J. Phys. Chem. 1983, 87,1757-1765
W
z
a X W
T
[I
v Flgure 4. I R spectra of hexane (700-1400 wavenumbers): A, freshly deposited Ar matrix (1:lOO): B, annealed matrix at 49 K; C, crystalline solid at 77 K; D, gas phase.
n-Pentane. The infrared data for n-pentane are contained in Figure 3 and Table V. Bands which disappear on annealing must correlate to a modes (C-type) of the anti rotamer, as was discussed previously. They are observed at 1342,1237,1137,1010,985,907,894,860,838,762, and 718 cm-I. The band at 894 cm-I (appearing at 900 cm-I in the Raman spectrum) has previously been assigned to
1757
a b mode, while from this analysis it can only be of a symmetry. All other bands are in agreement, except for the band at 718 cm-l, which has not been previously reported. Table VI contains our assignments, along with those of other workers. n-Hexane. Spectra and tabulated band maxima are presented in Figure 4 and Table VII. The same arguments which were used for butane apply for hexane. Bands which vanish on annealing are observed at 1342,1221,1136,1054, 903, 888, 867, 794, 755, and 741 cm-'. Their assignments and correlations to Raman bands can be found in Table VIII. From the rotational structure and the behavior of the correlated bands in the matrix, the band at 888 cm-I (A-type) is assigned to b symmetry and the bands at 1136 and 755 cm-' (B-type) are assigned to a symmetry. The remaining bands cannot be assigned on the basis of rotational structure. The bands at 1342,1221,1056,903, 867,794, and 741 cm-' could be of either a or b symmetry; however, they must correlate to gerade bands in the anti rotomer.
Conclusions Our study of the spectra of matrix-isolated, annealed matrix-isolated,crystalline, and gas-phase alkanes supports the previous assignments to the gauche conformations of those bands which appear only in the fluid phases. Selection rules for the anti rotamer give some indication of the symmetry classification of these bands. The structure of the bands in the gas phase provides additional information. These considerations generally support the classification of previous authors, which were largely based on normal-coordinate computations. Registry No. Propane, 74-98-6; butane, 106-97-8;pentane, 109-66-0; hexane, 110-54-3.
Mechanism of Porphyrin Ion Production from the Triplet State of Magnesium Octaethylporphyrin John F. Smalley,' Stephen W. Feldberg, Division of Chemical Sciences, Department of Energy and Environment, Brookhaven National Laboratoty, Upton, New York 11973
and Bruce S. Brunschwlg Department of Chemistry, Brookhaven National Laboratoty, Upton, New York 11973 (Received: September 16, 1981)
The production of ions from the first excited triplet state (T)of magnesium octaethylporphyrin (MgOEP or simply P) has been studied. Triplet-triplet annihilation is shown to be the major route for the production of the ions P+ and P-. Evidence is obtained indicating that the reaction of T with ground-state P is not a significant source of ions. Evidence is also presented indicating that the two triplets initially combine to form an excited charge-transfer complex. The relationship between the multiplicity of this charge-transfer complex and triplet quenching, delayed fluorescence, and ion formation is discussed.
Introduction
The study of photochemically initiated electron transfer has been a matter of continuing interest since it is a mechanism by which light energy can be converted to chemical or electrochemical Of particular in(1)(a) Blankenship, R.E.; Parson, W. W. Annu. Rev. Biochem. 1978, 47,635. (b) Seely, G. R. Photochem. Photobiol. 1978,27,639. 0022-365418312087- 1757$01.5010
terest are reactions between two excited states (e.g., triplet + triplet) which can effect electron transfer. An analagous reaction involving only one excited state (e.g., triplet + (2)(a) Sutin, N.J.Photochem. 1979,IO, 19. (b) Sutin, N.;Creutz, C. Pure Appl. Chem. 1980,52,2717. (3)Whitten, D. G. Acc. Chem. Res. 1980,13, 83. (4)Kawada, A.; Jarnagin, R. C. J. Chem. Phys. 1966,44,1919.Gary, L.P.;DeGroot, K.; Jarnagin, R. C. Ibid. 1968,49,1577. (5)Ballard, S. G.; Mauzerall, D. C. J. Chem. Phys. 1980, 72, 933.
0 1983 American Chemical Society
No. 10,
The Journal of Physical Chemistry, Voi. 87,
1758
TABLE I :
1983
Smalley et al.
Rate Constants at 298 K
solvent methanol ethanol 1-propanol 2-propanol dimethyl sulfoxide acetonitrile
0.551‘ 1.075‘ 1.988‘ 2.042‘ 1.996d 0.346d
32.63e 24.30e 20.1e
1.6 1.1 1.1 0.8 2.1
18.3= 46.68d 37.50d
1.5, 4.3f 0.7 0.4 0.4 0.1
5.7g 3.4 2.1 2.1 2.0 9.5h
13.4 6.9 3.7 3.6 3.7 21.3
0.43 0.49 0.56 0.58 0.54 0.45
5.7
3.2 2.0 5.2 1.4 25
22 1 13.1 8.0 8.2 5.3 33.4
0.26 0.25 0.25 0.63 0.26 0 76
‘
Dielectric constant a t 298 K. a Viscosity a t 298 K in centipoise. Lange, N. A . , Ed. “Handbook of Chemistry”, Handbook Publishers: Sandusky, OH, 1939. Reference 6. e West, R. C., Ed. “Handbook of Chemistry and Physics”, At 288 and 3 0 8 K in methanol, k , is 4.7 x 10’and CRC Press: Cleveland, OH, 1976. f See the caption for Figure 6. Calculated by assuming that h , in acetonitrile is 2.0 X l o 8 M s I . Reference 11. The 6.6 x l o 9 M-’s-’, respectively. average value of this ratio is 0.51 with a standard deviation of 20.06. J
-
--- -- --
-
-~ - ~-~ _____
-
__-_
BIMOLECULAR REACTIONS
UNI MOLECULAR REACTIONS
T+T -l--
I t
I
,y-T
P+tP- 1
- I s + _ p - p’i
lk2
I I
I k3
I
ki I
t
GROUND STATE
-_ I
t
r
P+P
Flgure 1. Diagram of the triplet decay mechanism.
