Direct Observation of Correlated Triplet Pair Dynamics During Singlet

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Direct Observation of Correlated Triplet Pair Dynamics During Singlet Fission Using Ultrafast Mid-IR Spectroscopy Christopher Grieco, Eric R. Kennehan, Hwon Kim, Ryan D Pensack, Alyssa N. Brigeman, Adam D. Rimshaw, Marcia M. Payne, John E Anthony, Noel C. Giebink, Gregory D. Scholes, and John B. Asbury J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11228 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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The Journal of Physical Chemistry

Direct Observation of Correlated Triplet Pair Dynamics During Singlet Fission Using Ultrafast Mid-IR Spectroscopy Christopher Grieco1, Eric R. Kennehan1, Hwon Kim2, Ryan D. Pensack2, Alyssa N. Brigeman3, Adam Rimshaw1, Marcia M. Payne4, John E. Anthony4, Noel C. Giebink3, Gregory D. Scholes2, John B. Asbury1* 1. Department of Chemistry, The Pennsylvania State University 2. Department of Chemistry, Princeton University 3. Department of Electrical Engineering, The Pennsylvania State University 4. Department of Chemistry, University of Kentucky

Abstract Singlet fission is an exciton multiplication mechanism in organic materials whereby high energy singlet excitons can be converted into two triplet excitons with near unity quantum yields. As new singlet fission sensitizers are developed with properties tailored to specific applications, there is an increasing need for design rules to understand how the molecular structure and crystal packing arrangements influence the rate and yield with which spin-correlated intermediates known as correlated triplet pairs can be successfully separated – a prerequisite for harvesting the multiplied triplets. Toward this end, we identify new electronic transitions in the mid-infrared spectral range that are distinct for both initially excited singlet states and correlated triplet pair intermediate states using ultrafast mid-infrared transient absorption spectroscopy of crystalline films of 6,13bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). We show that the dissociation dynamics of the intermediates can be measured through the time evolution of the mid-infrared transitions. Combining the mid-infrared with visible transient absorption and photoluminescence methods, we track the dynamics of the relevant electronic states through their unique electronic signatures and find that complete dissociation of the intermediate states to form independent triplet excitons occurs on time scales ranging from 100 ps to 1 ns. Our findings reveal that relaxation processes competing with triplet harvesting or charge transfer may need to be controlled on time scales that are orders of magnitude longer than previously believed even in systems like TIPS-Pn where the primary singlet fission events occur on the sub-picosecond time scale.

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Introduction Singlet fission is an exciton multiplication reaction whereby a singlet exciton can be converted to two triplet excitons with nearly unit quantum yield.1-4 The reaction has been characterized in a number of organic molecules and polymers,5-12 which promise to serve as sensitizers for photovoltaic cells. In this application, they may be used to surpass the Shockley-Queisser limit by eliminating thermalization losses associated with absorption of high energy photons particularly in the blue region of the electromagnetic spectrum.13-14 Proof of principle experiments have demonstrated the potential to use singlet fission to enhance the efficiency of functional devices.1417

However, the reported enhancements in terms of overall external quantum efficiency or power

conversion efficiency have been limited to date despite the use of systems such as lead chalcogenides to accept excited state energy from triplet states due to their high spin-orbit coupling. There remains a need to elucidate the nature of intermediate states and how their dynamics may influence the overall quantum yield for harvesting multiplied triplet excitons. It has proven challenging to separate the dynamics of intermediates, known as correlated triplet pairs (CTP), 9, 18-36 from separated triplet excitons using ultrafast visible transient absorption spectroscopy because both CTPs and triplet excitons have triplet character37-38 and overlapping spectral features. More recently, unique absorptions of CTPs have been reported in the near-IR spectral range,6, 26, 37, 39-46 and using electron paramagnetic resonance,47 leading to new insights about the mechanism of singlet fission. The separation of CTPs involves a type of intersystem crossing process,1 which is typically quite slow in organic crystals due to their small spin-orbit coupling. Such slow formation of separated triplets must compete with spin-allowed relaxation processes within the CTP intermediates before they separate. For example, recent work with amorphous TIPS-Pn nanoparticles demonstrated that internal conversion of CTPs can occur before they complete their separation process,48 which reduced the yield of triplets that survived long enough to fully separate.


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We recently used ultrafast mid-infrared (mid-IR) spectroscopy to examine the dynamics of electronic states involved in singlet fission in crystalline films of the model system, 6,13bis(triisopropylsilylethynyl) pentacene (TIPS-Pn).49 We monitored the time-dependent frequency of the alkyne stretch mode of the triisopropylsilylethynyl (TIPS) side groups of the molecules, which are coupled to the conjugated framework of their pentacene cores. The C-C stretch modes of the conjugated cores of molecules such as rubrene and perylenediimides can also be used to examine excited electronic states50 during singlet fission51 and excimer formation.52-53 Importantly, the frequency and line shape of the alkyne stretch mode of TIPS-Pn are temperature dependent, resulting in unique vibrational frequencies for both triplet excitons and hot ground state molecules. We used these unique frequencies to track the dynamics of triplet pair separation during the singlet fission reaction. The temperature dependence of the alkyne stretch also enabled us to track the dissipation of energy into the molecular crystals on ultrafast time scales to quantify relaxation processes that can compete with CTP separation.49 In our ultrafast vibrational studies to examine the dynamics of electronic states during singlet fission,49 we observed a broad electronic transition upon which the vibrational features were superimposed. In this work, we describe our investigation of the nature of this broad mid-IR electronic transition. We demonstrate that this broad feature provides a probe of the dynamics of CTP intermediates that is complementary to measurements of these states in the visible and near-IR spectral regions.6, 26, 37, 39-46, 54-55 Importantly, the broad electronic transition is measured simultaneously with the vibrational features of the electronic states involved in singlet fission, which provides an independent probe of CTP dynamics that can be compared quantitatively with the triplet separation and relaxation processes revealed through the vibrational modes.49 Furthermore, by combining ultrafast mid-IR transient absorption spectroscopy with visible transient absorption measurements, we show that the complete dissociation of CTP intermediates to form independent triplet excitons in crystalline films of TIPS-Pn occurs on time scales ranging from 100 ps to 1 ns. This time scale for completion of the singlet fission reaction is orders of 3 ACS Paragon Plus Environment


