J. Phys. Chem. 1996, 100, 627-632
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Photochemistry of Metalloporphyrins in Polymer Matrices. Laser Photolysis Studies of Chlorochromium(III) Tetraphenylporphyrin in Polystyrene Films in the Temperature Range 77-300 K Mikio Hoshino* and Norichika Tezuka The Institute of Physical and Chemical Research, Wako, Saitama 351-01, Japan
Masahiko Inamo Faculty of Education, Aichi UniVersity of Education, Kariya, Aichi 448, Japan ReceiVed: May 4, 1995; In Final Form: October 3, 1995X
Nanosecond laser photolysis studies of chloropyridinato- and chloroaquachromium(III) tetraphenylporphyrin, ClCrIIITPP(Py) and ClCrIIITPP(H2O), dissolved in polystyrene films have been carried out in the temperature range 77-300 K. The transient observed for ClCrIIITPP(Py) after laser pulsing is solely the excited states (the tripquartet, 4T1, and tripsextet, 6T1, states). The emission from the tripquartet state, 4T1, is detected upon laser excitation of ClCrIIITPP(Py). No photodissociation of axial pyridine is detected in the temperature range studied. The laser photolysis of ClCrIIITPP(H2O) in polystyrene films gives two transients; the excited states (4T1 and 6T1) of ClCrIIITPP(H2O) and the five coordinate ClCrIIITPP yielded by the photodissociation of axial H2O from ClCrIIITPP(H2O). The yields of the excited states (4T1 and 6T1 ) for both ClCrIIITPP(Py) and ClCrIIITPP(H2O) were found to increase with a decrease in temperature. The lifetimes and the relative yields of the excited states and the relative yields of the photodissociation of the axial ligands were measured in the temperature range 77-300 K. These results lead to the conclusions that (1) the energy gaps between the 4T1 and 6T1 states are 1.5 and 1.1 kcal mol-1 for ClCrIIITPP(Py) and ClCrIIITPP(H2O), respectively, (2) the full photodissociation of axial H2O from ClCrIIITPP(H2O) occurs from the singquartet state, 4S1, and (3) the photodissociation of the axial ligand is the major pathway for excitation energy dissipation at the 4S1 state of six-coordinate chromium(III) porphyrin.
Introduction Metalloporphyrins with paramagnetic metal centers such as Fe, Mn, Co, and Ni hardly exhibit their luminescence.1,2 However, copper(II), oxovanadium(IV) and chromium(III) porphyrins are known to show relatively strong luminescence particularly at low temperatures.3-5 The luminescence spectra and their lifetimes have been studied in order to elucidate the excited-state nature of these metalloporphyrins.3,4,6-8 For chromium porphyrins, the nature of the excited states has been interpreted in terms of weak coupling of the π excited states of porphyrin ligand and the d-electrons (S ) 3/2) of the central chromium atom.6 Figure 1 shows the electronic states and their transitions of chromium(III) porphyrins proposed by Gouterman and his co-workers.6 Recently, we have studied photochemical reactions of chromium(III) porphyrins.9,10 Six-coordinate chlorochromium(III) tetraphenylporphyrins, ClCrIIITPP(L) (L ) ligand), photodissociate the axial ligand L, leaving five-coordinate ClCrIIITPP which returns to ClCrIIITPP(L) by the recombination reaction with L. The dissociation of the axial L is assumed to occur at the 4S1 state of ClCrIIITPP(L). A novel photochemistry was found for nitritochromium(III) tetraphenylporphyrin, (NO2)CrIIITPP: (NO2)CrIIITPP undergoes photoinduced β-bond cleavage to yield NO and oxochromium(IV) tetraphenylporphyrin.11 As in the case of ClCrIIITPP(L), the reactive excited state of the β-bond cleavage is assumed to be the 4S1 state of the chromium porphyrin. Thus, the studies on the excited states become important for full understanding of the photochemistry of chromium porphyrins. X
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0627$12.00/0
Figure 1. Energy level diagram and the decay process of the excitation energy of chromium(III) porphyrins. Thermal processes are shown by broken arrows. The 4T1 and 6T1 states exhibit phosphorescence with the frequencies νp′ and νp′′, respectively.
