Article pubs.acs.org/JPCC
Ultrafast Triplet Formation in Thionated Perylene Diimides Andrew J. Tilley, Ryan D. Pensack, Tia S. Lee, Brandon Djukic, Gregory D. Scholes,* and Dwight S. Seferos* Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *
ABSTRACT: Perylene diimides (PDIs) are versatile n-type materials showing great promise in a number of optoelectronic applications. While the singlet manifold of PDI can be readily populated, triplet excited states are only accessible through complex multistep energy cascades or bimolecular sensitization. In this work, we have synthesized a series of thionated PDIs that display rapid intersystem crossing to triplet states. Significantly, the thionated PDIs are synthesized in one step from the parent compound using commercially available Lawesson’s reagent. Electrochemical and steady state optical absorption measurements show that the electron affinity and ionization potentials can be systematically tuned through successive sulfur atom substitution. Thin-film optical absorption measurements show how the number and regiochemistry of the thiocarbonyl groups influence π−π interactions in the solid state. Ultrafast transient absorption spectroscopy reveals rapid triplet formation that is independent of the degree of thionation, highlighting this approach as a facile means of accessing the triplet manifold of PDI.
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INTRODUCTION Second only to fullerene derivatives, perylene-3,4,9,10tetracarboxylic acid diimides (PDIs) are among the most promising negative charge-accepting and -transporting “n-type” materials for organic electronics due to their high molar absorptivity, excellent electron mobility and affinity, and high photochemical stability.1−6 An attractive feature of PDIs is that their electronic and materials properties can be tuned through synthetic modifications to the aromatic core and imide nitrogens.7−12 Substitution on the central aromatic rings, at the so-called bay positions, alters both electronic and materials properties, while substitution at the imide nitrogens controls only solubility and long-range molecular order.13−17 Chromophores that generate triplet states are very important in a number of optoelectronic applications.18−23 In organic photovoltaic devices, triplet excitons can potentially improve photocurrent as their long diffusion lengths maximize the probability for charge separation to occur at a donor−acceptor interface.24 Triplet-generating chromophores are also widely utilized in the production of singlet oxygen for photodynamic therapy25−27 and for photochemical upconversion.28−30 Thus, chromophores with large absorption coefficients that display efficient intersystem crossing to triplet states are highly sought after. Presently, the only reported methods of accessing the triplet manifold of PDI are through bimolecular triplet sensitization31 or by appending groups to the bay and imide positions which promote intersystem crossing (ISC) via a radical pair mechanism or by the so-called “heavy atom” effect.32−39 However, these approaches require the synthesis of challenging molecular architectures that facilitate multiple energy cascades © 2014 American Chemical Society
