FULL PAPER DOI: 10.1002/asia.201402653
Through-Space Conjugated Molecular Wire Comprising Three p-Electron Systems Yasuhiro Morisaki,* Naoya Kawakami, Shotaro Shibata, and Yoshiki Chujo*[a]
Abstract: A [2.2]paracyclophane-based through-space conjugated oligomer comprising three p-electron systems was designed and synthesized. The arrangement of three p-conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Fçrster mechanism. The energy transfer efficiency
and rate constants were estimated to be > 0.999 and > 1012 s 1, respectively. The key point for the efficient energy transfer is the orientation of the transiKeywords: energy transfer · FRET · ACHTUNGRE[2.2]paracyclophane · molecular wire · through-space conjugation
tion dipole moments. The time-dependent density functional theory (TDDFT) studies revealed the transition dipole moments of each stacked p-electron system; each dipole moment was located on the long axis of each stacked p-electron system. This alignment of the dipole moments is favorable for fluorescence resonance energy transfer (FRET).
Introduction Considerable attention has been paid to the unique structure of [2.2]paracyclophane consisting of two phenylenes stacked in proximity to each other.[1, 2] The introduction of certain p-conjugated units into the [2.2]paracyclophane skeleton provides a partially stacked structure of the resulting p-electron systems. The photophysical properties of such [2.2]paracyclophane-based systems with two stacked chromophores have been extensively investigated. The photoluminescence (PL) properties of the stacked p-electron system strongly depend on the length and stacked position of the p-electron system.[3] In other words, each partially stacked p-electron system with expanded conjugation absorbs and emits light independently. We have also focused on the [2.2]paracyclophane skeleton and synthesized [2.2]paracyclophane-containing through-space conjugated oligomers and polymers.[4] It was found that photoexcited energy and hole migrated through the stacked p-electron systems in a single polymer chain.[4d] To better understand the photoexcited energy transfer, through-space conjugated dimers comprising energy donor–acceptor p-electron systems (D1[5a] and D2[5b] in Figure 1) have been recently reported. The [2.2]paracyclophane scaffold appropriately aligned the transition dipole moments of the stacked p-elec-
Figure 1. Through-space conjugated oligomers and model compounds discussed in this study (R = C12H25). Energy band gaps (Eg) of M1–3 were estimated by their UV/Vis absorption edges in CHCl3 (1.0 10 5 m).
[a] Prof. Dr. Y. Morisaki, N. Kawakami, S. Shibata, Prof. Dr. Y. Chujo Department or Polymer Chemistry, Graduate School of Engineering Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan) Fax: (+ 81) 75-383-2607 E-mail: [email protected] [email protected]
tron systems, thus resulting in a high energy-transfer efficiency (FET > 0.999) with a high energy-transfer rate constant (kET) on the order of 1012 s 1. To expand the energytransfer system from two p-electron systems to three p-electron systems, in this study, a through-space conjugated trimer T1 consisting of three p-electron systems was synthe-
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402653.
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chain ends.[6] Photoexcited electron transfer occurred, resulting in charge separation; thus, the through-space conjugated oligomers transferred electrons through the stacked p-electron systems. This study moreover indicates that [2.2]paracyclophane-based through-space conjugated systems are promising candidates for single molecular wires that transfer energy and electrons depending on the design of the stacked p-electron systems.
