Multiexciton Dynamics Depending on Intramolecular Orientations in

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Spectroscopy and Photochemistry; General Theory

Multiexciton Dynamics Depending on Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs Hayato Sakai, Ryutaro Inaya, Hiroki Nagashima, Shunta Nakamura, Yasuhiro Kobori, Nikolai V. Tkachenko, and Taku Hasobe J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01184 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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The Journal of Physical Chemistry Letters

Multiexciton

Dynamics

Depending

on

Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs Hayato Sakai,*,a Ryutaro Inaya,a Hiroki Nagashima,b Shunta Nakamura,a Yasuhiro Kobori*,b,c Nikolai V. Tkachenko,*,d and Taku Hasobe*,a a

Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama, 223-8522, Japan

b

Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

c

Department of Chemistry, Graduate School of Science, Kobe University, Kobe 657–8501, Japan d

Laboratory of Chemistry and Bioengineering, Tampere University of Technology, 33720 Tampere, Finland

ACS Paragon Plus Environment

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ABSTRACT Pentacene dimers bridged by a phenylene at ortho and meta positions [denoted as o-(Pc)2 and m(Pc)2] were synthesized to examine intramolecular orientation-dependent multiexciton dynamics, especially focusing on the singlet fission (SF) and recombination from correlated triplet pairs (TT). Absorption and electrochemical measurements indicated strong intramolecular couplings of o-(Pc)2 relative to m-(Pc)2. Femtosecond and nanosecond transient absorption measurements successfully demonstrated the efficient SF in both dimers. In contrast, the dissociation process from the (TT) to the individual triplets [(2 × T)] was clearly observed in m-(Pc)2, which is in sharp contrast to a major recombination process in o-(Pc)2. Time-resolved electron spin resonance (TR-ESR) measurements demonstrated that the recombination and the dissociation proceed from quintet state of 5(TT) in m-(Pc)2. Rate constant of the SF was two orders of magnitude greater in o-(Pc)2 than that in m-(Pc)2 and were rationalized by enhanced electronic coupling between adjacent HOMOs of the Pc units.

Table of Content


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Singlet fission (SF), which involves the ultrafast splitting process of a singlet exciton into two triplet excitons with spin conservation, is an attractive and promising way to overcome the Shockley–Queisser limit on the efficiency of single-junction photovoltaics.1-3 Efficient SF requires the energy level matching between the energy of the lowest-lying singlet excited state E(S1) and energy of two triplet excited states 2E(T) [that is, E(S) ≥ 2E(T)]. SF populates correlated triplet pairs [(TT)] from a singlet excited state, which subsequently dissociates to generate individual triplet states [(2 × T)]. As the reverse process from [(TT)], triplet-triplet annihilation (TTA) inhibits to generate (2 × T) because of the competitive reaction with the dissociation process of (TT). Therefore, new methodologies to control photophysical processes from (TT) are definitely required for utilization of generated triplet excitons through high-yield SF. Pentacene is one of representative molecules to meet the above requirement as compared to other molecules. High triplet quantum yields through quantitative SF utilizing covalently linked molecular dyads4-13 and the related theoretical discussions14-16 were recently observed in addition to the occurrence of SF in crystalline and aggregate states of organic molecules.17-29 Moreover, the dissociation process from exchanged-coupled triplet pairs (i.e., quintet spin state [5(TT)]) to individual triplet states (2 × T) was observed by time-resolved electron spin resonance (TR-ESR).10,

30-32

However, there is no example regarding the evaluation of

intramolecular orientation-dependent multiexciton dynamics in covalently linked molecular dimers, which is associated with the dissociation and recombination from correlated triplet pairs (TT). In particular, no experimental studies have been performed to specify how the transferintegrals between the chromophores play roles in the efficient generations of 2 × T with inhibiting the energy-wasting TTA processes.


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In this study, we arranged and synthesized a series of pentacene dimers bridged by a phenylene linker at ortho and meta-positions [denoted as o-(Pc)2 and m-(Pc)2] to examine the multiexciton dynamics between recombination and dissociation from correlated triplet pairs (TT) generated by SF in pentacene dimers (Figure 1). Absorption and electrochemical measurements qualitatively suggested the strong intramoleclar chromophore coupling of o-(Pc)2 relative to m(Pc)2. Femtosecond and nanosecond transient absorption measurements combined with timeresolved electron spin resonance (TR-ESR) measurements quantitatively revealed the SFmediated multiexciton dynamics in these dimers.

Figure 1. Chemical structures of pentacene dimers and a reference compound in this study.