(for the radical pair mechanism) or the introduction of organometallic substituents at the bay positions to enhance spin−orbit coupling. As a result, the utilization of PDI triplet states in optoelectronic applications has remained largely unexplored. Group 16 elemental substitution is an effective means of modifying the optoelectronic properties of π-conjugated systems.40−43 One distinct advantage of this approach is that the frontier orbital energies can be altered without significantly perturbing the overall molecular structure. Interestingly, despite its rich chemical history, there is only one reported synthesis of group 16 substituted PDIs.44 In this patent by Facchetti and coworkers, sulfur atom incorporation was achieved by substitution of the carboxylic oxygens using Lawesson’s reagent, a commercially available reagent commonly used to thionate polyketones.45,46 Only trace amounts of material attributed to tri- and tetrathionated PDI were reported, likely due to challenges with solubility, and thus the photophysics and other properties of these compounds remain undetermined. Herein we report the synthesis, characterization, and ultrafast photophysics of the first full series of thionated PDI derivatives (Figure 1). The use of a branched 3-hexylundecyl side chain allows us to prepare significant quantities of mono-, di- (both regioisomers, vide infra), tri-, and tetrathionated PDIs that we have characterized and studied using NMR, optical absorption, and fluorescence spectroscopies, electrochemistry, and ultrafast transient absorption (TA) spectroscopy. Promisingly, we show that the PDI frontier energy levels can be systematically tuned Received: April 15, 2014 Published: April 18, 2014 9996
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heated at reflux for 18 h under argon. The resulting blue solution was cooled to room temperature and concentrated under reduced pressure to give a crude mixture of S1−S3. Separation by column chromatography (toluene/hexanes then CHCl3/hexanes) gave S1−S3 in the following yields. S1 (0.050 g, 10%), mp (decomp.) 108−110 °C, λmax (abs) = 574 nm: 1H NMR (500 MHz, CDCl3) δ 0.86−0.94 (m, 12H), 1.24−1.43 (m, 48H), 1.46−1.55 (m, 2H), 1.65−1.80 (m, 4H), 4.13−4.21 (m, 2H), 4.65−4.72 (m, 2H), 8.28 (d, J 8.53 Hz, 1H), 8.37−8.46 (m, 3H), 8.52−8.57 (m, 3H), 8.90 (d, J 8.28 Hz, 1H). 13C NMR (500 MHz, CDCl3) δ 14.31, 14.32, 14.35, 22.89, 22.92, 22.933, 22.934, 26.78, 26.83, 26.86, 26.91, 29.61, 29.62, 29.89, 29.92, 29.99, 30.0, 30.33, 30.35, 32.13, 32.15, 32.17, 33.72, 33.74, 36.2, 36.4, 39.2, 46.2, 122.4, 122.6, 122.8, 123.1, 123.19, 123.21, 125.32, 125.33, 126.8, 127.1, 128.8, 130.7, 130.8, 131.2, 132.7, 133.3, 133.65, 133.67, 135.8, 160.0, 162.7, 162.8, 191.9. FT-IR (neat, cm−1) 1695 (sym. CO stretch), 1682 (sym. CO stretch), 1652 (antisym. CO stretch), 1160 (CS stretch), 1125 (CS stretch). MALDIMS m/z 883.5 (C58H79N2O3S [M + H]+ requires 883.6). cis-S2 (0.066 g, 13%), mp (decomp.) 