Intra- to Intermolecular Singlet FissionM. Tuan Trinh,† Yu Zhong,† Qishui Chen,† Theanne Schiros,† Steffen Jockusch,† Matthew Y. Sfeir,‡

Michael Steigerwald,† Colin Nuckolls,*,† and Xiaoyang Zhu*,†

†Department of Chemistry, Columbia University, New York, New York 10027, United States‡Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States

*S Supporting Information

ABSTRACT: Singlet fission, the splitting of one singlet into two triplets, can potentiallyincrease the efficiency of optoelectronic devices beyond conventional limits. Among thesinglet fission molecules discovered to date, two mechanisms have emerged: intra- orintermolecular singlet fission. Here we show a combined intra- to intermolecular singletfission mechanism in the model system of diphenyl-dicyano-oligoene (DPDC). Excitationof DPDC to the first optically bright state leads to the ultrafast formation of anintramolecular triplet pair, which decays in 40 ps in the solution phase but can also splitcompetitively in 30 ps into two long-lived triplets (2×T1) on adjacent molecules in solidfilms. These findings suggest a design principle for efficient singlet fission: the independenttuning of singlet−triplet pair coupling and triplet pair splitting from intra- and intermolecular interactions, respectively.

■ INTRODUCTION

An optically excited singlet state is converted into two tripletstates in the energy and spin-allowed singlet fission process.1,2

Singlet fission was discovered 50 years ago,3,4 and recent surgesin research activities5−11 are driven by exciting prospects inoptoelectronic applications, such as photodetectors and solarcells.12−15 In photovoltaics, the power conversion efficiency of aconventional single-junction solar cell is constrained theoret-ically by the Shockley−Queisser limit of ∼31% due in a largepart to the loss of excess photon energy above thesemiconductor bandgap.16 When a singlet fission materialabsorbing at high photon energies is used in conjunction with alower bandgap semiconductor, the theoretical power con-version efficiency can be increased to 44%.17,18 Singlet fissionmaterials can also be used to increase the quantum efficiency ofphotodetectors from the conventional limit of 100% to 200%.12

A major obstacle to the realization of these potentials is theabsence of a large toolbox of singlet fission materials fromwhich engineers can make judicious choices in devicefabrication.In most molecular systems studied to date,1−11 singlet fission

has been attributed to an intermolecular mechanism in whichthe optically excited singlet state (S) on one molecule coupleswith neighboring molecules to form an intermolecular tripletpair, i.e., a biexciton (BE), also called a multiexciton (ME). TheBE in the solid state or an excimer in the solution19 can diffuseapart or form two individual triplets (2×T1). Such anintermolecular mechanism is believed to be responsible forefficient singlet fission in crystalline solids or aggregates oftetracene,7,20−22 pentacene,8,11,23 1,3-diphenylisobenzofuran,9

rubrene,24,25 carotenoids,10 and perylenediimide.26 Theoreticalstudies point to the essential role of intermolecular chargetransfer (CT) states in mediating the electronic couplingbetween S and BE.27−33

Intramolecular singlet fission, in which the BE and 2×T1

states are located on the same molecule, has been suggested forisolated polymers, including poly- or oligoenes.34−39 Musser etal. found that the triplet lifetime was longer in a solid film thanthat in the solution phase for polythiophene.34 These authorssuggested that the triplet pair separation via diffusion in thesolid film might explain the increased triplet lifetime. From thepoint of view of creating robust design principles, intra-molecular singlet fission is particularly attractive. The criticallyimportant electronic coupling between S and BE (with orwithout CT intermediates) can be more easily designed andcontrolled within a single molecule than between molecules. Apotential disadvantage with intramolecular fission is that thetriplet pair confined to the same molecule can easily annihilate(back to the singlet and then ground state), resulting in a shortlifetime of the triplet pair and limiting the charge extractiontime window. One may envision an “ideal” mechanism wherestrong S/CT/BE coupling leads to the ultrafast formation ofthe intramolecular BE state, while intermolecular electronicinteraction efficiently separates the triplet pair betweenneighboring molecules. Here we report the discovery of suchan intra- to intermolecular singlet fission mechanism in themodel system of modified oligoenes, α,ω-diphenyl-μ,ν-dicyano-oligoenes (DPDCs).We chose DPDC molecules (Figure 1) for several reasons. A