Results and Discussion Synthetic routes to the target through-space conjugated trimer T1 are shown in the Supporting Information. This system was designed to obtain a large overlap integral between an absorption band and a PL peak of adjacent p-electron systems; that is, it was designed for efficient fluorescence resonance energy transfer (FRET)[7, 8] from the M1 to the M3 moiety via the M2 moiety. Figure 2 A shows the absorption and PL spectra of T1 in dilute CHCl3 (1.0 10 5 m and 10 6 m, respectively). The PL spectrum was obtained by excitation at 290 nm; this wavelength mainly excites the M1 moiety. The absorption spectrum of T1 comprised the absorption spectra of the three stacked p-electron systems M1–3 (each spectrum is shown in the Supporting Information), whereas the PL spectrum exhibited a single-component emission at 525 nm with a PL lifetime (t) of 5.61 ns
Figure 2. (A) UV/Vis absorption spectrum of T1 in CHCl3 (1.0 10 5 m) and PL spectra of T1 in CHCl3 (1.0 10 6 m, excited at 290 nm). (B) UV/ Vis absorption spectrum of a mixture of M1–3 in CHCl3 (1.0 10 5 m) and PL spectra of M1–3 in CHCl3 (1.0 10 6 m, excited at 290 nm). The asterisks denote scattered light and overtone peaks.
sized (Figure 1). Using this system, an efficient unidirectional energy transfer could be achieved. Furthermore, the detailed mechanism of energy transfer was elucidated experimentally and theoretically. Recently, Guldi, Martn, and coworkers have reported [2.2]paracyclophane-based through-space conjugated oligomers containing zinc porphyrin and C60 at the oligomer
Chem. Asian J. 2014, 9, 2891 – 2895
Figure 3. Excitation spectra of T1 at 525 nm and D2 at 522 nm in CHCl3 (1.0 10 6 m). The asterisk denotes halftone peaks (at 262.5 nm and 261 nm).
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(c2 = 1.11).[9] This PL behavior was consistent with those of M3 (513 nm, t = 6.05 ns, c2 = 1.01) and D2 (522 nm, t = 5.67 ns, c2 = 1.16).[10] In the PL spectrum of T1, the emissions from the M1 and M2 moieties did not occur, and only the emission from the M3 moiety was observed as expected. On the other hand, Figure 2 B shows the PL spectrum of a mixture of M1–3 in CHCl3 (concentration of each compound = 1.0 10 6 m, excitation wavelength = 290 nm). Emissions from M1–3 were clearly observed at 318 nm, 401 nm, and 513 nm, respectively. By combining M1–3 with the [2.2]paracyclophane bridges, the emissions from the M1 and M2 moieties were quenched in T1. Figure 3 shows the excitation spectra of D2 and T1 in CHCl3 (1.0 10 6 m) monitored at 522 nm and 525 nm, respectively (the emission peak tops of the M3 moieties in D2 and T1, respectively). In the excitation spectrum of T1, the peak at around 300 nm increased compared to that in the spectrum of D2, thus indicating the contribution of the M1 moiety to the emission of T1. These results confirm that photoexcited energy was intramolecularly transferred from the M1 and M2 moieties to the M3 moiety. To estimate the photoexcited energy-transfer efficiency (EET) and mechanism of T1, density functional theory (DFT) studies were carried out. Figure 4 shows the molecular orbitals (MOs) of T1 calculated using the CAM-B3LYP long-rang corrected function with the 6-31G* basis set, and
Figure 5 shows the optimized structure of T1. The MOs comprised the frontier orbitals of M1–3; namely, HOMO-1/ LUMO, HOMO/LUMO + 1, and HOMO-2/LUMO + 2, corresponding to the frontier orbitals of the M3, M2, and M1 moieties. Their oscillator strengths (f) were calculated with the time-dependent DFT (TD-DFT), and the f values for HOMO-1/LUMO, HOMO/LUMO + 1, and HOMO-2/ LUMO + 2 were calculated to be 1.30 (l = 423 nm), 1.68 (l = 345 nm), and 0.56 (l = 305 nm),[11] consistent with the UV spectrum of T1. The TD-DFT studies revealed the transition dipole moments of each stacked p-electron system in T1 (Figure 5 and Table 1); each dipole moment was located on the long axis Table 1. Oscillator strength (f) and dipole moment of T1. Transition
l [nm]
f
HOMO-1 to LUMO HOMO to LUMO + 1 HOMO-2 to LUMO + 2
423 345 305
1.30 1.68 0.56
mx 4.2149 4.3143 2.3487
my 0.2786 0.3890 0.0782
mz 0.4289 0.5356 0.3655
of each stacked p-electron system. This alignment of the dipole moments is favorable for FRET,[8] and the orientation factors (k2) between the neighboring p-electron systems were calculated to be 3.52 and 3.68 for the M1/M2 and M2/
Figure 4. Molecular orbitals (top views) of T1 as well as model compounds M1–3 calculated at the CAM-B3LYP/6-31G* level. Calculations were carried out with the OCH3 substituent instead of OC12H25.