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magnitude longer than was previously believed for TIPS-Pn and related derivatives.36, 39-40 Our findings indicate that excited state deactivation pathways that compete with triplet harvesting or charge transfer56-57 in functional devices may need to be controlled on time scales far longer than previously realized to enable efficient harvesting of the multiplied triplet excitons,58 even in molecular systems like TIPS-Pn in which the primary singlet fission events occur on the picosecond time scale. Experimental and Computational Methods Solution samples were made by dissolving TIPS-pentacene in solvents such as toluene or carbon tetrachloride to obtain concentrations in the range 2 x 10-5 M – 2 x 10-4 M depending on the type of spectroscopy measurement made. For spectroscopy measurements, solutions were loaded into a homemade liquid cell consisting of sapphire or potassium bromide windows sandwiching a ~1.7 mm thick Viton o-ring, which was used as an optical spacer.

Some

spectroscopy measurements were made in a 1 cm pathlength quartz cuvette for more dilute solutions and in spectral regions for which transmittance of the solvent was sufficiently high. Film samples were made by spin-coating 20 mg/mL solutions of TIPS-Pn in dichloromethane onto 2.5 cm diameter CaF2 substrates at ~800 rpm. To obtain crystalline films, these amorphous films59 were then solvent vapor annealed using an apparatus previously described59 for ~2.5 hours. Some UV-Vis spectrophotometry was performed using a commercially available spectrometer (Beckman, DU 520; Brea, CA). Other absorbance spectra were collected using a home-built instrument, which was constructed as follows: The light source was a stabilized tungsten-halogen lamp (SLS-201, Thorlabs; Newton, NJ), which was collimated and passed through the sample. After encountering the sample, the beam was detected using a CCD spectrometer (USB-2000, Ocean Optics; Dunedin, FL).


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FTIR spectroscopy was performed using a commercially available instrument (Madison Instruments, Mattson Research Series; Middleton, WI) equipped with a liquid nitrogen cooled MCT detector. Nanosecond mid-IR spectroscopy was performed using a home-built flash photolysis system, similar to that described elsewhere.59-60 Visible excitation was tuned to 649 nm and the fluence was ~200 μJ/cm2 for solution measurements. For measuring kinetic decays, a 100 MHz MCT photodiode detector was used (Kolmar Technologies, KV104; Newburyport, MA) and transient absorption was measured over ~2.5 – 5.6 μm. For measuring spectra, a 16 MHz MCT photodiode detector was used (Kolmar Technologies, KMPV11-1-J1; Newburyport, MA) and monochromator slits were adjusted to achieve 40 nm effective spectral bandwidth. Nanosecond visible spectroscopy was performed using a different home-built flash photolysis system, similar to that described previously.59 The excitation source was the same as that used in nanosecond mid-IR spectroscopy. Nanosecond near-IR spectroscopy was performed using another home-built flash photolysis system, similar to that described elsewhere.61 The excitation source was the same as that used in nanosecond mid-IR spectroscopy. Depending on the wavelength range probed, InGaAs photodiode detectors were used (DET10N and DET05D, Thorlabs; Newton, NJ) having a 5 ns and 17 ns rise time, respectively. Ultrafast visible-pump/mid-IR probe transient absorption spectroscopy was performed using a home-built system as described elsewhere.62 The visible pump beam was tuned to 655 nm, while the probe beam was tuned over the range of ~4.4 – 4.9 μm. Ultrafast

2-color

visible-pump/visible-probe

spectroscopy

and

nanosecond

visible-

pump/visible-probe spectroscopy were performed using home-built systems as described previously.59 For solution samples the excitation (probe) wavelength was 642 nm (~505 nm), while for film samples the excitation wavelength was 655 nm (~520 nm). Time-resolved (ultrafast) photoluminescence spectroscopy was performed using a highspeed streak camera with ps time resolution (C10910, Hamamatsu; Bridgewater, NJ) synced to 5 ACS Paragon Plus Environment


