J . Phys. Chem. 1988, 92, 5982-5986
5982
A recent comparison of N O profiles at 100-200 km with modeling calculations has been presented by Cleary.,I The measurements were obtained from observations of solar scattering in the N O 7,6, and t band systems. The model was based on a total of 36 processes-1 1 photoabsorptive and 25 reactive. Good agreement was obtained between the observations and the model, in spite of the fact that the N(,D) + O(?) rate coefficient used was 5 x 1 0 - l ~ cm3 molecule-' s-l. As discussed above, the only reactions leading to NO formation are N(,D) and N(4S) interactions with 0,. With a drastic rethe only apparent duction in N O produced by N('D) + 02, compensating effect would be an increase in the N(4S) 0, rate coefficient. This is a well-studied reaction, and there is no reason to believe that the measured activation energy34of 6.3 kcal/mol is too large. A further aspect of the dominance of N(2D) quenching by O(3P) is that the electronic state of the oxygen product becomes of interest. Is it O(lD) or O('P)? If O('D) it will contribute to Oz(b'Zg+) generation by O(ID)/O, transfer and to the N, vibrational temperature by O(ID)/N2 transfer. Calculations by Abreu et al? indicate that quenching of O(lD) by O(3P) is relatively rapid, with a rate coefficient of 1 X lo-'' cm3 molecule-' s-l. Thermally, the quenching of O(lD) by O(3P) is the equivalent of O(3P) being the N(zD)/O(3P) product; in each
+
(34) Baulch, D. L.; Drysdale, D. D.; Horne, D. G.; Lloyd, A. C. Homogeneous Gas Phase Reactions of the H2-N2-02System; Butterworth: Leeds, 1973; Vol. 2.
case, -2 eV translational energy must be dissipated. However, even if the calculated O('D) O(3P) quenching rate coefficient is correct, it is only above 260 km that O(3P) becomes the dominant O('D) quencher. Thus, below this altitude it is important to know the N(,D) + O(3P) oxygen product. The experimental determination can be expected to be difficult.
+
Conclusions A direct comparison has been made between the N(,D) quenching rate coefficients for O(3P) and O, using the REMPI technique, whereby N(,D) is detected by a 2 1 photoabsorption and photoionization process at 269 nm. The study has k e n carried out over the 196-465 K temperature range in a discharge flow, and agreement with numerous previous studies on the N(2D) O2 reaction is excellent. A variety of reasons has been given as to why this determination is to be preferred over two earlier ones for N(,D) + O()P). The new value for this rate coefficient is 3.4 X lo-" exp(-l45/T) cm3 molecule-' s-l, which is an order of magnitude larger at 300 K than previous values, and a factor of 40 larger than the value currently used for this reaction in the terrestrial thermosphere. Considerable adjustments will be required in atmospheric models to accommodate this rapid quenching by O(3P), and it is unclear where changes will be needed.
+
+
Acknowledgment. This work was supported by a grant from the Aeronomy Section of the National Science Foundation. Registry No. N , 17778-88-0; 0, 17778-80-2.
Effects of Temperature and Solvent on Excited-State Deactivation of Copper( I I ) Octaethyl- and Tetraphenylporphyrin: Relaxation via a Ring-to-Metal Charge-Transfer Excited State Xinwei Yan and Dewey Holten* Department of Chemistry, Washington University, St. Louis, Missouri 631 30 (Received: February 19, 1988)
We have investigated the excited-state relaxation dynamics and pathways of copper(I1) octaethylporphyrin (CuOEP) and copper(I1) tetraphenylporphyrin (CuTPP) in noncoordinating solvents at temperatures between 295 and 77 K. The excited-state deactivation of CuOEP depends markedly on temperature and solvent. For example, the lifetime in methylcyclohexanevaries from 270 ns at 295 K to 10 p s at 150 K. The lifetime at 295 K varies from 100 ns to 1 p s with a change in solvent polarity. I n contrast, the lifetime of photoexcited CuTPP is 30-40 ns, essentially independent of temperature and solvent. These observations can be explained in terms of a model that includes the participation of a charge-transfer (CT) state, most likely a ring-to-metal (r,d) CT, in the deactivation of the tripdoublet ('T) excited state. Our results suggest that the CT excited state lies 600-800 cm-' above 'T in CuOEP and between *T and the quartet (4T)in CuTPP.