unique feature of oligoenes and similar carotenoid molecules isthe presence of triplet pair states (TT), also called excitedcovalent states, as suggested in theoretical studies.40−42 Thelowest singlet state in oligoenes is the “dark” S1 state, which isformed on sub-ps time scales from the relaxation of the

Received: December 18, 2014Revised: December 20, 2014Published: December 22, 2014

Article

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optically bright S2 state. While the S1 state can be considered asa doubly excited TT state, the binding energy between thesetwo closely interacting triplets is too high (>1 eV)40−42 topermit their separation into two individual triplets; as a result,S1 is only known to undergo nonradiative relaxation to S0.

37−39

However, there is another well-known dark state S*,43 which isenergetically between S1 and S2 and has been proposed as anintermediate for intramolecular singlet fission.37,38 While theexact nature of S* is still under debate,43 an interestinginterpretation is that S* is a geometrically twisted conformationof the TT state.44,45 The presence of a twist is expected tosignificantly weaken the coupling between the two triplets,permitting their separation. Our findings shown below suggestthat the triplet pair (BE) may be interpreted as S* which cansplit into two triplets on two adjacent molecules.One limitation of naturally occurring carotenoids is the

presence of side-chain methyl groups that may distort theplanar conjugated oligoene structure, reduce the extent ofconjugation, and prevent intermolecular close-packing. TheDPDC molecules are designed to overcome these limitations.46

The addition of the two cyano (−CN) groups planarizes theoligoene backbone, making intermolecular close packingfeasible and the extent of π-electron delocalization (thus, theoptical gap) easily tunable with molecular length. The −CNgroups are strong π-electron withdrawing46 and may introduceCT characters to the S2 state, potentially facilitate intra-molecular BE formation. Singlet fission has been reportedrecently for solid films of a related molecule, diphenylhexa-triene.47

■ EXPERIMENTAL SECTIONSample Preparation. The synthesis and characterization of

the DPDCn molecules have been reported elsewhere.46 Here nis the number of conjugated olefin in the molecule. All of thecompounds have exclusively trans-stereochemistry in theirdouble bonds. In this study, we chose five different lengths ofDPDCn molecules for investigation with n is an odd numberfrom 3 to 11 (Figure 1). The solid films were prepared by drop-casting of the saturated solution of DPDCn in chloroform ontosapphire coverslips. The optical density of the thin film samplesin the visible range was typically 0.1.Transient Absorption. The pump laser light (∼100 fs

pulse width, ∼200 nJ pulse energy) comes from an opticalparametric amplifier pumped by a Ti:sapphire femtosecondregenerative amplifier (800 nm, 1 kHz rep-rate). The probelight is a white-light supercontinuum (450−900 nm, ∼100 fs

pulse width). The pump and probe beams overlapped under asmall angle. The detection consists of a pair of high-resolutionmultichannel detector arrays coupled to a high-speed dataacquisition system. The pump laser pulse energy density on thesample was 20 μJ/cm2, and the probe pulse energy density wasapproximately 1 order of magnitude lower.

Triplet Sensitization Experiment.We obtained the tripletstate absorption spectrum of DPDC7 in the solution phase (inacetonitrile) using a nanosecond transient absorption setup(described elsewhere48) and sensitization. The excitation laserpulse (355 nm, 5 ns pulse width, 10 Hz) was from a Nd:YAGlaser (Spectra Physics). Thioxanthone (TX) was used as thesolution phase sensitizer. After photoexcitation, singlet excitonsare created in TX and then convert into triplet excitons viaintersystem crossing (ISC).49

Phosphorescence Measurement. We recorded phos-phorescence spectra from solid DPDCn using a modifiedspectrofluorometer (HORIBA Jobin Yvon, Fluorolog 3) inconjunction with a liquid nitrogen cooled Ge-diode detector(EO-817L, North Coast Scientific Corporation). The samplewas placed in a 4 mm Pyrex tube and in a liquid nitrogen dewarwith quartz windows and cooled down to 77 K.

