Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis and Photodynamics of Tetragermatetrathia[8]circulene Shuhei Akahori,† Hayato Sakai,‡ Taku Hasobe,‡ Hiroshi Shinokubo,† and Yoshihiro Miyake*,† †
Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan ‡ Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223-8522, Japan S Supporting Information *
ABSTRACT: Tetragermatetrathia[8]circulene has successfully been synthesized from tetraiodotetrathienylene through palladium-catalyzed germylation and rhodium-catalyzed intramolecular dehydrogenative cyclization. A single-crystal X-ray diffraction analysis elucidated that tetragermatetrathia[8]circulene has a highly symmetric, planar, and rigid structure. DFT calculations suggested that LUMO of tetragermatetrathia[8]circulene is stabilized due to σ*−π* interactions between σ* orbitals of the Ge−C bonds and π* orbitals of the thiophene rings. The excited dynamics of tetragermatetrathia[8]circulene have also been found to be significantly affected by the heavy-atom effect of the germanium atom. π-Conjugated compounds annulated with heteroles have attracted considerable attention due to their unique properties on the basis of their rigid structure and electronic perturbation by heteroatoms.1 Recently, silole and their annulated derivatives have been extensively studied because they exhibit high charge transport and photoluminescent properties (Figure 1a).2 These properties originate from the stabilized LUMO level owing to σ*−π* interaction between the σ* orbital of the silicon atom and the π* orbital of double bonds. Germole is the heavier group 14 congener of silole, and their annulated
derivatives are also applicable in organic light-emitting diodes and polymer solar cells.3 The germole-based compounds show a different electronic nature from that of silole-based compounds due to their atom size and electronegativity. Moreover, the heavy atom effect would be expected to modulate their excited-state property. These features indicate that the photophysical and electronic properties of the πconjugated compounds annulated with group 14 metalloles can be finely controlled by tuning bridging heteroatoms. Hetero[8]circulenes are one of the macrocyclic arenes in which central eight-membered rings are surrounded by eightfused benzenoids and heteroaromatic rings.4 For example, hetero[8]circulenes annulated by thiophenes,5−8 furans,9 pyrroles,10 and selenophenes5c have been synthesized successfully. Recently, we have reported the synthesis of tetrasilatetrathia[8]circulenes (2: R = Et) containing thiophenes and siloles, which are considered a cross-conjugated π-sysytem bridged by eight heteroatoms (Figure 1b).11 These compounds were synthesized in two steps from tetraiodotetrathienylene through palladium-catalyzed silylation12 and rhodium-catalyzed intramolecular dehydrogenative cyclization.13 In addition, the excited-state dynamics of 2 were evaluated by steadystate and time-resolved spectroscopic measurement, revealing an efficient intersystem crossing. As an extension of our study, we have expanded our synthetic strategy to tetragermate-
Figure 1. π-Conjugated systems annulated with group 14 metalloles.
Received: December 3, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03764 ���� ����� XXXX, XXX, XXX−XXX
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Organic Letters
central eight-membered ring is 1079.96°, which is close to that of a regular octagon (1080°). These results suggest that the tetragermatetrathia[8]circulene core is highly planar. The C−S bond lengths (1.6921(18)−1.6995(19) Å) are similar to those in 2 (1.70 Å).11 The C−Ge bonds in [8]circulene core (1.9258(19)−1.9385(19) Å) are also similar to those in the germoles and annulated germoles (1.93−1.96 Å).3 In addition, the eight C−C bond lengths (1.4327(25)−1.4964(25) Å) in the central eight-membered ring of 1 are quite longer than the typical CC double bond length as with those of 1. These results suggested that the structural contribution of a planar [8]radialene is stronger than that of cyclooctatetraene having D4h symmetry. The electrochemical property of 1 was examined by using cyclic voltammetry and differential pulse voltammetry (Figures S1 and S2). The circulene 1 exhibited two irreversible oxidation potentials, whereby tetrasilatetrathia[8]circulene 2 showed one irreversible wave. The first oxidation potential of 1 (0.67 V) is lower than that of 2 (0.74 V). On the other hand, no reduction peak was observed for 1 up to −2.35 V in THF. Electronic absorption spectra of 1 and 2 were measured in CH2Cl2 (Figure 3a). As compared to 2, the lowest energy