Introduction Copper(I1) porphyrins are among the most extensively investigated paramagnetic metalloporphyrins. These d9 complexes exhibit a diversity of photophysical behavior. Perhaps best known is the unusual low-temperature luminescence, which arises from the "tripdoublet" (,T) and quartet (4T) states in thermal equilibrium.' These two states are derived from the ring lowest excited triplet state, which is split due to the presence of the unpaired metal electron. The 2T/4T luminescence spectra, yields, and lifetimes measured between 10 and 80 K have been found to vary with temperature and macrocycle; the lifetimes range from -50 p s to 1 ms.'a2 More recently, picosecond transient absorption
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(1) (a) Smith, B. E.; Gouterman, M. Chem. Phys. Lett. 1968,2, 517-519. (b) Eastwood, D.; Gouterman, M . J. Mol. Specfrosc. 1969, 30, 437-458. (c) Gouterman, M.; Mathies, R. A,; Smith, B. E.; Caughey, W. S. J. Chem. Phys. 1970, 52, 3795-3802. (d) Ake, R. L.; Gouterman, M. Theor. Chim. Acta 1969, 15, 20-42.
0022-3654/88/2092-5982$01.50/0
measurements on a number of Cu(I1) porphyrins have shown that the excited-state decay times at 295 K are reduced from >10 ns to < l o 0 ps when an axial ligand is added to a four-coordinate species to make a five-coordinate ~ o r n p l e x . ~This ~ ~ dramatic lifetime reduction has been ascribed to a ring-to-metal chargetransfer (CT) state, which drops closer to or below ,T upon the formation of a five-coordinate ~ o m p l e x . ~ The dramatic change in lifetime with the state of axial ligation prompted us to consider whether a C T (or some other) state also might participate in the decay of 'T (and 4T) in noncoordinating solvents, especially at higher temperature. To this end, we have measured the excited-state decay kinetics of CuTPP and CuOEP (2) (a) Harriman, A. J. Chem. SOC.,Faraday Trans. I 1981, 77, 369-371. (b) Hoshino, M.; Seki, H. Chem. Phys. Lett. 1984, 110, 413-416. (3) Kim, D.-H.; Holten, D.;Gouterman, M. J. Am. Chem. SOC.1984,106,
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(4) Hilinski, E. F.; Straub, K. D.; Rentzepis, P. M. Chem. Phys. Lett. 1984, 111, 333-339.
0 1988 American Chemical Society
Copper( 11) Octaethyl- and Tetraphenylporphyrin 1 CuOEPlMethylcyclohexane
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5983 TABLE I: Excited-State Decay of CuOEP in Methylcyclohexane‘ temp, K lifetime
0.8 295 K a
Q)
0
C
:
I I
I I
I ,
I
0.4
3.4 f 0.3 p s 10 f 1 p s
Measured by decay of bleaching at 397 nm.
150 120 78
1
v)
400 f 30 ns 940 f 80 ns
TABLE 11: Kinetic Summary for CuOEP in 3-Methylpentane temp, K monomer lifetime,”p s aggregate lifetime,bps 295 1.0 f 0.1 220 4.5 f 0.5
0.6
a
270 f 25 ns
295 250 210 180 150
I
I
17 f 2 73 f 20 140 f 30
1.5 f 0.3 18 f 3 80 20
*
“Measured by decay of bleaching at 397 nm. Below 150 K, the lifetime is the major, longer lived component of a dual-exponentialdecay. bMeasured by decay of bleaching at 382 nm. 0.2
n
-
850
3jO
390
410
430
I
Wavelength (nm) Figure 1. Ground-electronic-stateabsorption spectrum in the Soret region of CuOEP in MCH at 295 K (solid) and 77 K (dashed). as a function of temperature and the polarity of the noncoordinating solvent. We have also compared the deactivation of the monomeric and aggregated forms of CuOEP at low temperature.
