Photoinduced Electron Transfer In Porous Organic Salt Crystals Impregnated With Fullerenes

Tetsuya Hasegawa a, Kei Ohkubo bc, Ichiro Hisaki a, Mikiji Miyata ad, Norimitsu Tohnai *a và Shunichi Fukuzumi *ce aDepartment of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, nhật bản. E-mail: tohnai bDepartment of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan cDepartment of Chemistry & Nano Science, Ewha Womans University, Seoul 120-750, Korea. E-mail: fukuzumi dThe Institute of Scientific and Industrial Retìm kiếm, Osaka University, Ibaraki, Osaka 567-0047, nhật bản eFaculty of Science & Technology, Meijo University, ALCA và SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aibỏ ra 468-8502, Japan

First published on 9th May 2016

Porous organic salternative text (POS) crystals composed of 9-(4-sulfophenyl)anthracene (SPA) & triphenylmethylamine (TPMA) were impregnated with fullerenes (C60 and C70), which were arranged in one dimensional cthua trận liên hệ. POS crystals of SPA và TPMA without fullerenes exhibit xanh fluorescence due to SPA, whereas the fluorescence was quenched in POS with fullerenes due to lớn electron transfer from the singlet excited state of SPA lớn fullerenes.

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Metal–organic frameworks (MOFs) have sầu attracted increasing attention as an interesting platkhung to lớn kiến thiết và develop artificial photosynthetic systems, because MOFs can contain light-harvesting & charge-separation units as well as catalytic units in a single crystal, providing the structural organization to lớn integrate these units of artificial photosynthesis inkhổng lồ a single crystal.1 There are many examples of MOFs in which energy và electron transfer has been investigated by fixing the distance và orientation between chromophores, which were precisely determined by single crystal X-ray crystallography.2–10

As compared with MOFs, porous organic salts (POSs) have advantage in terms of synthetic versatility, easier processing, more flexibility and better workability.11–15 In particular, POSs composed of ammonium cations and sulfonate anions have enabled various systematic designs of the resultant porous structures by changing the combination of ionic components using strong intermolecular interactions such as hydrogen bonds và electrostatic interactions.16,17 However, there has been no report on photoinduced electron transfer in POS crystals.

We report herein the first example of efficient photoinduced electron transfer in POS crystals composed of 9-(4-sulfophenyl)anthracene (SPA) và triphenylmethylamine (TPMA) impregnated with fullerenes (C60 and C70) as revealed by femtosecond laser flash photolysis and electronic paramagnetic resonance (EPR) measurements. Fullerenes were chosen as electron acceptors, because they are known to lớn undergo efficient electron-transfer reduction with small reorganization energies.18 The combination of SPA và TPMA was chosen in order lớn make enough space for accommodation of fullerenes in the porous structure.

SPA was synthesized according to lớn the literature (see the ESI†).19 The organic salternative text composed of SPA và TPMA yielded a pale yellow needle crystal of SPA/TPMA suitable for X-ray crystallographic analysis from an o-dichlorobenzene solution. The X-ray crystal structure of SPA/TPMA in Fig. 1 revealed that the crystal had a porous structure, which was hierarchically constructed (Fig. S1 in the ESI†). Four SPA molecules và four TPMA molecules assembled into lớn a tetrahedral supramolecular cluster via a cubic-like charge-assisted hydrogene bond.đôi mươi Anthracene moieties of SPA were jutting out in the tetrahedral direction và located around the core structure. The tetrahedral clusters accumulated along the c-axis by CH–π interactions between anthracene moieties và trityl groups to form a linear column structure differently from the previously formed diamondoid structures.21 The column structures were bundled by CH–π interactions between anthracene moieties lớn give sầu a bowl-like void space ahy vọng the columns. The void spaces were connected leading to a channel type porous structure having a bottlenechồng (Fig. S2 in the ESI†). Recrystallization solvents were incorporated there but disordered.

Fig. 1 Crystal structure of SPA/TPMA (left panel). The void space in SPA/TPMA (right panel).

The void space of SPA/TPMA from o-dichlorobenzene with a diameter of 9.3 Å (Fig. 1) is capable of accommodating fullerenes. Recrystallization of SPA/TPMA with C60 và C70 gave purple-red & red-brown needle crystals, respectively. X-ray studies revealed that the porous organic salts composed of SPA và TPMA were impregnated with fullerenes as shown in Fig. 2. The crystallographic parameters of SPA/TPMA, SPA/TPMA/C60 & SPA/TPMA/C70 are shown in Table S1 (ESI†). The void space is enlarged by impregnation of fullerenes khổng lồ be 10.8 Å & 11.1 Å in SPA/TPMA/C60 and SPA/TPMA/C70, respectively (Fig. S3 in the ESI†), because of comparatively weak interaction amuốn the columns. This indicates the flexibility of the POS crystals. Moreover, the bottlenechồng in SPA/TPMA/C60 và SPA/TPMA/C70 is also expanded lớn be 6.6 Å & 7.2 Å, respectively. Consequently, the distance between fullerenes gets closer in the porous structures.

Fig. 2 X-ray crystal structures of SPA/TPMA/C60 (upper panels) và SPA/TPMA/C70 (lower panels).