a mode-locked picosecond tunable laser (PL2210A laser and PG403 optical parametric generator, EKSPLA; Bozeman, MT). The excitation laser was tuned to 633 nm and was adjusted so that the film samples had an absorbed energy density of ~92 μJ/cm2. Photoluminescence spectra and kinetics were collected in the range of 640 to 1000 nm. Time-resolved (nanosecond) fluorescence spectroscopy was performed using a homebuilt spectrometer similar to that described elsewhere.63 In particular, a 30 Hz frequency-doubled Nd:YAG Laser (Surelite, Continuum; San Jose, CA)) was used to pump a modified dye laser (GL302, photon technology international; Edison, NJ) with an output of 642 nm with ~10 ns pulse duration. After sample excitation, the emission was dispersed using a monochromator (DK240, Spectral Products; Putnam, CT) and focused onto a 350 MHz silicon photodiode (DET210, Thorlabs; Newton, NJ). All spectra were measured with 2.5 nm effective bandwidth. A laser excitation fluence of ~200 μJ/cm2 was used for all solution measurements. For excitation energy calculations, several levels of theory were used. Density functional theory (DFT) was performed for the TIPS-Pn monomer. First, the structure was optimized with the B3LYP functional. Excitation energies were subsequently calculated using time-dependent DFT (TD-DFT) with the B3LYP or the long-range corrected (LCR) wPBE functional (LCR-wPBE). These calculations were performed with the cc-PVTZ basis set and under the polarized continuum model (PCM) for toluene. The basis set and functionals were selected based on previous computational studies.64-66 For multi-reference calculations of multi-exciton states, an ethynyl model of TIPS-Pn (with the triisopropylsilyl groups replaced by hydrogen atoms) was used. Complete Active Space Self Consistent Field (CASSCF) calculations were performed at the B3LYP/6-31G(d)67-68 optimized ground state (S0) and first excited bright state (S1) geometries. Following these calculations, Strongly Contracted N-electron Valence Perturbation Theory (SCNEVPT)69 was used to account for the dynamic correlation among the electronic configurations.


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Results and Discussion Ultrafast Mid-IR Spectroscopy of Isolated TIPS-Pn Molecules in Solution Ultrafast

mid-IR

transient

absorption

spectroscopic studies of singlet fission in TIPS-

in CCl4

Pn have not previously been reported. Therefore, we set out to establish the assignments of the mid-IR spectral features

110

–3

M

110

–5

M

that were used to directly probe the dynamics of CTP intermediates present during singlet fission. Figure 1 represents concentration dependent visible absorption spectra of TIPSPn molecules in CCl4 at two widely different concentrations. The absorption spectra exhibit

Figure 1. Absorption spectra of isolated TIPSPn molecules in CCl4 solutions at a range of concentrations. The absorption spectra show no indication of aggregation within this concentration range. The inset shows the structure of TIPS-Pn.

negligible change within the 110–5 to 110–3 M concentration range, indicating that the molecules do not aggregate under these conditions. TIPSPn solutions within this concentration range have been shown to not undergo diffusion-limited singlet fission.3, 49 Fluorescence spectra of a 210–4 M solution of TIPS-Pn in CCl4 appear in Figure S1 at several time delays. The spectra exhibit vibronic structure that complements the structure in the absorption spectrum and show time invariance, indicating that only excited state monomers were formed. This observation confirms that the TIPS-Pn molecules at this concentration did not form excimers or aggregates in solution. We desired to probe the excited singlet state dynamics of isolated TIPS-Pn molecules in the mid-IR spectral range. Therefore, a 2 x 10-4 M solution of TIPS-Pn in CCl4 was excited at 642 nm (~200 J/cm2) and the resulting transient absorption spectrum in the mid-IR was measured at ~10 ns time delay (Figure 2a). Because this concentration is below the limit for which diffusion-



mediated singlet fission has been observed,3, 49 the transient absorption spectrum reflects that of the first singlet excited state of isolated TIPS-Pn molecules. This represents the first report of a broad transient absorption band in isolated TIPS-Pn molecules that covers the full mid-IR spectral range. The mid-IR absorption band exhibits vibronic structure that is characteristic of electronic transitions of TIPS-Pn, similar to those observed in both absorbance and fluorescence spectra in Figure 1 and Figure S1. We demonstrated that the mid-IR transition appearing in Figure 2a results from the transient absorption of singlet excited states of TIPS-Pn by comparing the time scale for the decay of this absorption feature with the fluorescence decay as represented in Figure 2b and 2c. The decay

time of the mid-IR transient absorption feature matches quantitatively the fluorescence decay trace, which reflects the singlet excited state population dynamics. Comparisons of mid-IR transient absorption and fluorescence decay traces measured in other solvents of varying polarity appear in the Figure S2 and Figure S3. The solid curves overlaid on the data in Figure 2b and Figure 2c represent best single-exponential fit functions that were convoluted with the appropriate instrument response functions for each measurement.

Details about the fitting routine are

provided in the supporting information. As seen in Figure 2d, the lifetimes determined from 8 ACS Paragon Plus Environment

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the mid-IR transient absorption and fluorescence measurements were correlated in a 1:1 relationship for all solutions investigated, confirming that the mid-IR transition arises from the singlet excited state of isolated TIPS-Pn molecules. We will refer to this mid-IR absorption of singlet excited states as the prompt mid-IR absorption here and in the following discussion. In an effort to identify the nature of the final state involved in the prompt mid-IR absorption, we characterized other optically bright states that were energetically accessible in the visible, near-IR and mid-IR spectral ranges using either steady-state or time-resolved absorption spectroscopy. The absorption spectra for the ground (S0) and excited singlet (S1) and triplet (T1) states are shown in Figure 3a. Each excited state spectral assignment was based on analysis of their kinetic behavior, the details of which are provided in the supporting information. Molar absorption coefficients were also calculated (in M-1cm-1 units) and provided next to each spectrum. Details about these calculations are also provided in the supporting information. The transition energies were used to construct an energy level diagram for the singlet manifold, which is shown in Figure 3b (labeled Expt.). An energy level diagram from the experimental measurements including the triplet manifold is provided in Figure S7. From the energy level diagram, it was first considered if the final state of the mid-IR transition was either a singlet or a triplet. However, the moderate oscillator strength of the prompt mid-IR absorption of ~700 M-1cm-1 was used to rule out the possibility of a S1  Tn transition. We also used the perimeter-free electron orbital model70 and TD-DFT computations to eliminate the possibility that the prompt mid-IR absorption arose from a S1  Sn transition (see supporting information). In particular, the transition energies from the S1 state to higher-lying singlet states were much larger than the measured mid-IR transition (see Table S2). Considering that the final state involved in the prompt mid-IR absorption was not likely a singlet or triplet state, we considered that it might be a doubly excited state. The presence of doubly excited states in pentacene has been demonstrated using quantum chemical