WAVELENGTH (vvl
Flgure 3. Spectra of various species which are relevant to this work: (-) spectrum of P; (- - -) spectrum of P’ (Reference 21a. Fajer, J.; Forman, A,; Dolphin, D.; Felton, R. H. J . Am. Chem. SOC.1970, 9 2 , 3451); (. .) spectrum which is calculated for P‘ by shifting the spectrum of the monoanion of zinc etioporphyrin to the red by 20 nm (ref 14 and 15); (- -) spectrum of MV’ (Evans, A. K.; Dodson, N. K.; Rees, N. H.J . Chem. SOC.,Perkin Trans. 2 1976, 859).
,
.
I-
P. M . Tube
Light Source
Tope Drive
In fer f a c e
Digital Oscilloscope
Current to Voltage Converter
Figure 2. Block diagram of the dye laser flash photolysis apparatus.
ground state) would require a more energetic triplet to effect a given electron transfer. Thus, in principle one might effect electron transfer with lower energy photons. Usually, however, the reactions are highly exergonic (very negative AG) and virtually diffusion controlled, inviting explanation as to why the “inversion region” predicted by Marcus6 is not observed. The present work is a spectrophotometric study of the products of the flash photolysis of magnesium octaethylporphyrin (P) in a variety of solvents focusing on the fate of the triplet state (TI.Of particular interest are (a) the triplet-triplet reactions producing delayed fluorescence, (6)(a) Marcus, R. A. Discuss. Faraday SOC.1960, 29, 21; J. Chem. Phys. 1965,43, 2654. (b) Creutz, C.; Sutin, N. J.Am. Chem. SOC.1977, 99, 241.
ions (P’ and P-), or ground states and (b) the tripletground-state reaction which under appropriate conditions might be expected to produce ions. Evidence for the latter has been presented by Mauzerall and Ballard,5who used a conductance technique to study ion formation involving the triplet of zinc octaethylporphyrin (ZnOEP).
Theory The theoretical basis for the kinetic analysis of the data presumes the following mechanism for the formation and disappearance of the triplet: P + h + S
S
k,’
P
+ hu’
(RI)
(R2)
k,”
s-P
(R3)
k,”’
53-T
(R4) k 1‘
T+P-ZP k
T
+ P 2P+ + P-
(R5)
“
(R6)
Mechanism of Porphyrin Ion Production
- + k?l
2T
2T
kp
The Journal of Physical Chemistry, Vol. 87, No. 10, 1983
2P
(R7)
P+ P-
(R8)
kp
2T-S+P
(R9)
kx
T-P
P+ + P-
k3
(R10)
d[S]/dt = (k,”’/2)[TI2 - k,[S] k, = k,‘
ion production since the spectra of T, P, and P+ overlap. The concentration of ions produced (CI) may be expressed as
CI = k / s m0[ T ] ( [ P l O- [TI) dt
+ (k2”/2)Jm[TI2 0 dt (12)
CI = kl”[P]oJm[T] dt
+ (k2”/2 - k1”)Jm[Tl2 dt
(13)
2P
(R11) where S is the first excited singlet state of P. This mechanism is shown diagramatically in Figure 1. Reactions between the triplet and the ion products are ignored. However, the mechanism deduced from reactions R1-R11 adequately explains all of our observations. Following a light pulse, the rate of disappearance of the singlet is where
1759
(1)
+ k,” + k,”’
(2) Since the lifetime of S (s) is short compared to the lifetime of the triplet, the singlet concentration should be in steady state (d[S]/dt = 0) as the triplet decays
[SI = k;”[TI2/2k,
where CI is the concentration of either P+ or P- which would be produced in the absence of the annihilation reaction (reaction R11). With eq 4 one obtains
CI = k/[P],
(4)
.
-a - b[T] .”
d[T1 (15)
- kit)
-a
- b[Tl
or
c, = k,”[plo In ( z b
With the assumption7a
[PI = [PI, - [TI
( y + lT:o d[T1
JT;”
(3)
The equation describing the decay of the triplet following a light pulse is7a d[T]/dt = -a[T] - b[TI2
or
+ 1) + (k
Y = k,”/k,.
(R14
or
T + P-
1.801
--
is approximately 2.2 Vu and the triplet energy ( E T ) is only 1.8 eV,25our result might be expected on the basis of simple energetic arguments. Ballard and Mau~erall,~ however, argue that because of entropic factors reaction R6 can be a significant reaction path and their data for ZnOEP (whose AEredoxU and ET6’25 are nearly the same as those for MgOEP) appear to support this contention. From Table I1 it is clear that ion yields are at most -1370 of the total porphyrin concentration in methanol, which means that the maximum ion concentration is a somewhat lower percentage (