In the present study, the excited-state chemistry of chromium porphyrin is investigated with the use of polystyrene films as a matrix. The excited-state lifetimes of metalloporphyrins in polymer films are expected to be longer than those in fluid matrices because of the high viscosity of polymers, and, therefore, the excited states (the tripquartet, 4T1, and tripsextet, 6T , states) of chromium(III) porphyrins can be readily detected 1 by a conventional nanosecond laser system. Experimental Section Chloroaquachromium(III) tetraphenylporphyrin, ClCrIIITPP(H2O), was synthesized and purified according to literature.10,12 Reagent grade pyridine, benzene, and dichloromethane were supplied from Wako Pure Chem. Ind. Ltd., and polystyrene (MW 2-3 × 105) was from Scientific Polymer Products, Inc. © 1996 American Chemical Society
628 J. Phys. Chem., Vol. 100, No. 2, 1996
Figure 2. Transient and luminescence spectra of ClCrIIITPP(Py) in a polystyrene film detected at 50 ns after 355-nm laser pulsing at 300 K.
Sample films of ClCrIIITPP(H2O) were made as follows. A benzene solution (2 cm3) of polystyrene (ca. 5 g/50 cm3) was added into 1 cm3 of a dichloromethane solution of 5.0 × 10-4 M ClCrIIITPP(H2O). The mixed solution was stirred for 10 min. The solution was pipetted and cast over a slide glass. A thin film was obtained after drying it in a dark room for 3-4 h. Then, the film was kept in an oven for 1 day at 318 K. The polystyrene films of ClCrIIITPP(Py) were made in a similar manner. The dichloromethane solution of 5 × 10-4 M ClCrIIITPP(H2O) containing 10-2 M pyridine was mixed with the benzene solution of polystyrene and cast over a slide glass: ClCrIIITPP(H2O) is readily transformed to ClCrIIITPP(Py) in the solution. The thickness of the films was estimated as ca. 0.1 mm. The concentrations of ClCrIIITPP(H2O) and ClCrIIITPP(Py) in the films are estimated as ca. 2 × 10-3 M. Absorption spectra were recorded on a Hitachi 330 spectrophotometer. Laser photolysis was carried out by Nd-YAG laser (HY 500 from J. K. Lasers Ltd.) equipped with second (532 nm), third (366 nm), and fourth (266 nm) harmonic generators. All the photolysis studies were made by 366-nm laser pulses. The detection system of the transient spectra is described elsewhere.13 The temperature of a sample film was controlled by a cryostat (Model CF 10200 from Oxford Instruments). The film was firmly fixed on a thin quartz plate (1 mm in thickness) with a plastic past. Results ClCrIIITPP(Py). The absorption spectrum of ClCrIIITPP(Py) in a polystyrene film at room temperature shows the absorption peaks at 457 nm in the Soret band region and at 525, 570, and 610 nm in the Q band region. The spectrum is almost identical with that observed for ClCrIIITPP(Py) in toluene. Figure 2 shows the transient and luminescence spectra observed for ClCrIIITPP(Py) in the polystyrene film at 300 K at 50 ns after laser pulsing. The transient spectrum exhibits a strong negative absorption around 450-460 nm and a weak one around 560-620 nm, indicating that ClCrIIITPP(Py) is photobleached to give the transient species. The transient spectrum uniformly decays in the whole wavelength region studied according to first-order kinetics: the rate constant for the decay of the transient is determined as 6.5 × 106 s-1 at 300 K. The transient species is ascribed to the excited states (4T1 and 6T1) of ClCrIIITPP(Py) as will be stated later. No other transients are detected in the polystyrene films. The transient spectrum detected by laser photolysis of ClCrIIITPP(Py) at 77
Hoshino et al.
Figure 3. Logarithmic kPy(T) obtained from the decay of the transient absorption (O) and the emission (0), represented as a function of reciprocal temperature. Solid curve denotes the one calculated by eq 5.
