J. Phys. Chem. 1995,99, 14793-14797
14793
Dimer Radical Cation Formation in Poly(viny1pyrenes) Studied by Pulse Radiolysis Akira Tsuchida, Taiji Ikawa, and Masahide Yamamoto" Division of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606, Japan
Akito Ishida and Setsuo Takamuku The Institute of Scientific and Industrial Research, Osaka University, Yamadaoka, Suita, Osaka 565, Japan Received: April 14, 1995; In Final Form: July 17, 1995@
The intrapolymer dimer radical cation formed in poly(1-vinylpyrene) and in a novel polymer of poly(2vinylpyrene) was studied by nanosecond pulse radiolysis measurements. Both polymers gave the main transient absorption band at 510 nm in a 1,2-dichloroethane solution at room temperature. The band was ascribed to the full-overlap dimer radical cation of the pyrene chromophore from the transient absorption spectra of three pyrene dimer model compounds; 1,3-&( 1-pyrenyl)propane and 1,3-di(2-pyrenyl)propane gave the full-overlap dimer radical cation band at 5 10 nm, whereas 1-( l-pyrenyl)-3-(2-pyrenyl)propane gave the partial-overlap dimer radical cation band at 500 and 600 nm. On the other hand, 1-ethylpyrene and 2-ethylpyrene formed a distorted intermolecular dimer radical cation with absorption bands at (400, 510, and 580 nm.
Introduction The dimer radical cation is an important ionic intermediate of concentrated or polymer systems in photochemistry, electrochemistry, and radiation chemistry.'-6 The dimer radical cation is formed when an aromatic monomer radical cation associates with a neutral chromophore of the same kind. This dimer radical cation is stabilized by a charge resonance between the constituent chromophores, and is expected to have a sandwich structure. Several studies have been made on the structure, kinetics, and reaction mechanisms of the aromatic dimer radical cations.7-l0 The pyrene (Py) chromophore is well-known for the excimer formation, and its photophysical properties have been studied extensively."-19 Recently, we studied the charge-resonance (CR) band of inter- and intramolecular dimer radical cation of the P y by the near-IR laser photoly~is.'~The dimer radical cation of Py compounds showed a clear CR absorption band at around 1500 nm, whereas the locally-excited (LE) band of the dimer in the visible region overlapped with the triplet-triplet (T-T) absorption bands of the Py. In photoexcitation, a considerable amount of the triplet state formation of the Py chromophore is unavoidable by the intersystem crossing from the excited singlet state; this makes it difficult to observe the locally-excited (LE) band of the dimer radical cation in the visible region clearly. Furthermore, for polymer systems, efficient deactivation of excited chromophores by the excited singlet-singlet (S-S) annihilation also makes the laser photolysis measurement diffxult.20.2' In the present study, the intramolecular dimer radical cation for the Py-containing polymers and their low molecular weight dimer and monomer model compounds were measured by the pulse radiolysis. The pulse radiolysis is an effective method to obtain the transient absorption of ionic species in polymer systems. The structures of the Py dimer radical cations with different ring overlaps were discussed on the basis of the transient absorption spectra. Experimental Section Chemicals. Preparation and purification of 1- and 2-ethylpyrene (1EPy and 2EPy, respectively), 1,3-di(1-pyreny1)@
Abstract published in Advance ACS Abstracts, September 1, 1995.
propane (lPylPy), 1-(l-pyrenyl)-3-(2-pyrenyl)propane (1Py2Py), and 1,3-di(2-pyrenyl)propane (2Py2Py) have already been rep~rted.'~The purity of these monomer and dimer analogs of poly(viny1pyrenes) was evaluated to be more than 99.5% by a liquid chromatography. Poly( 1-vinylpyrene) (P1W y ) and poly(2-vinylpyrene) (p2VPy) were prepared by a radical polymerization of the corresponding vinylpyrenes initiated by azobisisobutyronitrile (AIBN) in benzene at 333 K.22 The molecular weights after several precipitations from a benzene solution into methanol were determined by GPC (TOSOH, HLC802UR, polystyrene standard) as Mw = 9.2 x lo3, M n = 6.9 x lo3 and Mw = 8.1 x lo3, M,,= 5.6 x lo3 for PlVPy and P2VPy, respectively. 2-Vinylpyrene was synthesized from 2-pyrenecarboxaldehyde by the Wittig reaction.23 The copolymer of 1-vinylpyrene(lVPy) with methyl methacrylate (MMA) (lVPy(x)IMMA, x (mole fraction of the vinylpyrene) = 0.03, 0.14, 0.65, and 0.85) and the copolymer of 2-vinylpyrene (2Wy) with MMA (2Wy(x)/h4MA,x = 0.03) were also prepared by the radical copolymerization of vinylpyrenes with MMA in the same manner as described above. The molecular weight of each copolymer after purification by precipitation ranged from 1 x lo4 to 4 x 10"; larger molecular weight copolymers were obtained with larger MMA feeds. The vinylpyrene fraction of each copolymer was determined by UV and NMR spectroscopy and elementary analysis. 1,2-Dichloroethane (DCE) used as a solvent for pulse radiolysis measurements was dried over CaH2 and purified by distillation several times. Pulse Radiolysis Measurements. The L-band linear accelerator at Osaka University was used as the electron pulse source. The pulse energy was 28 MeV and the pulse width was 8 ns. A pulsed 450-W xenon lamp (Osram, XBO-450) was used as the monitoring light source. The detection system consisted of a photomultiplier (Hamamatsu, R- 1477), monochromator (CVI-Laser, DIGIKROM-240), and a transient digitizer (Tektronix, 7912AD) connected to a personal computer. The details of the apparatus were as reported p r e v i o u ~ l y . ~ ~ The sample solution was freed from oxygen by argon bubbling for 20 min and sealed in a 1-cm Suprasil cell just
0022-365419512099-14793$09.00/0 0 1995 American Chemical Society
Tsuchida et al.