Grazing Incidence X-ray Diffraction (GIXD). We carriedout GIXD measurements at the National Synchrotron LightSource (NSLS) of Brookhaven National Laboratory onbeamline X-9 using 13.5 eV photon energy. The incident X-ray beam, kin, had a grazing incidence angle with the samplesurface. A Photonic Science wide-angle X-ray scattering(WAXS) detector (pixel size of 0.11 mm) at the NSLS,positioned a distance L from the sample, recorded the scatteredbeam, kout. This was converted into an image of the reciprocalspace (Q-space) with the scattering expressed as a function ofthe scattering vector Q = kout − kin. Here, the sample-to-detector distance L, calibrated with AgBH and LaB6 polycrystal-line standards, was 244.9 mm. The samples were measuredunder a vacuum to minimize damage to the films from theintense X-ray beam and eliminate X-ray scattering from air.

Time-Dependent Density Functional Theory (TD-DFT)Calculation. In the TD-DFT calculation, we estimated thetotal energies of the S0, S2, and T1 states for each of the DPDColigoenes using density functional theory (B3LYP functional, 6-31G** basis sets, and Jaguar50). For each molecule, weoptimized the geometry of the S0 state. Keeping the geometryfixed, we then calculated the vertical T1 energy with RODFTand the vertical S2 energy using TD-DFT. The singlet andtriplet energies for DPDCn with different numbers of π-bonds(n) are presented in Table S1 (Supporting Information) andFigure 2C. In each molecule, the vertical excited statescorrespond to the promotion of one electron from theHOMO of the ground state to the LUMO of the groundstate, which leaves two singly occupied orbitals. In the T1 state,these two electrons are high-spin coupled, and in the S2 state,they are low-spin coupled.

■ RESULTS AND DISCUSSIONFigure 2A shows the solution phase absorption spectra ofDPDCn (n = 3−11, the number of π-bonds in the oligoenebackbone, Figure 1). The peak absorption wavelength increasessystematically with n. Each absorption spectrum shows vibronicstructure. We take the lowest energy peak (0−0 transition) ineach as the optical gap and plot this as a function of n in Figure2C (open circles). The experimental transition energies are ingood agreement with calculations on DPDCn from TD-DFT

Figure 1. Structures of DPDCn (n = 3−11) molecules.

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(blue dots and line) detailed in the Supporting Information forvertical excitation to the first dipole-allowed singlet (S2). Alsoshown in Figure 2C are the calculated triplet (T1) energies (reddots and line), along with experimental values (open squares)obtained from phosphorescence of DPDCn (n = 5, 7, 9) thinfilms. In comparison to previous reports, the S2 transitionenergies of DPDCn are in good agreement with those ofdiphenylpolyenes.51−53 Interestingly, the T1 energies of DPDCnare lower than diphenylpolyenes or other polyenes. Forexample, our measured T1 energies of DPDC5, DPDC7, andDPDC9 are 1.34, 1.27, and 1.24 eV, respectively, while that ofall trans retinal is 1.7 eV.52 We attribute the difference to thestabilization of T1 by the −CN electron withdrawing groups.With our calculated S2 and T1 energies, we obtain theendoenergicity for singlet fission, ΔESF = 2ET1 − ES2, shownFigure 2D. The singlet fission energy barrier (0.15 eV) is thesmallest for DPDC7.Figure 2B shows absorption spectra of solid thin films of the

