Anion Photoelectron Spectroscopy of Rubrene: Molecular Insights into

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Anion Photoelectron Spectroscopy of Rubrene: Molecular Insights into Singlet Fission Energetics Hironori Tsunoyama† and Atsushi Nakajima*,†,‡ †

Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡ Keio Institute of Pure and Applied Sciences (KiPAS), Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *

ABSTRACT: Rubrene (C42H28, RUB) has been seen to be attractive as a promising building block for organic semiconductors. By means of gas-phase anion photoelectron spectroscopy, the adiabatic electron affinity for RUB molecules is determined to be 1.48 ± 0.03 eV, and the S0−T1 and S0−S1 transition energies of RUB are evaluated to be 1.16 ± 0.05 and 2.42 ± 0.05 eV, showing the possibility of singlet fission in terms of energy. The photoelectron spectra indicate that the vibronic coupling in RUB is similar in the neutral electronic states of S0, T1, and S1. Quantum chemistry calculation results demonstrate that the vibronic coupling in these states originates from their similarly restricted structural displacement upon photoexcitation. Molecular insights into energetics suggest the important role of a charge transfer state in singlet fission. triplet excitons,28−33 while the latter mediates the coupling between the singlet state and the TT state.34−39 The fundamental issue for analyzing the energetics for designing OFETs and PVCs is determining the experimental standard values for single molecules, such as ionization energy (Ei), electron affinity (EA), E(S1), and E(T1). In particular, for RUB, two isomeric forms are involved, which are the twistedTc backbone (D2) and the planar-Tc backbone (C2h),40 and the energetic criterion of the SF, E(S1) − 2 × E(T1) ≥ 0, is quite delicate, owing to it being close to zero. The Ei and EA are crucial in evaluating the performance of organic semiconductors, which are significantly affected not only by the electronic properties of each molecule but also by intermolecular interactions.41,42 Indeed, gas-phase spectroscopic measurements can concentrate on the former intrinsic property, enabling comparison with theoretical calculations; however, the EA of a single RUB molecule has not yet been experimentally determined and no simultaneous evaluation has been performed for E(S1) and E(T1), although gas-phase anion photoelectron spectroscopy (PES) could simultaneously provide this information on the basis of the relaxed selection rule from (RUB)− anions (D0) to the S1 and T1 states of the neutral. Along with these standard values, the high resolution of the gas-phase PES enables access to information on intramolecular structural rearrangements upon the photoexcitation

I. INTRODUCTION Organic semiconductors have attracted much attention due to their diverse industrial applications for flexible electronic devices, such as printed organic light emitting diodes and curved photovoltaic cells.1 Polyacenes, such as tetracene (Tc), pentacene (PEN), and rubrene (C42H28, 5,6,11,12-tetraphenylnaphthacene, RUB), have been one of the most widely studied organic semiconductors, because its thin film prepared by means of physical vapor deposition (PVD) has a well-ordered two-dimensional structure, exhibiting high field-effect hole mobility for the high performance of organic field-effect transistors (OFETs).2−5 As well as for OFETs, polyacenes are key organic materials for singlet fission (SF), in which a singlet excited state splits into two triplet states of about half the energy of the first excited singlet state (E(S1)).6 Having potential in extending the maximum conversion efficiency in conventional photovoltaic cells (PVCs),7−9 SF has been extensively investigated from viewpoints of photoelectric mechanism and material design involving polyacene derivatives 9−21 and related compounds.22−27 In the SF process, the energy of the generated triplet state (E(T1)) should satisfy the requirement of E(S1) − 2 × E(T1) ≥ 0, and the photoexcited molecule should electronically couple with neighboring molecules in the solid state. Regarding the mechanism of intermolecular SF, two intermediate states are involved: a correlated triplet pair (TT) state and a charge-transfer (CT) state. The former facilitates a spin-allowed transformation from a singlet exciton to two © XXXX American Chemical Society

Received: July 13, 2017 Revised: August 29, 2017 Published: August 29, 2017 A

DOI: 10.1021/acs.jpcc.7b06900 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C process starting from the anion D0 state, through vibrationresolved PES; electron detachments from the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) allow access to neutral S0 and S1/T1 states, respectively. Here we report the experimental determination of EA, E(S1), and E(T1) for RUB molecules by means of gas-phase anion PES. The PES indicate that the vibronic coupling upon the photoexcitation of (RUB)− is similar among the neutral electronic states of S0, T1, and S1, and that they show possible SF in terms of energy. Through the comparison of the experimental and computational results, detailed insights are provided for the geometric and electronic properties of RUB and its anion, unveiling the possible involvement of the CT state in the SF process.