calculations,71-73 and so we considered

(a) S0-S1

S0 absorption

whether such states might lie close in

4

ε~10

energy to the S1 state of TIPS-Pn. To test

3

ε~10

this, we performed Complete Active Space

S1-??

S1-Sn

3

Self

Consistent

Field

S0-Sn

(CASSCF)

S1 absorption

4

ε~10

ε~10

calculations at the B3LYP/6-31G(d)67-68 T1 absorption T1-Tn

optimized ground state (S0) and first excited

T1-Tm

4

5

ε~10

ε~10

bright state (S1) geometries of an ethynyl model of TIPS-Pn (with the triisopropylsilyl

(b)

these

calculations,

Strongly

S2

Contracted N-electron Valence Perturbation

ME S1

Theory (SC-NEVPT)69 was used to account

configurations.

calculated

transition

The

resulting

energies

S2 ME S1

~0.3

for the dynamic correlation among the electronic

~0.89

Following

~2.82

groups replaced by hydrogen atoms).

~1.93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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are

S0

summarized in the energy level diagram presented in Figure 3b (labeled Calc.). The calculated transition energies and oscillator strengths for each transition are tabulated in Table S3 and reveal that a state with significant double excited character lies higher in energy than the S1 state by a few tenths of an electron-volt, which is similar in energy to the mid-IR transition energy. We

S0 Expt.

Calc.

Figure 3. (a) Normalized absorbance spectra of ground state and first excited states (S1 and T1) in dilute TIPS-Pn solutions. Each spectrum is assigned a transition based on both optical energy gap values as well as kinetic decay behavior. The corresponding molar absorption coefficients of each transition are indicated for reference. (b) Energy level diagram for TIPS-Pn in solution on the left side was based on the absorbance spectra represented above. The energy level diagram on the right was obtained from multi-reference abinitio calculations of TIPS-Pn. The comparison with the experimental data indicates that the prompt mid-IR absorption is between the S1 state and a close-lying multi-excitonic (ME) state.

therefore assigned the final state of the prompt mid-IR absorption to be a doubly-excited state, 10 ACS Paragon Plus Environment

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which is labeled as a “multi-exciton (ME)” state in Figure 3b for consistency with previous literature.71-72 We note that there is a difference between the calculated and measured S1  ME transition energy as indicated in Figure 3b. Given the size of the TIPS-Pn molecule and the complexity of the multi-reference excited state energy calculations, we believe the energy difference to be in reasonable support of this assignment, especially considering that the oscillator strength calculated for the transition agrees favorably with the measured oscillator strength of the mid-IR transition (see Section S7). A more detailed examination of the precise assignment of the final state of the prompt mid-IR absorption and its implications for singlet fission materials is ongoing. Regardless of the precise assignment, the data presented herein indicate that the prompt mid-IR absorption can be used to track S1 states in TIPS-Pn.

Ultrafast Mid-IR Spectroscopy of Crystalline TIPS-Pn Films Having assigned the prompt mid-IR absorption to the S1 state of isolated TIPS-Pn molecules in solution, we investigated the time-dependence of this transition during singlet fission in crystalline films of TIPS-Pn. Figure 4a represents a Fourier transform infrared (FTIR) spectrum in the vicinity of the alkyne stretch modes of TIPS-Pn cast as a film that was fully solvent annealed to form the form-I 2D-brickwork phase using procedures that were reported previously.59, 74 We will refer to these films as crystalline TIPS-Pn films from this point forward. Figure 4b depicts a 2-dimensional frequency-time plot of the ultrafast mid-IR transient absorption signal of a crystalline TIPS-Pn film following excitation to the first singlet excited state by a 655 nm optical pulse with 65 J/cm2 absorbed excitation density. Similar to the time-resolved mid-IR transient absorption measurements of isolated TIPS-Pn molecules in solution, the mid-IR spectra measured in the crystalline film exhibit a broad electronic transition that appears with a pulse-



limited rise. Motivated by the solution

(a)

studies, we assign this broad absorption feature to the prompt mid-IR absorption of

Alkyne stretch

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Crystalline Film

(b)

the initially excited singlet states in the TIPSPn film. Super-imposed on the broad Prompt mid-IR absorption

electronic transition is a narrow vibrational feature corresponding to the alkyne stretch

(c)

(alkyne GSB)

Prompt mid-IR absorption 0.25 – 1 ps

mode of TIPS-Pn. This feature appears due (alkyne GSB)

to the reduction of the ground state

Persistent mid-IR absorption

population of TIPS-Pn molecules in the film 5 – 1000 ps

(termed a ground state bleach, GSB).49 Because

TIPS-Pn

molecules

are

coupled in the crystalline film, they undergo singlet fission,

59

leading to rapid loss of the

prompt mid-IR absorption of the S1 state in the film on the sub-picosecond time scale. This loss of the S1 state population is observed by the rapid decay of the prompt