K showed a weak absorption band longer than 650 nm. Except for the weak band, no marked difference in the transient spectra between 77 and 300 K was observed. The luminescence spectrum observed at 300 K for ClCrIIITPP(Py) in the polymer films at 50 ns after laser pulsing is broad with the peak at 840 nm. The decay of the luminescence follows first order kinetics with a rate constant of 6.4 × 106 s-1. The excited state responsible for the luminescence is considered to be the 4T1 state of ClCrIIITPP(Py), which thermally equilibrates with the 6T1 state. An earlier study of chromium porphyrins demonstrated that the 4T1 state of ClCrIIITPP in a 30:70 (v:v) mixture of 1-butanol and 3-methylpentane gives the luminescence spectrum with the peak at 818 nm at 300 K: the lifetime, τ, was estimated as τ < 6 µs.6 The lifetime of the luminescence becomes longer at lower temperatures and the luminescence intensity from ClCrIIITPP(Py) in the polymer film observed at 50 ns after pulsing becomes weak on going from 300 to 160 K. No appreciable luminescence was detected at 77 K. Figure 3 shows the logarithmic rate constants, kPy(T), for the decay of both the transient absorption and the luminescence spectra of ClCrIIITPP(Py) in polystyrene, represented as a function of reciprocal temperature. The rate constants for the decay of the transient are identical with those of the luminescence. The temperature dependence of the rate constant, kPy(T), is explained by simply assuming that the lowest excited sextet state, 6T1, of ClCrIIITPP(Py) is in thermal equilibrium with the second excited tripquartet state, 4T1 (see I in appendix)
kPy(T) ) [ks + 2/3kq exp(-∆Eqs/RT)]/[1 + 2/3 exp(-∆Eqs/RT)] (1) where ks, kq, and ∆Eqs are respectively the intrinsic decay rate constants of the 6T1 and 4T1 states and the energy separation between the two states. The solid curve in Figure 3 is the logarithmic kPy(T) calculated by eq 1 with the use of ks ) 5 × 104 s-1, kq ) 1.2 × 108 s-1, and ∆Eqs/R ) 751 K. The population ratios, nq/ns, of the 4T1 and 6T1 states are calculated as 0.06 and 3.9 × 10-5 at 300 and 77 K, respectively. This value suggests that the transient spectrum observed around 300 K is composed of the two species, 6T1 and 4T1 states. The spectrum measured at 300 K is similar to that at 77 K, indicating that the difference in the spin state between 6T1 and 4T1 scarcely causes the change in their optical spectra. Open circles in Figure 4 are the initial absorbance, D0(T), of the excited states detected for ClCrIIITPP(Py) at 480 nm
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J. Phys. Chem., Vol. 100, No. 2, 1996 629
Figure 4. Open circles are the absorbance D0(T) of the transient at 480 nm observed for ClCrIIITPP(Py) at 50 ns after 355-nm laser pulsing, represented as a function of temperature. Solid curve A is the one calculated by eq 2. Closed circles are the relative luminescence yields, Ir(T), of ClCrIIITPP(Py), represented as a function of temperature. Solid curve B denotes the one calculated by eq 6.
Figure 5. Transient and luminescence spectra of ClCrIIITPP(H2O) in a polystyrene film detected at 50 (O and b) and 300 ns (0).
SCHEME 1
observed at 50 ns after laser pulsing, represented as a function of temperature. With the increase in temperature, the absorbance decreases. This result suggests that the 4S1 state of ClCrIIITPP(Py) has a deactivation pathway with an activation barrier, ∆E. The absorbance, D0(T), is proportional to ΦΤ(6T1 + 4T1). Thus, we obtain (see II in the Appendix)
D0(T) ) D0(0)[1 + k0/kTq exp(-∆E/RT)]-1
(2)
Here D0(0), k0, and kTq are respectively the absorbance at T ) 0, a preexponential factor of the rate constant for the deactivation pathway, and the rate constant from 4S1 to 4T1. The solid curve A in Figure 4 is D0(T) calculated by eq 2 with the use of D0(0) ) 0.31, k0/kTq ) 5.59, and ∆E/R ) 703 K. The D0(T) values obtained experimentally are in good agreement with those calculated in the temperature range studied. The yield of the excited states, 6T1 and 4T1, is assumed to be unity at T ) 0. Thus, we obtain
ΦT(6T1 + 4T1) ) D0(T)/D0(0)
(3)
At an infinite temperature, the value Φ∞(6T1 + 4T1) is calculated as 0.15. The intensities, I0(T), of luminescence from ClCrIIITPP(Py) detected at 840 nm were measured at 50 ns after laser pulsing in the temperature range 130-300 K. The intensity represented as a function of temperature shows a bell-shape: with decreasing temperature, the intensity initially increases and decreases on going from 220 to 160 K. Luminescence was hardly detected below 160 K. Since (1) I0(T) decays according to the first order kinetics with the rate constant kPy(T) and (2) the spectral shape of the luminescence is almost identical in the temperature range studied, the relative luminescence yield, Ir(T), at temperature T is expressed by
