5261
J. Phys. Chem. 1984, 88, 5261-5264
The Photochemical Properties of Rhodium( I I I ) Phthalocyanine Cation Radicals G. Ferraudi,* S. Oishi, and S. Muraldiharan Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 465.56 (Received: January 30, 1984; In Final Form: April 17, 1984)
Stable rhodium(II1) phthalocyanine cation radicals, Rh(pc)(CH30H)Xt with X = C1 or Br, were generated by one-electron chemical or electrochemicaloxidations of the corresponding rhodium(II1) phthalocyanines,Rh(pc)(CH,OH)X, and characterized by means of their EPR and optical spectra. Some transitions in the optical spectra of the radicals were assigned by comparison with similar transitions in the spectra of the parent phthalocyanines. Moreover, the photochemical properties of the radicals were investigated by continuous, laser, and flash photolysis. Irradiations in the UV bands, Xexcit = 250 or 320 nm, induce the photoreduction of the radicals (back to the parent phthalocyanine) by abstraction of hydrogen. Photolysis in the visible region, Xclcit = 525 nm, induces the photodecomposition of the macrocycle. The mechanisms of these photoprocesses are discussed by comparison with known photochemical transformations of related compounds.
Introduction The electrochemical oxidation or reduction of a number of metallophthalocyanines (I) produces species wth a radical nature
X=CI,Br
and, in many cases, with considerable stability.'-lo Phthalocyanine radicals have also been generated by chemical and photochemical mean~,~-lO and it is possible to regard them as common intermediates in redox reactions of metallophthalocyanines. These species, related to porphyrin radicals,"J2 are very likely radicals with an unpaired electron in a ligand-centered molecular orbital. Moreover, chemical and electrochemical evidence suggest that there are minor differences between the nuclear configurations of the radicals and the parent metallophthalocyanine~.~~~~~ Such (1) Cahill, A. E.; Taube, H . J . Am. Chem. SOC. 1951, 73, 2487. (2) Rollman, L. D.; Iwamoto, R. T. J . A m . Chern. SOC.1968, 90, 1455. (3) Lever, A. B. P.; Minor, P. C.; Wilshire, J. P. Inorg. Chem. 1981, 20, 2550. (4) Dolphin, D.; James, B. R.; Murray, A,;Thornback, J. R. Can.J. Chem. 1980, 58, 1125. ( 5 ) Philip, G.; Ingram, D. J. E.; Bennett, J. E. J . Am. Chem. SOC.1957, 79, 1870. (6) Clack, D. W.; Yandle, J. R. Inorg. Chem. 1972, 11, 1738. (7) Ferraudi, G.; Srisankar, E. V. Inorg. Chem. 1978, 17, 3164. (8) Ferraudi, G. Inorg. Chem. 1979, 18, 1005. (9) Muralidharan, S.; Ferraudi, G. J . Phys. Chem. 1983, 87, 4877. (10) Muralidharan, S.;Ferraudi, G.; Schmatz, K. Inorg. Chem. 1982,21, 2961. (1 1) Barley, M.; Becker, J. Y.; Domazetis, G.; Dolphin, D.; James, B. R. J . Chem. Soc., Chem. Commun. 1981, 982. (12) Harel, Y.; Manassen, J. J . Am. Chem. SOC.1977, 99, 5817. (13) Prasad, D. R.; Ferraudi, G. J . Phys. Chem. 1982, 86, 4037.
0022-3654/84/2088-5261$01.50/0
structural features make it possible to establish interesting correlations between the properties of the phthalocyanines and the realted radicals produced by adding or removing one electron to them. Since the phthalocyanine radicals are intermediates in photochemical reactions and have sufficiently large lifetimes which allow them to participate in sequential biphotonic processes: we have investigated in this work thier photochemical properties. For this purpose we have used particularly stable cation radicals, Rh(pc)(CH,OH)Xt with X = C1 or Br, which can be compared with their parent phthalocyanines (I).