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Figure 6. Results of energy transfer through T1.
ies. In both cases, the kET of direct energy transfer were sufficiently large on the order of 1010 s 1. However, we should conclude that the stepwise energy transfer from the M1 to the M3 via the M2 moiety is dominant according to the larger kET values. The theoretical calculations described above were carried out on the assumption that Fçrster-type[7] energy transfer occurred, disregarding the Dexter-type[11] energy transfer. Recently, Knorr and Harvey reported the triplet-state energy transfer, which is performed by the Dexter mechanism, using platinum-containing through-space conjugated copolymers.[12] The kET values were calculated to be on the order of 104–105 s 1. Moreover, the photoexcited electron transfer rate constant through the stacked p-electron systems was estimated to be on the order of 109 s 1.[6] From these rate constant values, it can be concluded that efficient photoexcited energy transfer occurred dominantly by the Fçrster mechanism in this study.
Figure 5. Optimized structure of T1 calculated at the CAM-B3LYP/631G* level.
M3 pairs (Figures S29 and S30, Supporting Information), respectively. The maximum k2 value is theoretically 4; therefore, the obtained k2 values of 3.52 and 3.68 were sufficiently large, indicating ideal orientations for FRET between the neighboring p-electron systems in T1. The spectral overlap integrals (J) of the M1/M2 and M2/M3 pairs were calculated from their absorption and PL spectra, and the Fçrster radii (R0) for the M1/M2 and M2/M3 pairs were calculated to be 42.0 and 58.6 , respectively. Finally, the EET and rate constants (kET) were estimated; the results are shown in Figure 6, and the detailed data are summarized in Figures S29 and S30 in the Supporting Information. Both the EET (from the M1 to the M2 moieties and from the M2 to the M3 moieties) were > 0.999, and their kET values were 1.10 1013 s 1 and 3.54 1012 s 1, respectively. In this system, the direct energy transfer from the M1 to the M3 moieties is possible. Thus, as shown in Figure 6, the EET and kET values from the M1 to the M3 moieties in T1 were also calculated to be 0.95–0.96 and 4.1–5.1 1010 s 1, respectively. The maximum values (EET = 0.96 and kET = 5.1 1010 s 1) were estimated from the step structure of T1 (Figure S31, Supporting Information), and the minimum values (EET = 0.95 and kET = 4.1 1010 s 1) were estimated from the stair structure, as shown in Figure 4. The kET of the step structure was larger than that of the stair structure owing to the shorter path length between the M1 and the M3 moiet-
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Conclusions In conclusion, we designed and prepared a [2.2]paracyclophane-based through-space conjugated oligomer, in which three p-conjugated systems were aligned and stacked. The arrangement of p-conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Fçrster mechanism. The key point for the efficient energy transfer is the orientation of the transition dipole moments between the adjacent p-electron systems as well as their proximity to each other. When the p-conjugated compounds with welloverlapped PL spectrum and absorption band are connected using [2.2]paracyclophane, an efficient energy transfer is readily achieved. A combination between the energy and electron transfer is also possible by designing the energy levels of p-conjugated systems in addition to the energy band gaps. The construction of through-space conjugated systems comprising four and five p-electron systems is currently underway for long-distance unidirectional energy transfer.
