Solvent effects on the magnetic field dependent recombination kinetics of the Zn-porphyrin—viologen dyad radical ion pair state

Chemtcal Phystcs North-Holland

162 (1992)

155-163

Solvent effects on the magnetic field dependent recombination kinetics of the Zn-porphyrin-viologen dyad radical ion pair state Vladimir Ya. Shafirovich,

Elena E. Batova

Institute ofChemlca1 Physics. .-lcademy ofSclences, 142432 Chernogolovka, Russia

and Peter P. Levin Institute ofChemlca1 Physics. Academy of Sciences. 117334 Moscow. Russia Recetved

13 June 1991

Magnettc field dependent recombination kinetics of the triplet radtcal patr state (RIPS) of the semirtgtd Zn-porphyrin-viologen (P-Ph-VI’+ ) dyad m which &-porphyrin (P) and viologen (VI’+ ) linked by the semtrigid spacer (Ph) contammg a 1,4phenylene motety has been studied by nanosecond laser flash photolysts technique m sixteen solvents of wtdely varying properties. The mfluence of solvent viscosity, the we of solvent molecules and the specific solvatton on the trtplet RIPS recombmatton kmettcs m an external magnetic field has been found and discussed in terms of the hyperfine coupling mechanism including an exchange mteractton modulated by the stochastic spacer motion m low magnetic fields and the spu-orbit coupling induced intersystem electron transfer m high magnetic fields.

1. Introduction The role of the solvent in the intramolecular spinallowed electron transfer (ET) in covalently-linked donor-acceptor systems has been usually interpreted in terms of ET theories. Considerable efforts have been made to determine the solvent reorganization energies and their dependence on the optical (e,,) and static (es) dielectric constants and molecular size effects [ I-71. However, different approaches should be used for the interpretation of the solvent effects on the spinforbidden magnetic field sensitive ETs for which the role of the solvent is not so obvious [g-lo]. Pronounced solvent viscosity and polarity effects on the triplet state yields in an external magnetic field have been found for the polymethylene-linked radical ion pairs generated by the photoinduced intramolecular ET in compounds of the type pyrene-( CH2),,-N,Ndimethylaniline [ 1 1- 13 1. These effects have been interpreted in terms of the electron-nuclear hyperfine interaction (hfc) mechanism including a spin 0301-0104/92/$

05.00 0 1992 Elsevter Science Publishers

exchange interaction (J) modulated by the solvent dependent stochastic motion of the methylene chain. Solvent effects on recombination kinetics of triplet RIPS formed by the photoinduced intramolecular ET in bifunctional molecules benzophenone- ( CH1 ) nN,N-diphenylamine in an external magnetic field have been attributed to conformational difference in RIPS on account of the complex formation of the radical ions with the solvent [ 141. Recent studies on the intramolecular ETs in the Znporphyrin-viologen dyads (P-Sp-Vi’+ ) in which Znporphyrin ( P ) and viologen (Vi’+ ) linked by a flexible or semirigid spacer (Sp) have shown an understandable role of media [ 15-251. The photochemical behaviour of this kind of supramolecules is very complex on account of the flexibility of the spacer between the porphyrin and viologen moieties. Both the singlet and the triplet excited states of the porphyrin are important for charge separation. In polar solvents the ET rates estimated from the fluorescence lifetimes are poor, dependent on solvent polarity and viscosity, and decrease drastically in moderately po-

B.V. All rtghts reserved.

1: Ya Shafirowh et al. /Solvent effects on the magnetlcfield dependent krnetrcs

156

Fig. 1. Molecular structure of the P-Ph-VI*+ dyad.

lar tetrahydrofurane [25]. The ET rates from the porphyrin triplet excited state do not correlate to solvent polarity. The lifetimes of the triplet radical ion pair state (RIPS) are sufficiently longer than those of the corresponding singlet one and are strongly perturbed by an external magnetic field [ 17-20.22-241. The recombination rates of the triplet RIPSs in an external magnetic field are sensitive to their microenvironment. In order to elucidate solvent effects on the photoinduced intramolecular ETs we have extensively studied the photochemical behaviour of semirigid Znporphyrin-viologen dyad (P-Ph-Vi’+ ) in which Znporphyrin (P) and viologen (Vi” ) are linked by a semirigid spacer (Ph) containing 1,4-phenylene moiety (fig. 1) in media of widely varying solvent polarity (e,=2-1 lo), viscosity (r1=0.32-1000 cP) and nature.