The detail synthetic procedure and characterization of o-(Pc)2 are shown in the experimental section and Scheme S1 and Figures S1-S3 in Supporting Information (SI). The synthetic details of m-(Pc)2 were previously reported by our group.7 To investigate the electronic interactions between two pentacene units via a phenylene linker, the cyclic voltammetry (CV) was employed in CH2Cl2 (Figure 2). The details concerning the oxidation and reduction potentials including PcRef are summarized in Table 1. The split and successive oxidation peaks of o-(Pc)2 (0.76 V and 0.87 V vs. SCE) are observed as shown by the horizontal arrow in Figure 2a. The splitting


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energy of 0.11 eV of o-(Pc)2 is interpreted to be caused by the electronic coupling between adjacent HOMOm levels where the subscript “m” represents the TIPS-pentacene (Pc) monomer unit. Alternately, this splitting would be associated with the inequivalent energy levels in HOMOms. This may occur by internal rotations of the bulky TIPS groups near the adjacent Pc in o-(PC)2 as indicated by Figure 3a, denoting that HOMOm is affected by the adjacent TIPS group with a strong electronic interaction. (See: temperature-dependent 1H NMR as show in Figures S4-S6 in SI). However, no apparent oxidation peak splitting is seen in m-(Pc)2 at 0.85 V vs. SCE in Figure 2b, which potential is approximately similar to that of Pc-Ref (0.86 V vs SCE) in Figure 2c,7 although a slight peak broadening is observed in comparison to the corresponding line shape of the oxidation peak in Pc-Ref. This indicates the significant attenuation of the intramolecular coupling in m-(Pc)2 on the HOMOm levels. It should also be noted that the integrated area under anodic surface wave of o-(Pc)2 is approximately similar to that of m-(Pc)2. Contrary to the above-mentioned oxidation processes, the voltammograms for the reductions of o-(Pc)2 and m-(Pc)2 exhibit no splittings as shown in Figures 2a and 2b, which are attributable to the first and second reduction peaks of Pc-Ref (Figure 2c). Thus, the significantly smaller coupling in the LUMOms between the monomers are conclusive both in the o-(Pc)2 and m-(Pc)2 dimers than the HOMOm coupling to result in the splitting of 0.11 eV and less, respectively. Based on the above results, we can conclude the stronger intramolcular chromophore interactions of o-(Pc)2 relative to m-(Pc)2. Additionally, the split and broader absorption bands of o-(Pc)2 and m-(Pc)2 in the range from 300 to 350 nm relative to Pc-Ref are due to the dipole-dipole interactions between two Pc chromophores (Figures S7 and S8 in SI).


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0.11 V

Figure 2. Cyclic voltammograms of (a) o-(Pc)2 (red line), (b) m-(Pc)2 (blue line) and (c) Pc-Ref (black line) in CH2Cl2 with 0.1 M nBu4NPF6 as supporting electrolyte. Scan rate: 100 mV·s–1. Table 1. Summarized Oxidation and Reduction Potentials of Pentacene Derivatives. Ered1 (V vs SCE)

Ered2 (V vs SCE)

Eox1 (V vs SCE)

Eox2 (V vs SCE)

o-(Pc) 2

–1.09

–1.59

0.76

0.87

m-(Pc)2

–1.03

–1.46

0.85

–

Pc-Ref

–1.07

–1.51

0.86

–

The splitting energies (ΔDS) derived from Davydov splitting were estimated by the absorption spectra (o-(Pc)2 : 2000 cm–1 and m-(Pc)2 : 1130 cm–1 (Table S1 in SI)).33, 34 This trend is coincided


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with the electrochemical measurements. The above characterizations of the single ΔDS values indicate the existences of single conformers in the corresponding dimers although the broader absorptions in Figure S8A in SI would be associated with inhomogeneous broadening by the internal rotations of the bulky TIPS groups in o-(Pc)2.

