J. Phys. Chem. 1983, 87,28-29
28
Complexing of Porphyrins in Supersonic Jets Uzl Even and Joshua Jortner Department of Chemlstfy, Tel Aviv University, 69978 Tel Avlv, Israel (Received: October 25, 1982)
Complexes of zinc octaethylporphyrin with medium-sized molecules (water, methanol, acetonitrile, benzene, and pyridine) were synthesized in pulsed supersonic jets of He. Information on microscopic spectral shifts of these large complexes was inferred from laser-induced fluorescence spectroscopy.
The complexing of porphyrins' is a subject of considerable interest in the areas of physical chemistry and biophysical diagnosis. The attachment of various ligands (L) to metal porphyrins (MP) in s ~ l u t i o nand ~ * ~in lowtemperature solids4is manifested in the appearance of new spectral features of the MP-L complexes. We would like to draw attention to the potential of supersonic expans i o n ~ "for ~ the production of internally cold, collision-free, large, molecular complexes. We have applied the techniques of laser spectroscopy in seeded supersonic expans i o n ~ "for ~ the synthesis, identification, and exploration of excited-state energetics of isolated, ultracold complexes of porphyrins with medium-sized molecules (L = water, methanol, acetonitrile, benzene, and pyridine), providing a new approach for the elucidation of solvent perturbat i o n ~on~ porphyrins, as explored from the microscopic point of view. We have studied the laser-induced fluorescence (LIF) excitation spectra of the complexes of zinc octaethylporphyrin (ZnOEP) with medium-sized molecules. The ZnOEP-L complexes were synthesized in pulsed supersonic jets of He seeded with the two molecules which form the complex. A solid sample of ZnOEP was heated in the nozzle chamber to 350 "C to yield a vapor pressure of 0.1 torr and seeded into a mixture of He + L. The partial pressure of L was pL = 0.1-1 torr, while the stagnation pressure of He was p = 1200-2000 tom. The He + ZnOEP + L mixture was expanded through a circular pulsed nozzle (diameter D = 600 pm)*. A nitrogen-pumped dye laser (spectral resolution 0.3 cm-') crossed with supersonic expansion at the distance X = 15 mm (X/D= 25) down the nozzle. The LIF spectra were measured as previously described.6 Figure 1 shows the LIF spectrum of ZnOEP in a jet of He. The spectral features, whose energies and relative intensities are independent of the stagnation pressure in the range p = 1200-2500 torr, are assigned to the electronic-vibrational excitation of the So S1 transition (the Q band)g of the bare ZnOEP. No spectral features are exhibited at wavelengths above 5600 A. The lowest-energy spectral features consist of a triplet (Figure 1)exhibiting
-
(1) D. Dolphin, Ed., 'The Porphyrins", Academic Press, New York, 1978. (2) J. R. Miller, and G. D. Dorough, J. Am. Chem. SOC.,74, 3977 (1952). (3) G. T. McCrew, Ph.D. Thesis, John Hopkins University, Baltimore, MD, 1967. (4) G. Jansh and M. Noort, Spectrochim. Acta, Part A , 32,747 (1978). ( 5 ) D. H. Levy, Annu. Rev. Phys. Chem., 31, 197 (1980). (6) A. Amirav. U. Even, and J. Jortner, J . Chem. Phys., 75, 3770 (19811. ( 7 ) A. Amirav, U. Even, and J. Jortner, J . Chem. Phys., 75, 2489 (1981). (8) U. Even, J. Magen, and J. Jortner, C h e n . Phys. Lett. 88, 131 (1982). (9) M. Gouterman, ref 1, Vol. 111, pp 1-165.
TABLE I: Excitation Energies of Zinc Octaethylporphyrin (ZnOEP) and ZnOEP-L Complexes 6u,a$b
cm-
molecule bare ZnOEP
ZnOEP-benzene ZnOEP-methanol ZnOEP-water ZnOEP-acetonitrile ZnOEP-pyridine
-6.7 0 t 9.0 t 66 +I35 - 120 -182 - 232 -82 - 276 - 143 -497 - 347
AV,'
assign-
cm-'
mentd
0-0
0-0 e I
0
0 0 149
0 133 0 150
(0)
(0) (0) ti,
(0) bi
(0) vi
a The energies of the spectral features are presented with respect to the main 0-0 spectral feature (at 6 u = 0 ) of the bare ZnOEP molecule. The main 0-0 spectral feature of the bare molecule is located at 5588.7 A , Accuracy of 6 v values for the 0-0 triplet of bare ZnOEP is i- 1 cm-', while the accuracy of the energies of all other spectral bands is i 1 0 cm-'. Energy relative to the electronic origin of each complex. ( 0 ) labels the electronic origin of the complex, while v , corresponds to the 135-cm-' intramolecular vibrational excitation of the porphyrin ring. e The weak 66-cm.' band may be due to out-of-plane vibrational excitations of the ethyl groups.
the most intense spectral feature at 5588.7 A, together with the two additional features at 5590.8 and at 5585.0 A. The individual spectral features are characterized by widths (fwhm) of 3 cm-' that are essentially due to unresolved rotational broadening.6 The relative intensities of the components of the triplet are independent of the stagnation pressure, so that they cannot be attributed to sequence and/or hot bands. Such complex structure is unique for ZnOEP, not being exhibited in the electronic spectra of porphyrins which do not contain side groups, e.g., free-base porphyin8 and zinc tetrabenzoporphyrin'O in supersonic jets. The triplet structure of ZnOEP can be attributed either to the electronic origin together with low-frequency vibrational motion of the eight ethyl groups or, alternatively, to the vibrationless 0-0 excitation of three distinct conformers, which differ in the relative orientations of the ethyl groups. We prefer the latter interpretation, as the spectrum of ethylbenzene" does not reveal any low-frequency vibrational modes. The spectroscopic congestion of the components of the triplet, originating from conformer splitting, becomes more severe at higher energies, prohibiting the resolution of the individual components (10) U. Even, J. Jortner, and J. Friedman, J. Phys. C h e n . 86, 2273 (1982). (11)J. B. Hopkins, D. E. Powers, and R. E. Smalley. J . C'hem. Phys.. 72, 5039 (1980).