Figure 1. Mass spectrum of rubrene measured immediately after sample heating, in which monomeric rubrene anions are accompanied by H2O complexes, and the number of H2O (m) is also shown. The inset shows the mass spectrum after successive production for 5 h.

II. EXPERIMENTAL METHODS Details of the experimental apparatus used have been described in previous papers.43−45 Briefly explained, the sample (without any purification processes) of RUB (purity 99%, purchased from Aldrich) was heated to 240 °C. The sample vapor was mixed with helium carrier gas at 50−80 atm. This mixture was pulsed out through an Even−Lavie valve46 into a vacuum chamber at a 10 Hz repetition rate to form a well-cooled molecular beam. The anionic species were produced by the attachment of slow secondary electrons generated by highenergy electron impact (∼200 eV) at the condensation zone in a free jet. They were then extracted to a linear time-of-flight (TOF) mass spectrometer, mass-selected, and photodetached using the second (532 nm, 2.33 eV), third (355 nm, 3.49 eV), and fourth (266 nm, 4.66 eV) harmonic outputs of another Nd3+:YAG laser. The kinetic energies of the detached photoelectrons were measured using a magnetic bottle-type photoelectron spectrometer. The electron binding energies were then obtained using the energy-conservation relationship hν = EBE + EKE, where hν, EBE, and EKE are the photon energy, electron binding energy, and electron kinetic energy, respectively. EKE was analyzed with TOF, and calibrated by the transition of Au− (2S1/2 ← 1S0).47,48 The photoelectron signal was typically cumulated for 20,000 laser shots.

at the mass of 532.7 u. The relative abundances of these byproducts were suppressed after the successive production for several hours (see inset in Figure 1). It is thought to be that the series of (RUB + 18m)− is (RUB)− with water molecule adducts. Taking into account how water contamination can be removed from RUB by vacuum sublimation, it is reasonable that the time dependent mass spectra of (RUB)− reflect the purification process of RUB through heating in the source. On the other hand, compared to PEN molecules, (PEN)− anions were accompanied by much more portions of byproducts, leading to +16 and +30 mass increase, which could be a result of oxidation. It is expected that both (PEN + 16)− and (PEN + 30)− are oxidized species of (PEN)−; in particular, the latter is assignable to the anion of 6,13pentacenequinone, which is a known major impurity of PEN under atmospheric conditions.54,55 In fact, the PES for (PEN + 30)− is identical to that for the 6,13-pentacenequinone anion. Considering that the 6,13-pentacenequinone contaminant is often contained in PEN,56 the impurity-free RUB may suggest that RUB is chemically more stable and easier to use than PEN as a practical organic material. B. Photoelectron Spectra of (RUB)−. Figure 2 shows the PES of (RUB)− anions measured with different wavelengths, and the spectra of (RUB)− exhibit an almost photon-energy-

III. COMPUTATIONAL METHODS Quantum chemistry computations were carried out for RUB at the B3LYP level49,50 using 6-311G(d) basis sets,51 implemented in the Gaussian 09 program package.52 A convergence criterion of 10−8 Hartree for the total energy was adopted for selfconsistent field calculations. Vibrational frequency analyses were performed at each stationary point during geometry optimizations, and the geometries were allowed to relax through vibrational modes with imaginary frequencies until no further imaginary frequencies were found. EA was calculated as the energy difference between the ground state of the anion and that of the neutral (zero-point vibration energies were corrected). Contour plots of molecular orbitals at an isosurface value of 0.04 were drawn using the Gabedit program.53 IV. RESULTS AND DISCUSSION A. Mass Spectra of Generated Anions. Figure 1 shows the mass spectra of generated anions, where the main peaks were assigned to (RUB)−. As seen in Figure 1, (RUB)− anions were accompanied by neighboring mass products, whose masses exhibit 18 mass intervals relative to that of (RUB)−,