2. Experimental The dyad, containing Br- as a counter anion, was prepared by the methods described previously [ 261. The product was dissolved in methanol and an excess of NHdPF6 was added to replace the counter anion to PF;. The precipitate was collected. washed with water and dried under vacuum at room temperature for one day. The synthesized dyad gave one spot on TLC (Rf: 0.39; saturated aqueous KN03: HzO: CH,CN = 1: 4 : 5; silica gel). Spectroscopic data of the dyad were consistent with the assigned structure. Commercially available “chemically pure” grade dioctyl phthalate (~=64 cP), poly(ethylene glycol) (PEG. MW = 600. ‘I= 154 cP, Loba Chemie), Triton X-100 (~~315 cP, Merck) and glycerol (?/=lOOO cP) were used without further purification. Other organic solvents being best available grade from com-

mercial suppliers were dried by conventional methods and twice distilled. Sample concentrations were (2-5 ) x 1Op5 M (nanosecond laser flash photolysis) and (0.5-2) x 1Op6 M (microsecond flash photolysis, absorption and fluorescence measurements). Absorption spectra were recorded with a Specord M40 spectrophotometer, and fluorescence spectra were recorded with an Aminco Bowman spectrofluorimeter. Relative fluorescence yields (&) were estimated by comparing integrated yields of the two emission bands near 6 14 and 660 nm with that of Znmeso-4-benzoxyphenyl-tris (4-tolyl ) porphine (ZnBTP). The absorption spectra and decay kinetics of the intermediates were recorded by nanosecond laser flash photolysis using a PRA LN-102 dye laser (420 nm ) pumped by a LN- 1000 nitrogen laser as an excitation source [27]. Kinetic curves were averaged over 128 laser pulses by a Biomation 6500 waveform recorder coupled to an Apple IIe microcomputer. In magnetic field effect experiments, the sample holder was placed between the two pole pieces of a permanent magnet. The distance between the pole pieces can be varied and the magnetic field strength (B) was up to 0.24 T. The experiments were conducted in a 1 cm quartz cell with a water jacket kept at 1O-60” C. Microsecond flash photolysis studies were made with a conventional equipment. A combination of glass-light filters was used to pick out the region 550650 nm. Pulse energy of xenon lamp to excite the sample = 5 m.I, flash duration = 5 us. The experiments were conducted in an 8 cm quartz cell at room temperature. Before the laser flash photolysis studies the cells were carefully evacuated on a vacuum pump line up to 1Op3 Torr and filled with argon. The relative yields of 3P-Sp-Vi’i (&) and P+ ‘Sp-Vi + * (&) were estimated by comparing the transient absorption of the dyads with that of the ZnBTP-benzylviologen system [ 261. It was assumed that the dyad rate constants of the internal conversion of ‘P to the ground state, the intersystem crossing of ‘P to ‘P and the radiation lifetime of ‘P were equal to those of ZnBTP [ 28 1.

V Ya.Shafirowchet al. /Solventeffectson the rnagnetlcfielddependentkmetrcs 3. Results

rather than on the dielectrical

157

properties

of solvents.

3.2. Spectroscopic andfluorescence properties

3.1. General features

of

P-Ph- Vi”+

The photochemical behaviour of P-Ph-Vi’+ has been studied in organic solvents with widely varying properties. In polar organic solvents the dyad in the [ P-Ph-Vi”+ ] [Br- I2 form is quite soluble. To enhance the dyad solubility in the solvents of middle and low polarity PF; has been used as a counter anion. However, the dyad being even in the [P-PhVi’+ ] [PF; ]? form is completely insoluble in nonpolar saturated hydrocarbons. The aggregate formation which results in a drastic broadening and a shape change of the Soret band occurs in aromatic (benzene, toluene) and chloro (CH,Cl, CH2Cl,) hydrocarbons. The aggregates are destroyed by the addition of macrocyclic ligands, e.g. of dibenzo-1 %crown6. The dyad solubility decreases drastically in moderately polar sterically hindered solvents, e.g. in 2,5dimethyltetrahydrofuran and diethyl ether. Hence, in weak and nonpolar solvents the dyad solubility seems to be dependent on the specific solvation of the dyad