Results Nature of the Rhodium(II4 Phthalocyanine Cation Radicals. We have observed that one-electron oxidations of Rh(pc)(CH30H)Xf (X = C1 or Br) in deaerated acetonitrile produce the corresponding rhodium(II1) cation radical. The oxidation of the phthalocyanines with 1 equiv of Ce(IV) proceeds quantitatively toward the formation of the radical, but excess Ce(IV) carries the reaction beyond the one-electron-oxidized specie^.'^ An equally smooth oxidation takes place when rhodium(II1) phthalocyanines are oxidized at a Pt anode under an applied potential of 1.6 V. The exchange of 0.97 f 0.03 faraday for each mole of oxidized phthalocyanine was determined from current vs. time curves and proved that the phthalocyanines experienced a quantitative one-electron oxidation in these electrolytses. Moreover, such a facile oxidation was in agreement with the observation, in cyclic voltammetry, of a reversible wave ( q I 2= 0.87 V vs. SCE) which was assigned to the couple Rh(pc)(CH,OH)X+/R~(~C)(CH~OH)X.'~ The EPR and optical spectra of the rhodium(II1)-ligand radicals (Figure 1) are independent of the procedure followed for the preparation of the radicals. The EPR spectra are very close to the EPR spectrum of the free electron and exhibited g 2.002 and line widths 6-10 G. Such features in the EPR spectrum point to a species whose structure can be described as a phthalocyanine-centered radical with an unpaired electron in a aHOM0.17 Furthermore, the optical spectra exhibit intense ab-
-
(14) Lever, A. B. P.; Pickens, S. R.; Minor, P. C.; Licoccia, S.; Ramaswamy, B. S.; Magnell, K. J . A m . Chem. SOC.1981, 103, 6800. (15) Neither the spectral changes nor the absence of free halides, CI- or Br-, in solutions of the cation radicals is consistent with substitution of the axial ligands during or after the oxidation of the phthalocyanines. -0.72 V vs. SCE) which was (16) We observed another wave (e1 assigned to the couple Rh(pc)(CH30HjX/Rh(pc)(CH30H)X-.These potentials for the oxidation and reduction of the rhodium(II1) phthalocyanines obey Lever's linear relationship between the redox potential and the reverse of the ionic potential of the metal, e.g. the ratio of the charge to the ionic radius.I4 (17) The labeling and ordering of the molecular orbitals in this work follow previous reports,18 and the abbreviations HOMO and LUMO stand for the highest occupied and lowest unoccupied molecular orbitals, respectively.
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0 1984 American Chemical Society
5262 The Journal of Physical Chemistry, Vol. 88, No. 22, 1984
Ferraudi et al.
16_G
log E
h
. 4.0 2 .o
f1-10.0
0 5 IO 15 20 25 [2-propono1] XIO? M
2
2'0310
390
470
550
630
710
790
V 01
6
$m
9
A, nm
.......
0.4 0.2
0.0
Figure 1. Comparison between the spectra of rhodium(II1) phthalocyanines, (a) Rh(pc)(CH,OH)CI, and the corresponding radical cation, (b) Rh(pc)(CH,OH)CI+,in CH,CN. The inset shows the EPR spectrum
of the phthalocyanine radical.
A, nm
Figure 2. Difference transient spectra (a) in 337-nm laser flash photolysis of Rh(pc)(CH,OH)Clt in deaerated CH,CN. The inset shows the dependence of the rate constant for the decayof the short-lived intermediate, A, 500 nm, on 2-propanol concentration. Part b shows the spectrum ( 0 ) 800 ms after the flash irradiation of Rh(pc)(CH,OH)Clt and (0) before the flash irradiation. The inset gives a normalized trace for the reformation of the Q band at Xow = 630 nm with u = ( A - A o ) / ( A ,Ao),where A,, and A , are the optical densities at zero and infinite re-
TABLE I: Product Yields in the Photochemistry of Rhodium(II1) Phthalocyanine Cation Radicals
-
10,
nm
A,,,
einstein dm-3 s-l
@:
254
4.5 x 10"
350 525
2.2 x 10"
$da
Rh(pc)(CH30H)Brt (2.1 0.2) x 10-2 (2.4 f 0.2) X (2.0 f 0.1) x 10-2b
*
510-4
action times, respectively.