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Acknowledgements This work was partially supported by Grant-in-Aid for Young Scientists (A) (No. 24685018) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
[5]
[6] [1] a) C. J. Brown, A. C. Farthing, Nature 1949, 164, 915 – 916; b) D. J. Cram, H. Steinberg, J. Am. Chem. Soc. 1951, 73, 5691 – 5704. [2] a) Cyclophane Chemistry: Synthesis Structures and Reactions (Ed.: F. Vçgtle), Wiley, Chichester, 1993; b) Modern Cyclophane Chemistry (Eds.: R. Gleiter, H. Hopf), Wiley-VCH, Weinheim, 2004. [3] a) W. J. Oldham, Jr., Y. J. Miao, R. J. Lachicotte, G. C. Bazan, J. Am. Chem. Soc. 1998, 120, 419 – 420; b) G. C. Bazan, W. J. Oldham, Jr., R. J. Lachicotte, S. Tretiak, V. Chernyak, S. Mukamel, J. Am. Chem. Soc. 1998, 120, 9188 – 9204; c) S. Wang, G. C. Bazan, S. Tretiak, S. Mukamel, J. Am. Chem. Soc. 2000, 122, 1289 – 1297; d) J. Zyss, I. Ledoux, S. Volkov, V. Chernyak, S. Mukamel, G. P. Batrholomew, G. C. Bazan, J. Am. Chem. Soc. 2000, 122, 11956 – 11962; e) G. P. Bartholomew, G. C. Bazan, J. Am. Chem. Soc. 2002, 124, 5183 – 5196; f) G. P. Bartholomew, G. C. Bazan, Synthesis 2002, 1245 – 1255. [4] a) Y. Morisaki, Y. Chujo, Angew. Chem. Int. Ed. 2006, 45, 6430 – 6437; Angew. Chem. 2006, 118, 6580 – 6587; b) Y. Morisaki, Y. Chujo,
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[7] [8] [9] [10] [11] [12]
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Polym. Chem. 2011, 2, 1249 – 1257; c) Y. Morisaki, Y. Chujo, Chem. Lett. 2012, 41, 840 – 846; d) Y. Morisaki, S. Ueno, A. Saeki, A. Asano, S. Seki, Y. Chujo, Chem. Eur. J. 2012, 18, 11251 – 11257. a) Y. Morisaki, N. Kawakami, T. Nakano, Y. Chujo, Chem. Lett. 2014, 43, 426 – 428; b) Y. Morisaki, N. Kawakami, T. Nakano, Y. Chujo, Chem. Eur. J. 2013, 19, 17715 – 17718. a) A. Molina-Ontoria, M. Wielopolski, J. Gebhardt, A. Goulumis, T. Clark, D. M. Guldi, N. Martn, J. Am. Chem. Soc. 2011, 133, 2370 – 2373; b) M. Wielopolski, A. Molina-Ontoria, C. Schubert, J. T. Margraf, E. Krokos, J. Kirschner, A. Gouloumis, T. Clark, D. M. Guldi, N. Martn, J. Am. Chem. Soc. 2013, 135, 10372 – 10381. T. Fçrster, Naturwissenschaften 1946, 33, 166 – 175. Principles of Fluorescence Spectroscopy, 3rd ed. (Ed.: J. R. Lakowicz), Springer, New York, 2006, pp. 443 – 475. Figure S28 in Supporting Information. PL decay plots of M3 and D2 are shown in Figure S22 and S26, respectively. D. L. Dexter, J. Chem. Phys. 1953, 21, 836 – 850. S. Clment, T. Goudreault, D. Bellows, D. Fortin, L. Guyard, M. Knorr, P. D. Harvey, Chem. Commun. 2012, 48, 8640 – 8642. Received: June 12, 2014 Published online: July 23, 2014
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Through-Space Conjugated Molecular Wire Comprising Three p-Electron Systems Yasuhiro Morisaki,* Naoya Kawakami, Shotaro Shibata, and Yoshiki Chujo*[a]
Abstract: A [2.2]paracyclophane-based through-space conjugated oligomer comprising three p-electron systems was designed and synthesized. The arrangement of three p-conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Fçrster mechanism. The energy transfer efficiency
and rate constants were estimated to be > 0.999 and > 1012 s 1, respectively. The key point for the efficient energy transfer is the orientation of the transiKeywords: energy transfer · FRET · ACHTUNGRE[2.2]paracyclophane · molecular wire · through-space conjugation
tion dipole moments. The time-dependent density functional theory (TDDFT) studies revealed the transition dipole moments of each stacked p-electron system; each dipole moment was located on the long axis of each stacked p-electron system. This alignment of the dipole moments is favorable for fluorescence resonance energy transfer (FRET).