Figure 3. Optimized structures of (A) o-(Pc)2 and (B) m-(Pc)2 calculated by DFT methods atω B97XD/DGTZVP level of theory. The center-to-center distance (red) and torsion angle (blue) between two pentacene units are shown. To obtain optimized structures of o-(Pc)2 and m-(Pc)2, the density functional theory (DFT) calculations were performed with Gaussian 09 program at the ωB97XD/DGTZVP level of theory (Figure 3 and Figures S9 and S10 in SI).7, 14, 15, 35-38 As schematically shown in Figure 3 and Figures S9 and S10 in SI, the torsion angles between two long axes of pentacenes in o-(Pc)2 and m-(Pc)2 were found to be 56° and 143°, respectively. The center-to-center distance between


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two pentacene units in o-(Pc)2 (ca. 7.50 Å) is much smaller than that in m-(Pc)2 (ca. 16.8 Å). The smaller distance may contribute to the strong interaction of o-(Pc)2. Additionally, the energies and molecular orbitals (MOs) of o-(Pc)2 and m-(Pc)2 were also estimated (Figures S9 and S10 in SI). To examine the orientation-dependent singlet fission processes of o-(Pc)2 and m-(Pc)2, transient absorption measurements were performed in toluene solution by a 600 nm femtosecond laser pulse (full width at half maximum, fwhm = 100 fs). The steady-state absorption and fluorescence spectra are shown in Figure S7 in SI. First, we determined the triplet-triplet (T–T) absorption spectra of these pentacene dimers by the energy transfer from anthracenes to these dimers (the detail calculation process and Figures S11 and S12 in SI) in toluene. Based on the results, the molar absorption coefficients (εT-T) were determined to be 156,000 M–1 cm–1 at 517 nm for o-(Pc)2 and 188,000 M–1 cm–1 at 526 nm for m-(Pc)2. The singlet-singlet (S–S) absorption spectra of pentacene dimers were separately assigned from the S-S absorption spectrum of PcRef (Figure S13 in SI). Figure 4A shows the femtosecond transient absorption (fsTA) spectra and time profiles of o-(Pc)2 in toluene. In transient spectra, the S–S absorption spectra were observed after irradiating by a pump laser light. After a few picoseconds, the characteristic T–T absorption band derived from the pentacene units at 520 nm develops, whereas the S–S absorption bands at 470 and 555 nm decay within ca. 15 ps. Therefore, in o-(Pc)2, ultrafast intramolecular SF occurred due to the strong intramolecular chromophore coupling. The photophysical behavior of m-(Pc)2 is in sharp contrast with that of o-(Pc)2. Namely, the transient spectra and corresponding time profile at 520 nm (Figure 4B) indicated the much slower SF process as compared to o-(Pc)2. The SF rate constants (kSF) were determined to be 1.2 × 1011 s–1 for o-(Pc)2 and 2.1 × 109 s–1 for m-(Pc)2 by fitting to decay curves corresponding to the T-T


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absorption band (520 nm) using multi-exponential fits and assigning the formation time constant to the SF. Finally, the triplet quantum yields of (TT) in these dimers (Φ(TT)) were quantitatively calculated to be 99% for o-(Pc)2 and 94% for m-(Pc)2, respectively (see: the calculation processes in SI). It should be noted that a ‘(TT)’ pair is regarded as a pair of two correlated triplet excitons (i.e., theoretical maximum value of Φ(TT) : 100%).

Figure 4. fsTA spectra (left) and corresponding time profiles (right) of (A) o-(Pc)2 and (B) m(Pc)2 in toluene. The excitation wavelength was 600 nm.


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Figure 5. nsTA spectra (left) and corresponding time profile (right) of (A) o-(Pc)2 and (B) m(Pc)2. The measurements were performed in toluene and excitation wavelength was 600 nm. Next, to study the deactivation process from (TT) to the ground state, nanosecond transient absorption (nsTA) measurements were performed by a 600 nm picosecond laser pulse (full width at half maximum, fwhm = 25 ps) (Figure 5). In both dimers, we observed only decay processes of T-T absorption of pentacenes with increasing time. However, two different lifetime


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components were seen in the time profiles. For example, in o-(Pc)2, the relative ratio of initial and faster species (τT1 = 0.113 μs, 90%) is much larger than that of second and longer one (τT2 = 23.0 μs, 10%). This trend is in sharp contrast with that of m-(Pc)2 because the short lifetime component (τT1 = 0.170 μs, 44%) is smaller than that of longer one (τT2 = 20.0 μs, 56%). The initial and second components should be responsible for the correlated triplet (TT) and individual triplet states (2 × T), respectively. Consequently, we also evaluated apparent quantum yields of individual triplet states (ΦT) by the second component in the nsTA spectra. Here we supposed that the εT-T values in (TT) states are approximately to the same as those in (2 × T) states. Based on the εT-T values of these dimers, the yields of ΦT in o-(Pc)2 and m-(Pc)2 were estimated to be 20% and 105%, respectively. Noted that these apparent values may include the individual triplet species (2 × T) mediated by the competitive process between TTA and SF. It is quite difficult to differentiate the two triplet states such as triplet pairs (TT) from the individual triplet states (2 × T) by the transient absorption measurements although the recombination or dissociation processes from triplet pairs (TT) proceeds. To exactly assign the triplet species in the secondary component, time-resolved electron spin resonance (TR-ESR) measurements were performed as shown in Figure 6A and 6B for o-(Pc)2 and m-(Pc)2, respectively. TR-ESR spectra of these dimers were obtained in toluene at 77 K following excitation with a 532 nm laser pulse (pulse width: 5 ns). In m-(Pc)2, the TR-ESR spectra (Figure 6B) showed two components around the center field (336 mT) for g = 2; 1) narrow spectra (inner component) with the peak splitting of ~15 mT as indicated by the blue 5(TT) and 2) the outer spectrum components possessing the 41 mT separations as indicated by the red 3T. To assign these two components, the spin nutation measurements were performed, as shown in Figure 6C.