Figure 4: (a) FTIR spectrum of a fully solvent annealed crystalline TIPS-Pn film highlighting the alkyne stretch region. (b) Transient absorption spectrum of a TIPS-Pn film showing a broad photoinduced absorption (PIA) feature and the ground state bleach (GSB) of the alkyne asymmetric stretch mode. (c) Time-integrated slices of the transient absorption spectrum at early (0.25 – 1 ps) and later (5 – 1000 ps) time delays. Note the differences in absorption intensity of the PIA feature.

mid-IR absorption in Figure 4b. Figure 4c displays mid-IR transient absorption spectra that were obtained by averaging spectra measured over several time points within the 0.25 – 1 ps time window (top) and 5 – 1000 ps time window (bottom). The comparison of the spectra indicates that the mid-IR transient absorption feature undergoes a change in spectral shape and amplitude over this time range. We will refer to the transient absorption feature represented in the 5 – 1000 ps spectrum as the persistent mid-IR absorption here and in the following discussion. We note that the amplitude of the alkyne stretch GSB provides an internal reference indicating the concentration of TIPS-Pn molecules that were excited out of their ground electronic states within 12 ACS Paragon Plus Environment

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the indicated time range. Therefore, the data indicate that the transition giving rise to the persistent mid-IR absorption has a significantly smaller absorption cross section in comparison to the prompt mid-IR absorption of the S1 state. We verified that the rapid loss of the prompt mid-IR absorption on the sub-picosecond time scale in Figure 4b resulted from singlet fission by measuring the dynamics of the triplet T1  Tn transition in the visible spectral range. Figure

(a)

mid-IR

vis T1Tn

5a reproduces the prompt mid-IR absorption spectrum plotted on an energy scale from 0.25 0.25 – 1 ps

– 0.28 eV alongside the transient absorption arising from the T1  Tn transition between 2 eV and 3 eV probe energies. Figure 5b

(b)

represents the transient absorption kinetics traces of the singlet excited state dynamics

Prompt mid-IR abs.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 ns

measured using the prompt mid-IR absorption at 0.27 eV (2177 cm-1) with the triplet population dynamics measured in the visible at 2.39

eV

(520

nm).

The

(c)

mid-IR

comparison

demonstrates that the ultrafast loss of the prompt

mid-IR

absorption

occurs

synchronously with the growth of the triplet population, confirming that the rapid loss of singlet population results from singlet fission. Singlet fission is known to occur with unit quantum yield and on the 100 fs timescale in pentacene and crystalline TIPS-Pn films.36, 75-

Figure 5: (a) Ultrafast transient absorption spectra measured in the mid-IR (0.25-0.28 eV) and visible (2-3 eV) spectral ranges. The dotted lines indicate probe energies at which the kinetics traces in (b) were measured. The comparison of kinetics traces reveals that the mid-IR absorption of the singlet excited state decays synchronously with the growth of the triplet population on the ~100 fs time scale. However, the mid-IR transient absorption persists into the 100 ps time scale. The spectrum of this persistent mid-IR absorption in (c) differs from the singlet absorption spectrum in (a). This absorption is assigned to correlated triplet pairs (see discussion).



76

However, the persistent mid-IR absorption signal does not decay completely on the sub-

picosecond time scale as would be expected if this signal arose exclusively from singlet excitons in the crystalline film. Instead, the spectral shape and absorption cross section of the

(a)

persistent mid-IR absorption changes relative to the prompt mid-IR absorption of the S1 state. In the following, we consider several possible origins of the persistent mid-IR absorption

(b)

feature. Residual Singlet Exciton Hypothesis. The persistent mid-IR absorption feature has an amplitude at 10 ps (Figure 5b) that is 10% relative to the initial prompt mid-IR absorption

(c)

signal around the zero-time delay. The most obvious possible explanation of this persistent mid-IR absorption is that it might arise from a population of singlet excited states that could not undergo singlet fission, possibly due to singlet trapping77 or disorder in the film. We used fluorescence to test this hypothesis because it is a more sensitive technique than mid-IR transient absorption spectroscopy. Fluorescence can be detected with a zero background, and the optical transition has a greater transition dipole moment than the midIR transition (Figure 3a). Therefore, we

Figure 6: (a) Absorbance spectra for an amorphous and crystalline film of TIPS-Pn. Red vertical line represents the pump wavelength (633 nm) used for photoluminescence measurements. (b) Ultrafast fluorescence spectra for an amorphous and crystalline film of TIPS-Pn. Spectra were obtained by time-integrating up to 150 ps time-delay. The excitation wavelength was 633 nm, and the absorbed energy density was ~92 2 μJ/cm . The corresponding spectrally-integrated fluorescence decay traces are represented in (b) for the amorphous film and crystalline films. No discernible singlet fluorescence signal was detected in the crystalline film, indicating that the persistent mid-IR signal did not arise from residual singlet excitons.