DPDCn molecules. We observe a significant broadening of theabsorption peak in the solid film as compared to thecorresponding spectrum in the solution. While the trend ofdecreasing optical gap with increasing n and the presence ofvibronic progression are still observed in the solid film for n =3−7, the spectra of n = 9 and 11 are broader and show no clearvibronic features. The latter results from a lack of crystallinity inthese solid films, vide inf ra. For DPDC7 thin film, the T1 energyis 1.27 eV from phosphorescence (see below) and the S2 state(lowest energy peak in the green spectrum in Figure 2B) is∼2.34 eV; this gives an energy barrier for singlet fission in filmof ΔESF ≈ 0.2 eV.We use transient absorption (TA) spectroscopy to probe

singlet fission. Figures 3A shows pseudocolor (−ΔT/T) plotsof TA spectra for DPDC7 molecules in toluene solution asfunctions of probe wavelength and pump−probe delay time. Inthe experiment, the pump pulse at 450 nm excites the DPDC7molecules to the first optically bright state (S2); after acontrolled time delay (Δt), a white-light probe pulse follows

the dynamics from the time-dependent ground state bleaching(GB) and excited state absorption (ESA).The TA spectra (Figure 3A) consist of GB at ≤500 nm and

ESA in three spectral regions that are very similar to those fromthe extensively studied carotenoids and related molecules.43 Incarotenoids, the ESA from S2 lies in the near-IR range of∼800−1100 nm,43,54,55 while ESA peaks of S* and S1 are in thevisible range of ∼500−700 nm with S* on the blue side ofS1.

37,43 In Figure 3A, we assign the short-lived ESA peak at≥850 nm to the initially populated S2 state; it rises with thelaser pulse and decays in ≤0.5 ps. We assign the intense peak at∼580 nm and a weak overlapping peak at ∼680 nm to the BEor dark S* and S1 states, respectively. Figure 3B compares threeTA spectra (at pump−probe delays: Δt = 0.0, 0.4, and 1.8 ps)with a spectrum of T1 obtained from sensitization experiment(circles, see Figure S1 in the Supporting Information). The BEpeak is similar to and slightly red-shifted (by ∼30 nm) from theT1 peak; in contrast, S1 is ≥100 nm red-shifted from T1. Thissupports the notion that S* is a triplet pair state withcharacteristics of two individual triplets,37,38,43 e.g., due to theproposed twisting which weakens electronic coupling of thetriplet pair.44,45 The strongest support for the triplet pair,1(TT), interpretation comes from results below for solidDPDC7 film where BE is shown to further split into two tripletson adjacent molecules. Figure 3C shows kinetic profiles atindicated probe wavelengths (vertical cuts from Figure 3A). Asexpected, the formation of S2 is concurrent with the bleach ofS0 within experimental resolution, as determined by the laserpulse and the thickness of the sample cuvette. In contrast, theformation of either BE or S1 is delayed by ∼0.5 ps, suggestingthat both come from the relaxation of the optically bright S2state. The S2 decay dynamics is complex, with an initialnonexponential decay in <0.5 ps, with ∼1/4 of the residualsignal on longer time scales. This complex dynamics can be

Figure 2. Absorption spectra of DPDCn with different numbers of π-bonds (n = 3, 5, 7, 9, 11) in a toluene solution (A) and in solid thinfilm (B). (C) Calculated S2 (blue dots) and T1 (red dots) excitationenergies as a function of n from TD-DFT. The open circles are fromthe lowest energy peak positions in panel A; the open squares are fromthe lowest energy peak in the phosphorescence spectrum fromDPDC7 thin film. (D) The energy barrier for singlet fission, ΔESF =2ET1 − ES2, from the TD-DFT results in panel B.