Introduction Considerable attention has been paid to the unique structure of [2.2]paracyclophane consisting of two phenylenes stacked in proximity to each other.[1, 2] The introduction of certain p-conjugated units into the [2.2]paracyclophane skeleton provides a partially stacked structure of the resulting p-electron systems. The photophysical properties of such [2.2]paracyclophane-based systems with two stacked chromophores have been extensively investigated. The photoluminescence (PL) properties of the stacked p-electron system strongly depend on the length and stacked position of the p-electron system.[3] In other words, each partially stacked p-electron system with expanded conjugation absorbs and emits light independently. We have also focused on the [2.2]paracyclophane skeleton and synthesized [2.2]paracyclophane-containing through-space conjugated oligomers and polymers.[4] It was found that photoexcited energy and hole migrated through the stacked p-electron systems in a single polymer chain.[4d] To better understand the photoexcited energy transfer, through-space conjugated dimers comprising energy donor–acceptor p-electron systems (D1[5a] and D2[5b] in Figure 1) have been recently reported. The [2.2]paracyclophane scaffold appropriately aligned the transition dipole moments of the stacked p-elec-
Figure 1. Through-space conjugated oligomers and model compounds discussed in this study (R = C12H25). Energy band gaps (Eg) of M1–3 were estimated by their UV/Vis absorption edges in CHCl3 (1.0 10 5 m).
[a] Prof. Dr. Y. Morisaki, N. Kawakami, S. Shibata, Prof. Dr. Y. Chujo Department or Polymer Chemistry, Graduate School of Engineering Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan) Fax: (+ 81) 75-383-2607 E-mail: [email protected] [email protected]
tron systems, thus resulting in a high energy-transfer efficiency (FET > 0.999) with a high energy-transfer rate constant (kET) on the order of 1012 s 1. To expand the energytransfer system from two p-electron systems to three p-electron systems, in this study, a through-space conjugated trimer T1 consisting of three p-electron systems was synthe-
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402653.
Chem. Asian J. 2014, 9, 2891 – 2895
2891
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
Yasuhiro Morisaki et al.
chain ends.[6] Photoexcited electron transfer occurred, resulting in charge separation; thus, the through-space conjugated oligomers transferred electrons through the stacked p-electron systems. This study moreover indicates that [2.2]paracyclophane-based through-space conjugated systems are promising candidates for single molecular wires that transfer energy and electrons depending on the design of the stacked p-electron systems.
Results and Discussion Synthetic routes to the target through-space conjugated trimer T1 are shown in the Supporting Information. This system was designed to obtain a large overlap integral between an absorption band and a PL peak of adjacent p-electron systems; that is, it was designed for efficient fluorescence resonance energy transfer (FRET)[7, 8] from the M1 to the M3 moiety via the M2 moiety. Figure 2 A shows the absorption and PL spectra of T1 in dilute CHCl3 (1.0 10 5 m and 10 6 m, respectively). The PL spectrum was obtained by excitation at 290 nm; this wavelength mainly excites the M1 moiety. The absorption spectrum of T1 comprised the absorption spectra of the three stacked p-electron systems M1–3 (each spectrum is shown in the Supporting Information), whereas the PL spectrum exhibited a single-component emission at 525 nm with a PL lifetime (t) of 5.61 ns
Figure 2. (A) UV/Vis absorption spectrum of T1 in CHCl3 (1.0 10 5 m) and PL spectra of T1 in CHCl3 (1.0 10 6 m, excited at 290 nm). (B) UV/ Vis absorption spectrum of a mixture of M1–3 in CHCl3 (1.0 10 5 m) and PL spectra of M1–3 in CHCl3 (1.0 10 6 m, excited at 290 nm). The asterisks denote scattered light and overtone peaks.