510-4 (7.0 i 0.5) X lo-)
The well-established spectra of the photolytes, Rh(pc)(CH30H)X+, and the products, Rh(pc)(CH,OH)X, show sufficiently pronounced differences which have been used for the spectrophotometric investigation of the photoreaction. This has shown that the disappearance of the radical and the formation of the phthalocyanine take place with a 1:l stoichiometry. Furthermore, the participation of the solvent in the photochemical processes induced with 320- or 254-nm irradiations was confirmed by the mass spectrometric detection of succinonitrile. Although the analysis of succinonitrile was too laborious and open to experimental errors for the determination of quantum yields, concentrations of this product were found in a good stoichiometric relationship with the other species in eq 1. Quantum yields for
320- and 254-nm photolyses are reported in Table I. It must be pointed out that neither the photodecomposition of the phthalocyanine ligand nor the photosubstitution of the axial ligands had significant yields in UV photolyses (Table I). We have observed, however, that 550-nm photolysis induces the photodecomposition of the radi~a1s.l~Indeed, the irradiation bleaches the solutions at wavelengths corresponding to the absorption bands of the radicals, namely 530 and 710 nm, without the restoration of the typical Q band of the phthalocyanine macrocycles, namely at 645 nm. Quantum yields for the photoreduction and photodecomposition of the cation radicals (Table I) show that the photodecomposition is the most significant photoprocess in 550-nm photolysis. Despite the simple stoichiometry of the photoreduction (eq I), this reaction proceeds by a complex mechanism which has been investigated by laser and conventional flash photolysis. Spectra, determined with various delays after the 350-nm laser flash irradiations of the radicals, point to the presence of two intermediates. The difference spectrum of a short-lived intermediate, obtained at times shorter than 20 ps, has A,, N 500 nm (Figure 2) and is converted into a long-lived intermediate with a rate that obeys a first-order rate law. The rate constants, k = (2.5 f 0.3) X lo7 s-l and k = (2.0 f 0.3) X lo7 SKI, were determined for the disappearance of the short-lived intermediates produced in photolyses of Rh(pc)(CH,OH)Cl+ and Rh(pc)(CH30H)Brf, respectively. Moreover, the rate of disappearance of the short-lived intermediate in 2-propanol-acetonitrile mixtures increased with 2-propanol concentration (Figure 2). This process exhibited a second-order rate law, k = (2.0 f 0.2) X IO8 M-' s-l , with first-order dependences on the concentrations of 2-propanol and intermediate. The reactivity described above for the short-lived intermedite and the results of continuous photolysis can be equated to the abstraction of hydrogen from acetonitrile and/or 2-propanol. The long-lived intermediate was detected at reaction times longer than 30 ps and exhibited a spectrum where the Q band of the phthalocyanine macrocycle was absent (Figure 2a). Therefore, the formation of the final phthalocyanine product, Rh(pc)(CH,Oh)X, was easily followed by means of the 645-nm, absorbance growth in flash photolysis (Figure 2b). Flash irradiations of Rh(ph)(CH30H)Brt in 2-propanol-acetonitrile mixtures, [2-propanol] I0.2 M, revealed that the rate law for the
(18) Schaffer, A. M.; Gouterman, M.; Davidson, E. R. Theor. Chim. Acta 1973, 30, 9.
(19) Irradiations at 680 nm also induce the decomposition of the cation radicals.
254 350 525
8.3 x 10-7
510-5
Rh(pc)(CH3OH)CIt (3.1 f 0.2) X lo-* (3.6 f 0.2) X 2.2 X 10" (2.9 f 0.3) X 8.3 x 10-7 510-5 4.5 X lod
(7.6 i 0.5)
X
aQuantum yields for the radical reduction, $,, and for the decomposition of the macrocycle, @d. Each quantum yield corresponds to an average of more than five determinations. Photolyses were carried out under Ar, and the radicals were obtained by oxidation of the corresponding phthalocyanines with CeIVunless indicated. Radicals produced by electrochemical oxidation of the phthalocyanines. sorption bands (A, = 710 and 530 nm) which are in agreement with the proposed structure of a a-phthalocyanine radical (Figure 1).
