Article pubs.acs.org/JPCA
Properties of Triplet-Excited [n]Cycloparaphenylenes (n = 8−12): Excitation Energies Lower than Those of Linear Oligomers and Polymers Mamoru Fujitsuka,*,† Chao Lu,† Takahiro Iwamoto,‡ Eiichi Kayahara,‡,§ Shigeru Yamago,‡,§ and Tetsuro Majima*,† †
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan Institute for Chemical Research, Kyoto University, Uji 611-0011, Japan § CREST, Japan Science and Technology Agency, Tokyo 102-0076, Japan ‡
ABSTRACT: Cycloparaphenylenes (CPPs), a class of hoop-shaped conjugated macrocycles, have attracted the attention of researchers in various fields because of their interesting properties. Although their properties in the singlet-excited state have been reported, there is no systematic information on the triplet-excited state. In the present study, the properties of triplet-excited [n]CPP (n = 8−12, where n denotes the number of phenyl rings) were comprehensively investigated. The phosphorescence peak shifted to the shorter-wavelength side with increasing ring size of the CPPs, indicating smaller triplet energy for smaller CPPs. It was found that the triplet energy of a smaller CPP is even smaller than those of poly(p-phenylene)s, indicating that small hoop-shaped conjugated macrocycles are effective in realizing low-band gap materials. By applying laser flash photolysis, the Tn−T1 absorption spectra of CPPs were obtained, from which the sizedependence of energy levels of higher triplet states were also determined. Generation of singlet oxygen by the energy transfer from a triplet-excited CPP was confirmed. The generation yield became smaller with increasing size of the CPP in accordance with the triplet yield. From these observations, the size-dependence of the deactivation pathways is explained.
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INTRODUCTION
show various interesting properties. For example, CPPs can form supramolecular inclusion complexes with fullerenes.16−19 The smaller HOMO−LUMO gaps found with smaller CPPs are also interesting, because this tendency is contrary to that of linear oligomers and polymers.4,12 The quinoidal character of smaller CPPs, which is essential for charge conduction and photoexcitation of π-conjugated oligomers and polymers, has also been reported.20−22 In a previous study, we investigated the singlet-excited state properties, such as fluorescence lifetime and yield of CPPs.23 It was revealed that the fluorescence quantum yield varied significantly with the size of the ring. In particular, the fluorescence quantum yields of smaller CPPs were small (0.084 for [8]CPP in THF, for example), indicating that the singletexcited state deactivates by processes other than radiative process. Intersystem crossing and internal conversion should be considered to be the deactivation pathways; however, the contributions of these processes are unknown. Very recently, Kamat et al. reported some triplet properties of [9] and [12]CPPs.24 However, the triplet energy was not reported, and the precise size-dependence of triplet-excited CPPs has not been systematically investigated.
Hoop-shaped conjugated macrocycles have attracted the attention of researchers in various fields because of their interesting properties resulting from their distorted and strained aromatic systems and radially oriented p-orbitals.1−3 Cycloparaphenylenes (CPPs, Figure 1) are typical hoop-shaped conjugated macrocycles and can be regarded as the shortest armchair carbon nanotubes, while their synthetic methods were established rather recently.4−15 It has been reported that CPPs
Received: May 13, 2014 Revised: June 7, 2014
Figure 1. Molecular structure of [n]CPP (n = 8−12). © XXXX American Chemical Society
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dx.doi.org/10.1021/jp504689q | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
In the present study, we comprehensively investigated the properties such as triplet energy, lifetime, and yields of tripletexcited [n]CPPs (n = 8−12, n denotes the number of phenyl rings), by means of various spectroscopic methods. It was revealed that triplet properties showed substantial ring-size dependence, and the origin of the size dependence is discussed. In addition, it was also found that small hoop-shaped conjugated macrocycles are effective for the realization of low-band gap materials.