Figure 2. Photoelectron spectra of rubrene anions taken with different wavelength photons. Final states and transition energies are denoted in the spectra. AD stands for autodetachment. B

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C. Computational Results and Comparison with Experiment. As reported for RUB neutrals in the gas phase and in the bulk, two isomeric forms exist: the twisted-Tc backbone (D2) and the planar-Tc backbone (C2h), where the twisted D2 isomer is more stable than the planar C2h.40 Similarly, as with the RUB neutrals, our DFT-based geometry optimizations obtained D2 and C2h structures for RUB anions, as shown in Figure 3 (see the Supporting Information for the

independent feature, although vibration progressions become less prominent at higher photon energies; this behavior originates from (1) lower energy resolution of the TOF electron analyzer, resulting from higher kinetic energies of the corresponding photoelectrons with higher photon energies, and (2) increased contributions of autodetachment processes through photoexcitation to high-lying excited states of the anions. In the following, each EA and vertical detachment energy (VDE) were taken from the spectra with the lowest possible photon energy, as photoelectron resolution is better at lower photon energies. The 532 nm spectrum of (RUB)− (Figure 2, top) has partially resolved vibrational bands, which allow for the strict determination of RUB’s adiabatic EA (1.48 ± 0.03 eV) from the 0−0 transition (S0(v′=0) ← D0(v″=0)). The VDE of (RUB)− is considered to be equivalent to the adiabatic EA, because the 0−0 transition is the most significant. Furthermore, another peak was observed in the 355 nm spectrum, while two other peaks were observed in the 266 nm spectrum, around 2.64 ± 0.04 and 3.90 ± 0.04 eV in electron binding energy; these photodetachments correspond to electronic excited states of neutral RUB. The former is a triplet state of T1, while the latter is a singlet state of S1. The difference between the two different 0−0 transition energies gives 1.16 ± 0.05 eV for the S0−T1 and 2.42 ± 0.05 eV for the S0−S1 transition energies of RUB. Indeed, these values are in line with those measured via phosphorescence (1.14−1.15 eV)56,57 and fluorescence (2.35− 2.36 eV),58−60 respectively. As described previously,61 the agreement between our experimental values and those in solvent/on interfaces indicates that the electronic properties of RUB are not largely modified by intermolecular and molecular−substrate interactions. Strictly speaking, the excited T1 and S1 states seem more favorably stabilized by the solvent/ interface compared to the ground S0 state, which results in redshift values of 0.01−0.02 eV for T1−S0 and 0.06−0.07 eV for S1−S0. Furthermore, the energy differences between S0−S1 and S0−T1 show that singlet fission (SF) is energetically possible, since a singlet excited state exothermically converts into two triplet states; hence, the energetic criterion of the SF, E(S1) − 2 × E(T1) ≥ 0, is satisfied. The energetics are similar to those of pentacene (S0−S1, 2.18 eV; S0−T1, 0.96 eV),55 which also exhibits SF.6 As shown in Figure 2, the electronic states consist of progressions of vibrational modes; the main peaks, assignable to S0, T1, and S1, are concurrent with two successive peaks of 0−1 and 0−2. The intervals between the peaks are ∼170 meV for S0 and T1, while they are ∼180 meV for S1. Since the values are very close to the calculated frequencies in each electronic state of ν150 (179 meV (1447 cm−1)) and ν150 (173 meV (1392 cm−1)) (see the Supporting Information) for a neutral RUB molecule in D2 symmetry, it can be assumed that the vibrational mode is excited upon photodetachement. In fact, these vibrational modes are along the structural displacement between the anion and the neutral, as discussed later on. Note that the similar vibrational profiles for the three states show that the structural displacements from the anionic ground states are similar: a similar vibronic coupling upon the photodetachment. In addition, the highest intensity of the 0− 0 transition for the three states of neutral RUB indicates small structural displacement upon photoexcitation, showing small vibronic coupling in the transition. In the following subsection, molecular-level descriptions for similar vibrational excitations are provided with the aid of computational methods.