Ir(T) ) ∫0 I0(T) exp(-kPy(T)t) dt ∞
(4)
Closed circles in Figure 4 are the plot of Ir(T) represented as a function of temperature. The luminescence of ClCrIIITPP(Py) originates from the 4T1 state. The luminescence yield, ΦL, is represented by
ΦL ) kTq(kTq + kS0)-1kL[kq + ks(χ)-1]-1
(5)
where kL is the rate constant for the radiative process from the to the 4S0 state, kS0 is the rate constant for the deactivation process from the 4S1 to the 4S0 state, and χ is the population ratio of the 4T1 and 6T1 states (see the Appendix). Here again, the thermal equilibrium between 6T1 and 4T1 is assumed to hold during the course of the decay of the 4T1 state. Equation 5 is rewritten as 4T 1
Ir(T) ) RΦL ) RkTq(kTq + kS0)-1[kq + ks(χ)-1]-1 ) β[1 + 5.59 exp(-703/T)]-1[5.0 × 104 exp(751/T) + 1.2 × 108]-1 (6) where R and β are constants independent of temperature. The solid curve B in Figure 4 is Ir(T) calculated by eq 6 with the use of β ) 6.2 × 108. The relative luminescence intensities measured in the temperature range 150-300 K are moderately in agreement with those calculated. The decay pathways for the excited states of ClCrIIITPP(Py) are represented in Scheme 1. ClCrIIITPP(H2O). The absorption spectrum of ClCrIIITPP(H2O) in a polystyrene film at room temperature exhibits absorption peaks at 450 nm in the Soret band region and at 560 and 610 nm in the Q band region. The spectrum is similar to that observed for ClCrIIITPP(H2O) in a toluene solution. Figure 5 shows the transient absorption spectra and luminescence spectrum observed after laser pulsing. The transient spectrum detected at 50 ns after laser pulsing decays according to first-order kinetics with the rate constant of 1.0 × 107 s-1, leaving long-lived transient spectrum, which decays within 20 µs. The decay of the long-lived transient does not follow firstorder kinetics. We tentatively analyzed the decay according to second-order kinetics.
630 J. Phys. Chem., Vol. 100, No. 2, 1996
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Figure 6. Logarithmic kAq(T) represented as a function of reciprocal temperature. Solid curve is the one calculated by eq 9.
The results mentioned above indicate that the laser photolysis of ClCrIIITPP(H2O) gives rise to the formation of the two transient species; a faster decay component and a slower decay component. The luminescence from ClCrIIITPP(H2O) exhibits a peak at 830 nm. Probably, the luminescence originates from the 4T1 state of ClCrIIITPP(H2O). The luminescence intensity was found to be much weaker than that from ClCrIIITPP(Py). The decay of the luminescence follows first-order kinetics with the rate constant 9.6 × 106 s-1. This value is almost identical with that of the faster decay component of the transient species, indicating that the transients are ascribed to the 6T1 and 4T1 states of ClCrIIITPP(H2O), which are in thermal equilibrium each other. The slower decay component shows a negative peak at 450 nm and a positive one around 430 nm. On the basis of an earlier study on photodissociation of H2O from ClCrIIITPP(H2O) in toluene, the transient is concluded to be a five-coordinate species ClCrIIITPP.10 Thus, we concluded that photodissociation of H2O from ClCrIIITPP(H2O) occurs even in a polystyrene film at room temperature.
ClCrIIITPP(H2O) + hν f ClCrIIITPP + H2O
(7)
The decay of the long-lived transient is shown by
ClCrIIITPP + H2O f ClCrIIITPP(H2O)
(8)
Laser photolysis of ClCrIIITPP(H2O) in polystyrene was carried out at low temperatures. Below 240 K, the photodissociation of H2O is completely suppressed and the transient species observed is solely the faster decay component. The luminescence from the 4T1 state became too weak to be detected below 240 K by the present laser system. Figure 6 shows the plot of the logarithmic decay rate constants, kAq(T), of the faster decay component of ClCrIIITPP(H2O) in polystyrene, represented as a function of reciprocal temperature. The rate constant decreases with a decrease in temperature. Since the 4T1 and 6T1 states are in thermal equilibrium, the rate constant has the same expression as eq 1. The rate constants kAq(T) obtained experimentally are well reproduced by the following equation:
kAq(T) ) [8.1 × 104 + (2/3)8.5 × 107 exp(-548/T)] × [1 + 2/3 exp(-548/T)]-1 (9) The solid line in Figure 6 is logarithmic kAq(T) calculated by eq 9. Figure 7 shows the absorbances, DF(T) and DS(T), of the faster (6T1 + 4T1) and slower decay components, represented as a
Figure 7. Plot of DF(T) (O) and DS(T) (0) vs T. Solid curves denote the ones calculated by eqs 10 and 13 (see text).