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EXPERIMENTAL SECTION
Materials. A series of [n]CPPs (n = 8−12) was synthesized according to the procedures reported in the previous papers.11,12 In the present study, solvents for the optical measurements were of the spectroscopic grade or the best grade available. Nanosecond Laser Flash Photolysis. The transient absorption measurement during the laser flash photolysis was carried out using a nanosecond photoreaction analyzer system (Unisoku, TSP-1000). After Ar bubbling of the sample, the sample was irradiated by third harmonic generation (355 nm) of ns Nd:YAG laser (Quantel, Brilliant). A pulsed Xe arc lamp (Ushio, UXL-451-0, 450 W) was employed as a monitor light source. The monitor light passed through the sample solution was focused on an entrance slit of a monochromator (Unisoku, MD200) and detected with a photomultiplier tube (Hamamatsu Photonics, R2949) and a transient digitizer (Tektronix, TDS580D). The transient absorption spectra were measured using a photodiode array (Hamamatsu Photonics, S3904− 1024F) with a gated image intensifier (Hamamatsu Photonics, C2925−01) as a detector. Phosphorescence Measurements. Phosphorescence spectra of the sample were measured using Horiba FluoroMax-4P. For detection of phosphorescence, the sample was excited with pulsed light and detected using a gate between 50 μs and 1 ms after excitation, typically. Near-IR Emission Measurements. The third harmonic generation (355 nm) from an ns Nd:YAG laser (Quantel, Brilliant) was used for the excitation of the sample. Emission from the sample cell was focused on to an entrance slit of a monochromator (Koken Kogyo, SG-100) and then detected by a near-IR photomultiplier tube module (Hamamatsu Photonics, H10330A-75). After amplification by a 350 MHz amplifier unit (Stanford Research Systems, SR445A), the output of the photomultiplier was recorded with a gated photon counter (Stanford Research Systems, SR400) controlled by PC. The generation yield of singlet oxygen (ΦO2) was determined by comparing the emission intensity with that of the reference with matched absorbance at the excitation wavelength.
Figure 2. Normalized phosphorescence spectra of [n]CPP (n = 8−12) in EEET at 77 K. Excited at 350 nm. Phosphorescence spectra were measured by applying gate between 50 μs and 1 ms after excitation, typically.
peak, as listed in Table 1. The separation between the main and second peaks was ∼1300 cm−1, which was almost the same as Table 1. Fluorescence (λfl) and Phosphorescence (λph) Peak Positions in EEET at 77 K and S1 and T1 energies (E(S1) and E(T1), respectively) of [n]CPP (n = 8−12) n
λfl (nm)
8 9 10 11 12
495,a 462,a 440, 428, 421,
E(S1) (eV)b 530 489 466 453 446
2.51 2.68 2.82 2.90 2.95
λph (nm) 671 633 610, ∼660 600, 651 591, 638
E(T1) (eV)c 1.85 1.96 2.03 2.07 2.10
a
Peak position was estimated by derivative of the spectrum. Estimated from the fluorescence peak position. Although E(S1) can be estimated from the intersection of normalized absorption and fluorescence spectra, unclear (0,0) absorption band of CPPs makes this difficult. cEstimated from the phosphorescence peak position.
b
that of fluorescence observed under the same conditions. In the cases of [8]CPP and [9]CPP, the second peak was obscure and appeared as a shoulder. The most prominent feature of the phosphorescence of CPPs is the ring-size-dependence of the peak position: The phosphorescence peak appeared at a shorter wavelength with an increase in the ring size of the CPP. This tendency is contrary to that of linear oligomers, including oligo(pphenylene)s, which exhibit a peak on the longer-wavelength side with an increase in the chain length.26 The present result indicates that the triplet (T1) energy (E(T1)) of a CPP becomes higher with an increase in the size of the ring. In Table 1, the E(T1) values estimated from the phosphorescence peak positions are listed. The singlet (S1) energy (E(S1)) also showed a similar ring-size dependence (Table 1). The variation in the E(T1) of the present [n]CPP (n = 8−12) is 0.25 eV, which is smaller than that for E(S1) (0.44 eV). This is the same tendency as for other linear oligomers such as oligo(pphenylene)s and oligothiophenes.26−29 The E(T1) values of the polymers of 2,5-disubsituted phenylene and ladder-type poly(p-phenylene) are reported to be 2.1−2.2 eV.26 Thus, the E(T1) of a CPP becomes close to that of poly(p-phenylene)s with an increase in n. From the viewpoint of lowering E(T1),