The SPA/TPMA crystal exhibits fluorescence due lớn the SPA moiety under photoirradiation (Fig. 3).22 In contrast, SPA/TPMA/C60 and SPA/TPMA/C70 exhibited no fluorescence probably because of electron transfer from the singlet excited state of SPA lớn the fullerene. The occurrence of the photoinduced electron transfer in SPA/TPMA/C60 crystals was confirmed by femtosecond laser flash photolysis measurements. The observed transient absorption spectra are shown in Fig. 4, where the transient absorption band at 580 nm of SPA (1SPA*) was observed upon femtosecond laser excitation & the decay is accompanied by an increase in the new absorption b& at 700 nm due lớn the radical cation of SPA (SPA˙+).23 The concomitant formation of C60˙− was also observed at 1030 nm.18 Thus, photoinduced electron transfer from 1SPA* to C60 occurred in SPA/TPMA/C60 to produce the charge-separated (CS) state (SPA˙+/TPMA/C60˙−).

Fig. 3 Colour change of a SPA/TPMA crystal from o-dichlorobenzene under photoirradiation.

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Fig. 4 Transient absorption spectra of SPA/TPMA/C60 dispersed in the KBr pellet recorded at 0.6, 1.2, 4.0, 19 and 350 ps after femtosecond laser excitation at 393 nm.

The rate constant of photoinduced electron transfer from 1SPA* to lớn C60 was determined by the fast decay of absorbance at 600 nm due to lớn 1SPA* khổng lồ be 7.3 × 1011 s−1 (Fig. 5a). The decay time protệp tin of absorbance at 700 nm due to SPA˙+ exhibited two components as shown in Fig. 5b, where the fast component afforded a rate constant of 7.3 × 1011 s−1, which agrees with the rate constant of electron transfer from 1SPA* to C60, và the slower component afforded a rate constant of 1.9 × 1010 s−1. This value is larger than the decay rate constant of C60˙− (5.7 × 109 s−1) determined by the slower decay of absorbance at 1030 nm (Fig. 5c). The faster decay of SPA˙+ than the decay of C60˙− indicates that electron transfer from TPMA khổng lồ SPA˙+ occurred to produce the CS state (SPA/TPMA˙+/C60˙−), which decayed with a rate constant of 5.7 × 109 s−1 & a CS lifetime of 180 ps. Electron transfer from TPMA khổng lồ SPA˙+ is indeed energetically feasible, because the one-electron oxidation potential of TPMA (1.20 V vs. SCE) is lower than that of SPA (1.40 V vs. SCE) as shown in their cyclic voltammograms (Fig. S5 in the ESI†).

Fig. 5 Decay time profiles of absorption at (a) 600 nm, (b) 700 nm & (c) 1030 nm of SPA/TPMA/C60 in the KBr pellet.

Similarly a transient absorption spectrum of the CS state of SPA/TPMA/C70 was observed by femtosecond laser flash photolysis measurements (Fig. S6 in the ESI†). The rate constant of photoinduced electron transfer from 1SPA* lớn C70 was determined by the fast decay of absorbance at 600 nm due khổng lồ 1SPA* lớn be 2.6 × 1011 s−1. The CS lifetime of SPA/TPMA˙+/C70˙− was also determined by the slower decay of absorbance at 1030 nm due to C70˙− lớn be 4.4 × 109 s−1, which corresponds khổng lồ the lifetime of 230 ps.

Upon photoirradiation (λ > 340 nm) of single crystals of SPA/TPMA/C60 in an ESR cavity at 77 K, we could observe a signal at g = 2.0025 as shown in Fig. 6. This signal can be assigned khổng lồ the superposition of TPMA˙+ và C60˙−. In addition, we could observe sầu a signal due khổng lồ a triplet CS state, for which the zero-field splitting (D) value was determined lớn be 49 G. On the basis of this value, the distance between TPMA˙+ và C60˙− is estimated lớn be 8.3 Å,24 which is in good agreement with that found in the crystal structure (8.1 Å in Fig. 7).

Fig. 6 ESR spectrum recorded upon photoirradiation (λ > 340 nm) of crystals of SPA/TPMA/C60 at 77 K.

Fig. 7 The centre-to-centre distance between TPMA & C60 in the SPA/TPMA/C60 crystal.

In conclusion, porous organic salt (POS) crystals composed of 9-(4-sulfophenyl)anthracene (SPA) và triphenylmethylamine (TPMA) were successfully impregnated with fullerenes (C60 & C70) lớn reveal the X-ray crystal structures. Photoexcitation of POSs impregnated fullerenes resulted in efficient electron transfer from 1SPA* to fullerenes, followed by hole transfer from SPA˙+ to lớn TPMA lớn produce the final CS states (SPA/TPMA˙+/C60˙− và SPA/TPMA˙+/C70˙−), which decayed with lifetimes of 180 and 230 ps, respectively. This work has demonstrated the excellent ability of POSs impregnated with fullerenes for efficient photoinduced electron transfer. The tunability and crystalline structures of POSs impregnated with various electron acceptors provide a promising platsize khổng lồ develop artificial photosynthetic systems.

This work was supported by Grants-in-Aid (No. 16H02268 lớn S. F., No. 25288036 lớn N. T., và No. 16K13964, 26620154 and 26288037 to lớn K. O.) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT); ALCA và SENTAN projects to lớn S. F. from JST, nhật bản.

Notes và references