0022-3654/83/2087-0028$01.50/00 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87,No. 1, 1983 29
Letters
1,
\ ,
ZnOEP
, b I D I N f
5680 5720 5760 5800 Wavelength
(8)
ACETONITRILE
5500
5550
5600
9
W A V E L E N G T H &)
Figure 1. Fluorescence excitation spectrum in the range 5580-5780 A of ZnOEP seeded in pulsed supersonic expansions of He. ZnOEP was heated in the nozzle chamber to 350 O C and mixed with He at p = 1500 torr. The mixture was expanded through a 600-pm nozzle. The dye laser (spectral resolution 0.3 cm-l) crossed the supersonic jet at X = 15 mm downstream. The fluorescence excitation spectrum corresponds to the fluorescence intensity vs. the laser wavelength. All spectra are normalized to the laser intensity. The main component of the electronic origin is labeled 0-0,while (135) labels the average position of the prominent vibrational excitation at 135 cm-'.
of the v1 = 135 cm-l intramolecular vibrational excitation of the porphyrin ring. This spectral congestion limits the resolution of the spectra of the complexes of ZnOEP, which is determined by the energetic spread A N 20 cm-l of the members of the triplet. The spectral resolution for ZnOEP complexes in the jet exceeds by 1to 2 orders of magnitude that accomplished in solution spectroscopy of porphyrin complexe~.~~~ Figure 2 shows the LIF spectra of ZnOEP expanded in He doped with benzene, water, methanol, acetonitrile, and pyridine, which reveal new spectral bands whose characteristics are as follows: (1) Their energies depend on the nature of L. (2) The relative intensities in each spectrum are independent of pL. These new bands, whose peak energies are summarized in Table I, are attributed to So S1 excitations of the ZnOEP-L complexes. The lowest-energy spectral band in each spectrum is attributed to the electronic origin (0) of the So SI transition of the ZnOEP-L complex, which reveals an ill-defined structure presumably due to conformer splitting. The energies, 6v, of the electronic origins, relative to the electronic origin of the bare ZnOEF, constitute the microscopic solvent shifts induced by the binding of L to the porphyrin (Table I). The red spectral shifts, reflecting the stabilization of the S1state, fall into three categories: (i) small 16vI for van der Waals binding of benzene, (ii) moderate 16vI for the binding of polar molecules, e.g., water, methanol, and acetonitrile, and (iii) large 16vl for the "chemical" binding of pyridine via its lone pair. The spectral shift for the isolated ZnOEP-pyridine complex in the jet is comparable to the value 6v = -454 cm-' observed for the zinc porphyrin-pyridine complex in a low-temperature n-octane m a t r i ~ .However, ~ supersonic beam spectroscopy is superior to low-temperature, solid-state spectro~copy,~ as the latter technique is complicated by inhomogeneous broadening and phonon broadening effects.
-
-
ZnOEPiWATER
nn
(0)
A
Ij"
/
Jli."v" IUVL
,-
ZnOEPtBENZENE
, ;
_-
7 n CIFP
5660 5700 W A V E L E N G T H (i)
5620
Flgure 2. Fluorescence excitation spectra in the range 5580-5800 A of the bare ZnOEP molecule and of the complexes of ZnOEP with benzene, methanol, water, acetonitrile, and pyridine in pulsed supersonic expansions of He. ZnOEP was heated in the nozzle chamber to 350 OC and mixed w b He (p = 1500 torr) or with an He 4- L mixture (p = 1500 torr; pL 1 torr). The ZnOEP L He mixture was expanded through a 600-pm nozzle. Other experimental conditions were described in Figure 1. The lower spectrum corresponds to the bare molecule, while the other spectra exhibit the spectral features of the Zn0EP.L complexes, with L being marked on each individual spectrum. The electronic origin of each complex is labeled by (0).
-
+ +
A second, high-energy, spectral band is exhibited in the LIF spectra of some complexes (Figure 2 and Table I) at energies 6 v = 130 f 10 t o 150 f 10 cm-l above the electronic origin of ZnOEP-L (L = water, acetonitrile, and pyridine). These values of 6v are close to the 6v = 135 cm-' vibrational frequency of the bare ZnOEP. Accordingly, the high-energy bands are assigned to an intramolecular vibrational excitation of ZnOEP in the ZnOEP-L complexes. The present study demonstrates the potential and promise of supersonic beam spectroscopy for the interrogation of excited-state energetics and dynamics of large molecular complexes of porphyrins. Acknowledgment. This research was supported in part by the United States-Israel Binational Science Foundation, Jerusalem (Grant No. 2641) and by the United States Army through its European Research Office. U.E. acknowledges partial support of this research by the Committee for Basic Research of the Israel National Academy of Sciences, Jerusalem.
