J. Phys. Chem. 1996, 100, 8867-8874
8867
Transient Raman Spectroscopic Investigations on RuIITPP(L)2 (L ) Pyridine and Piperidine) and RuIITPP(CO)(py) in Various Solvents: Alternation of Excited Charge Transfer Depending on Axial Ligation Sae Chae Jeoung and Dongho Kim* Spectroscopy Laboratory, Korea Research Institute of Standards and Science, Taejon 305-600, Korea
Dae Won Cho† and Minjoong Yoon* Department of Chemistry, Chungnam National UniVersity, Taejon 305-764, Korea
Kwang-Hyun Ahn* Department of Chemistry, Kyung Hee UniVersity, Yongin 449-701, Korea ReceiVed: December 5, 1995; In Final Form: February 26, 1996X
Transient Raman spectra with an excitation of nanosecond pulses at 416 nm were first recorded for RuIITPP(L)2 (L ) pyridine and piperidine) and RuIITPP(CO)(py) in various solvents such as benzene, tetrahydrofuran, pyridine, and piperidine. This work is intended to investigate the nature of the charge transfer states accompanied by photoexcitation. It was found that the electronic structure of the lowest energy charge transfer (CT) state is largely dependent on not only the nature of axial ligands attached to the central metal but also the solvents employed. As for RuIITPP(L)2 (L ) pyridine and piperidine) in neat pyridine and piperidine solvents, the transient species were observed to be different in structure depending on the axial ligand. Several reasons for this observation were considered, including the discussion on the nature of the CT states such as (π,d), (d,d), (dπ,π*(L)) (L ) axial ligand), and (dπ,π*(ring)) states, and the configuration interaction between the two eg(π*) orbitals. The present transient Raman spectroscopic studies on photoexcited RuIITPP(pip)2 has manifested the Jahn-Teller effects, which may result in a diamond distortion of the porphyrin ring superimposed by a minor rectangular distortion. This major B2g distortion accounts for the observation of a variety of unusual Raman spectral features in photoexcited RuIITPP(pip)2. Meanwhile, the introduction of a CO ligand to RuII porphyrin also altered the nature of the CT state in nonligating and weakly ligating solvents. It was observed that the CO ligand of the carbonylated RuII porphyrin was substantially replaced by the solvent molecules such as pyridine and piperidine in the excited state. These experimental results were interpreted in terms of the change in the charge distribution among the porphyrin ring, the central metal and the axial ligand in the metal-to-ring (dπ,eg(π*)) CT state. The relevance of our findings to the photodynamics observed by the previous picosecond transient absorption measurements of photoexcited Ru(II) porphyrins is discussed in terms of the conformational changes of photoexcited species.
Introduction The photophysics and redox properties of ruthenium(II) porphyrins (RuIIP) in the excited as well as the ground states have attracted much attention1 as models for the biologically important iron porphyrin complexes and as a good candidate for the solar energy conversion. Ruthenium(II) has d6 electronic configuration with filled metal dxy and dπ(dxz,dyz) orbitals, and their energies are comparable to the highest filled porphyrin ring orbitals, a1u(π) and a2u(π). Thus, it has been expected2 that the interaction of the filled dπ orbitals with the π-electronic system of the porphyrin ring results in the formation of the (dπ,π*(ring)) metal-to-ring charge transfer (CT) state in the same energy range as the lowest ring triplet T1(π,π*) state. Among the various important aspects in the photophysics of RuII porphyrins, the marked influence of the two axial ligands on the photokinetics has drawn special attention. The effects of the two axial ligands on the photophysical properties have been extensively studied by a variety of experimental meth* To whom correspondence should be addressed. † Present address: Department of Chemistry, Seonam University, Kwangchi-dong, Namwon, Chunbuk 590-170, Korea. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03597-0 CCC: $12.00