Figure 3. Optimized structures of the (a) twisted Tc backbone (D2) and the (b) planar Tc backbone (C2h) isomers calculated with B3LYP/ 6-311G(d), where the D2 anion is 0.21 eV more stable than the C2h anion.

total energies and Cartesian coordinates). The D2 anion is 0.21 eV more stable than the C2h anion, and the EAs for the D2 and C2h isomers were calculated as 1.43 and 1.38 eV, respectively. The more stable D2 anion shows better agreement with the experimental EA value of 1.48 eV, which enables us to provide the following discussion on the basis of the DFT results. Figure 4 shows the Kohn−Sham HOMO and LUMO of RUB molecules. The HOMO and LUMO are mainly delocalized along the π orbitals of the Tc backbone. Upon the photodetachment of (RUB)−, electron detachment from the LUMO of RUB causes the structural displacement from the anionic D0 to neutral S0 states, while electron detachment from the HOMO of RUB causes the structural displacement from the anionic D0 states to the neutral T1 and S1 states. Displacements upon excitation are clearly identified in Figure 4c: the changes in C−C bond lengths are almost the same for D0−S1 and D0−T1 in contrast to the behavior for C−C bond length variation for D0−S0, in addition to the negligible changes for C−H bond lengths. As shown in Figure 2, all final states of S0, T1, and S1 exhibit intense 0−0 transitions concurrent with a few vibrational progressions, implying that the structural displacement is relatively small. Our current calculations consistently show small structural changes in the Tc backbone of RUB upon electron detachment (Figure 4c). Parts e, f, and g of Figure 4 show the Franck−Condon (FC) simulation for the S0 ← D0, T1 ← D0, and S1 ← D0 transitions from (RUB)− to RUB. The calculated transition probability of the 0−1 band relative to the 0−0 band is approximately one-half (p0−1/p0−0 = 0.5) for S0 ← D0, while those for T1 ← D0 (p0−1/p0−0 = 0.33) and S1 ← D0 (p0−1/p0−0 = 0.13) decrease. In addition, the other vibrational modes are simultaneously excited through the S0 ← D0 transition, whereas the FC factors dominate the ν150 mode for T1 ← D0 and S1 ← D0 transitions. A similar decrease in the intensity ratio between 0−0 and 0−1 transitions to the S0, T1, and S1 excited states is found in the experimental PES (Figure C

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Figure 4. Results for quantum chemistry calculations: HOMOs and LUMOs of RUB neutrals for the (a) twisted Tc backbone (D2) and the (b) planar Tc backbone (C2h). (c) Bond length changes of C−C and C−H upon excess electron injection from twisted RUB anion (D2), where the positions of C and H atoms are indicated as bond indexes of 1−11 and 1′−5′ in the molecular framework in part d. Franck−Condon simulation for (e) the S0 ← D0, (f) T1 ← D0, and (g) S1 ← D0 transitions of (RUB)−, in which Gaussian-broadened spectra are drawn with a full width at halfmaximum of 0.04 eV.

2). Moreover, small structural displacement between the neutral RUB and the ionized RUB+ has been similarly seen in the PES for the photoionization process for RUB neutral molecules.62 The small structural displacement (vibronic coupling) upon the detachment and attachment of an electron implies that the solid-state RUB favors hole and electron mobility,63 since hole and electron transports are detached from structural disordering; small structural displacements upon charging cause smaller reorganization energies due to negligible structural rearrangements between the two charged states. Figure 5 summarizes the energy differences of electronic states for twisted D2 and planar C2h structures for RUB, together with another theoretical result.21 In all electronic states, the D2 isomer is calculated to be more stable than the C2h isomer. In fact, the PES for (RUB)− consistently shows that there is no state inversion between twisted D2 and planar C2h structures, considering that the state inversion would result in spectral broadening concurrent with the vibrational progressions of torsional vibrational modes of the Tc backbone (100− 200 cm−1) (see the Supporting Information). Although all calculated values are underestimated compared with the experimental ones, the experimental results for RUB molecules guarantee in satisfying an energetic constraint for the SF, since the S1 state splits into two T1 states within the excitation energy from the S0 state to the S 1 state. Furthermore, the experimentally obtained EA value allows for a revision of the estimated CT energy, E(CT), in the bulk (see the Supporting