Figure 3. Transient absorption for DPDC7 in solution. (A) 2Dpseudocolor (−ΔT/T) plots of TA spectra as functions of probewavelength and pump−probe delay (Δt). The pump laser wavelengthwas 450 nm. The missing data points around 800 nm was due tosaturation caused by high probe intensity. The ESA for BE, S1, and S2are indicated. (B) TA spectra at different pump−probe delays. Opencircles are the triplet induced absorption obtained via sensitization (seethe Supporting Information). (C and D) Kinetic profiles for S2 probedat 860 nm (red), S1 at 700 nm (green), BE at 580 nm (blue), andbleaching at 493 nm (light blue, scaled by a factor of −1). The dottedcurve is the instrument response.

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attributed to the interconversion between S2 and BE. This ismost obvious in Figure 3D: on the longer time scale, thedominant species, BE, decays with a lifetime of 40 ± 4 ps,matching that of ground state recovery. On this time scale, theweaker S2 and S1 signals exactly track that of BE. This can beaccounted for by the interconversion between S2 and BE; bothcan decay to S1 and, finally, to S0. Supporting this, we note thatthe 40 ps lifetime of BE is 5−6 orders of magnitude shorterthan the μs lifetime of a single triplet obtained in a sensitizationexperiment (see Figure S1 in the Supporting Information). Weattribute the much shorter lifetime of BE, i.e., a stronglycorrelated triplet pair, to their easy annihilation to reform S2.We conclude that intramolecular singlet fission takes place inDPDC7 in solution with the fission yield of 160%. The fissionyield was determined based on the bleaching amplitudes at 0and 2 ps delay times. At the latter delay time, S2 and S1 decayshave almost completed.We now turn to DPDC7 solid thin film and establish the

successful separation of the intramolecular triplet pair intointermolecular triplets. Figure 4A shows a 2D pseudocolor plot

of TA spectra from a DPDC7 thin film. We see three majorchanges from the solution spectra: (i) the GB and ESA peaksare all red-shifted in the solid state, and (ii) all ESA peaksoverlap, especially for S1 and BE at 630 nm; (iii) in contrast toESA, the GB signal is much longer lived and extends to wellbeyond nanoseconds. The long lifetime of bleaching signal isconsistent with the formation of uncorrelated triplets (T1), as isunambiguously verified by phosphorescence emission at ∼1000nm (two peaks at 950 and 1055 nm, Figure 4B; see Figure S2,Supporting Information, for phosphorescence spectra from

DPDC5 and DPDC9). The phosphorescence signal decays witha time constant of ∼15 μs (inset in Figure 4B).The presence of much longer-lived triplets only in the solid

film, not in the solution phase, is a signature of intermolecularsinglet fission reported for most molecular solids/aggre-gates.7−11,19−23 Intersystem crossing from S2 to the low-lyingT1 should occur slower and similarly in both solution and solidfilm, in contrast to results presented here, and is excluded. Wealso exclude thermal artifacts which might result in a long livingsignal,22,56 as the observed GB dynamics is independent of laserpower and substrates (see Figure S6, Supporting Information).Figure 4C shows ESA kinetic profiles at 630, 765, and 1020

nm from the DPDC7 thin film. The initial excitation creates theoptically bright S2 state (olive triangles and line) andsubsequently decays on ultrafast time scales into BE and S1states. While the signal at 765 nm is S1, that at 630 nmrepresents a mixture of BE and S1 (see global fitting analysis inthe Supporting Information). Within the limited timeresolution (∼100 fs), BE seems to be formed on a fastertime scale than S1 is. On longer time scales (ps), S2 and S1 showsimilar decay dynamics that can be attributed to theinterconversion between S2 and BE and the decay of both viaS1 to S0, similar to that of the solution phase. Note that therelaxation via the S2 → S1 → S0 channel is one of the two decaychannels available to BE. The second decay channel is singletfission, i.e., the splitting of the intramolecular triplet pair intotwo triplets on adjacent molecules.The most critical evidence for the intra- to intermolecular