trathia[8]circulene 1 (R = Et). Here, we disclose the structure and photophysical properties of 1. Tetragermatetrathia[8]circulene 1 was synthesized from tetraiodotetrathienylene 3 in two steps as shown in Scheme 1. Treatment of 3 with 10 equiv of Et2GeH2 in the presence of Scheme 1. Synthesis of 1
20 mol % of Pd(Pt-Bu3)2 and 8 equiv of N,N-diisopropylethylamine (i-Pr2NEt) in THF afforded the corresponding tetrakis(diethylgermyl)tetrathienylene 4 in 78% for 18 h. We then conducted the intramolecular cyclization of 4 by rhodium-catalyzed dehydrogenative C−H germylation.14 The reaction of 4 in the presence of 15 mol % of [RhCl(cod)]2 and 45 mol % of 1,2-bis[bis(pentafluorophenyl)phosphino]ethane L13e in toluene at 140 °C for 20 h provided the corresponding tetragermatetrathia[8]circulene 1 in 27% yield, while the use of 1,2-bis(diphenyl)phosphinoethane (dppe), which is the best ligand for the synthesis of 2, afforded no formation of 1 with formation of tetrathienylene. These results suggest that electron-deficient ligand L may suppress the cleavage of carbon−germanium bonds and the choice of ligands is quite important for applying transition-metal catalysis to the transformation of heavier heteroles. The compound 1 is stable in the solid state under air and soluble in common organic solvents such as CH2Cl2, CHCl3, THF, and toluene. Recrystallization of 1 from 1,2-dichloroethane/hexane afforded a single crystal suitable for X-ray diffraction analysis, which unambiguously confirmed the molecular structure of 1 (Figure 2).15 The mean plane deviation of the [8]circulene core in 1 is 0.020 Å, and the sum of the inner angles of the
Figure 3. (a) UV−vis absorption spectra of 1 and 2 in CH2Cl2 and simulated absorption spectra calculated by the TD-DFT method at the B3LYP/6-31G(d) level (red lines: 1, black line: 2). (b) Emission spectra of 1 (red dash line) and 2 (black dash line) at room temperature and 1 (blue line) and 2 (green line) at 77 K in toluene.
absorption band of 1 was slightly blue-shifted. The molar extinction coefficient (ε) at λmax of 1 is very small at 366 nm (418 M−1cm−1), which corresponds to the 0−0 transition. This feature is in good agreement with the calculated absorption energies and oscillator strengths of the model compounds for 1 and 2 by the time-dependent density functional theory (TDDFT) calculations at the B3LYP/6-31G(d) level of theory (Figure S3). The value of the oscillator strength for the S0−S1 transition at 355.46 nm is zero, of which the major contribution is the HOMO−LUMO transition. These results suggest that the lowest energy absorption band of 1 around 350 nm originates from the symmetry-forbidden transition as was the case of 2, while the HOMO−LUMO gap of 1 is wider than that of 2. In addition, on the basis of the TD-DFT calculations, the high energy absorption band of 1 around 270 nm consists of two similar transitions, which were assigned to the combination of five transitions, respectively.16 The HOMO level of 1 was calculated to be −5.24 eV from onset value of the oxidation wave. The optical band gap of 1 was 3.38 eV, thus indicating the LUMO level of 1 to be −1.86 eV. DFT calculations suggested that LUMO of 1 is stabilized due to σ*−π* interactions between σ* orbitals of the two exocyclic σ bonds on the germanium atoms and π* orbitals of
Figure 2. (a) Top and (b) side views of the molecular structure of 1 (atomic displacement parameters set at 50% probability; all hydrogen atoms are omitted for clarity). B
DOI: 10.1021/acs.orglett.7b03764 ���� ����� XXXX, XXX, XXX−XXX
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Organic Letters the thiophene rings. However, the LUMO level of 1 was higher than that of 2 (−2.14 eV),11 indicating that σ*−π* interactions in 1 occur less effectively than 2. This is probably due to the less effective interaction between π* orbital of the thiophene ring and σ* orbital of germanium having the larger principal quantum number than silicon as well as the longer distance between the two orbitals in 1. Such a relationship is similar to that between typical siloles and germoles.2c Figure 3b shows the emission spectra of 1 and 2 in toluene. The emission of 1 at room temperature appeared at 379 nm upon excitation at 300 nm. The fluorescence emission quantum yield (ΦF) and fluorescence lifetime (τF) was 0.0076 and 2.5 ns, respectively. In contrast, 1 exhibited phosphorescence at 524 nm in frozen toluene at 77 K upon excitation at 310 nm. The phosphorescence quantum yield (ΦP) was 0.084. The emission wavelength of tetragermatetrathia[8]circulene 1 is shorter than that of 2. These results are also in line with wider HOMO−LUMO gap of 1 than that of 2. To reveal the excited state property of 1, we evaluated the photophysical dynamics of 1 at room temperature (Table 1).
