ChemComm COMMUNICATION
Cite this: Chem. Commun., 2013, 49, 2998 Received 11th January 2013, Accepted 21st February 2013
A boron dipyrromethene–phthalocyanine pentad as an artificial photosynthetic model† Jian-Yong Liu,a Yingsi Huang,a Roel Menting,b Beate Ro ¨ der,b Eugeny A. Ermilovbc a and Dennis K. P. Ng*
DOI: 10.1039/c3cc00262d www.rsc.org/chemcomm
A silicon(IV) phthalocyanine with two axial p-phenylene-linked boron dipyrromethene and monostyryl boron dipyrromethene moieties has been prepared. The resulting pentad absorbs strongly in most of the UV-visible region and serves as an artificial photosynthetic antenna–reaction centre model.
There has been considerable interest in the development of artificial photosynthetic models that can mimic the primary events of natural photosynthesis, including light harvesting, photoinduced multistep electron transfer and catalysis.1 The studies are important not only for photochemical conversion of solar energy into fuels,1a,e but also for construction of various optoelectronic devices.2 Various chromophores and redox-active units have been chosen and assembled to construct these systems, particularly porphyrins and fullerenes, which have been studied extensively. As part of our continuing interest in phthalocyanine (Pc) and boron dipyrromethene (BDP) derivatives as versatile functional dyes, we thought that these compounds are also excellent building blocks for this application. In fact, both classes of dyes show strong absorptions in the UV-visible region and exhibit tunable redox and photophysical properties. A substantial number of Pc- and BDP-based light-harvesting and electron donor–acceptor systems have been reported.3,4 However, conjugates of these chromophores remain very rare.5 Recently, we have prepared two silicon(IV) phthalocyanines with two axial BDP or monostyryl BDP (MSBDP) moieties, which undergo predominantly a photoinduced energy and electron
transfer process in toluene respectively.5a,b The slight modification of the BDP skeleton can switch the photophysical pathway of the system. Based on this finding, we have extended the study by preparing another analogue in which the silicon centre is conjugated to two p-phenylene-linked BDP and MSBDP moieties via the styryl end. This pentad absorbs strongly in most of the UV-visible region. Upon excitation at the peripheral BDP units, it undergoes sequential energy and electron transfer, thus mimicking the photosynthetic antenna–reaction centre complex. The synthesis and photoinduced processes of this interesting BDP–Pc pentad are reported below. Scheme 1 shows the synthetic route used to prepare this pentad. The p-phenylene-linked bis-BDP 1 was first prepared by
a
Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China. E-mail:
[email protected]; Fax: +852 2603 5057; Tel: +852 3943 6375 b ¨r Physik, Photobiophysik, Humboldt-Universita¨t zu Berlin, Newtonstr. Institut fu 15, D-12489 Berlin, Germany c ¨r Physiologie, Campus Benjamin Franklin, Charite´ – Universita¨tsmedizin Institut fu Berlin, Thielallee 71, D-14195 Berlin, Germany † Electronic supplementary information (ESI) available: Experimental details and characterisation data, normalised absorption and DAF spectra of 2, 6 and 7, comparison of the fluorescence spectra of 2, 4 and 5, transient absorption spectra of 2 in toluene and 4 in chloroform, and their corresponding ground-state recovery profiles, electrochemical data for 2 and 5 and 1H NMR spectra of the new compounds. See DOI: 10.1039/c3cc00262d
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Scheme 1
Synthesis of pentad 4.
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Communication
Fig. 1
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Structures of the reference compounds 5–7.