Experimental Section Cu(I1) porphyrins were purchased from Porphyrin Products and Aldrich. Ground-state absorption spectra of the compounds agreed with the published spectra, and their purity was checked further by TLC. Spectral grade solvents were employed for all measurements. Transient absorption spectra and kinetics were acquired as described e l ~ e w h e r e . ~The Cu(I1) complexes were typically excited with 10-ns, 532-nm pulses containing 25-50 mJ at 10 Hz. Samples (=2 X M) in 1-cm square cuvettes were deoxygenated either by bubbling with dry nitrogen or by repeated freeze-pump-thaw cycles on a vacuum line. Ground-state absorption spectra were measured on a Cary 219 or a Perkin-Elmer 330 spectrophotometer. The temperature of the samples for both ground-state and transient-state absorption measurements was controlled with an Oxford Instruments DNl704 optical cryostat. The solvents 3-methylpentane (3-MP) and methylcyclohexane (MCH) used for the temperature-dependence studies both make low-temperature glasses. Results Ground-State Absorption Spectra. Room temperature and liquid nitrogen temperature near-UV absorption spectra of CuOEP in MCH are shown in Figure 1. The room temperature spectrum (solid) contains a strong Soret band at 397 nm with a weak (1,O) vibrational component near 375 nm. The Soret band is reduced substantially in amplitude and slightly red-shifted at 77 K. These spectral changes are accompanied by the appearance of a prominant, broader band at 380 nm. This new band, which starts to appear as the temperature is lowered through the freezing point of the solvent, can be ascribed to an aggregate of CuOEP. The blue shift is consistent with the known effect of exciton coupling ( 5 ) Tait, C. D.; Holten, D.; Barley, M.; Dolphin, D.; James, B. R. J . Am. Chem. SOC.1985, 107, 1930-1934.
between face-to-face (*-stacked) porphyrins: Previous 77 K ESR and luminescence measurements on copper(I1) mesoporphyrin dimethyl ester support the interpretation that the species absorbing near 380 nm is an aggregate (possibly a dimer).’ We have observed similar spectral behavior for CuOEP in 3-MP, but aggregation at 77 K is more substantial than in MCH. Above the freezing points of the solvents (near 150 K) there is only a minor contribution of the 380-nm feature, indicating that essentially all of the molecules are monomeric. For CuTPP, on the other hand, we have observed no evidence of aggregation in either solvent at 77 K. The Soret band is at 412 nm at 295 K, red-shifting to 417 nm at 77 K. There is no indication of a blue-shifted band due to an aggregate, probably due to steric hindrance of *-stacking by the phenyl rings. Temperature Dependence of the Excited-State Spectra and Kinetics of CuOEP. Soret-region transient difference spectra measured immediately after excitation of CuOEP with a IO-ns 532-nm flash are presented in Figure 2. The 150 K spectrum in MCH (Figure 2A) contains only a minor dip near 380 nm due to bleaching of the Soret band of the aggregate. This dip is absent in the 295 K spectrum. Similar results are observed for CuOEP in 3-MP (Figure 2B). In agreement with the low-temperature ground-state absorption spectrum in 3-MP, the 78 K difference spectrum in this solvent contains a strong bleaching of the aggregate’s 380-nm band and a smaller bleaching due to the monomer near 400 nm. The difference spectrum measured at 220 K, which is well above the freezing point of the solvent, contains only the sharp bleaching of the 400-nm monomer band. The decay kinetics of photoexcited CuOEP in both solvents depend strongly on temperature. At all temperatures between 150 and 295 K the decay of bleaching of the 397-nm Soret band of CuOEP in M C H obeys single-exponential kinetics. Some typical decay curves and computer fits are shown in Figure 3. Note the increasing time scale of the abscissa as the temperature is reduced. Since the data and the fits are virtually indistinguishable on these plots, the original digitizer traces are shown in the inset to each panel. The time constants are listed in Table I. An Arrhenius plot of the data (Figure 4) gives an activation energy of approximately 800 cm-’. The kinetic data obtained for CuOEP in 3-MP are summarized in Table 11. At 150 K and below, where aggregation becomes significant, a dual-exponential decay is measured at 397 nm. The major component, which has the longer lifetime, can be ascribed to the monomer. The smaller, faster component can be associated with the aggregate, whose 380-nm Soret band bleaching tails into the 400-nm region. Clean single-exponential decay of the ag( 6 ) Gouterman, M.; Holten, D.; Lieberman, E. Chem. Phys. 1977, 25, 139-153. (7) (a) Konishi, S.; Hoshino, M.; Imamura, M. J . Phys. Chem. 1982, 86, 4888-4892. (b) Konishi, S.; Hoshino, M.; Imamura, M. Chem. Phys. Lett. 1983, 94, 261-269.
5984
Yan and Holten
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 01
!
-
*\.
15
CuOEP/Methylcyclohexane
h
0,
