Two-Photon Absorption in Pentacene Dimers: The Importance of the

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Two-Photon Absorption in Pentacene Dimers: The Importance of the Spacer Using Upconversion as an Indirect Route to Singlet Fission Eleonora Garoni,†,‡ Johannes Zirzlmeier,§ Bettina S. Basel,§ Constantin Hetzer,∥ Kenji Kamada,*,‡ Dirk M. Guldi,*,§ and Rik R. Tykwinski*,∥,⊥ †

Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, Milan 20133, Italy IFMRI, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan § Department für Chemie und Pharmazie & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstrasse 3, 91058 Erlangen, Germany ∥ Department für Chemie und Pharmazie & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Henkestrasse 42, 91054 Erlangen, Germany ⊥ Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada ‡

S Supporting Information *

Recently, acenes, in general, and acene dimers, in particular, have been at the focal point of studies concerning fundamental aspects of SF.13−18 Use of dimeric acene systems is motivated by a number of incentives. On one hand, the intervening spacer can be used to place the pentacene chromophores in specific geometric orientations. On the other hand, coupling between the acenes can be modulated by varying the nature of the spacer, i.e., conjugated, nonconjugated, or cross-conjugated. Considering the fixed proximity between the tethered acenes, SF proceeds in an intramolecular fashion (iSF) in dilute solutions, which eliminates the needs for concentrated solutions or crystalline materials that are required for intermolecular SF. In this study, we show that iSF is initiated from a singlet excited state accessed by two-photon absorption (TPA), rather than through a traditional route of direct one-photon absorption (OPA).19 In a proof of concept, iSF in pentacene dimers 2 and 3 (Figure 1) is enabled through NIR irradiation at 775 nm, a wavelength where neither dimer exhibits linear absorption of light. Analyses of 2 and 3 using the Z-scan and

ABSTRACT: In this proof of concept study, we show that intramolecular singlet fission (iSF) can be initiated from a singlet excited state accessed by two-photon absorption, rather than through a traditional route of direct one-photon excitation (OPE). Thus, iSF in pentacene dimers 2 and 3 is enabled through NIR irradiation at 775 nm, a wavelength where neither dimer exhibits linear absorption of light. The adamantyl and meta-phenylene spacers 2 and 3, respectively, are designed to feature superimposable geometries, which establishes that the electronic coupling between the two pentacenes is the significant structural feature that dictates iSF efficiency.

B

y virtue of their ability to control and manipulate light, nonlinear optical (NLO) materials have inspired chemists, physicists, and materials scientists for decades. Early work often focused on frequency doubling and tripling, or on electro-optic effects including light induced changes in refractive index and absorption.1−3 More recent efforts target, for example, optical limiting or two-photon absorption/emission.4−6 NLO effects in molecular materials have been shown to primary depend on the extension of π-conjugation,7 the electron distribution in molecules with electron donor and/or acceptor substitution,8 and open-shell electronic structure.9 In spite of the fact that pentacene and its derivatives feature an extended π-conjugated framework, studies exploring their potential as NLO materials are rather scarce, mainly due to a susceptibility to photo-oxidation.10 The two-photon absorption (TPA) behavior of TIPS-pentacene (1a) has been described giving rise to, however, rather poor performances.9 Using films of crystalline pentacene, Zhang and Xiao demonstrate significant enhancement of optical nonlinearities induced by singlet fission (SF),11 as measured by the optical Kerr effect (OKE). Zhao et al., on the other hand, have reported that derivatives of pentacene and azapentacene exhibit strong reverse saturable absorption (at 532 nm); the overall performance rivals that of graphene.12 © 2017 American Chemical Society

Figure 1. Molecular structures of derivatives discussed in this study; inset shows a structural overlay of spacer units (phenylene linker in blue, adamantyl linker in orange).20 Received: August 4, 2017 Published: September 15, 2017 14017