treating 2,4-dimethylpyrrole with terephthalaldehyde via sequential condensation catalysed by trifluoroacetic acid (TFA), oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and complexation with BF3Et2O according to a typical procedure for BDP synthesis.6 This compound then underwent Knoevenagel condensation reaction with one equiv. of 4-hydroxybenzaldehyde to give dyad 2 in 38% yield. Finally, 2 was treated with silicon(IV) phthalocyanine dichloride (3) in the presence of pyridine in toluene to afford pentad 4 in 82% yield. This compound was purified by silica gel column chromatography followed by size exclusion chromatography and recrystallisation. For comparison, Pc 5,5a BDP 67 and 4-hydroxystyryl BDP 7 (Fig. 1) were also prepared. Compound 7 was prepared by mono-condensation of 6 with 4-hydroxybenzaldehyde. The experimental details and characterisation data for all the new compounds are given in ESI.† To reveal the electronic interactions among the chromophores in pentad 4, the absorption spectrum of 4 was recorded in toluene and compared with those of the dyad 2 and the reference compounds 5–7. The data are compiled in Table 1. As shown in Fig. 2, the spectrum of 4 shows several strong Q-band absorptions at 681 nm due to the Pc core, 586 nm due to the MSBDP bridges, and 507 nm due to the peripheral BDP units, as well as a strong B band at ca. 350 nm for these chromophores. The Q bands of BDP and MSBDP moieties are comparable with those of the dyad 2 (507 and 584 nm respectively), but red-shifted by 4 and 14 nm, respectively, as compared to those of the reference compounds 6 and 7. This observation suggests that these two chromophores may have ground-state interactions both in the pentad 4 and dyad 2 (Fig. S1, ESI†). Due to the fact that the absorptions of the different chromophores in 4 are well separated, they could be selectively excited. Upon excitation at 470 nm, where only the BDP moiety absorbs, Table 1
Fig. 2 Normalised electronic absorption spectra of pentad 4 and the reference compounds 5–7 in toluene.
the dyad 2 gave a strong fluorescence emission at 590 nm due to the MSBDP moiety (Fig. 3). The BDP emission at ca. 510 nm as shown in the spectrum of 6 became very weak. The fluorescence quantum yield for the BDP emission decreased from 0.57 (for 6) to 0.007, and the value for the MSBDP emission (FF = 0.66) was slightly lower than that of 7 (FF = 0.78) when it was directly excited (Table 1). The results indicated the presence of an efficient photoinduced energy transfer from the excited BDP unit to the MSBDP entity in 2. It was supported by the excitation spectrum, which resembled the absorption spectrum (Fig. S2, ESI†). In fact, both MSBDP 7 and an equimolar mixture of 6 and 7 in toluene did not give this emission band. By comparing the normalised absorption and excitation spectra in the BDP region (430–540 nm), the energy transfer quantum yield (FENT) was estimated to be 94%. The energy transfer process in dyad 2 was also studied using time-resolved fluorescence spectroscopy. In toluene, the singlet excited state of both the reference compounds 6 and 7 decayed mono-exponentially with a lifetime of 3.11 0.02 and 3.46 0.02 ns respectively (Fig. S3, ESI†). For the dyad 2, upon excitation at the BDP moiety, the fluorescence decay of the MSBDP unit was detected, which also followed a mono-exponential manner with a lifetime of 3.82 0.02 ns. This value is slightly longer than that of MSBDP 7, which may be attributed to the additional BDP substituent in the dyad. Upon direct excitation at the MSBDP moiety, a strong emission band at 590 nm was also observed and the decay-associated fluorescence (DAF) spectrum, which showed only one decay component with a lifetime of 3.84 0.02 ns, was essentially the same as that obtained upon BDP-part excitation (Fig. S4, ESI†). The very similar lifetimes suggested that electron
Electronic absorption and fluorescence data for dyad 2, pentad 4 and the reference compounds 5–7 in toluene
FFa
lem (nm) Comp.
lmax (nm) (log e)
BDP-part excitation
MSBDP-part excitation
Pc-part excitation
BDP-part excitation
MSBDP-part excitation
Pc-part excitation
2 4 5 6 7
507 507 683 503 572
513,b 590c 513,b 589c — 512 —
590c 590c — — 581
— —d 687 — —
0.007,b 0.66c 0.001,b 0.001c — 0.57 —
0.70c 0.001c — — 0.78
— —d 0.60 — —
(5.03), 584 (5.04) (5.24), 586 (5.26), 681 (5.28) (5.37) (4.99) (5.07)
a With reference to rhodamine 110 in ethanol (FF = 0.94)8 (for BDP-part emission), rhodamine 6G in ethanol (FF = 0.95)9 (for MSBDP-part emission) or meso-tetraphenylporphyrin in N,N-dimethylformamide (DMF) (FF = 0.11)10 (for Pc-part emission). b For BDP-part emission. c For MSBDP-part emission. d Pc-part emission was not observed.