Table 1 Solvent effects on the relative yields of the P-Ph-VI’+ constant of the ‘P-Ph-Viz+ decay (k,) at 20°C Solvent

dtoxane dioctyl phthalate ethyl acetate tetrahydrofuran pyrtdme acetone ethanol methanol dtmethylformamide acetomtrtle glycerol dtmethylsulfoxide formamtde Trtton X- 100 poly(ethylene glycol)

Ga)

2.2 5.1 6.0 7.6 12.3 20.7 24.5 32.6 36.7 37.5 42.5 46.7 110

n (CP)“’

1.1 64 0.44 0.46 0.95 0.32 1.1 0.55 0.80 0.34 1000 2.2 3.3 320 150

fluorescence

1 sore, (nm ) dyad

ZnBTP

425 427 423 425 432 423 424 423 42-l 423 428 430 427 428 428

425 427 423 425 432 423 424 423 427 423 428d’ 430 f) 428 428

The absorption spectra of P-Ph-Vi2+ are equal to the sum of the component chromophores. Porphyrin dominates the visible absorption spectrum, while viologen contributes strongly ( EZ 1 x 1O4M- ’ cm- ’ ) at 260 nm only. No noticeable charge transfer bands are observed in the UV-visible absorption spectrum. The dyad absorption peaks in visible region as well as their intensity ratios and fwhm are very close to those of ZnBTP being used as a reference and are dependent on the solvent only. Hence, the attached viologen does not perturb the ground and first singlet excited states of the porphyrin. In agreement with the previous studies for Zn-porphyrins [29-321 a redshift of both Soret (table 1) and Q bands demonstrates the prominent role of specific (chemical, shortrange) solute-solvent interactions rather than nonspecific (universal, long-range) ones determined by the solvent polarity and polarizibility [ 341. The val-

(or), ‘P-Ph-VI’+

(&)

and ‘[P+‘-Ph-Vi+‘]

(h,

@a) and the rate

orb’

@Tb’

@Rb’

QRb’

QTb)

k,b’ (10’s_‘)

0.8

0.6 0.5 I.0 1.0

0.2 0.2 0.3 0.8 I.0

0.2 0.2 0.2 0.2 0

0.4 0.4 0.5 0.4 0

0.4 0.2 c) 0.6 I.0

3.4 3.2 4.1 4.1 _

0.5 0.4 0.4 1.0 0.5 0.6” 1.0 0.5B’ 1.0 0.6

0.3 0.2 0.1 1.0 0.3 0.5 1.0 0.3 1.0 0.2

0.2 0.2 0.3 0 0.2 0.1 0 0.2 0 0.4

0.5 0.4 0.4 0 0.5 0.6 0 0.5 0 0.2

c) c) c) 1.0 c)

‘) All values are at 20°C. taken from ref. [ 321. b, Esttmated accuracy ? IO-30%. Cl &T( 0.1 probably arises from traces of an unattached Zn-porphyrin. d, ZnTPP. ” &was measured relative to that of ZnTPP. Qr=0.7 ‘I. r) Low solubility of ZnBTP and ZnTPP. *) &was esttmated from the fluorescence time-resolved experiments [ 251