DOI: 10.1021/jacs.7b08287 J. Am. Chem. Soc. 2017, 139, 14017−14020

Communication

Journal of the American Chemical Society

All three TPA spectra show similar features including vibrational fine structure in the range of 760 to 900 nm. A comparison with the corresponding OPA spectra (Figures 2a and S5−S7)21 reveals good correlation between the TPA bands and the 1Lb bands of OPA in Platt’s notation.22 Notable discrepancies include a 0.02−0.07 eV blue shift relative to the corresponding 0−0 transitions and features at around 730−740 and >900 nm, which are absent in the OPA spectra of 1b and 2. Only 3 shows an OPA transition at 368 nm that matches the corresponding TPA transition at 742 nm. For dimer 2, the magnitude of the TPA cross section is nearly twice of that found for 1b. From this, it can be concluded that the two pentacenes of 2 are spectroscopically independent. For dimer 3, TPA is slightly larger than for 2, and the peaks at 810 and 870 nm are broader than in 2, likely the result of electronic communication mediated by the cross-conjugated23 meta-phenylene linkage. The TPA cross section (σ(2)) of 1b at 870 nm is 42 ± 5 GM (1 GM = 10−50 cm4 s molecule−1 photon−1), in good agreement with previous measurements with 1a.9 The corresponding absorption bands for 2 and 3 at 870 nm show σ(2) of 75 ± 13 and 110 ± 15 GM, respectively. Overall, these σ(2)-values are rather small, when compared to the values reported for molecular building blocks of similar molecular weight.24 Photoexcitation at 610 and 775 nm was used for pump− probe experiments with 1b, 2, and 3 (in THF), which corresponds to on- and off-resonant conditions, respectively, based on the pentacene ground-state absorptions (Figure 2a). With photoexcitation at 610 nm, on-resonance experiments into the pentacene ground state are in sound agreement with recent investigations (Figure 3D−F). For 1b, the differential absorption spectra corroborate intersystem crossing of the singlet excited state (S1) with a lifetime of 15.1 ns to afford the

pump−probe techniques corroborate that iSF results from nonlinear absorptions, whereas “monomer” 1b does not show SF under analogous conditions. The adamantyl and metaphenylene spacers of 2 and 3, respectively, are designed such that their overall geometry is superimposable. Thus, the electronic coupling between the two pentacenes is the only significant difference between the two dimers (Figure 1, inset). TPA spectra of 1b, 2, and 3 have been measured in tetrahydrofuran (THF) between 720 to 1050 nm (Figure 2b).

Figure 2. (a) OPA spectra of 1b (orange), 2 (green), and 3 (blue), and (b) TPA spectra of 1b (filled/open circles), 2 (filled/open squares), and 3 (filled/open triangles); in THF. Data shown with filled and open symbols are measured by different procedures (see Experimental Section).

Figure 3. Differential absorption changes obtained upon femtosecond pump−probe experiments at 775 nm with 8 μJ per pulse excitation for 1b (A), 2 (B), and 3 (C), and at 610 nm with 400 nJ per pulse excitation for 1b (D), 2 (E), and 3 (F); all measurements in argon-saturated THF at room temperature with time delays between 0 and 5500 ps.25 14018

DOI: 10.1021/jacs.7b08287 J. Am. Chem. Soc. 2017, 139, 14017−14020

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Journal of the American Chemical Society triplet excited state (T1) with a lifetime of 41.2 μs (Figures S8 and S9).13 Fingerprint absorptions of the singlet and triplet excited state are identified at 447 and 507 nm, as well as 466 and 499 nm, respectively. In the more strongly coupled phenylene-linked dimer 3 (in comparison to 2), slow intersystem crossing is replaced by a biphasic process corresponding to iSF. The singlet excited state (S1S0) maxima at 448 and 506 nm convert into the triplet excited state 1(T1T1) maxima at 469 and 501 nm with lifetimes of 26.7 and 117.8 ps, respectively, in reasonable agreement with our earlier work (Figure S11).15 The (S1S0) to 1(T1T1) conversion is much slower (818.9 ps) in the weakly coupled, adamantyl-linked dimer 2, which is slightly slower than that reported for the (S1S0) to 1(T1T1) transition in benzonitrile (Figure S10).13 By virtue of weak electronic couplings between the two pentacene moieties in 2, sequential population of 5(T1T1) and (T1 + T1) follows and precedes the ground state recovery, which lasts several tens of microseconds.26 Off-resonant photoexcitation experiments at 775 nm were then explored (Figure 3A−C). Comparison of the spectra from off- and on-resonant excitation shows that analogous singlet excited-state characteristics are found for 1b (S1), as well as for 2 and 3 (S1S0). The singlet excited state maxima [(S1) and (S1S0)] centered at ca. 450 and 505 nm are the most notable. From these comparisons, it is concluded that TPA must be operative en route to formation of the singlet excited state for 1b, 2, and 3. Independent confirmation of TPA is provided by the quadratic dependences of laser power measurements, while monitoring the differential absorption changes, for example, at 450 nm stemming from the singlet excited states as shown in Figures 4 and S12. It is noteworthy that the overall yields of

