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J. Phys. Chem. 1996, 100, 860-868
Photophysics of Lanthanide Triple Decker Porphyrin Sandwich Complexes Lisa L. Wittmer and Dewey Holten* Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130 ReceiVed: September 6, 1995X
Ground state and time resolved excited state optical absorption spectra and luminescence spectra are reported for La2(OEP)3, Eu2(OEP)3, and Ce2(OEP)3 (OEP ) 2,3,7,8,12,13,17,18-octaethylporphyrinate). The porphyrin macrocycles in these molecules are strongly coupled due to their mean separation of e3.5 Å. The triple deckers show many optical properties analogous to those found for double decker porphyrin sandwich complexes, including features not exhibited by porphyrin monomers. Following excitation, the lowest 1(π,π*) state of La2(OEP)3 decays in ∼10 ps predominantly to the phosphorescent 3(π,π*) state, which lives for ∼10 µs. The lowest 1(π,π*) state of Eu2(OEP)3 also decays rapidly and with high yield to the lowest 3(π,π*) state, which has a rather short lifetime of 630 ps. The lack of luminescence from Eu2(OEP)3 is ascribed to deactivation of the 3(π,π*) state through one or more low-energy ligand-field states of the metal ion. Ce2(OEP)3 is also nonluminescent. Unlike La2(OEP)3 and Eu2(OEP)3, the 1(π,π*) lifetime of 7 ps for Ce2(OEP)3 largely reflects deactivation within the singlet manifold. The decay of 1(π,π*) to the ground state likely involves low-lying (f,f) states, although a charge transfer state may play a role. The small (∼20%) population of 3 (π,π*) that forms from 1(π,π*) also decays rapidly (τ ∼ 300 ps), again via ligand-field states of the metal ions. These combined results further demonstrate the importance of (i) π-π interactions in defining the electronic properties of chromophores brought within van der Waals distance and (ii) metal-centered excited states in determining the relaxation rates and pathways following photoexcitation of tetrapyrrole complexes.
Introduction Identifying the nature of electronic interactions between porphyrinic macrocycles in close proximity is critical to assessing the novel characteristics of multichromophoric chemical species, including the bateriochlorophyll dimer of the photosynthetic reaction center and linked porphyrins in assemblies. The investigation of bis(porphyrin) sandwich complexes has helped reveal the origin of the distinctive properties of the reaction center special pair such as its low-energy electronic absorption band and the near-IR absorptions observed when it is oxidized.1-8 The π-π interactions evident in certain porphyrin arrays impart interesting optical, redox, electron transfer, and conductivity properties to these complexes.1-11 Further study of these interactions may lead to a better understanding of the functions of porphyrin complexes in biological systems and of the utility of porphyrin assemblies in building optoelectronic devices. Triple decker porphyrin sandwich complexes have been synthesized which serve as excellent probes into the extent of the π-π interactions and which provide the next step beyond the double deckers toward extended assemblies.1a-c,5a,11 The triple deckers have two trivalent octadentate metal cations sandwiched between three porphyrin macrocycles, with a mean spacing between the core atoms of adjacent macrocycles of e3.5 Å.1a-c The trisporphyrinates provide the added complexity of another macrocycle participating in π-electron delocalization while retaining a systematic means of varying the extent of π-π interactions via a change in the ionic radius of the metal ions. In this paper we exploit these properties by studying the photophysical behavior of three lanthanoid triple deckers, La2(OEP)3, Eu2(OEP)3, and Ce2(OEP)3. (OEP ) 2,3,7,8,12,13, 17,18-octaethylporphyrinate.) We assess the optical properties of the compounds relative to monomers and double deckers in terms of the interactions between the macrocycles. We also X
Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0860$12.00/0
describe the excited state relaxation pathways and dynamics in terms of the involvement of the metal ions. Experimental Section La2(OEP)3, Eu2(OEP)3, and Ce2(OEP)3 were kindly provided by Drs. D. Bocian and J. Duchowski.11 The compounds were chromatographed on predried activity I basic alumina using toluene as the eluent. Ground state absorption spectra were recorded on a Perkin-Elmer 330 spectrophotometer. Emission spectra were recorded on a Spex Fluorolog 2 spectrofluorometer using an optically chopped (200-400 Hz) excitation light and an RCA C30956E Si avalanche photodiode connected to a Standford Research SR530 lock-in amplifier. Low-temperature studies utilized an Oxford Instruments cryostat. Transient absorption data were acquired on picosecond and femtosecond spectrometers by exciting with a 0.5 mJ, 30 ps, 532 nm pulses or 100 µJ, 150 fs, 582 nm pulses, respectively.12a,b Spectra were acquired in overlapping 150 nm intervals using white light probe pulses encompassing the blue to near-IR regions. The picosecond apparatus allowed studies to 12 ns and probing from 450 to 1000 nm, whereas on the femtosecond spectrometer spectra could be obtained to 3 ns and from 400 to 900 nm. Longer time scale experiments employed degassed samples excited with 90 mJ, 10 ns, 532 nm pulses from a Q switched Nd:YAG laser.12c Results Electronic Ground State Absorption. Figure 1 shows the room temperature ground state electronic absorption spectra of La2(OEP)3, Eu2(OEP)3, and Ce2(OEP)3. These spectra agree with those reported previously.1a-c,11 Peak positions are summarized in Table 1. For comparison, Figure 2 shows ground state absorption spectra of a typical porphyrin monomer, ZnOEP, and a typical bisporphyrinate, Th(OEP)2.2b,4d Spectra of the La(III), Eu(III), and Ce(IV) double deckers are very similar to that of the Th(IV) complex except for the exact positions of © 1996 American Chemical Society
Photophysics of Lanthanide Porphyrins
J. Phys. Chem., Vol. 100, No. 2, 1996 861
Figure 1. Ground state electronic absorption spectra of three lanthanide triple decker sandwich complexes in toluene at 295 K: (A) La2(OEP)3, (B)Ce2(OEP)3, and (C) Eu2(OEP)3. The 400-850 nm region has been multiplied by a factor 14.