118−120 °C, λmax (abs) = 616 nm: 1H NMR (400 MHz, CDCl3) δ 0.89−0.97 (m, 12H), 1.29−1.43 (m, 48H), 1.46−1.55 (br. s, 2H), 1.66−1.77 (m, 4H), 4.47−4.58 (m, 4H), 7.73 (d, J 8.52 Hz, 2H), 7.97 (d, J 8.16 Hz, 2H), 8.22 (d, J 7.97 Hz, 2H), 8.51 (d, J 8.28 Hz, 2H). 13 C NMR (500 MHz, CDCl3) δ 14.35, 14.37, 22.94, 22.97, 26.89, 26.94, 29.7, 29.86, 29.95, 30.0, 30.4, 32.18, 32.19, 33.7, 36.4, 46.1, 121.9, 122.7, 123.0, 124.4, 126.3, 126.6, 130.8, 132.0, 132.7, 135.4, 159.5, 191.4. Partial gCOSY (500 MHz/500 MHz, CDCl3) δ 1H/1H 8.20/7.31, 7.95/7.60. Partial gHSQC (500 MHz/125 MHz, CDCl3) δ 1H/13C 8.18/135.2, 7.93/130.6, 7.59/122.5, 7.30/121.7. Partial gHMBC (500 MHz/125 MHz, CDCl3) δ 1H/13C 8.19/191.2 (3JH,C), 8.19/131.9 (3JH,C), 8.19/126.3 (3JH,C), 7.94/159.4 (3JH,C), 7.94/132.6 (3JH,C), 7.94/126.2 (3JH,C), 7.94/122.6 (3JH,C), 7.60/132.6 (2JH,C), 7.60/124.3 (3JH,C), 7.60/122.8 (3JH,C), 7.31/131.9 (2JH,C), 7.31/126.4 (3JH,C), 7.31/124.3 (3JH,C). Partial ROESY (500 MHz/500 MHz, CDCl3) δ 1 H/1H 8.18/7.32, 7.94/7.60. FT-IR (neat, cm−1) 1682 (sym. CO stretch), 1664 (antisym. CO stretch), 1157 (sym. CS stretch), 1124 (antisym. CS stretch). MALDI-MS m/z 899.4 (C58H79N2O2S2 [M + H]+ requires 899.6). trans-S2 (0.091 g, 17%), mp (decomp.) 152−153 °C, λmax = 615 nm: 1H NMR (400 MHz, CDCl3) δ 0.87−0.96 (m, 12H), 1.24−1.43 (m, 48H), 1.48−1.59 (m, 2H), 1.71−1.79 (m, 4H), 4.62−4.69 (m, 4H), 8.18 (d, J 8.61 Hz, 2H), 8.27 (d, J 8.21 Hz, 2H), 8.48 (d, J 8.04 Hz, 2H), 8.79 (d, J 8.35 Hz, 2H). 13C NMR (500 MHz, CDCl3) δ 14.35, 14.37, 22.94, 22.97, 26.9, 27.0, 29.7, 29.87, 29.95, 30.01, 30.4, 32.18, 32.19, 33.7, 36.4, 46.2, 122.2, 122.4, 122.9, 124.4, 126.3, 126.6, 130.9, 132.0, 132.7, 135.2, 159.6, 191.2. Partial gCOSY (500 MHz/500 MHz, CDCl3) δ 1H/1H 8.13/7.37, 7.97/7.51. Partial gHSQC (500 MHz/125 MHz, CDCl3) δ 1H/13C 8.13/135.2, 7.96/ 130.1, 7.51/122.4, 7.37/122.1. Partial gHMBC (500 MHz/125 MHz, CDCl3) δ 1H/13C 8.13/191.1 (3JH,C), 8.13/131.8 (3JH,C), 8.13/126.2 (3JH,C), 7.96/159.5 (3JH,C), 7.96/132.5 (3JH,C), 7.96/ 126.2 (3JH,C), 7.96/122.6 (2JH,C), 7.51/131.8 (2JH,C), 7.51/124.3 (3JH,C), 7.51/122.8 (3JH,C), 7.37/132.5 (2JH,C), 7.37/126.4 (3JH,C), 7.37/124.3 (3JH,C). Partial ROESY (500 MHz/500 MHz, CDCl3) δ 1H/1H 8.13/7.38, 7.97/7.52, 7.51/7.38. FT-IR (neat, cm−1) 1683 (sym. CO stretch), 1661 (antisym. CO stretch), 1159 (sym. CS stretch), 1122 (antisym. CS
Figure 1. Thionated PDIs studied in this work.
through successive sulfur atom substitution, yet thionated PDIs undergo rapid and highly efficient ISC to triplet states that is independent of the degree of thionation. Thus, this appears to be a facile one-step means of accessing the triplet excited state of PDIs that does not require extensive syntheses or coupling to a transition metal complex.