To assign the TPA peaks and the excited-states promoted by the off-resonant excitation in the pump−probe experiments, ab initio molecular orbital calculations at the TDA-CAM-B3LYP// 6-31+G(d) level in THF with a polarization continuum model (PCM)27 have been done for model compounds 1b′−3′ (to reduce computational costs iBu were replaced by Me groups). For 1b′, S1 is an OPA-allowed state with B2u (1La) symmetry followed by TPA-allowed S2 (B1g or 1Ca) and OPA-allowed S3 (B3u or 1Lb), which are nearly degenerate (3.40 and 3.45 eV, respectively, Table S1). The symmetry assignments of the OPA bands agree with those previously determined for pentacene,28 and the calculated excitation energies (2.23 eV for S1 and 3.45 eV for S3) are analogous to those determined for pentacenes (2.15 eV for S1 and 3.22 eV for S3) using the same approximation method (TDA) but with a slightly different level of theory (B3LYP).29 The TPA transition is therefore assigned to the TPA-allowed S2 state (see the spectral simulation in Figure S16).30 The overlap of the transition energies of the OPA peak (440 nm) and the TPA peak (870 nm) can be explained by the nearly degenerate features of TPAallowed S2 and OPA-allowed S3 states. The TPA transition to S2 consists of the sum of the possible transition paths (S0 → i → S2), where i is the intermediate state. The largest contribution derives from i = S1 because the transition energy to S1 is the closest to the photon energy of excitation, based on the Sum-Over-State model.31 Nevertheless, the magnitude of the S0 → S1 → S2 path is small because the angle between the transition dipole moments of the TPA transition (M01 (S0 → S1) and M12 (S1 → S2)) are orthogonal (90.2°), irrespective to their moderate magnitudes (|M01| = 8.0 D and |M12| = 11.7 D). Similar conclusions are reached for 2′ and 3′ (Table S2). From the orbital correlation of the dimers to the monomer (Figure S15), S5 and S6 are assigned as degenerate TPA-allowed states. Simulated TPA spectra for 2′ and 3′, decomposed by the destination states of their TPA transitions, show significant amplitude for the components to S5 and S6 (Figure S16). In conclusion, TPA (upconversion) has been used, for the first time, to induce iSF in pentacene dimers. This result is significant in that the SF triplet formation (downconversion) results from irradiation with a wavelength at which the pentacene dimers are transparent to light via OPA. This expands the spectral range for SF to the NIR, beyond the typical absorptions via OPA in the higher energy UV−vis region, albeit high intensity light is required for the TPA process. The nature of TPA transitions has been established by measuring Z-scan spectra and through theoretical calculations. Even though the geometries of the dimers are essentially identical, the more strongly coupled dimer (3) has more intense two-photon transitions, as well as faster singlet fission than the weakly coupled dimer (2). These results emphasize the importance and relationship between TPA, singlet fission, and the extent of intramolecular electronic coupling via the spacer linking two pentacene moieties.

Figure 4. Plot of the differential absorption changes of the singlet excited states [(S1) and (S1S0)] of 1b, 2, and 3 obtained upon fs pump−probe experiments at 775 nm excitation versus the energy per pump pulse.