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reasoned that if residual singlet excited states in the crystalline film had sufficient concentration to cause the persistent mid-IR absorption feature with 10% of the initial amplitude, then fluorescence from these singlet states should be clearly observed. We used ultrafast fluorescence spectroscopy using a 633 nm excitation laser with a 15 ps pulse width and 1 kHz repetition rate along with a streak camera/spectrograph combination to measure the fluorescence from the TIPS-Pn film. We first verified that we could measure fluorescence from singlet excited states in an amorphous film of TIPS-Pn because such films are disordered and likely to contain regions where singlet excitons can become trapped. We used recently reported spin casting methods to prepare crystalline and amorphous films59 whose absorption spectra are represented in Figure 6a for reference. The corresponding fluorescence spectrum of the amorphous film that was time-integrated from 0 - 150 ps is represented in Figure 6b and indeed exhibits the characteristic spectrum of the singlet excited state emission. However, the spectrum of the crystalline TIPS-Pn film measured under identical conditions exhibited no discernible fluorescence that could be distinguished from the noise floor. Furthermore, we measured the fluorescence decay kinetics of the amorphous TIPS-Pn film by integrating the emission signal from 643 – 818 nm and verified that the fluorescence from the amorphous film could be clearly observed (Figure 6c). However, in the crystalline film, no discernible fluorescence signal was detected outside of the excitation pulse envelope described by the instrument response function (IRF) in Figure 6c. We recall from our solution studies described above that the time-dependence of the prompt mid-IR transient absorption feature of singlet excited states was quantitatively correlated with the decay of the fluorescence emission of TIPS-Pn molecules (Figure 2). Therefore, the absence of a measureable fluorescence signal on the 10 ps to 1 ns time scale in the crystalline TIPS-Pn film demonstrates that the persistent midIR transient absorption did not arise from singlet excited states.



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Triplet Fusion to Reform Singlet Exciton Hypothesis. We also considered the possibility that the persistent mid-IR absorption signal at 10 ps and longer time scales might have arisen from singlet excitons that reformed due to fusion of triplets, possibly giving rise to delayed fluorescence. Delayed fluorescence has not

Probe ε = 0.27 eV bimolecular reformation (power = 2)

been observed in TIPS-Pn films because reformation of singlet excitons by triplet fusion is endergonic, although triplets can fuse to reform

bimolecular decay

CTP states.38 Nonetheless, we measured the

(power = ½)

excitation energy density dependence of the persistent mid-IR absorption to test whether it might arise from this source. In Figure 7, the amplitudes of the persistent mid-IR absorption measured at 10 ps time delay are plotted versus the excitation density on a logarithmic scale,

Figure 7: Dependence of the persistent mid-IR transient absorption signal on incident energy density, shown for 10 ps and 100 ps time delays. Square, linear, and square-root functions (lines) are overlaid on the data. The comparison demonstrates that the persistent mid-IR signal does not arise from reformation of singlet excitons by triplet fusion.

revealing that the signal varies linearly with excitation density. The fusion of independent triplet excitons to reform singlets would lead to quadratic excitation density dependence. Therefore, the linear behavior excludes this possibility. One might consider that the persistent mid-IR signal at 10 ps arose from geminate recombination of pairs of triplets that never separated. However, we exclude this possibility on the basis of the arguments given above in the Residual Singlet Exciton Hypothesis discussion. In particular, if a sufficient density of singlet excitons were reformed by geminate triplet pair fusion to produce a persistent mid-IR absorption with 10% of the original magnitude, then we would have been able to measure fluorescence from these singlet states. Finally, we note that triplet excitons undergo bimolecular annihilation under the experimental conditions used here.78 Consequently, the excitation density was sufficient to observe triplet-triplet fusion within their lifetime. We conclude therefore that the persistent mid-IR absorption does not arise from singlet states in the crystalline TIPS-Pn film. 16 ACS Paragon Plus Environment

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Mid-IR Probe of CTP Intermediates in Crystalline TIPS-Pn Films Comparison of the kinetics traces in Figure 5b reporting the time-dependence of the persistent mid-IR absorption versus the T1  Tn transition reveals that the persistent mid-IR absorption decayed to zero amplitude at 1 ns time delay while the visible absorption did not. This indicates that the electronic states giving rise to the persistent mid-IR absorption are a type of intermediate species that form during the singlet fission reaction. Recognizing that the persistent mid-IR absorption arises from intermediates in the singlet fission reaction, we considered a ‘one CTP intermediate’ kinetic model to quantitatively describe the mid-IR and visible transient absorption data. We considered the sequence of elementary steps adapted from previous studies of singlet fission37-38, 79 as described in Section S10, k1 S1S0   1 (TT )

k2 k2

k3 T1  T1   S0*  T1

(eqn 1)

where S1S0 represents a singlet exciton, T1 + T1 represents triplet excitons that successfully separated from the CTP intermediates, and S0* represents hot ground state molecules. Here, 1

(TT) is used to represent CTP intermediates in the kinetic model. We introduce this nomenclature

to refer to CTP intermediates because it allows us to establish a framework needed to consider the more complex two CTP intermediate kinetic model described below. Because triplet-triplet annihilation does not necessarily result in loss of both triplet states, we assumed in the kinetic model that one triplet of the pair survived the annihilation step and would be available to diffuse further or to be annihilated at a later time. This assumption affected the time constant of the annihilation process in the kinetic model, which is not critical to the analysis describe below. We used a nonlinear least squares fitting routine to fit the triplet absorption dynamics (kinetics trace in Figure 8a labeled ‘T1Tn’) and the persistent mid-IR absorption (kinetics trace labeled ‘mid-IR’) using a common set of parameters for both traces. As described in Section S11, this approach allowed us to determine the parameters of the kinetic model in a highly constrained



way, resulting in the fit parameters provided in

One CTP Intermediate Model

Table S5. The smooth curves overlaid on the data (circles) in Figure 8a show that the kinetic model provides an adequate description of both the

Abs. (Norm.)

