbetween PDI-B molecules, a property that is important forsinglet fission to occur. This should not be confused withexcimer formation in the crystal that has been seen in otherrylene systems.29,30 Because of the unique molecular packing ofPDI-B, offering lower levels of electronic interaction, excimerformation is not likely. This is also confirmed in the shape ofthe PL spectrum; excimers exhibit red-shifted and extremelybroadened PL spectra. The slight degree of broadening in thePDI-B crystalline film, coupled with the lack of red-shift,indicates that excimer formation is not a consideration in thissystem.The TR-PL, Figure 3b, normalized in each case to optical

density, pump power, and collection time also supports singletfission in the crystalline PDI-B film. The TR-PL traces of PDI-B both in solution and in the polymer matrix are of similarintensity and are characterized by single-exponential decayswith similar time constants of τPL ≈ 4.7 and 5.5 ns,respectively. In contrast, PL from the crystalline PDI-B thinfilm is two-orders of magnitude lower in intensity and decaysover one-order of magnitude faster, with τPL ≈ 0.17 ns, whichis consistent with singlet fission. Note that the τ1 ≈ 2.5 pssinglet fission time cannot be resolved in the TR-PLmeasurement due to the limited time resolution (∼20 ps,SI7). Instead, τPL ≈ 0.17 ns time constant is consistent withthe triplet annihilation time determined in TA measurement,160 ps. Note that PL decay in the crystalline thin film cannotbe described by a single exponential, and there is a weak butlonger-lived component on the 1.0s ns time scale. This slowcomponent may come from the annihilation of a smallpopulation of triplets that have spatially separated in parts ofthe film with favorable intermolecular coupling for tripletenergy transfer.The contrasting PL decay dynamics of PDI-B in crystalline

thin film from that in the solution phase or in the polymermatrix establishes that the fast decay in the former is not aresult of intersystem crossing, a unimolecular process, butis due to singlet fission. The two-orders of magnitude increasein triplet generation rate in crystalline thin films of PDI-B fromthose in thin films of other PDIs can be attributed to the morefavorable energetics (Δ) for singlet fission in the former.Specifically, the closest planar analog to our PDI-B is the PDIwith four phenyl groups attached to the perylene core, i.e.,N,N-bis(n-octyl)-2,5,8,11-tetraphenylperylene-3,4:9,10-bis-(dicarboximide), reported by Wasielewski and co-workers.16

The singlet fission time in the solid film of this molecule is τSF= 180 ps, which is nearly 2 orders of magnitude slower thanthe τSF = 2.4 ps observed here. Our results suggest that thetuning of singlet and triplet energies by molecular strain is ageneral strategy to control singlet fission energetics providedthat intersystem crossing is not too fast to compete. Thisprincipal can be applied to a large library of stablechromophores that are not typically considered for singletfission due to their unfavorable energetics for this process, thusgreatly expanding the molecular toolbox for singlet fission.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.9b05357.

Synthetic details for PDI-B and structural character-ization; details regarding sample preparation andspecifics of optical characterization (steady-state andtransient absorption); details on molecular energydeterminations using electrochemistry, DFT calcula-tions, TD-DFT calculations, and molecular orbitals(PDF)

■ AUTHOR INFORMATIONCorresponding Authors*liutf@shnu.edu.cn.*cn37@columbia.edu.*xyzhu@columbia.edu.ORCIDRoberto Costantini: 0000-0003-1017-949XJue Wang: 0000-0001-6843-9771Raul Hernandez Sanchez: 0000-0001-6013-2708Hexing Li: 0000-0002-3558-5227Colin P. Nuckolls: 0000-0002-0384-5493Xiaoyang Zhu: 0000-0002-2090-8484Author Contributions#F.S.C.-B. and T.F.L. contributed equally.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the U.S. Department of Energy,Grant DE-SC0014563. F.G. was supported by a Feodor LynenFellowship of the Alexander von Humboldt Society. R.C. wassupported by the SIR Grant SUNDYN [Nr RBSI14G7TL,CUP B82I1500090001] of the Italian Ministry of EducationUniversities and Research (MIUR). R.H.S. acknowledges thesupport from the Columbia Nano Initiative PostdoctoralFellowship. H. Li, S. Xiao and T. Liu acknowledge financialsupport from National Natural Science Foundation of China(21761142011, 21772123, 51502173) and Shanghai Govern-ment (18JC1412900, 18DZ2254200) for the synthetic efforts.

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