function of temperature T:DF(T) was measured at 445 nm and DS(T) at 435 nm at 50 ns after a laser pulse. Since the value of DF(T) increases with a decrease in temperature, it is suggested that the 4S1 state of ClCrIIITPP(H2O) possesses an excitation energy dissipation process with an activation barrier. The yield can be formulated similarly to eq 2: the plot of DF(T) vs T is reproduced by
DF(T) ) 0.43[1 + 16.8 exp(-800/T)]-1
(10)
The activation barrier for the energy dissipation process at the 4S state is obtained as 1.6 kcal mol-1. 1 The value of Ds(T) decreases with a decrease in temperature, indicating that the photodissociation of H2O is suppressed at low temperatures. The suppression of the photodissociation of H2O causes the increase in ΦΤ(6T1 + 4T1) at low temperatures. The assumption6 that no internal conversion 4S0 r 4S1 occurs at the 4S1 state leads to
Φdis(T) ) kdis(kdis + kTq)-1
(11)
where Φdis is the yield for photodissociation and kdis and kTq are respectively the rate constants for photodissociation of H2O and for the formation of the 4T1 state from the 4S1 state. Since the photodissociation of H2O is considered to be an activation process, kdis ) k0 exp(-∆Ediss/RT), we obtain
Φdis(T) ) (1 + kTq/kdis)-1 ) [1 + kTq/k0 exp(∆Ediss/RT)]-1 (12) Since the value of Φdis is proportional to the value of Ds(T), eq 12 is rewritten as
Ds(T) ) Ds(∞)(1 + kTq/k0)[1 + kTq/k0 exp(∆Ediss/RT)]-1 (13) The solid line of Ds(T) in Figure 7 is calculated by eq 13 with the use of Ds(∞)(1 + kTq/k0) ) 0.236, kTq/k0 ) 6.0 × 10-2, and ∆Ediss/R ) 800. It is noteworthy that the value of ∆Ediss is identical with that of the activation barrier for the energy dissipation process at the 4S1 state. From eqs 12 and 13, Φdis(T) is formulated as
Φdis(T) ) Ds(T)[Ds(∞)(1 + kTq/k0)]-1
(14)
The photodissociation yield at an infinite temperature, Φdis(∞), is calculated as Φdis(∞) ) (1 + kTq/k0)-1 ) 0.94.
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The decay of the long-lived transient, ClCrIIITPP, follows second-order kinetics. The bimolecular rate constant, kbi, is shown by
kbi ) ka∆(λ)d
(15)
Here ka is the apparent decay rate constant measured at the wavelength λ, ∆(λ) is the difference in the molar absorption coefficient between ClCrIIITPP(H2O) and ClCrIIITPP, and d is the thickness of the film. With the use of the values d ) 0.01 cm and ∆(λ) ) 6.0 × 104 M-1 cm-1 at 435 nm,10 the values of kbi are obtained as 3.5 × 109, 1.6 × 109, 4.3 × 108, and 1.5 × 108 s-1 at 300, 280, 260, and 240 K, respectively. The Arrhenius plot gave the expression for kbi:
kbi ) 1.3 × 1015 exp(-3850/T)
(16)
The preexponential factor, 1.3 × 1015 M-1 s-1, and the activation energy for kbi, 7.7 kcal mol-1, are markedly larger than those of the diffusion process in fluid solutions.14 It seems necessary to compile more detail studies for full understanding of the recombination reactions in polymer matrices. Discussion The excited states of chromium(III) porphyrins in solutions have been investigated theoretically and experimentally on the basis of the detailed observation of luminescence in the temperature range 300-4.3 K.6 The energy diagram shows that, because of the spin-forbidden process 2T1 r 4S1, the 4S1 state converts to the 4T1 state with high efficiency close to unity. The energy gap between the 4T1 and 6T1 states estimated from the luminescence spectra is as small as ca. 520 cm-1(1.5 kcal mol-1).6 Thus, the two states are considered to achieve the Boltzmann distribution. The laser photolysis studies of ClCrIIITPP(Py) in polymer films revealed that the transient observed is the excited states with the decay rate constant 6.5 × 106 s-1 at 300 K. The excited-state spectrum has a peak around 480 nm, similarly to the excited triplet spectra of other metallotetraphenylporphyrins with a diamagnetic central metal. The temperature dependence of the decay rate constants is explained on the basis of the thermal equilibrium between the 4T1 and the 6T1 state. The energy gap between the two states is evaluated as 1.5 kcal mol-1. This value is in good accord with that (520 cm-1) estimated from the emission spectra originating from the 4T1 and the 6T1 state of chromium(III) tetraphenylporphyrin.6 The photochemical properties of ClCrIIITPP(Py) in polymer films are markedly different from those