14794 J. Phys. Chem., Vol. 99,No. 40,1995
IEPy 2EPy H CHI
1Py2Py
H H
ZPyZPy H H
CHI
IVPy(x)/MMA
PlVPy
2VPy(x)/MMA
P2VPY
Figure 1. Molecular structures of the Py compounds used in this study with their abbreviations.
0 400
before the measurement. All samples were measured in DCE solvent at room temperature (298 K).
Results and Discussion Figure 1 shows the Py compounds used in this work with their abbreviations. When two chromophores are connected by C3 methylene chains, they easily form an excimer overlap either sandwich or partial. The molecular structure of aromatic vinyl polymers to satisfy this condition enables the efficient excimer formation in solution.25 The dimer radical cation that takes a similar conformation to that of the excimer is also an important ionic intermediate of the aromatic vinyl polymers. Todesco et al. studied the excimer formation of PlVPy by time-dependent fluorescence measurements and concluded that there are two excimers with a full- and partial-overlap structures of the Py chromophores.14 Since the Py group of PlVPy is substituted at the non-C2-symmetry position, two rotamers of the excimer can be formed by the rotation of a Py moiety. On the other hand, for a novel polymer of P2VPy whose Py group is substituted at the C2-symmetry position, only the full-overlap excimer is allowed to be formed.23 The existence of several overlap conformations of the Py chromophores in their excimer and dimer radical cationic states have also been studied using the dimer compound^.^^"-'^^'^ These rotamers have different absorption bands and lifetimes. Therefore, to thoroughly understand the properties of the aromatic association formed in poly(vinylpyrenes), spectroscopic differentiation of each rotamer is necessary. As shown in Figure 1, three-dimer model compounds of the polymers were prepared to afford the different ring overlap structures for the intramolecular dimer radical cations. The MMA unit was copolymerized to reduce the pyrene diad in a polymer chain. Monomer Model Compounds. Figure 2 shows the transient absorption spectra of the monomer analog lEPy measured by pulse radiolysis at 30 and 150 ns after a pulse excitation in the DCE solvent. A large absorption band at 460 nm is ascribed to the monomer radical cation of Py.26 The excited triplet state of Py (3Py*)has been reported to have absorption bands at around 390,410, and 520 nm.27,28As this figure indicates, the contribution of this 3Py*to the spectra is negligible in this experimental condition, and therefore the spectra represent almost the pure absorption spectra of 1EPy radical cation (1EW'). In pulse radiolysis, the solvent is initially excited by an electron pulse and the radical cationic state of the solute is mostly produced by the subsequent hole transfer from the radical cation of the solvent.' DCE is a good solvent to form a solute cation and therefore the 3Py*formation was a minor process. The transient decay figure inserted in Figure 2 shows that 1EPy" disappeared in the time scale of microseconds at this
800 Wavelendh / nm
800
Figure 2. Transient absorption spectra of lEPy (1.4 x mom) in DCE obtained at 30 ns (closed circles) and 150 ns (open circles) after a pulse. Insert: transient decays measured at 460 and 580 nm.
400
600 Wavelength / nm
800
Figure 3. Transient absorption spectra of lEPy (1.0 x mom) in DCE obtained at 30 ns (closed circles) and 150 ns (open circles, absorbance x 2) after a pulse. Insert: transient decays measured at 460 and 580 nm.
concentration of lEPy (1.4 x mom) probably by the recombination with anionic species. Figure 3 shows the transient absorption spectra at 30 and 150 ns after excitation for a higher concentration of 1EPy (1.0 x mom). The monomer radical cation band of lEPy'+ at 460 nm decayed within 200 ns and several new bands appeared at ca. 2200 nm due to the small charge resonance stabilization energy.7b Therefore, the bands at 500 and 600 nm of 1Py241'+ are ascribed to the partialoverlap dimer radical cation. Figure 7 shows the transient absorption spectra of 2Py2Py'+, which can take only the full-overlap conformation of the Py rings because they are substituted at the C2-symmetry position. The dimer band of 2Py2Py'+ is located at 5 10 nm, which is the same position as that for lPylPy'+. As for the dimer conformation of lPylPy'+, two rotamers of partial (one benzene ring overlap of Py chromophores) and full (four benzene rings) overlap structures are possible. However, the same position of the dimer band at 510 nm for lPylPy'+ and 2Py2Py'+ indicates that lPylPy'+ takes only the full-overlap form. This is consistent with the finding of the CR band measurements,'b where both l P y l W + and 2Py24r'+ gave a CR band at the same peak wavelength 1600 nm with the same oscillator strength. Therefore, the partial-overlap conformation of the Py rings with only one benzene ring overlap is not stable enough to be formed. It is interesting that the intermolecular Py dimer radical cations of lEPy and 2EPy (Figures 3 and 4, respectively) described above have the dimer band at