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RESULTS AND DISCUSSION Triplet Energy of CPPs. For phosphorescence measurements, the employment of a low temperature glassy matrix and the removal of fluorescence by applying a gate are usually essential. Additionally, in this study, the EEET solvent (a 2:2:1:1 mixture of ethyl iodide, ether, ethanol, and toluene) was employed to enhance the intersystem crossing yield due to the external heavy atom effect.25 Figure 2 shows the normalized phosphorescence spectra of CPPs in EEET at 77 K, which exhibited a peak at 671−591 nm depending on the ring size. In the cases of [10]CPP, [11]CPP, and [12]CPP, an additional peak was observed on the longer-wavelength side of the main B
dx.doi.org/10.1021/jp504689q | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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estimated on the basis of relative actinometry method32 using the reported value for [12]CPP as a reference24 and the intersystem crossing yield discussed in later section. Decay kinetic profiles of the triplet-excited states were measured with sufficiently low excitation laser power in order to minimize the contribution of triplet−triplet annihilation. The decay profiles were reasonably fitted to a single exponential function, from which the lifetimes of the triplet state (τT) were estimated as summarized in Table 2. The τT values were about 60 μs for all CPPs, indicating no clear size dependence for triplet lifetimes. Because the observed peaks can be attributed to electronic transitions from T1 to higher triplet states, the observed peaks represent energy differences between T1 and higher triplet states. By assuming that the peaks at around 700 and 400 nm are due to Tn−T1 and Tn′−T1 transitions, respectively, the energy levels of Tn and Tn′ can be calculated. In Figure 4, the
the lowest limit of E(T1) in the case of linear oligomers is that of a polymer. However, the hoop-shaped conjugated macrocycles exhibited even lower E(T1)s as well as E(S1)s, which became smaller with decreases in the ring size. From these facts, it can be pointed out that small hoop-shaped conjugated macrocycles are effective for the realization of low-band gap materials, which are useful in various applications such as light energy conversion and conductive materials.30 Triplet Lifetimes of CPPs. Figure 3 shows transient absorption spectra at 100 ns after excitation during the laser
Figure 3. Transient absorption spectra of [n]CPP (n = 8−12) in THF at 100 ns after laser excitation during laser flash photolysis using 355 nm-nanosecond laser.
flash photolysis of [n]CPP (n = 8−12) in THF. The transient absorption spectra show spectral patterns similar to each other, i.e., peaks appeared at around 700 and 400 nm. The 700 nm peak resembles that of hexaphenyl.31 In detail, both the CPP peaks tended to shift to the shorter-wavelength side with an increase in the ring size, as summarized in Table 2. The extinction coefficients of the absorption bands (Table 2) were
Figure 4. Ring-size dependence of S1-, T1-, Tn-, and Tn′-excitation energies of [n]CPP (n = 8−12). Arrows indicate that the indicated level is lower limit.
energy levels of these states as well as S1 and T1 are plotted against n of [n]CPP. For Tn′ of [11]CPP and [12]CPP, the levels indicated in Figure 4 are lower limits because the absorption peaks should be located at wavelengths shorter than that of the present spectral region. It is obvious that all energy levels of excited states became higher with an increase in the ring size, which is the reverse of linear oligomers. In addition, all energy levels change extensively when n is small, that is, the change in the energy level seems to be almost saturated for a larger n such as n = 12. In addition to the absorption bands at around 700 and 400 nm, a small band was observed between them. Interestingly, the band shifted to a longer wavelength with an increase in the size of the ring, probably indicating that the origin of the band is similar to that of linear oligomers. Triplet Yield. It is known that, in the presence of molecular oxygen in a solvent, a triplet-excited solute (3S*) deactivates by energy transfer generating singlet oxygen (1O2) according to eq 1,
Table 2. Absorption Peak Positions of Transient Absorption Spectra (λTT) and Triplet Lifetime (τT) of [n]CPP (n = 8− 12) in THF n 8 9 10 11 12
λTT (nm)a,b 708 (48000), 671, 678 (39000), 676 (37000), 668 (31000)d,
400 389 (25000)d 472, 383 487,