in toluene solutions. In the solution, ClCrIIITPP(Py) photodissociates the axial pyridine to give the five-coordinate ClCrIIITPP. The quantum yields for the photodissociation of pyridine in toluene at 298 K were obtained as 0.57 and 0.61 at the excitation wavelengths of 532 and 355 nm, respectively. However, no photodissociation of pyridine is detected in the polystyrene film by present nanosecond laser photolysis. Probably, rigid polymer, which surrounds ClCrIIITPP(Py), prevents the full dissociation of pyridine. The yield of the excited states, 4T1 and 6T1, increases with a decrease in temperature, indicating that the 4S1 state of ClCrIIITPP(Py) has an energy wasting process with an activation barrier of 1.4 kcal mol-1. This process is considered to be the temporal dissociation of pyridine at the 4S1 state of ClCrIIITPP(Py), followed by the rapid geminate recombination to regenerate ground ClCrIIITPP(Py). The quantum yield, Φ(6T1 + 4T1), at an infinite temperature is estimated as 0.15. Thus, the maximum yield for the temporal dissociation of pyridine is calculated as
TABLE 1: Decay Rate Constants, kq and ks, and ∆Eqs of ClCrIIITPP(Py) and ClCrIIITPP(H2O) in Polystyrene Filmsa kq, s-1 III
ClCr TPP(Py) ClCrIIITPP(H2O) a
1.2 × 10 8.5 × 107 8
ks, s-1
∆Eqs, kcal mol-1
5 × 10 8.1 × 104
1.5 1.1
4
Experimental errors are within (10%.
0.85. This value is moderately in agreement with the dissociation yield of pyridine determined in a toluene solution at 298 K. The transient spectrum of ClCrIIITPP(H2O) in polystyrene film detected after 355 nm laser pulsing is composed of two species: one is the excited states, 6T1 + 4T1, and the other is the five-coordinate ClCrIIITPP yielded by the photodissociation of axial H2O. On the basis of the excited state lifetime measurements in the temperature range 77-300 K, the energy gap between the 6T1 and 4T1 states is obtained as 1.1 kcal mol-1. In Table 1 are listed the decay rate constants, ks and kq, of 6T and 4T determined for ClCrIIITPP(H O) and ClCrIIITPP1 1 2 (Py). Because 4S0 r 4T1 and 4S0 r 6T1 are respectively the spin-allowed and the spin-forbidden decay process, the rate constant kq is ca. 3 order of magnitude larger than ks. No marked differences in the rate constants between ClCrIIITPP(H2O) and ClCrIIITPP(Py) can be seen. The yield of the excited states, 6T1 + 4T1, measured for ClCrIIITPP(H2O) in the temperature range 77-300 K indicates that the 4S1 state returns to the ground ClCrIIITPP(H2O) through the deactivation process with an activation barrier of 1.6 kcal mol-1. The process is caused by the dissociation of an axial H2O. The yield for photodissociation of H2O at an infinite temperature is estimated as 0.95. This value is in good accord with that (0.94) determined for ClCrIIITPP(H2O) in a toluene solution at 298 K. Both ClCrIIITPP(H2O) and ClCrIIITPP(Py) efficiently photodissociate the axial ligand in toluene solutions. In polymer films, ClCrIIITPP(H2O) is able to undergo full photodissociation of H2O. However, no full photodissociation of axial pyridine was observed for ClCrIIITPP(Py). The difference in the photochemical behavior between ClCrIIITPP(H2O) and ClCrIIITPP(Py) in polymer films is presumably interpreted in terms of the size effect of the axial ligand. It is suggested that, since the polystyrene net firmly surrounds ClCrIIITPP(Py), the axial pyridine photodissociated from ClCrIIITPP(Py) cannot escape from the net and, therefore, readily recombine with ClCrIIITPP to regenerate ClCrIIITPP(Py) within a laser pulse (20 ns). The molecule, H2O, photodissociated from ClCrIIITPP(H2O) is much smaller than pyridine and is able to escape from the hydrophobic polystyrene net, leading to the full photodissociation of H2O. Appendix (I) The decay of the sum of the populations of the 4T1 and states (nq and ns) is expressed as
6T 1
-d(nq + ns)/dt ) kPy(T)(nq + ns)
(I-1)
where kPy(T) is the decay rate constant at temperature T. Equation I-1 is rewritten as
-d(nq + ns)/dt ) -dnq/dt - dns/dt ) kqnq + ksns
(I-2)
Here kq and ks are respectively the intrinsic decay rate constants of the 4T1 and the 6T1 state. With the use of the energy separation, ∆Eqs, between the two states, the population ratio χ, nq/ns, is shown by
632 J. Phys. Chem., Vol. 100, No. 2, 1996
χ ) nq/ns ) 2/3 exp(-∆Eqs/RT)
Hoshino et al.
