J. Phys. Chem. C 2007, 111, 14881-14888
14881
Perturbation of Electronic States and Energy Relaxation Dynamics in a Series of Phenylene Bridged ZnII Porphyrin Dimers Sung Cho,† Min-Chul Yoon,† Chul Hoon Kim,‡ Naoki Aratani,§ Goro Mori,§ Taiha Joo,*,‡ Atsuhiro Osuka,*,§ and Dongho Kim*,† Center for Ultrafast Optical Characteristics Control and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, Department of Chemistry, Pohang UniVersity of Science and Technology, Pohang, 790-784, Korea, and Department of Chemistry, Graduated School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan ReceiVed: December 12, 2006; In Final Form: July 30, 2007
We have investigated the perturbed electronic states and energy relaxation dynamics of a series of phenylene bridged ZnII porphyrin dimers to reveal the effects of phenylene bridges by using computational and timeresolved spectroscopic methods. The electronic states of ZnII porphyrin dimers are largely perturbed by the interchromophore interactions that can be controlled by the phenylene bridges; linking position and number of phenyl rings. On the basis of our observations, we can gain further insight into the dipole-dipole interactions and through-bond/through-space electronic exchange interactions between the neighboring porphyrin moieties, which provides a firm basis for further understanding the modification of photophysical properties caused by the phenylene bridge in multiporphyrin assemblies as artificial light-harvesting apparatus.
I. Introduction The efficient light-energy capture and storage leading to photoinduced electron-transfer processes between constituent pigments in the natural photosynthetic system are the root of high efficiency in light conversion to chemical energy.1,2 There have been continuous efforts to mimic the natural photosynthetic system for the realization of highly efficient molecular photonic and electronic devices such as molecular wires, switches, transistors, and artificial light-harvesting apparatus in order to exploit the high efficiency in natural photosynthesis.3 In this context, numerous porphyrin assemblies have been synthesized and investigated to reveal the photophysical properties pertinent to the realization of molecular devices because of the fascinating nature of the porphyrin unit; intense electronic absorption and emission, photochemical stability, tunable optical and redox properties by appropriate metalation, and β- or meso-carbon substitution.4-11 The functionality of molecular devices is generally introduced by the combination of various porphyrin constituents with different electronic natures. One of the key factors in determining the light-energy and electron-transfer processes in molecular devices is the strength of interchromophore interactions between constituent moieties. Through the extensive research works, it is generally accepted that the interchromophore coupling strength depends mainly on the energy difference, interchromophore distance, relative orientation, and electronic communication.6,12 In this regard, it is relevant to design and to prepare the molecular architectures systematically by considering the influence on the ground and excited electronic states depending on the coupling strength between the two neighboring porphyrin moieties. Thus, it becomes prerequisite to understand the dipole and electronic * To whom correspondence should be addressed. E-mail: thjoo@ postech.ac.kr;
[email protected];
[email protected]. † Yonsei University. ‡ Pohang University of Science and Technology. § Kyoto University.
exchange interactions between the adjacent porphyrin units by controlling the nature of the bridge, the linking position, and the length of the bridge to connect the two porphyrin moieties. With these objectives in our mind, we have synthesized a series of ZnII porphyrin dimers bridged by ortho- (OPD), meta(MPD), and para-phenylene (PPD), 2,7-naphthalene (NPD), 4,4′-biphenylene (BPD), and 4,4′′-para-terphenylene(TPD), as shown in Scheme 1. The ZnII porphyrin dimers exhibit welldefined structures with various center-to-center distances and relative orientations between the adjacent ZnII porphyrin units. To explore the effect of the perturbed electronic states induced by the interchromophore interactions, we have investigated the energy relaxation dynamics of the dimers. Transient absorption (TA) is the most widely used technique in energy transfer because it offers high time resolution and relative ease of measurement.5,6 A TA signal consists of three contributions, ground-state bleaching recovery, stimulated emission, and photoinduced absorption, which make unambiguous interpretation difficult. Alternatively, time-resolved fluorescence (TRF) has also been used extensively because it gives information on the emitting state exclusively. Up-conversion of the fluorescence is used to achieve femtosecond time resolution of several tens of femtoseconds.13 Because the fluorescence from both S2 and S1 states is observed, ZnII porphyrin is ideally suited to investigate the electronic-state dynamics by monitoring the decay of the S2 state fluorescence as well as the rise of the S1 state fluorescence, which allows us to examine the extent of perturbation on the electronic states and the possibility of intervention by other electronic states located between the S2 and S1 states.6,13 II. Experimental Section Sample Preparation and Steady-State Absorption and Fluorescence Measurements. The phenylene bridged ZnII porphyrin dimers have been synthesized by the Suzuki-Miyaura coupling reaction (Supporting Information). 5,10,15,20-tetraphe-
10.1021/jp068522+ CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
14882 J. Phys. Chem. C, Vol. 111, No. 40, 2007
Cho et al.