sized (Figure 1). Using this system, an efficient unidirectional energy transfer could be achieved. Furthermore, the detailed mechanism of energy transfer was elucidated experimentally and theoretically. Recently, Guldi, Martn, and coworkers have reported [2.2]paracyclophane-based through-space conjugated oligomers containing zinc porphyrin and C60 at the oligomer
Chem. Asian J. 2014, 9, 2891 – 2895
Figure 3. Excitation spectra of T1 at 525 nm and D2 at 522 nm in CHCl3 (1.0 10 6 m). The asterisk denotes halftone peaks (at 262.5 nm and 261 nm).
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(c2 = 1.11).[9] This PL behavior was consistent with those of M3 (513 nm, t = 6.05 ns, c2 = 1.01) and D2 (522 nm, t = 5.67 ns, c2 = 1.16).[10] In the PL spectrum of T1, the emissions from the M1 and M2 moieties did not occur, and only the emission from the M3 moiety was observed as expected. On the other hand, Figure 2 B shows the PL spectrum of a mixture of M1–3 in CHCl3 (concentration of each compound = 1.0 10 6 m, excitation wavelength = 290 nm). Emissions from M1–3 were clearly observed at 318 nm, 401 nm, and 513 nm, respectively. By combining M1–3 with the [2.2]paracyclophane bridges, the emissions from the M1 and M2 moieties were quenched in T1. Figure 3 shows the excitation spectra of D2 and T1 in CHCl3 (1.0 10 6 m) monitored at 522 nm and 525 nm, respectively (the emission peak tops of the M3 moieties in D2 and T1, respectively). In the excitation spectrum of T1, the peak at around 300 nm increased compared to that in the spectrum of D2, thus indicating the contribution of the M1 moiety to the emission of T1. These results confirm that photoexcited energy was intramolecularly transferred from the M1 and M2 moieties to the M3 moiety. To estimate the photoexcited energy-transfer efficiency (EET) and mechanism of T1, density functional theory (DFT) studies were carried out. Figure 4 shows the molecular orbitals (MOs) of T1 calculated using the CAM-B3LYP long-rang corrected function with the 6-31G* basis set, and
Figure 5 shows the optimized structure of T1. The MOs comprised the frontier orbitals of M1–3; namely, HOMO-1/ LUMO, HOMO/LUMO + 1, and HOMO-2/LUMO + 2, corresponding to the frontier orbitals of the M3, M2, and M1 moieties. Their oscillator strengths (f) were calculated with the time-dependent DFT (TD-DFT), and the f values for HOMO-1/LUMO, HOMO/LUMO + 1, and HOMO-2/ LUMO + 2 were calculated to be 1.30 (l = 423 nm), 1.68 (l = 345 nm), and 0.56 (l = 305 nm),[11] consistent with the UV spectrum of T1. The TD-DFT studies revealed the transition dipole moments of each stacked p-electron system in T1 (Figure 5 and Table 1); each dipole moment was located on the long axis Table 1. Oscillator strength (f) and dipole moment of T1. Transition
l [nm]
f
HOMO-1 to LUMO HOMO to LUMO + 1 HOMO-2 to LUMO + 2
423 345 305
1.30 1.68 0.56
mx 4.2149 4.3143 2.3487
my 0.2786 0.3890 0.0782
mz 0.4289 0.5356 0.3655
of each stacked p-electron system. This alignment of the dipole moments is favorable for FRET,[8] and the orientation factors (k2) between the neighboring p-electron systems were calculated to be 3.52 and 3.68 for the M1/M2 and M2/
Figure 4. Molecular orbitals (top views) of T1 as well as model compounds M1–3 calculated at the CAM-B3LYP/6-31G* level. Calculations were carried out with the OCH3 substituent instead of OC12H25.