(I-3)
From eqs I-2 and I-3, we obtain
-d(nq + ns)/dt ) (ks + kqχ)/(1 + χ)(nq + ns)
(I-4)
Acknowledgment. This work is partly supported by a Grantin-Aid on Priority-Area-Research “Photoreaction Dynamics” from the Ministry of Education, Science, and Culture, Japan (06239267). M. I. acknowleges a Grant in Aid from the Ministry of Education, Science, and Culture, Japan (06303005).
Equations I-1 and I-3 give the rate constant kPy(T).
kPy(T) ) (ks + kqχ)/(1 + χ) ) [ks + kq(2/3) exp(-∆Eqs/RT)]/[1 + 2/3 exp(-∆Eqs/RT)] (I-5) (ΙΙ) Since the 4S1 state goes to both excited states, 6T1 and 4T , and the ground state, 4S the sum of the yields, Φ (6T + 1 0, Τ 1 4T ), for the formation of the 6T and 4T states at the 1 1 1 temperature T is formulated as
ΦT(6T1 + 4T1) ) kTq/(kTq + kS0)
(II-1)
where kTq and kS0 are, respectively, the rate constants for the decay of the 4S1 state to the 4T1 and the 4S0 state. The assumption that the deactivation process, 4S0 r 4S1, is an activation process gives an expression for kS0 as
kS0 ) k0 exp(-∆E/RT)
(II-2)
Equation II-1 is rewritten as
ΦT(6T1 + 4T1) ) [1 + k0/kTq exp(-∆E/RT)]-1 (II-3)
References and Notes (1) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. III, Chapter 1. (2) Becker, R. S.; Allison, J. B. J. Phys. Chem. 1963, 67, 2662-2669. (3) Becker, R. S.; Allison, J. B. J. Phys. Chem. 1963, 67, 2669-2675. (4) Smith, B. E.; Gouterman, M. Chem. Phys. Lett. 1968, 2, 517519. (5) Gouterman, M.; Mathies, R. A.; Smith, B. E. J. Chem. Phys. 1970, 52, 3795-3802. (6) Gouterman, M.; Hanson, L. K.; Khali, G.-E.; Leenstra, W. R. J. Chem. Phys. 1975, 62, 2343-2353. (7) Hoshino,; M. Seki, H. Chem. Phys. Lett. 1984, 110, 413-416. (8) Ake, R. L.; Gouterman, M. Theoret. Chim. Acta 1969, 15, 20-42. (9) Yamaji, M.; Hama, Y.; Hoshino, M. Chem. Phys. Lett. 1990, 165, 309-314. (10) Inamo, M.; Hoshino, M.; Nakajima, K.; Aizawa, S.; Funahashi, S. Bull. Chem. Soc. Jpn. in press. (11) Yamaji, M.; Hama, Y.; Miyazaki, Y.; Hoshino, M. Inorg. Chem. 1992, 31, 932-934. (12) Summervill, D. A.; Jones, R. D.; Hoffman, B. M.; Bassolo, F. J. Am. Chem. Soc. 1977, 99, 8195-8202. (13) Hoshino, M.; Imamura, M.; Watanabe, S.; Hama, Y. J. Phys. Chem. 1984, 88, 45-49. (14) Calvert, J. G.; Pitts, Jr., J. N. In Photochemistry, John Wiley: New York, 1966.
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