ods: electrochemistry,1 UV-vis absorption, emission,2 resonance Raman,3 and picosecond transient absorption.4 These studies unequivocally demonstrated that the CO ligand accepts π-electron density from the metal dπ orbitals lowering their energies relative to the π-HOMO’s of the porphyrin ring. This π-back-bonding model has provided a firm basis to explain the dynamics of photoexcited RuII porphyrins. Figure 1 shows the electronic states and relaxation pathways of Ru(II) porphyrins proposed by Rodriguez and his co-workers.4a RuIIP(L)2 (P ) TPP or OEP and L ) pyridine or piperidine) has been known to have a (dπ,π*(ring)) CT state as the lowest excited state which is lower in energy than the porphyrin ring T1(π,π*) state. Thus, this CT state serves as an important decay route in the deactivation process of photoexcited RuIIP(L)2.4 The proposed (dπ,π*(ring)) CT state for photoexcited RuIIP(L)2 (L ) pyridine and piperidine) is relatively short-lived with different lifetimes depending on the axial ligands (15 ns in RuIITPP(py)2 vs 2 ns in RuIITPP(pip)2). On the contrary, when one of the ligands is CO, a longer lived T1(π,π*) state is the lowest excited state because π-back-bonding to CO raises the energy of the (dπ,π*(ring)) CT state. While the temperature dependence of the excited state lifetimes for RuIIP(L)2 is negligible, the lifetimes of photoexcited RuIIP(CO)(L) (P ) TPP or OEP and © 1996 American Chemical Society
8868 J. Phys. Chem., Vol. 100, No. 21, 1996
Jeoung et al.
Figure 1. Energy level diagram and the decay process for photoexcited RuIITPP(L)2 (L ) pyridine and piperidine) (A) and Ru(II)TPP(CO)(L) (L ) pyridine and piperidine) (B). The lifetimes denoted were adopted from the ref 4a.
L ) pyridine or piperidine) exhibit a strong temperature dependence. The reported lifetime of 134 µs, for example, for photoexcited RuIITPP(CO)(py) at 77 K is about 6 times as long as the lifetime at room temperature. This temperature dependence is attributed to the presence of an appreciable activation barrier (∆E) between the T1(π,π*) state and the proposed (dπ,π*(ring)) CT state in the decay route for photoexcited RuIIP(CO)(py) (Figure 1). As for the photoinduced conversion of RuIIP(CO)(L) in strong basic solvents like pyridine and piperidine to RuIIP(L)2, the quantum yield was also found to be temperature dependent.4 This dependence was explained by suggesting that the dissociative state for photodecarbonylation is the proposed CT state. Although the dynamics of photoexcited RuII porphyrins have been investigated by the transient absorption and luminescence spectra,4 the vibrational spectroscopic studies on the structural changes accompanied by photoexcitation have not yet been reported. This report presents the first observation of the nanosecond transient Raman spectra in order to examine the nature of the excited state of RuII porphyrins. We have focused our attention on RuIITPP(CO)(py) and RuIITPP(L)2 (L ) pyridine and piperidine) in various solvents such as benzene, tetrahydrofuran (THF), pyridine, and piperidine. We also pursued to find out not only the effects of the solvents on the photodecarbonylation of the carbonylated RuII porphyrins but also the nature of the dissociative state. Experimental Section RuIITPP(CO)(py) was purchased from Porphyrin Products (Logan, UT) and used without further purification. RuIITPP(L)2 (L ) pyridine and piperidine) was synthesized and then recrystallized according to the published procedures.1b,g,2c,5 The transient Raman spectra6 were recorded by using the 416 nm pulses, generated by hydrogen Raman shifting of the third (355 nm) and second (532 nm) harmonics, respectively, from a nanosecond Q-switched Nd:YAG laser. The Raman spectra were collected with a HR 640 spectrograph (Jobin-Yvon), a gated intensified photodiode array detector (Princeton Instruments IRY700), and a pulse generator (Princeton Instrument FG100). In all the measurements, the sample solution was flowed through a glass capillary (0.8 mm i.d.) at a rate sufficient to ensure that each laser pulse encounters a fresh volume of the sample. We carefully checked the sample decomposition by comparison with the ground state Raman spectra of the fresh sample. Control of photon densities at the sample was necessary to obtain the Raman spectra of ground and excited state species. Laser power densities were controlled in one of the two ways
Figure 2. Transient resonance Raman spectra of RuIITPP(py)2 in pyridine with an excitation of 416 nm nanosecond pulses: (a) low power (ca.