Figure 5. Energy level diagrams of D0 (anion) and S0, T1, and S1 (neutral) for the rubrene molecule in two isomeric forms of twisted Tc and planar Tc backbones. Calculated values with B3LYP/6-311G(d) are shown in eV for the twisted Tc backbone (D2; left side) and the planar Tc backbone (C2h; right side) with the experimental values (center). Calculated energy differences (ΔE) between the two isomers in each state are also shown. Calculated values in square parentheses [ ] show calculations with ωB97/cc-pVDZ.21 Numbers in normal parentheses ( ) show uncertainties in experimental values: 1.48(3) represents 1.48 ± 0.03.

D

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The Journal of Physical Chemistry C Information); our E(CT) estimation is 1.96 eV using the calculated Coulombic interaction (−0.71 eV)21 of molecular ion pairs between (RUB)+@(RUB)n and (RUB)−@(RUB)n, which stand for positive and negative polarons consisting of an ion core and surrounding molecules, respectively. Our revised estimation is smaller than previously calculated (2.66 eV),21 implying that the CT states may largely be contributing to the SF process. However, in the solid state, it is important to take into account the electronic coupling between neighboring RUB molecules as well as the effect of geometrical packing.40 In fact, it has been recently reported that a symmetry-breaking intermolecular mode plays an important role in electronic coupling between S1 and TT.33 Therefore, as demonstrated with other oligoacene clusters,43−45,64 size-dependent investigation on RUB clusters may help to bridge the gap between the single molecule and the bulk state, where photoelectron spectroscopy for RUB cluster anions would provide systematic information on the evolution of the electronic states.

Nanocluster Assembly Project of Japan Science and Technology Agency (JST) in 2009−2016, and also is partly supported by JSPS KAKENHI of Scientific Research (A) Grant Number 15H02002.

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V. CONCLUSIONS We have conducted anion photoelectron spectroscopy for the RUB molecule and have precisely estimated the adiabatic EA of 1.48 ± 0.03 eV. We have also defined the S0−T1 and S0−S1 transition energies of RUB as 1.16 ± 0.05 and 2.42 ± 0.05 eV, respectively, and these values show importance in increasing the theoretical accuracy of designing relevant derivatives. The vibration-resolved PES indicates that the equilibrium structures in the neutral S0, T1, and S1 states are similar among each other, which implies similar vibronic coupling in the anion to neutral transition. The DFT-based calculation results demonstrate that the vibronic coupling originates from the similar, small structural displacement upon electron injection to RUB. The current characterization of the single RUB molecule will help in the understanding of the electronic behaviors of organic semiconductors in bulk and thin films, particularly emphasizing the role of a CT state in SF.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06900. Full citation of ref 52, calculated vibrational modes of 1436 and 1470 cm−1 (Figure S1), calculated torsional vibrations of 100−200 cm−1 (Figure S2), scheme for E(CT) (Figure S3), and total energies and Cartesian coordinates (Table S1) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-45-566-1697. ORCID

Hironori Tsunoyama: 0000-0002-0332-5324 Atsushi Nakajima: 0000-0003-2650-5608 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Mr. Y. Kitade and Dr. N. Ando for their initial experimental contributions. This work is partly supported by a program entitled “Exploratory Research for Advanced Technology (ERATO)” as Nakajima Designer E

DOI: 10.1021/acs.jpcc.7b06900 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b06900 J. Phys. Chem. C XXXX, XXX, XXX−XXX