c) 1.o c) 1.0 0.2

3.2 4.0 4.3 1.5 0.55 4.2 0.55

158

L: Ya. Shafirorrch et al /Solvent ejfects on the magnetlcfield dependent kmetm

ues of the red-shift are the largest in pyridine and dimethylsulfoxide in which quite stable complexes of Zn-tetraphenylporphin with two and one solvent molecules have been identified [ 29-321. The dyad fluorescence spectra are very close to those of ZnBTP and are dependent on the solvent nature only. Two emission bands (around 6 14 and 660 nm) of the porphyrin as well as their ratio and fwhm are unaffected by the attachment of the viologen to the porphyrin. In many cases the fluorescence yields (@r) of P-Ph-Vi’+ measured relatively to that of ZnBTP are less than unity, showing that the viologen moiety functions as a quencher for the singlet excited state of the porphyrin (table 1). The value ( 1 -@r) represents the contribution of a new route of the ‘P decay in the dyads and seems to be equal to the yield of the singlet RIPS formed by the intramolecular ET in ‘P-Sp-Vi2 +. Efficiency of ‘P quenching does not correlate with solvent viscosity and polarity. In terms of the scale used the relative &value implies the contribution of the ‘P+“P intersystem crossing. The highest contribution of this route is observed in solvents which, being nucleophiles, can form complexes with the central zinc atom as an electrophilic center [ 29-32,341. It should be mentioned that the relative & values are close to those calculated from the fluorescence lifetimes [ 25 1.

3.3. Time-resolved measurements

transient absorption

Laser photoexcitation of P-Ph-Vi2+ in polar organic solvents [ 221 and micellar solutions [ 231 results in the appearance of the triplet excited state, 3PPh-Vi’+ and RIPS, P+‘-Ph-Vi+‘. The formations of 3P-Ph-Vi2+ and P+‘-Ph-Vi+ ’ occur within the response time (% 10 ns) of the apparatus [ 27 1. Immediately after the laser pulse, the well-known absorptions of 3P-Ph-Vi’+ and P+ ‘-PhVi+’ with characteristic maxima at 470 and 410,620 nm, respectively, are observed (fig. 2). In solvents which can form complexes with porphyrin the formation of P+ . -Ph-Vi+ . does not occur and 3P-PhVi’+ is observed only. The sums of the 3P-Ph-Vi’+ (&) and RIPS (eR), relative yields being equal to the corresponding @rvalues (table 1 ), manifest that the RIPS precursor is the very short-lived ‘P-Ph-

006 h

I

I

400

450

500

550

600

650

700

WAVELENGTH/nm

Fig. 2. Transient absorption spectra recorded various times after laser excltatlon (1~420 nm) of P-Ph-Vi*+ in acetone at 20°C. (0 ) Immediately after a laser flash; (0 ) 80 ns after a laser flash.

02 -

0.06

B 470nm

002

0

Lm

0

I

I

I

05

10

15

TIME/p

I -

0

12

TIME/y

Fig. 3. Absorption changes observed at 470 and 620 nm following laser excitation (I= 420 nm) of P-Ph-V?+ (A) m acetone and (B) tetrahydrofuran at 20°C. Lower curves: B=O; upper curves: B=0.24 T; dotted lines: absorption of the long-hved ‘PPh-Vi’ +,

Vi2+, i.e. RIPSs of this type are born in the triplet state, “[P+‘-Sp-Vi+‘]. The decay kinetics of 3P-Ph-Vi’+ is biexponential with a short and a long-lived component (fig. 3 ), The relative amplitudes of these components and hence the relative yields of a short ( @+DT) and a long-lived ( DT) 3P-Ph-Vi2+ depend on the solvent nature (table 1). The Qr values are close to unity in the solvents which favour the complex formation between the porphyrin and solvent molecules. The fast decay of the 3P absorption at 470 nm is followed by appearance of the additional RIPS absorption at 4 10 and 620 nm (fig. 3 ) . Hence, the decay of 3P-Ph-Vt ‘2+ in nanosecond time scale is ascribed to the intramolecular ET from 3P to Vi’+ [ 221:

159

V. Ya. Shafirovrch et al. /Solvent efects on the magnetrc field dependent kmetlcs

3p_ph_vi2+45+3[p+‘-ph-Vi+’

solvent polarity and increase in viscous glycerol. The RIPS recombination is poorly temperature dependent in the temperature range 1O-60’ C. P+ ‘-PhVi+’ exhibits the close to zero activation energies, E, (table 2).