singlet fission mechanisms comes from ground state bleaching.Figure 4D compares the time profiles of GB (right axis) for S0at 555 nm (green circles) and ESA (left axis) for S1 at 765 nm(red circles) and S1+BE at 630 nm (blue squares). The decay inS1 at 765 nm is not single exponential; a biexponential fit (graycurve) gives a fast component (75%) with a lifetime of 2.5 ±0.5 ps and a slow component (25%) with a lifetime of 20 ± 5ps. The fast S1 decay matches exactly the recovery of S0 on thesame time scale, as expected from S1 being the intermediate forexcited state decay. After the initial recovery on the 2.5 ps timescale, the magnitude of GB increases again with a time constantof ∼30 ps, which closely follows the decay of BE. We take thisas evidence for intermolecular singlet f ission. The separation of atriplet pair, BE, localized on one molecule to two uncorrelatedtriplets on two molecules increases the number of bleachedmolecules seen by the probe laser pulse.The proposed mechanism for singlet fission in the DPDC7

solid film, in comparison to the photophysical process insolution, is summarized in Scheme 1. Initial excitation createsthe optically bright S2. In the solution phase, the dark BE[=1(TT)] and S1 states are formed on the ultrafast time scalefrom S2. S1 decays to the ground S0 state in ∼1 ps. The dynamicequilibrium between S2 and BE, combined with the ultrafast S2→ S1 → S0 decay channel, results in an effective S2 lifetime of40 ± 4 ps. For the solid film, in addition to the decay dynamicsfor individual DPDC molecules in the solution, BE can undergointermolecular triplet transfer, i.e., singlet fission, on thecompetitive time scale of 30 ps. These time constants areobtained from global fitting, as briefly presented below and inthe Supporting Information. We emphasize that the 15 μslifetime of the T1 state after intermolecular triplet pairseparation is 5−6 orders of magnitude longer than the 40 pslifetime of the intramolecular triplet pair, despite the fact thatthe annihilation of two T1 back to the BE and S2 is energeticallyfavorable. This illustrates an advantage of the intra- to

Figure 4. Transient absorption and phosphorescence for DPDC7 film.(A) 2D pseudocolor (−ΔT/T) spectra as functions of probewavelength and pump−probe delay time. The pump laser wavelengthwas 450 nm. The ESA for BE, S1, and S2 are indicated. (B)Phosphorescence spectrum at 77 K, with excitation at 335 nm. Thedecay dynamics (inset) at 1000 nm can be approximated by a single-exponential lifetime of 15 μs. (C) ESA dynamics for BE + S1 at 630nm, S1 at 765 nm, and S2 at 1020 nm. (D) A comparison of ESA at630 nm (blue squares) and 765 nm (red circles), left axis, and GB at555 nm (green circles), right axis, as a function of pump−probe delay.The solid gray curve is a biexponential fit to the ESA signal, and thedashed black curve is from the kinetic model (see the SupportingInformation).

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intermolecular singlet fission mechanism, in which the spatialseparation of 1(TT) to 2×T1 slows down the reverse of thesinglet fission process. A long T1 lifetime is desirable for theharvesting of triplets in solar cells or other optoelectronicdevices. The intra- to intermolecular singlet fission mechanismin Scheme 1 is likely responsible for singlet fission in theaggregates, but not in the solution, of zeaxanthin (a carotenoidanalogue) reported by Wang and Tauber.10

We carry out a global fitting analysis based on the kineticmodel in Scheme 1, focusing on dynamics in the 0.4−100 pstime window (see Figures S3 and S4 and associated text in theSupporting Information). While both S1 and BE can decay to S0with rates of 1/τS1 and 1/τBE, respectively, BE can also undergointermolecular fission with a rate of 1/τSF to form two tripletson adjacent molecules. On this time scale, we treat the fractionof BE, γ = [BE]/([S1]+ [BE]), as a fitting parameter andneglect the decay of the separated triplets (15 μs lifetime). Weassume the following spectral components: (1) GB of the S0 →S2 transition, (2) ESA of the singlet S1, and (3) ESA of thetriplets. To minimize the number of variables, we do notdistinguish the ESA spectra from the triplet pair on onemolecule (BE) and uncorrelated triplets on two molecules(2×T1), as is justified by the closeness in the absorption spectraof the two. From the fitting (detailed in the SupportingInformation, Figures S3 and S4), we obtain the followingparameters: γ = 0.55; τS1 = 2.5 ± 0.5 ps; τBE = 40 ± 10 ps; τSF =30 ± 8 ps; and a singlet fission quantum yield of 70% (numberof triplets per absorbed photon) for excitation at 450 nm.Both theoretical calculations on the DPDC7 molecule and