Figure 2. Ground state electronic absorption and fluorescence spectra of (A) a typical OEP-based monomeric metalloporphyrin, ZnOEP, and (B) a typical OEP-based double decker, Th(OEP)2.
TABLE 1: Electronic Ground State Absorption Data for the Triple Deckersa compound
B(0,0)
Q′′
Q(1,0)
Q(0,0)
Q′ manifoldb
La2(OEP)3 Ce2(OEP)3 Eu2(OEP)3
388 386 385
∼490 ∼490 ∼490
532 528 534
570 567 578
630, -, 738 640, -, 755 658, -, 805
a Wavelengths in nanometers. b There appear to be at least three bands in the Q′ manifold.
the bands.1a-c,3a,b,4a,b The absorption spectra of the triple deckers, like those of the double deckers, contain (π, π*) absorption bands in the same spectral regions as those found for monomeric metalloporphyrins: the intense near-UV B(0,0) band with a shoulder due to B(1,0), and the two weaker bands, Q(1,0) and Q(0,0), between 500 and 600 nm. The spectra of the triple deckers, like the double deckers, also contain bands not seen in the spectra of porphyrin monomers: a very weak band between the Q and B bands and several weak bands to the red of the Q bands. In analogy to the double deckers, we designate the new features to the blue and red of the monomerlike Q bands as the Q′′ and Q′ bands, respectively.4c-e Additionally, the ground state absorption spectra of both the bis- and trisporphyrinates contain a sloping base line, suggestive of the presence of additional weaker features underlying the Soret and Q bands. The lowest energy absorption band in the Q′ manifold of the triple deckers, like double deckers such as Th(OEP)2, may represent the Q′(0,0) origin transition, and this band is better resolved and stronger in the triple deckers (Figures 1 and 2B). The energies of the Q′ absorption features decrease with the ionic radius of the metal ion (and the distance between the mean planes of the macrocycles) (Table 1). This same dependence has been observed for a series of double deckers.1d,4e Emission Behavior. Emission from La2(OEP)3 in degassed toluene is shown in Figure 3. The luminescence behavior is analogous to that of Th(IV) double deckers.4e We assign the prominent emission at 950 nm as phosphorescence from the lowest 3(π,π*) state, which will be denoted 3T′(π,π*). The higher-energy, temperature-dependent feature at 805 nm is likely
Figure 3. Corrected luminescence spectra of La2(OEP)3 in degassed toluene at 180, 240, 260, 275, 285, 295, and 300 K ((0.1 K, temperatures increase from right to left on the temperature axis). The delayed fluorescence peak at 805 nm is barely discernable at 180 K but gains intensity as the temperature is increased to 300 K. The phosphorescence peak at 950 nm maintains a constant position and intensity as the temperature is varied.
delayed fluorescence from the 1Q′(π,π*) state, formed by thermal repopulation from 3T′(π,π*). Similar to Th(OEP)2 (Figure 2B), this broad (fwhm ∼100 nm) fluorescence for La2(OEP)3 may represent overlapped vibronic bands with the origin transition lying closer to the Q′(0,0) absorption transition near 740 nm. Thus, like the double deckers,4c-e there is a substantial (3000-4000 cm-1) shift between the maxima in the Q′ absorption and fluorescence manifolds. The temperature dependence of the fluorescence intensity for La2(OEP)3 (Figure 3) illustrates that at low temperature thermal repopulation of 1Q′(π,π*) is so unfavorable that no fluorescence is observed. The amplitude and position of the phosphorescence at 950 nm are approximately independent of temperature. An activation energy of ∼2800 cm-1 is obtained for the triplet-tosinglet thermal repopulation process from an Arrhenius plot of the peak amplitude of the 805 nm emission feature (Figure 4). In contrast to La2(OEP)3, degassed samples of Eu2(OEP)3 and Ce2(OEP)3 show no discernable phosphorescence or fluorescence at room temperature. The quantum yield of emission for these two triple deckers was calculated to be