(1)

1*

The corresponding rate constant (k,) is poorly perturbed by solvent polarity (table 1). The effect of rl is noticeable. The lowest k, values are observed in viscous glycerol and PEG. The slow decay of the 3P absorption at 470 and 620 nm (fig. 3b) results in the dyad ground state formation and in highly diluted solutions the corresponding rate constants are close to those of ZnBTP (=5xlO’s-’ [26]). Thus, the formation of P+ ‘-Sp-Vi+ ’ occurs by two pathways: (i) the very fast ET occurring in the very short-lived 3P-Sp-Vi2+ within the time < 10 ns; (ii) the fast ET in ‘P-Sp-Vi2+ with characteristic lifetimes 20-200 ns. The sum of the total relative yields of RIPS ( QR) and @= is equal to df within the experimental accuracy (table 1). Hence, all RIPSs observed in microsecond time scale are the triplet-derived ones [ 22-241. The kinetics of the RIPS decay (fig. 3) fits to the first-order law (k,) and can be ascribed to the backward intramolecular ET within RIPS [ 22-241: .

‘[P’.-Ph-Vi+‘]LP_Ph_Vi’+

3.4. Magnetic field effect on the ‘[P’.-Ph-

In an external magnetic field (B< 0.24 T) the lifetime of RIPS is drastically increased (table 2, fig. 3 ). The maximum values of the magnetic field effect, k,(B=O)/k,(B=0.24T) areobservedinviscoussolvents and independent of the solvent polarity. The k,(B=0.24 T) value depends poorly on the solvent nature. The lowest value of k,(B=0.24 T) is observed in PEG. In the strong magnetic field (B=0.24 T) the E, values are close to zero (table 2). The magnetic field dependences of the k, values versus B judging by their shapes can be described by the following parameters: B,,,, magnetic field strength at which the k, value has a maximum; Bl,2, magnetic field strength at which the magnetic field effect attains half of its saturation value (table 2). Solvent polarity does not affect the B,,, and B, ,* values (fig. 4a). Specific solvation of RIPS by cyclic ethers results in the broadening of the magnetic field dependences with the corresponding shifts of the B,,,

(2)

The solvent nature affects the RIPS recombination rates (table 2). The k, values do not correlate with Table 2 The recombination dyads at 20°C

rate constants

Solvent

dtoxane toluene b, droctyl phthalate ethyl acetate tetrahydrofuran acetone ethanol methanol acetonitrrle glycerol d’ formamrde poly( ethylene glycol) d’

(k,) and characteristic

magnetic

kI a’ (106s-‘)

field parameters

B,,.

k,(B=O)

B=O

B,,,

B=0.24

1.0 1.0 1.7 1.4 1.o 1.4 1.5 1.1 1.4 3.3 1.9 1.3

1.1 1.1 1.7 1.5 1.1 1.5 1.5 1.2 1.5 3.3 1.9 1.3

0.55 0.48 0.65 0.40 0.50 0.38 0.60 0.43 0.38 0.31 0.45 0.14

T

VI +‘/

recombination

kJBc0.24 2.0 2.0 2.6 3.6 2.0 3.6 2.5 2.6 3.6 10.6 4.2 9.3

aJ Estrmated accuracy + 5- 10%. b, 0.4 M solution of dibenzo-18-crown-6 ” The range of the k, plateau. ” [P-Ph-Vi’+] [Br-Cl-] [22].

of the triplet RIPS of the Zn-porphyrin-vtologen

(mT )

B,,, (mT)

T) >

E, (kcal/mol) B=O

7 7 3” 5 7 5 7 7 5 0 3 0 in toluene.

50 50 35 32 50 32 32 38 32 14 34 14

0.0

B=0.24 0.4

-0.5

0.0

-0.2 1.5

0.0 2.0

0.0

0.0

T

V Ya. Shafirowch et al. /Solvent effects on the magnetic field dependent kmetu

160

and B, 12in high fields (fig. 4b). The narrowing of the magnetic field dependences with the co~esponding shifts of the B,,, and B,/, in low fields occurs in viscous solvents (fig. 4~). Hence. the shape of the magnetic field dependences is modulated by solvent viscosity and specific solvation of RIPS only.