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Figure 6. (A, B) TR-ESR spectra of (A) o-(Tc)2 and (B) m-(Tc)2 in toluene at 77 K. (C) Fourier transformed spin nutation spectra of m-(Tc)2. Inset shows the spin echo intensities measured as a function of initial microwave pulse length T in the T - τ - π/2 - τ - π -τ - echo sequences for which the field strengths are indicated by the arrows in panel A. (D) Upper: Time profiles of the TR-ESR signals of the quintet state at 341 mT in o-(Tc)2. The field position is indicated by the arrow in panel A. Lower: Time profile of the quintet state (blue) at B0 = 341 mT and the triplet state (red) at 355 mT, as indicated by the arrow in panel B for m-(Tc)2. Experimental results and fittings are shown by the scatters and the lines, respectively.

The ratio of the nutation frequencies of the inner component (19.5 MHz) to the outer component (11.7 MHz) is 1.7 ≈ √3. Thus, the inner and outer components were assigned to the sublevel


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transitions of ms = 0 −> ±1 in the quintet state (S = 2) and the transitions of ms = 0 −> ±1 in the dissociated (2 × T) triplet (S = 1), respectively, as consistent to the previous reports.10, 30-32 Therefore, the TR-ESR spectra suggest that the initially formed 1(TT) mixes with 5(TT) in the ms = 0 sublevel in the presence of the magnetic field, resulting in the E/A spin polarization pattern and that 5(TT) is followed by the dissociations to (2 × T).30 In o-(Pc)2 on the other hand (Figure 6A), only the inner component corresponding to 5(TT) was observed at the early delay times. Also, this component quickly disappeared within 1000 ns, while both of 5(TT) and 3T were longlived in m-(Pc)2. Delay time dependences of the 5(TT) and 3T components are displayed in Figure 6D at the magnetic field strengths of 341 mT and of 355 mT, respectively. In both dimers, 5(TT) (blue traces) appeared at 200 ns and disappeared with increasing time. The individual triplet states (2 × T) were observed only in m-(Pc)2, as shown by the red trace. The 5(TT) decay is followed by the generation of a pair of independent triplet states [(2 × T)] from the blue traces as shown in the lower traces in Figure 6D. Based on the above results, it is concluded that the dissociation process to the individual triplets (2 × T) took place from the quintet state 5(TT) in m-(Pc)2, whereas the dissociation did not occur in o-(Pc)2, indicating that the recombination to the ground state dominates the quintet decay. We analyzed the time profiles (solid lines in Figure 6D), by solving coupled rate equations taking into account kinetics of 1) the separation of 5(TT) into (2 × T) with the rate constant of kDISS, 2) the subsequent back-reaction (ktq) from (2 × T) to 5(TT), and 3) the geminate recombination processes of 5(TT) –> S0S0 represented by the rate constant of kREC, as detailed in the SI. The resultant recombination rate kREC = 2.8 × 106 s–1 in o-(Pc)2 is larger than kREC = 2.5 × 105 s–1 in m-(Pc)2 and is coincident with the initial decay traces of the triplet transient


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absorptions of o-(Pc)2 and m-(Pc)2 in Figure 5, demonstrating that the quintet state 5(TT) plays an important role both for the recombination and dissociation processes from (TT). The electronic interactions are anticipated to be stronger both in the HOMOm−HOMOm’ and LUMOm−LUMOm’ couplings (THH and TLL, respectively) between the monomers in o-(Pc)2 than those in m-(Pc)2 from Figure 3. According to the charge-transfer (CT) mediation mechanism for the SF process, the electronic couplings (V) are described for the reactions of S1S0 −> TT and S1S0

Multiexciton Dynamics Depending on Intramolecular Orientations in

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