(a)

From the kinetic modeling, we obtain the

Pop. Density

mid-IR absorption kinetics.

(c)

population dynamics of the singlet excitons S1S0, the CTPs 1(TT) and the separated triplets T1 + T1 that are represented in Figure 8b. The population

CTP separation

0.5

(b)

ultrafast visible T1Tn transition and the persistent

T1 Tn (520 nm)

1

mid-IR 2

T1 + T1 1

S1S0

S1S0

fission in crystalline films of TIPS-Pn revealed that the few picosecond rise of the amplitude of the T1  Tn transition results from the separation of CTPs.37-38, 54 The kinetic model demonstrates that the decay component of the persistent mid-IR absorption kinetics that appears on the few picosecond time scale can be described by the

(TT)

1(TT)

T1 + T1

T

Vibrational 0.1

dynamics are overlaid on the mid-IR absorption kinetics for comparison. Previous studies of singlet

1

0

Vibrational Pop. Dynamics

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

1

10

100

1000

Time (ps)

Figure 8. (a) Comparison of the fast decay of the broad mid-IR absorption on the sub-ps time scale with the rise of the triplet absorption feature (T1  Tn) at 520 nm. The few picosecond decay of the mid-IR absorption coincides with the slow rise of the T 1  Tn transition, which results from CTP separation. (b) A kinetic model involving one CTP intermediate was used to simultaneously fit the triplet absorption and mid-IR CTP signals to extract the underlying population dynamics, which are overlaid on the mid-IR kinetics. These are compared to the population dynamics in (c) from analysis of the vibrational features of the triplet and hot ground electronic states from Reference 49.

same CTP separation process. The plateau in the kinetic model that appears around the 10 to 100 ps time scale arises from the back reaction by which separated triplet excitons can fuse to reform CTPs. Furthermore, in Figure 8c, we reproduce the triplet formation dynamics obtained from a recent ultrafast vibrational spectroscopy study of singlet fission in crystalline TIPS-Pn films.49 The 3.5 ± 0.6 ps time constant describing the appearance of the alkyne stretch of triplets is in quantitative agreement with the 3.2 ± 0.9 ps rise of the T1  Tn transition and the decay of


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the persistent mid-IR absorption reported here. This comparison supports the conclusion that the decay of the persistent mid-IR absorption on the few picosecond time scale results from CTP separation. We considered whether the persistent mid-IR absorption on the 10 ps to 1 ns time scale could have arisen from an alternate source. The broad mid-IR absorption is reminiscent of transient absorption signatures in the near-IR that have been associated with excimers of perylene derivatives.53, 80-83 Excimers have been implicated as intermediates in the singlet fission reaction in pentacenes.71, 84-85 However, the fast rate of CTP formation and high yield of singlet fission in crystalline TIPS-Pn36,

75-76

indicates that the transient population of excimers would not be

sufficient to explain the persistent mid-IR absorption on the 10 ps to 1 ns time scale. Therefore, we rejected the presence of excimers as a plausible explanation for the persistent mid-IR signal. The kinetic modeling procedure revealed that it is essential to include the back reaction whereby triplet fusion can reform CTPs in order to describe the experimental data, particularly on the 10 to 100 ps time scale. We show in Figure S12 that the data cannot be adequately fit without inclusion of this back reaction. Because singlet fission is exergonic in TIPS-Pn, triplet fusion does not lead to reformation of singlet excitons, consistent with the lack of fluorescence emission in the crystalline TIPS-Pn film (Figure 6). These findings reveal that CTPs persist in the TIPS-Pn film for extended periods of time. Note that the rate of the back reaction describing the fusion of triplets to reform CTPs in the kinetic model is restricted by spin statistics to be one-third of the rate of the forward reaction. Referring to equation(1), this means that k2 

1

3

k2 .38 This restriction constrained the amplitude

of the CTP population on the 10 ps and longer time scale, which caused the deviation of the kinetic model in comparison to the persistent mid-IR absorption data around 10 ps as seen in Figure 8b. We found that loosening this requirement permitted an accurate fit of the mid-IR data on the 10 ps and longer time scale. However, this change of the kinetic model implied that triplet



fusion to form a net singlet CTP state had a higher probability than would be expected for randomly distributed spins. Such a model implied that the triplet fusion process involved some degree of ‘spin memory’ of the triplets, which would enable them to reform net singlet CTPs with higher probability on the 10 ps and longer time scale. To explore this possibility, we developed a second kinetic model involving two CTP Time (ps) -1

0

1

2

3

k1 S1S0   1 (TT )

k2 k2

1

(T ...T )

k3 k3

Borrowing assignments from Reference 37, the

Population Density

10 of singlet 10 fission 10 in10TIPS-Pn. 10 37 intermediates based on recent observations from near-IR studies 2

k4 T1  T1   S0*  T1 1

spin. The rate equations for this two-intermediate model are described in Section S10 and their application to fit the visible and mid-IR kinetics traces is described in Section S12. A similar nonlinear least squares fitting routine was used to simultaneously fit the T1  Tn triplet absorption kinetics trace and the persistent mid-IR absorption kinetics as represented in Figure 9a. Figure 9b depicts the population dynamics of the S1S0 singlet

A(a.u.)

non-interacting 1(T…T) CTPs that retain net singlet

(eqn 2)

Two CTP Intermediate Model T1Tn (520 nm)

1

0.5 0.5

CTP separation

mid-IR

(b) 0 2

A(a.u.)

model distinguished between interacting 1(TT) and

Abs. (Norm.)