SCHEME 1: Optimized Molecular Structures of Phenylene Bridged ZnII Porphyrin Dimers and Monomera
a
t-Butylphenyl groups at meso-phenyls are removed to minimize a job size and to show clear 3D geometries.
nyl ZnII porphyrin (ZnIITPP) was purchased from Sigma-Aldrich Corporate. Toluene (Sigma-Aldrich, anhydrous, 99.8% purity), pyridine (Sigma-Aldrich, HPLC grade, ∼99.9% purity) and tetrahydrofuran (THF; Merck, HPLC grade, ∼99.8% purity) solvents were purchased and used without further purification. UV-visible absorption spectra were recorded with a Shimadzu UV-2400 spectrometer, and steady-state fluorescence spectra were taken on a Shimadzu RF-2500 fluorometer. Femtosecond Time-Resolved Fluorescence Experiments. The femtosecond fluorescence up-conversion apparatus was used for the time-resolved spontaneous fluorescence.6b,13b The beam source for the B- and Q-states’ fluorescence was a home-built cavity-dumped Kerr lens mode-locked Ti:sapphire oscillator.6 The second harmonic of the fundamental pulses generated in a 100-µm-thick BBO crystal served as pump pulses. Residual fundamental pulses were used as gate pulses. Three pairs of fused silica Brewster angle prisms compensated group velocity dispersions of the fundamental pulses prior to the secondharmonic generation, the second harmonic around 410 nm, and the residual fundamental pulses. The pump beam was focused to a 500-µm-thick cuvette containing sample solution by a 5-cmfocal-length plano-convex lens. The cuvette was mounted on a motor-driven state and moved constantly back and forth across the beam to minimize photodegradation. Collecting the fluorescence and focusing it into a BBO crystal for the frequency
conversion was achieved by a reflective microscope objective lens (Coherent). We used two kinds of mixing BBO crystals to improve the signal-to-noise ratio. We measured the S2 fluorescence by using a 0.5-mm crystal to prevent a pulse broadening in mixing crystal. Because the S1 emission rate was slow compared with the S2 fluorescence, we used a 1-mm crystal to measure the S1 fluorescence due to the enhance the signal-tonoise ratio. Therefore, there were two instrument response functions (IRFs) with different time scales. The full width at half-maximum (fwhm) of the cross-correlation functions between the scattered pump pulses and the gate pulses were 80 and 120 fs for S2 and S1 fluorescence, respectively. Nanosecond Time-Resolved Fluorescence Decay Profiles. A time-correlated single-photon counting (TCSPC) system was used for the spontaneous fluorescence decay measurement.6 The system consisted of a cavity-dumped Kerr-lens mode-locked Ti:sapphire laser pumped by a continuous-wave Nd:YVO4 laser (Coherent, Verdi). The second-harmonic pulses of the fundamental pulses generated in a 1-mm-thick BBO crystal served as an excitation source. Residual fundamental pulses were used as a trigger source detected by a fast photodiode. The excitation beam was focused onto a 10-mm-thick cuvette containing sample solution by a 5-cm-focal-length lens with s-polarization. The fluorescence from the sample was collected and focused to a monochromator (Acton research) by 2” plano-convex lens
Phenylene Bridged ZnII Porphyrin Dimers pair and detected by the MCP-PMT (Hamamatsu). The full width at half-maximum (fwhm) of the instrument response function obtained by a dilute solution of coffee cream (diffuser) was typically ∼70 ps in our TCSPC system. To obtain the isotropic fluorescence decay profiles, we measured the fluorescence decays at magic angle (54.7°) using a linear polarizer between monochromator and 2” lens pair. The number of fluorescence photons per unit time was always maintained to