Photochemical Reactivity of the Cation Radicals. The phthalocyanine cation radicals, in solutions protected from oxygen, moisture, and light, proved to be stable for more than 3 days. However, they are photoreactivity, and UV continuous photolyses (Aexcit = 254 or 320 nm) of the radicals Rh(pc)(CH,OH)X+ (X = C1 or Br) in deaerated acetonitrile induce their reduction according to hv
Rh(pc)(CH30H)X+ + CH3CN Rh(pc)(CH,OH)X
+
+ H+ + '/2(CH2CN)z
(1)
AeXci, = 320 or 254 nm
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5263
Rhodium(II1) Phthalocyanine Cation Radicals
- blU ... . .. r + - ~ u ~ ~ - -eP ........
I+
-
__
--
radical indicate that the a-a* transition between a-HOMO and a*-LUMO orbitals in the phthalocyanine must be red-shifted with respect to the similar transition in the parent phthalocyanine (Figure 3). Therefore, the transition that gives intensity to the Q band, ,,A 645 nm, in rhodium(II1) phthalocyanines is expected to be equivalent to a similar allowed transition in the rhodium(II1) radical, and this must be placed at lower photonic energies. Such a bathochromic shift can be justified in terms of the different Coulombic and exchange contributions that determine the state energies in each species.23 So a transition to the lowest lying 2aa* can be assigned to absorptions near 710 nm the spectrum of the rhodium(II1) radical (Figure 1). For such assignment to be correct the energy gap between the a-HOMO and the a*-LUMO must be -116 kJ, a value that is in close agreement with those reported for phthalocyanines, namely 130 & 10 kJ.'* Similar calculations reveal that a second occupied a-orbital with appropriate symmetry must be placed -48 kJ below the r-HOMO, and as a consequence of the energy of this orbital, another a-a*-allowed transition is expected at -21 6 kJ, namely 5 16 nm. This result signals that an allowed a-a* transition must be placed at X 550 nm in the absorption spectrum of the radical. Hereafter it seems possible to establish a one to one correlation between some a-a* transitions in the phthalocyanines and the corresponding transitions in phthalocyanine radicals (Figure 3). The photochemical reactivity of the rhodium(II1) cation radicals is not surprising if one considers that reactive states, similar to the na* in the parent phthalocyanines,1°must be available in these species (Figure 3). Our results in UV continuous photolysis can be explained in terms of a mechanism where the formation of the phthalocyanine involves a short-lived intermediate, A, and a long-lived intermediate, B, as indicated in eq 2-7.
-
-
Figure 3. Energy diagram for a correlation between some selected electronic states of the phthalocyanine, Rh(pc)(CH,OH)X, and related n-ligand radical, Rh(pc)(CH,OH)X+. The insert shows a correlation between orbitals for the phthalocyanine (a), the n-ligand radical (b), and the N,,-ligand radical (c). absorbance growth was zero order on 2-propanol concentration and second order on the transient concentration. Measurements of the reaction rate vs. the intermedite concentration gave a ratio of the rate constant to the extinction coefficient, k / t = (5.3 f 0.3) X lo6 cm SKI, at 645 nm.
Discussion The spectral properties, optical and EPR, of the rhodium(II1) cation show that these species can be regarded as a-ligand-centered radicals. In the optical spectrum, the intensity of the absorption bands is very close to those in parent phthalocyanines where major contributions to the spectrum are provided by phthalocyaninecentered AT* transitions.1° This suggests that also major contributions to the spectrum of the cation radicals come from a-a* transitions between molecular orbitals whose character is largely bestowed by ligand orbitals. It is possible to obtain some additional information on the nature of the electronic transitions in the spectrum of the cation radicals by comparing their energies with the energies of the related transitions in the spectrum of the parent phthalocyanines. This comparison can be greatly simplified under the assumption that the differences between the nuclear configurations of the 32-a-electron phthalocyanine and the 3 1-*-electron cation radical must be smalLZ0 With this assumption, the expressions for the state energies2' of the phthalocyanine and cation (20) The reasons for this approximation have been discussed e1~ewhere.l~ and they are in good agreement with various experimental observation^.'^^^^ (21) The state energies can be expressed by means of the equations for a close- and an open-shell configurationzz
Wopsn=
Xh," + X A,, - X K,,,W.s*) n m