4. Discussion

0

/ 005

I

010

015

020

015

020

I

B/T

OX

005

010

B/T 0

5

10

B/mT

0 B/T

Fig. 4. Solvent polarity, specific salvation and wscosity effectson the magnetic field dependences of k, versus B at 20°C: (A): x ethyl acetate (~,=6.0); 0 -acetone ($=20.7); 0 - acetonitrde (t,=37.5). (B): x - 0.4 M solution of dibenzo-18-crown-6 in toluene (t,=2.4); 0 - dioxane (~,=2.2); 0 - tetrahydrofuran (~,=7.6). (C): x - dloctyl phthalate (q=64 cP); 0 poly(ethyleneglyco1) (q=15OcP) [23]; O-glycerol (q=lOOO CP) [23].

The phot~hemical behaviour of the P-Ph-Vi*+ molecules in general should be attributed to the intramolecular ETs in different ensembles of conformers which do not equilibrate on the nanosecond time regime [ 21-251. Two subensembles denoted as “closed” and “opened” conformers have been postulated for the interpretation of the fluorescence decay profiles. The most stable conformations of P(CH,),-Vi’+ with the center-to-center distance 11.1 and 15.2 A were also predicted by computer modeiling (2 I J. However, the apparent averaging of the NMR spectra of P-(CH2)3_,-Vi2+ suggests that the ensemble of conformers is in dynamic equilibrium in the microsecond time scale [ 351. The closed and opened conformers have also been important for the interpretation of the decay of the porphyrin triplet excited state [ 22-241. The rates of the transitions between the “closed” and “opened” subensembles may be roughly estimated as those of the decay of the short-lived ‘P-PhVi2+ which can be assigned to the conformational transition “opened”+“closed” being followed by the fast ET in [ ‘P-Ph-Vi’+ lci [ 22 1. Solvent polarity insignificantly perturbs these rates. The effects of solvent viscosity lend a support of the conformational transition as the rate determining step of the 3P-PhVi2+ decay (table 1). Size and rigidity of solvent molecules seem to be even more important than macroscopic viscosity. The k, value in PEG is less than that in more viscous glycerol. The molecules of Triton X-100 are more rigid than those of PEG and in Triton X- 100 the intramolecular ET in 3P-Ph-Vi’+ does not occur even in millisecond time scale. The specific solvation of the dyad by cyclic ethers and especially the formation of Zn-porphyrin complexes with solvent (pyridine, etc) results in the appearance of 3P-Sp-Vi’f m . which the intramolecular ET does not occur. The very pronounced effects of specific

V. Ya. ShaJirovlch et al /Solvent effects on the magnetlcjield dependent klnetm

solvation have been observed in the intramolecular quenching of ‘P by Vi2+ as well [25]. The specific solvation may prevent the formation of a sandwich conformation with a large electron exchange interaction [ 22-241. One may expect a similar time scale for the intramolecular dynamics in the RIPS. The utilization of the same simple approach for the RIPS recombination is very attractive as well [ 22-241. Two conformers (“opened’ and “closed’) with quite different distances between the dyad fragments have been assumed to be responsible for the 3 [P+ *Ph-Vi+ . ] recombination behaviour in polar liquid media [ 22-241. In the opened conformer with a long distance between the P+’ and Vi+. moieties the effective constant of the hfc, A,r,> 2J whereas in the closed one with a short distance between the P and Vi’+ moieties A,r, << 2J. The mechanism of the 3 [P+ ‘-Ph-Vi+ ’ ] Clrecombination is one-step spin-forbidden transition (intersystem ET with the rate constant ksoc) of the RIPS to the ground state induced by the SOC (Scheme 1) [ 22-241. This kind of the intersystem ET is very important for the recombination of the triplet radical ion pairs. the main state of which is the contact one, e.g. charge transfer triplet exciplexes in nonpolar solvents [ 27 1. The contribution of k,,, to the observed rate constant of the RIPS recombination (k,) can be estimated from the limit value of k, at high B, when intersystem ET is the limiting step of the RIPS recombination [ 22-24,27 1. The corresponding heavy-atom effect has been observed in a strong magnetic field, while it has been dramatically diminished in zero magnetic field [ 19,231. The k,(B=0.24 T) values being temperature independent do not depend significantly on the solvent nature (table 2). It