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EXPERIMENTAL SECTION Synthesis of P−S4. N,N′-Di(3-hexylundecyl)-perylene3,4:9,10-tetracarboxylic Acid Bisimide (P). A mixture of perylene-3,4,9,10-tetracarboxylic dianhydride (1.348 g, 3.436 mmol), 3-hexylundecylamine (2.661 g, 10.42 mmol) (refer to Supporting Information (SI) for synthesis), imidazole (14.00 g, 205.6 mmol), and zinc acetate (0.520 g, 2.834 mmol) was stirred at 160 °C for 24 h under argon. After 24 h the reaction mixture was allowed to cool to room temperature, diluted with CHCl3 (200 mL), washed with H2O (2 × 100 mL) and sat. aq. NaCl (1 × 100 mL), dried (MgSO4), filtered, and concentrated under reduced pressure to give a purple solid. The purple solid was purified by column chromatography (100% CHCl3) to afford the title compound as a deep red solid (2.401 g, 81%), mp 128−129 °C, λmax (abs) = 526 nm. 1H NMR (400 MHz, CDCl3) δ 0.85−0.93 (m, 12H), 1.22−1.45 (m, 48H), 1.46− 1.55 (m, 2H), 1.64−1.73 (m, 4H), 4.10−4.19 (m, 4H), 8.30 (d, J 8.3 Hz, 4H), 8.47 (d, J 7.9 Hz, 4H). 13C NMR (500 MHz, CDCl3) δ 14.28, 14.30, 22.87, 22.91, 26.75, 26.80, 29.6, 29.9, 30.0, 30.3, 31.8, 32.1, 33.7, 36.2, 39.0, 122.5, 123.1, 125.5, 128.7, 130.6, 133.5, 162.7. FT-IR (neat, cm−1) 1695 (CO stretch), 1651 (CO stretch). HRMS (DART)+ m/z 867.6051 (C58H79N2O4 [M + H]+ requires 867.6040). S1−S3. A solution of Lawesson’s reagent (0.473 g, 1.169 mmol) and P P (0.507 g, 0.585 mmol) in toluene (70 mL) was 9997
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Figure 2. Partial 1H NMR spectra of P, S1, S3, and S4 (panel A) and partial 1H−1H ROESY spectra of cis-S2 and trans-S2 (panels B and D) recorded in CDCl3 at 25 °C. R denotes the 3-hexylundecyl side chain.
stretch). MALDI-MS m/z 899.4 (C58H79N2O2S2 [M + H]+ requires 899.6). S3 (0.128 g, 24%), mp (decomp.) 121−123 °C, λmax (abs) = 663 nm: 1H NMR (300 MHz, CDCl3) δ 0.89−1.00 (m, 12H), 1.31−1.42 (m, 50H), 1.63−1.79 (m, 4H), 4.37−4.50 (m, 2H), 4.84−4.96 (m, 2H), 7.30 (d, J 8.65 Hz, 1H), 7.33−7.41 (m, 2H), 7.59 (d, J 8.27 Hz, 1H), 8.00−8.05 (m, 2H), 8.10 (d, J 8.08 Hz, 1H), 8.25 (d, J 8.29 Hz, 1H). 13C NMR (500 MHz, CDCl3) δ 14.36, 14.38, 14.39, 14.41, 22.96, 22.97, 22.98, 26.93, 26.97, 26.99, 27.04, 28.6, 29.7, 29.96, 29.99, 30.01, 30.03, 30.35, 30.38, 32.19, 32.21, 32.24, 33.65, 33.72, 36.48, 36.55, 46.2, 54.4, 122.1, 122.4, 122.6, 122.7, 122.9, 123.0, 124.15, 124.23, 126.4, 126.6, 128.23, 128.25, 131.0, 131.4, 132.1, 132.8, 135.4, 136.3, 136.4, 159.7, 187.3, 187.4, 191.3. FT-IR (neat, cm−1) 1683 (sym. CO stretch), 1662 (antisym. CO stretch), 1154 (sym. CS stretch), 1116 (antisym. CS stretch). MALDIMS m/z 915.4 (C58H79N2OS3 [M + H]+ requires 915.5). S4. Using a procedure similar to the one described above, a solution of Lawesson’s reagent (2.352 g, 5.815 mmol) and P (1.005 g, 1.159 mmol) in toluene (150 mL) was heated at reflux for 50 h. The resulting deep blue solution was cooled to room temperature and concentrated under reduced pressure to give a blue solid. Purification by column chromatography (toluene/hexanes) gave S4 (0.316 g, 29%) and S3 (0.140 g, 13%) as deep blue solids. Mp (decomp.) (S4) 115−116 °C. λmax (abs) (S4) = 706 nm. 1H NMR of S4 (300 MHz, CDCl3) δ 0.92−1.00 (m, 12H), 1.29−1.41 (m, 50H), 1.68−1.79 (m, 4H), 4.76−5.00 (br. S, 4H), 7.17 (d, J 8.72 Hz, 4H), 7.99 (d, J 8.24 Hz, 4H). 13C NMR (500 MHz, CDCl3) δ 14.38, 14.40, 22.96, 22.97, 26.96, 27.01, 29.7, 29.96, 29.97, 30.3, 32.19, 32.20, 33.5, 36.5, 54.3, 121.9, 122.3, 123.0, 127.6, 130.6, 135.8, 186.6. FT-IR (neat, cm−1) 1155 (sym. CS stretch), 1116 (antisym.