palladium-catalyzed germylation and rhodium-catalyzed cyclization. A single-crystal X-ray diffraction analysis revealed that tetragermatetrathia[8]circulene 1 has highly symmetric, planar, and rigid structure. We have also revealed the difference in photophysical and electrochemical properties between 1 and the corresponding sila-analogue 2. In addition, we have demonstrated that tetraiodotetrathienylene 3 is a versatile precursor for hetero[8]circulenes containing thiophene rings. Further work is currently in progress in our group to synthesize new hetero[8]circulenes incorporating the heavier group 14 elements.
Table 1. Photophysical Parameters of 1 and 2a
CCDC 1588724 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
ΦF τFc (nm) ΦISC ΦIC kF (s−1) kISC (s−1) kIC (s−1) ΦPd τp (ms)d S1−S0 (eV) T1−S0 (eV) S1−T1 (eV) ε0−0 (M−1cm−1)
1
2b
0.0076 2.5 0.32 0.67 3.0 × 106 1.3 × 108 2.7 × 108 0.084 98 3.40 2.38 1.02 418 (at 366 nm)
0.012 0.92 0.41 0.58 1.3 × 107 4.6 × 108 6.3 × 108 0.0075 235 3.23 2.26 0.97 170 (at 384 nm)
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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.orglett.7b03764. xperimental procedures, as well as spectroscopic and crystallographic data (PDF) Accession Codes
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Taku Hasobe: 0000-0002-4728-9767 Hiroshi Shinokubo: 0000-0002-5321-2205 Yoshihiro Miyake: 0000-0003-0247-531X Notes
Measurements were conducted in toluene at room temperature. S1− S0: determined from the absorption and fluorescence spectra, T1−S0: determined from the phosphorescence spectrum, ΦIC = 1−ΦF−ΦISC, kF = ΦF /τF, kISC = ΦISC /τF, kIC = ΦIC /τF. bReference11. cMeasured in THF. dMeasured at 77 K. a
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas “pi-System Figuration (No. 2601)” (JSPS KAKENHI Grant Nos. 26102003 and 17H05162) and “Precisely Designed Catalysts with Customized Scaffolding (No. 2702)”(JSPS KAKENHI Grant No. 16H01013) from MEXT, Japan.
The intersystem crossing quantum yield (ΦISC = 0.32) was determined by 1O2 phosphorescence measurements utilizing energy transfer from triplet-excited state of 1 to O2 (Figure S5).17 The internal conversion (IC) quantum yield (ΦIC = 0.67) was determined by subtracting ΦF and ΦISC from the unity. The rate constant values kF, kISC, and kIC were calculated on the basis of fluorescence lifetime and corresponding quantum yields. The rate constant of intersystem crossing (kISC) is 2 orders of magnitude greater than that of fluorescence emission (kF). This result derives from the symmetry forbidden nature of the S0−S1 transition. The ΦISC value of 1 is smaller than that of 2, while the phosphorescence quantum yield (ΦP) value of 1 (ΦP = 0.084) is more than 10 times larger than that of 2 (ΦP = 0.0075). These results suggest that the larger heavy-atom effect of the germanium atom in 1 than that of the silicon atom in 2 has a significant effect on the photodynamics of hetero[8]circulenes. In summary, we have synthesized tetragermatetrathia[8]circulene 1 from tetraiodotetrathienylene 3 through 4-fold
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DOI: 10.1021/acs.orglett.7b03764 ���� ����� XXXX, XXX, XXX−XXX