161

is not surprizing on account of the system being near the top (AG z - 1.5 eV [ 23 ] ) of the bell-shaped energy gap dependence of ksoc versus the intersystem ET free energy [27]. The lowest k,(B=0.24T) is observed in PEG. The long chain molecules of PEG may prevent the formation of the RIPS conformation with a large SOC coupling which may be of another symmetry than the exchange one. SOC is usually anisotropic and enhances in the perpendicular conformation in accordance with the urgency of the overlap between the molecular orbitals of different nature [ 36 1. The conformational concept was drawn from the solvent effects on the recombination of the triplet exciplexes with complete charge transfer [ 271. In zero magnetic field the relative contribution of the intersystem ET depends on the solvent nature that implies the appearance of the alternative route of the RIPS recombination which may be controlled by the conformational transition or the intersystem crossing (ISC) (scheme 1). Previous studies of the 3[ P+ ‘-Ph-Vi+ ’ ] recombination in polar organic solvents and micellar solutions have assumed that the RIPS recombination may be controlled by the conformational transition [ 2224]. However, from this point of view it is very difficult to explain the effects of solvent viscosity and temperature on the k, values as well as on the shape of the magnetic field dependences (table 2, fig. 4). This problem can be easily solved in terms of the interplay of the spin and molecular dynamics which can be described by the so-called static ensemble approximation (SEA) established by Schulten et al. [ 37-391 for the separation of molecular dynamics (governing the “spectrum of the exchange interaction”) and spin dynamics. The essentials of this treatment have been presented in a qualitative form by Staerk et al. [ 13,401. The stochastic ensemble with

Scheme 1.

162

C’Ya. Shafirovrch et al. /Solvent effects on the rnagnetrcJeld dependent kmetrcs

the distance (v) and hence time-dependent Hamiltonian characterized by J=J[ r( t) 1, is approximated by isolated subensembles with the constant J and thus time-independent Hamiltonian. The exchange interaction Jgives rise to dependence of the ~amiltonian on the conformation of the flexibly linked radical ions. In this approximation the exchange interaction occurs through space (solvent ) and depends exponentially on the distance [ 4 11. The strongly exponential dependence of the exchange interaction on distance implies an asymmetrical “motional deformation” and “motional shifting” of the exchange spectrum and the co~esponding magnetic field dependence with changing intramolecular mobility which can be modulated by the viscosity and temperature [ 1 l- 13 1. The SEA predicts the corresponding shift of B,,, to the high fields as well as the decrease of the k, value in zero magnetic field with increase of the solvent viscosity [ 1 l- 13,37-401. In the agreement with the SEA predictions the required viscosity effects on B,,, are observed in dioctyl phthalate, glycerol and PEG (fig. 4c ) . The relatively small k, (B= 0)value in viscous PEG implies the limits of the SEA predictions and an importance of solvent molecule size. The specific solvation of RIPS also modulates the stochastic motion of radical ion moieties and appears in “motional deformation” and “motional shifting” of the magnetic field dependences (figs. 4a and 4b and table 2). Parameters of magnetic field dependences are unaffected by solvent polarity (fig. 4a). It means that the intramolecular dynamics in the subensemble of the opened conformers is important for the shape of the magnetic field dependences. In the opened conformers intramolecular dynamics are not modulated by the Coulombic interactions because of the long distance between the radical ions.

5. Conclusion The recombination of the triplet radical ion pair state of the porphyrin-viologen dyads occurs via two most important routes: (i ) the spin-forbidden transition (intersystem electron transfer) stimulated by the spin-orbit coupling in the subensemble of the closed conformers with large enough exchange interaction between the radical centers; (ii) the pathway controlled by the hyperfine coupling induced S-T

transition modulated by the exchange interaction in the subensemble of the opened conformers. In zero magnetic field the relative contribution of the second route depends on the solvent nature. The change of intramolecular mobility of radical ion moieties which can be modulated by the solvent viscosity, the size of solvent molecules and the specific solvation appears in an asymmetrical “motional deformation” and “motional shifting” of the exchange spectra and the corresponding magnetic field dependences. The spin-orbit coupling induced transition is responsible for the RIPS recombination in a strong magnetic field in which the rate constant of the RIPS recombination depends weakly on the solvent nature.

References I R.A. Marcus and N. Sutin, Blochem.

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