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Fig. 3 Fluorescence spectra of dyad 2, pentad 4 and BDP 6 in toluene with equal absorbance at the excitation position at 470 nm.
transfer did not take place to quench the excited MSBDP in 2. The picosecond transient absorption spectra of 2 in toluene showed a strong ground-state bleaching for the MSBDP moiety both upon excitation at the BDP and MSBDP parts (Fig. S5, ESI†). The corresponding decay times were found to be 3.8 0.1 and 3.7 0.1 ns, which correlate well with the measured fluorescence lifetimes. By contrast, the MSBDP-part emission of pentad 4 was significantly quenched. Upon excitation at the BDP moiety at 470 nm, virtually no fluorescence emission due to MSBDP was observed (Fig. 3). When the MSBDP unit was directly excited at 544 nm, the MSBDP-part emission at ca. 590 nm was also greatly reduced compared with that for dyad 2 (Fig. S6, ESI†). The fluorescence quantum yield decreased from 0.70 (for 2) to 0.001 (for 4) (Table 1). The emission band at ca. 690 nm due to the Pc core was not observed. These results suggested that the first excited singlet state of the MSBDP moiety in 4 is strongly quenched by the attached Pc core likely by photoinduced electron transfer. In addition, upon excitation at the Pc core at 613 nm, while the reference compound 5 gave a strong emission peak at 687 nm (FF = 0.60), no fluorescence was detected for the pentad 4 (Fig. S7, ESI†). Since energy transfer from the excited Pc core to the MSBDP moiety is energetically unfavourable, the fluorescence quenching of pentad 4 should be mainly due to the electron transfer process between the Pc and MSBDP entities. To better understand this process, the electrochemical properties of dyad 2 and Pc 5 in DMF were studied using cyclic voltammetry and the data are given in Table S1 (ESI†). Unfortunately, the data for pentad 4 could not be obtained due to its low solubility in DMF. Nevertheless, as the BDP–MSBDP dyad and the Pc core do not have significant ground-state interactions in pentad 4, it can be assumed that it retains the electrochemical properties of 2 and 5. As shown in Table S1 (ESI†), the dyad 2 has a lower oxidation potential than Pc 5, suggesting that the BDP–MSBDP unit is a better electron donor than Pc. The Pc core, however, is easier to be reduced than the BDP–MSBDP moiety according to their first reduction potentials. According to the Rehm–Weller equation11 and by
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Communication using these electrochemical data and the S0 - S1 transition energy of Pc 5 (1.81 eV) and MSBDP 7 (2.15 eV), the free-energy change (DG1) of charge-separation in 4 was estimated to be 0.11 eV (via 1Pc*) and 0.45 eV (via 1MSBDP*), respectively (ESI†), showing that the photoinduced electron transfer from the BDP–MSBDP moiety to the Pc core is a thermodynamically favourable process upon excitation at both the Pc and MSBDP units. The picosecond transient absorption spectra of 4 were also recorded in toluene. No bleaching signals were observed even at short delay time after excitation at the BDP-, MSBDPor Pc-part. This observation indicated that depopulation of the excited states of this pentad proceeds extremely fast. Nevertheless, we were able to observe the bleaching signals when the solvent was changed to chloroform (Fig. S8, ESI†). In addition, a weak absorption band at ca. 620 nm was also observed, which is one of the characteristic bands for Pc .5b,12 The decay of this absorption and the ground-state recovery of the three components proceeded mono-exponentially with a characteristic time of 27 ps (Fig. S9, ESI†), which can be attributed to the lifetime of the charge-separated state. In conclusion, we have prepared a novel pentad of Pc and BDP derivatives. This compound exhibits strong absorptions covering a wide region in the solar spectrum. Upon excitation at the peripheral BDP units, the energy is transferred to the MSBDP moiety, which is followed by an electron transfer to the Pc core. This pentad thus serves as an artificial photosynthetic antenna–reaction centre model.
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