experimental measurements on DPDC7 film give an energybarrier for singlet fission of ΔE = 2ET1 − ES2 ≈ 0.2 eV (Figure2). Thus, we predict that the singlet fission rate should beassisted by the excess photon energy. Figure 5 compares the

TA results for DPDC7 thin film excited at hν = 2.25−3.54 eV.The picosecond decay dynamics of S1 (ESA at 765 nm, Figure5A) is independent of pump photon energy, consistent with thefact that the dark S1 state is formed on the sub-ps time scalefrom the relaxation of S2.

37−39 The most obvious effect ofphoton energy is on GB dynamics (Figure 5C). The initialrecovery of the GB signal on the ∼2.5 ps time scale is due tosinglet recombination (S1 → S0) and, in agreement with Figure5A, is independent of excitation photon energy. In contrast, thesubsequent increase in GB on longer time scales (∼30 ps),attributed to intra- to intermolecular singlet fission, dependsstrongly on pump photon energy, The rate of intermolecularsinglet fission is approximately proportional to the negativeslope of the GB signal change on the time scale of 10−40 ps,while the yield of singlet fission is approximately proportionalto the GB signal at longer times (∼400 ps). Both the singletfission rate and yield increase with increasing photon energy, asexpected from the activated process due to the ∼0.2 eV energybarrier. The singlet fission yield in DPDC7 thin films increasesfrom 44% at hν = 2.25 eV to 74% at hν = 3.54 eV, Figure 5B.Note that, for each GB kinetic profile on the long time scale,there is an oscillatory component on top of the GB dynamics.The oscillation has a time period of ∼120 ps and dephaseswithin ∼300 ps. These oscillations are the well-known acousticwaves intrinsic to pulsed laser measurements on thin films,57 asdetailed in the Supporting Information, Figure S9.Since the energetics for singlet fission depends on the

conjugation length of the DPDCn molecules, we expect thesinglet fission yield to also depend on n, as confirmed in Figure6A (see Figure S7, Supporting Information, for TA spectra for n= 3−11). The negligible singlet fission yield in DPDC3 can beattributed to the large energy barrier (ΔESF ∼ 0.6 eV, Figure2D). As expected from ΔESF, the singlet fission yield firstincreases with n, maximizes for n = 7, and decreases again for n= 9 and 11. Interestingly, theory predicts that ΔESF for n = 9 or

Scheme 1. Summary of DPDC7 Photophysics in the Solution(Upper) and Solid Thin Film (Lower)a

aIn the lower panel, blue and red represent two different DPDC7molecules. The solid arrows represent optical excitation and dashedarrows subsequent dynamics.

Figure 5. TA results for DPDC7 thin film excited at different photonenergies (hν). (A) ESA at 765 nm (S1) for hν = 2.25−3.54 eV. Thetime profiles at different hν values are normalized at the peak ΔT/Tvalue near time zero. (B) Singlet fission yield (solid circles) as afunction of pump photon energy. (C) GB at 555 nm (averaged in arange of 550−560 nm) for hν = 3.54 (green), 2.76 (blue), and 2.53 eV(orange). The time profiles at different hν are normalized at the peakbleaching value near time zero.