CS stretch). MALDI-MS m/z 931.4 (C58H79N2S4 [M + H]+ requires 931.5).
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RESULTS AND DISCUSSION The nonthionated precursor PDI (P) was used as a control molecule throughout this study and was synthesized according to a modified literature procedure.47,48 Briefly, commercially available perylene-3,4,9,10-tetracarboxylic dianhydride was treated with three equivalents of 3-hexylundecylamine49 in molten imidazole, which after chromatographic purification afforded P in 81% yield. To prepare the thionated PDIs, P was treated with Lawesson’s reagent (LR), which affects thionation at the imide carboxyl positions. This reagent was recrystallized from toluene to improve the thionation yields.50 Both the stoichiometry of LR and reaction time control the extent of thionation (see the Experimental Section). The most intriguing aspects of the synthesis are the extent of thionation as well as the regioisomers that occur when two thionation reactions take place. When P was treated with 5.0 equiv of LR in toluene and heated at reflux for 50 h, tetrathionated S4 was isolated in 29% yield (Figure S1, SI). This reaction also produces trithionated S3 in 13% yield. These derivatives were separated by silica gel chromatography and characterized by NMR, FT-IR, and MALDI-TOF mass spectrometry. 1H NMR spectroscopy was used to help identify S3 and S4 (Figure 2A). When P was treated with 2.0 equiv of LR in toluene and heated at reflux for 18 h, monothionated S1 and an isomeric mixture of dithionated PDIs (13% of cis-S2 and 17% of transS2) are prepared. We were able to separate isomers cis-S2 and trans-S2 using conventional silica gel chromatography; however, structural assignment is not possible using 1D 9998
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Figure 3. Optical absorption spectra of PDIs in room-temperature chloroform solution (panel A) and as-cast film (panel B). The annealed film absorption spectra (solid lines) of P and S4 are shown in panel C, while cis-S2 and trans-S2 are compared in panel D. The corresponding solution spectra are represented by dashed lines. Films were prepared by spin-casting 10 mg/mL CHCl3 solutions onto glass slides and were annealed until evolution of the spectral features ceased. S4, cis-S2, and trans-S2 were annealed at 80 °C, while P was annealed at 100 °C. Annealed film absorption spectra for the other thionated PDIs can be found in Figure S5 (SI).
Table 1. Reduction Potentials, HOMO/LUMO Energies, and Fluorescence Quantum Yields of Each PDI PDI
E1/2−1 (V)
E1/2−2 (V)
optical band gap (eV)a
P S1 cis-S2 trans-S2 S3 S4
−0.68 −0.55 −0.48 −0.51 −0.36 −0.23
−0.91 −0.72 −0.57 −0.61 −0.45 −0.33
2.25 2.06 1.91 1.90 1.78 1.64
HOMO (eV)b,d
LUMO (eV)d
−5.92 −5.85 −5.78 −5.74 −5.77 −5.76
−3.67 −3.80 −3.87 −3.84 −3.99 −4.12
(−6.23) (−6.15) (−6.09) (−6.08) (−6.04) (−5.98)
(−3.76) (−3.88) (−3.99) (−3.97) (−4.07) (−4.15)
Φfc 0.97