(S1) and (S1S0), the factor b of the quadratic function in Figure 4, follow the same trend as the Z-scan measurements, namely phenylene spacer (3) > adamantyl spacer (2) > TIBS (1b). At longer time scales, the triplet excited states evolve from a slow intersystem crossing for 1b and fast iSF for 2−3, respectively. In ananalogous fashion, quadratic dependences observed for the singlet excited states (S1) and (S1S0) were also derived for the corresponding triplet excited states 1(T1T1) in the form of the differential absorption changes at the 1(T1T1) maximum for 2 and 3 (Figures S13 and S14).

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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/jacs.7b08287. Addition experimental, spectroscopic, and calculation data (PDF) 14019

DOI: 10.1021/jacs.7b08287 J. Am. Chem. Soc. 2017, 139, 14017−14020

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(18) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M. T.; Choi, B.; Xia, J.; Taffet, E. J.; Low, J. Z.; Miller, J. R.; Roy, X.; Zhu, X. Y.; Steigerwald, M. L.; Sfeir, M. Y.; Campos, L. M. J. Am. Chem. Soc. 2015, 137, 8965. (19) Clearly, the use of TPA for solar energy capture via singlet fission would be plagued by the issue of intensity, because the optical intensity needed for TPA (MW/cm2 to GW/cm2) is far from that of sunlight (mW/cm2), i.e., a difference of at least >1010. (20) The comparison in the inset of Figure 1 is based on the crystal structures of meta-diethynylbenzene and 1,3-diethynyladamantane derivatives available from the CCDC; see Figure S17 for details. (21) Emission of 1b in THF (Φ = 0.30) is considerably stronger than for dimers 2 (Φ = 0.03) and 3 (Φ = 0.02), see refs 13 and 15, respectively, for details. (22) Platt, J. R. J. Chem. Phys. 1949, 17, 484. (23) Gholami, M.; Tykwinski, R. R. Chem. Rev. 2006, 106, 4997. (24) Kamada, K. Proc. SPIE 2004, 5516, 97. (25) Data points around (i) the maximum absorption (∼650 nm) for Figure 3A−C and (ii) the excitation wavelength (∼610 nm) for Figure 3D−F have been masked due to (i) low white light intensity and (ii) saturation of the detector in the region of the excitation wavelength. (26) Laser power measurements show linearity of the differential absorption changes within the 0 to 400 nJ range at 610 nm excitation. (27) This level of theory overestimates the transition energies to some degree more than previous calculations with the more sophisticated CASSCF-based methods (refs 13 and 15). Nevertheless, the order and nature of the excited states by the approximated method are consistent with the previous results. (28) Birks, J. B. Photophysics of Aromatic Molecules; WileyInterscience: London, 1970. (29) Wang, Y.-L.; Wu, G.-S. Int. J. Quantum Chem. 2008, 108, 430. (30) It should be noted that the symmetry breaking transition to the OPA-allowed states cannot be completely excluded as an alternative to the observed two-photon transition. (31) He, G. S.; Tan, L.-S.; Zheng, Q.; Prasad, P. N. Chem. Rev. 2008, 108, 1245.

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Dirk M. Guldi: 0000-0002-3960-1765 Rik R. Tykwinski: 0000-0002-7645-4784 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding is gratefully acknowledged from the Emerging Fields Initiative “Singlet Fission” supported by Friedrich-AlexanderUniversität Erlangen-Nürnberg, the Cluster of Excellence Engineering of Advanced Materials, “Solar Technologies Go Hybrid”, The Natural Sciences and Engineering Research Council of Canada (NSERC), and by a Grant-in-Aid for Scientific Research on Innovative Areas “Photosynergetics” JP26107004 (K.K.) from MEXT, Japan and JP25248007 (K.K.) from JSPS. B.S.B. gratefully acknowledges financial support through a Ph.D. scholarship from the “Studienstiftung des deutschen Volkes”.

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DOI: 10.1021/jacs.7b08287 J. Am. Chem. Soc. 2017, 139, 14017−14020