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11 is only slightly higher than that for n = 7, but the singletfission yield decreases significantly from ∼70% for DPDC7 to35 and 18% for DPDC9 and DPDC11, respectively. The singletfission yields here are a net result of intramolecular triplet pair(BE) formation and intermolecular triplet pair splitting. WhileBE formation is an inherent property of the molecules, theseparation of the triplet pair must compete with the relaxationof BE to S0 and the rate depends on the intermolecularcoupling and, thus, crystallinity of the solid. As shown bygrazing incidence X-ray diffraction (GIXD) images in Figure6B−D, the DPDCn films are highly crystalline for n ≤ 7 butamorphous for n ≥ 9 (see Figure S8, Supporting Information,for GIXD data from all samples). Compared to DPDC7, thelack of crystallinity in DPDC9 and DPDC11 films leads toweaker intermolecular electronic interaction and slower rates ofintra- to intermolecular BE → 2×T1 transition.

■ SUMMARYWhile the mechanism established here comes from analysis ofsinglet fission in DPDC molecules, oligoenes or polyenes maynot be the best candidates for highly efficient singlet fission dueto the endothermic process and the S1 → S0 loss channel.However, the intra- to intermolecular singlet fission should beof general significance to other potential candidates where anintramolecular biexciton may split into an intermolecular tripletpair. This mechanism suggests a new design principle for singletfission: intramolecular electronic structure for the efficientformation of an intramolecular triplet pair (i.e., biexciton) uponoptical excitation and intermolecular electronic interaction forthe separation (fission) of the biexciton state into twoindividual triplets on adjacent molecules. For intramolecularsinglet fission design, we note that the BE peak intensitydominates over that of S1 in DPDC7 (Figure 3A), whileprevious TA studies on carotenoids showed similar S* whichwas suggested as the triplet pair state and S1 intensities.37,43

The propensity in DPDCn to preferentially form BE over S1may be attributed to the two electron withdrawing groups(−CN) that introduce CT character to the optically excitedstate and assist the localization of triplets. This issue deserveshigh-level theoretical investigation. For intermolecular design,we point out the need for sufficient HOMO−HOMO andLUMO−LUMO coupling to facilitate intermolecular triplettransfer, i.e., efficient Dexter energy transfer.58 Such designcriteria are different than those outlined for intermoleculartriplet pair formation, where balanced HOMO−HOMO,LUMO−LUMO, and HOMO−LUMO couplings are neededfor efficient intermolecular singlet fission.1,2

■ ASSOCIATED CONTENT*S Supporting InformationDetails on experimental methods, sample preparation, addi-tional experimental results, data analysis, and TD-DFTcalculations. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: cn37@columbia.edu.*E-mail: xyzhu@columbia.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported before June 30, 2014, as part of theprogram “Center for Re-Defining Photovoltaic EfficiencyThrough Molecule Scale Control”, an Energy Frontier ResearchCenter funded by the U.S. Department of Energy, Office ofScience, Office of Basic Energy Sciences under Award NumberDE-SC0001085; most experimental work on DPDCn moleculesand initial data analysis was carried out during this time period.After June 30, 2014, this work was supported by the NationalScience Foundation grant DMR 1321405 (to X.Z.); newexperimental analysis presented in this manuscript and controlexperiments on other oligoene molecules were carried outduring this time period. Research carried out in part at theCenter for Functional Nanomaterials, Brookhaven NationalLaboratory, which is supported by the U.S. Department ofEnergy, Office of Basic Energy Sciences, under Contract No.DE-AC02-98CH10886. X.Z. thanks Prof. Valy Vardeny for theteaching on picosecond acoustics. We thank D. Nordlund atSLAC National Accelerator Laboratory for help withsynchrotron measurements.

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Figure 6. π-bond (n) dependence of singlet fission yields andcrystallinity in DPDCn films. (A) Singlet fission yield as a function of n.The excitation wavelengths are 320 nm (n = 3), 420 nm (n = 5), 450and 490 nm (n = 7), 500 nm (n = 9), and 525 nm (n = 11). (B−D) 2-D GIXD patterns, measured at an incident angle of 0.7°, for (B)DPDC5, (C) DPDC7, and (D) DPDC9 thin films.

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