Chem. Asian J. 2014, 9, 2891 – 2895
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Figure 6. Results of energy transfer through T1.
ies. In both cases, the kET of direct energy transfer were sufficiently large on the order of 1010 s 1. However, we should conclude that the stepwise energy transfer from the M1 to the M3 via the M2 moiety is dominant according to the larger kET values. The theoretical calculations described above were carried out on the assumption that Fçrster-type[7] energy transfer occurred, disregarding the Dexter-type[11] energy transfer. Recently, Knorr and Harvey reported the triplet-state energy transfer, which is performed by the Dexter mechanism, using platinum-containing through-space conjugated copolymers.[12] The kET values were calculated to be on the order of 104–105 s 1. Moreover, the photoexcited electron transfer rate constant through the stacked p-electron systems was estimated to be on the order of 109 s 1.[6] From these rate constant values, it can be concluded that efficient photoexcited energy transfer occurred dominantly by the Fçrster mechanism in this study.
Figure 5. Optimized structure of T1 calculated at the CAM-B3LYP/631G* level.
M3 pairs (Figures S29 and S30, Supporting Information), respectively. The maximum k2 value is theoretically 4; therefore, the obtained k2 values of 3.52 and 3.68 were sufficiently large, indicating ideal orientations for FRET between the neighboring p-electron systems in T1. The spectral overlap integrals (J) of the M1/M2 and M2/M3 pairs were calculated from their absorption and PL spectra, and the Fçrster radii (R0) for the M1/M2 and M2/M3 pairs were calculated to be 42.0 and 58.6 , respectively. Finally, the EET and rate constants (kET) were estimated; the results are shown in Figure 6, and the detailed data are summarized in Figures S29 and S30 in the Supporting Information. Both the EET (from the M1 to the M2 moieties and from the M2 to the M3 moieties) were > 0.999, and their kET values were 1.10 1013 s 1 and 3.54 1012 s 1, respectively. In this system, the direct energy transfer from the M1 to the M3 moieties is possible. Thus, as shown in Figure 6, the EET and kET values from the M1 to the M3 moieties in T1 were also calculated to be 0.95–0.96 and 4.1–5.1 1010 s 1, respectively. The maximum values (EET = 0.96 and kET = 5.1 1010 s 1) were estimated from the step structure of T1 (Figure S31, Supporting Information), and the minimum values (EET = 0.95 and kET = 4.1 1010 s 1) were estimated from the stair structure, as shown in Figure 4. The kET of the step structure was larger than that of the stair structure owing to the shorter path length between the M1 and the M3 moiet-
Chem. Asian J. 2014, 9, 2891 – 2895
Conclusions In conclusion, we designed and prepared a [2.2]paracyclophane-based through-space conjugated oligomer, in which three p-conjugated systems were aligned and stacked. The arrangement of p-conjugated systems in an appropriate order according to the energy band gap resulted in efficient unidirectional photoexcited energy transfer by the Fçrster mechanism. The key point for the efficient energy transfer is the orientation of the transition dipole moments between the adjacent p-electron systems as well as their proximity to each other. When the p-conjugated compounds with welloverlapped PL spectrum and absorption band are connected using [2.2]paracyclophane, an efficient energy transfer is readily achieved. A combination between the energy and electron transfer is also possible by designing the energy levels of p-conjugated systems in addition to the energy band gaps. The construction of through-space conjugated systems comprising four and five p-electron systems is currently underway for long-distance unidirectional energy transfer.
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
Acknowledgements This work was partially supported by Grant-in-Aid for Young Scientists (A) (No. 24685018) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
[5]
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