(a) 0

0.2

Pop. Density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

S1S0 1

1(T…T)

0.1

T1 + T1

1

(TT)

0 0

0.1-1

10

10

101

1002

3 1000

10 Time10(ps) 10 10 Figure 9. (a) Best Time fit curves (ps)from the two CTP intermediate kinetic model overlaid on the persistent mid-IR absorption and T1  Tn kinetics traces. (b) The population dynamics of singlet excitons S1S0, two CTP 1

1

intermediates (TT) and (T…T) and fully separated triplets T1 + T1 are overlaid on the mid-IR absorption kinetics to show the improved quality of the fit.

exciton, the interacting 1(TT) and non-interaction (T…T) CTP intermediates, and the fully separated triplet states T1 + T1 obtained from the kinetic

1

modeling that are overlaid on the mid-IR absorption kinetics for comparison. The fit parameters corresponding to these population dynamics are summarized in Table S6. Similar to the oneintermediate kinetic model, this model predicts the persistence of CTPs into the 10 ps to 1 ns time scale, but the greater flexibility of the model allowed it to describe the amplitude of the CTP 20 ACS Paragon Plus Environment

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population on this time scale. Mathematically, this was simply the result of having more fitting parameters. But, the physical meaning underpinning the mathematics was significant. Both kinetic models therefore predicted that triplets dissociated from CTPs retained some memory of their original spins, allowing them to reform CTPs with higher probability than would be predicted from randomized spin statistics. Regardless of which kinetic model best describes the experimental data, the modeling procedure provides important insight about the time scale over which CTP intermediates persisted in the crystalline TIPS-Pn film, which extends into the 100 ps to 1 ns time scale. This finding indicates that excited state deactivation pathways that can compete with triplet harvesting or charge transfer56-57 may need to be controlled on time scales far longer than previously realized, even in molecular systems like TIPS-Pn. The duration of this time period may explain recent observations that subtle changes in packing structure comparing amorphous versus crystalline TIPS-Pn nanoparticles can have a significant influence on the overall yield of triplet separation.58 While the origin of the persistent mid-IR transition measured on the 1 ps and longer time scale can be assigned to CTP intermediates, the nature of the transition remains to clearly established. It is possible that it results from mixing of the S1 and multi-exciton71-73 states involved in the prompt mid-IR transition with the states of the CTP intermediates because both possess singlet character. Such mixing would allow the CTP intermediates to borrow oscillator strength from the bright transition from the S1 state. In this case, the absorption coefficient of the persistent mid-IR transition could be used to estimate the mixing of these states, and its time-dependence would be a direct probe of their separation, consistent with the comparison represented in Figure 8. If this assignment is correct, it implicates the one-intermediate mechanism, for it is difficult to explain why non-interacting CTPs would still exhibit such a mid-IR transition if they had already partially separated. In either case, elucidating the nature of this transition and comparison to the experimental findings reported here will likely provide new insight into the mechanism of triplet exciton separation from CTP intermediates. This is the subject of ongoing work. 21 ACS Paragon Plus Environment


Conclusions We used ultrafast mid-IR spectroscopy to directly probe the dynamics of correlated triplet pair intermediates that formed following singlet fission in TIPS-Pn through their broad electronic transitions in the mid-IR spectral region. The nature of the mid-IR electronic transition and its assignment were established by examination of isolated molecules of TIPS-Pn in solution. Singlet excited states exhibited broad mid-IR transitions whose decay lifetimes matched those determined through time-resolved fluorescence measurements. In films, this absorption was observed to decay on the sub-picosecond time scale that closely matched the rise of the T1  Tn triplet absorption feature in the visible spectral range. Following the decay of the singlet absorption in the mid-IR, a residual signal persisted into the 100 ps to 1 ns timescale, which was assigned to the absorption of CTP states that formed as the primary intermediates following singlet fission. This persistent mid-IR transient absorption revealed that CTP intermediates survived into the 100 ps to 1 ns time scale, even in the crystalline TIPS-pn film in which the primary formation of CTPs occurred on the sub-picosecond time scale. This finding suggested that relaxation processes that can compete with CTP separation may need to be controlled over time scales far longer than previously realized in materials like TIPS-Pn that undergo exergonic singlet fission.

Associated Content Supporting Information Detailed experimental methods, fluorescence spectra and kinetics in solution, fitting procedures, extinction coefficients, power dependence of decay processes, computational methods, kinetic analysis and modeling.

This material is available free of charge via the Internet at

http://pubs.acs.org.


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Author Information Corresponding Author J.B.A.: [email protected]

Acknowledgements C.G., A.R. and J.B.A. thank the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-SC0008120 for support of this work. C.G., A.R. and J.B.A. thank Lasse Jensen for helpful discussions. G.D.S. acknowledges the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-SC0015429. R.D.P. and G.D.S acknowledge partial support for this work through the Princeton Center for Complex Materials, a MRSEC supported by NSF Grant DMR 1420541. JEA and MMP thank the National Science Foundation (CMMI-1255494) for support of organic semiconductor synthesis. A.N.B. and N.C.G. were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-SC0012365.

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Direct Observation of Correlated Triplet Pair Dynamics During Singlet

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