Fast non-exponential intramolecular electron transfer reactions in pentanediol solutions

Volume 150, number 3,4

CHEMICAL PHYSICS LETTERS

FAST NON-EXPONENTIAL INTRAMOLECULAR IN PENTANEDIOL SOLUTIONS

16 September 1988

ELECTRON TRANSFER REACTIONS

Dan HUPPERT, Varda ITTAH, Asnat MASAD and Edward M. KOSOWER School of Chemistry, Sackler Facu1t.yqfExact Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel

Received 20 June 1988

Activationless, solvent-controlled fast intramolecular electron transfer (ET) kinetics arc reported for three N-arylaminonaphthalenesulfon-N,N-dimethylamides over a wide temperature range in three pentanediols. A stretched exponential decay (WilliamsWatts) using the longitudinal relaxation time, q, tits the results. The temperature dependence of the observed dynamics is interpreted.

1.

Introduction

tation. The relaxation time of a correlation function, C(t),

Interest in the role of the solvent in intramolecular electron transfer has heightened in recent years, due to new experimental and theoretical results [ l- 131. Until recently, analysis of the results has been based on a dielectric continuum model for the solvent. Within this model, microscopic relaxation can be related to the bulk dielectric susceptibility function of the pure solvent. For Debye solvents (exponential relaxation), the experimental rates of intramolecular electron transfer processes are related to the longitudinal relaxation time, TV, X=(&/G)%,

(1)

in which tD is the Debye dielectric relaxation time of the solvent, es is the static dielectric constant and E, is the high-frequency dielectric constant. However, more refined experimental measurements have shown that there are deviations from the simple model. Non-exponential decay is found for the fluorescence of the initially formed excited state of 4-dimethylaminobenzonitrile (DMABN) in several alcoholic and nitrile solvents [ 2,3 1. Certain classes of molecules exhibit time-dependent fluorescence shifts in polar solvents. A non-equilibrium distribution of the solvent molecules is formed around a probe solute molecule as a consequence of the instantaneous change in its dipole moment on exci-

C(t)=[v(t)-zqm)]/[zqo)-~(ccl)])

(2)

in which F( t ) is the mean fluorescence frequency of the spectrum at time t, for coumarin 153 (~~153) in some solvents is non-exponential with a value between TLand zD [4]. The non-exponential relaxation of the correlation function, C(t), can be rationalized using the mean spherical approximation (MSA) introduced by Wolynes [ 5 ] and recently applied by Rips et al. [ 6 1. The MSA method yields energy relaxation response functions, S(t), like that of C(t):

s(t)=[E(t)-E(co)ll[E(O)-E(co)l,

(3)

in which E(t) is the free energy of solvation at time t. The free energy, E(t), depends on the ratiop=R,/ Ri (R, is the radius of the solvent and R, that of the solute). On this basis, Maroncelli and Fleming [ 7,8] explored the behavior of S(t) by varying p at fixed E(O)/_!?(m) andbyvaryingE(O)/E(co) atfixedp. S(t) should be correlated with C(t) as found experimentally. The relaxation time varies between T,_ and rD for E(O)/E(m) B50. Another approach to the explanation of non-exponential solvent relaxation behavior involves a molecular dynamics simulation [ 81. Simulation of the dynamics of water suggests that the time scale for solvent relaxation decreases as the distance from the

0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )

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CHEMICALPHYSICSLETTERS

solute increases. Since the first-shell molecules contribute significantly to the total stabilization energy, the relatively slow response of these molecules gives an intrinsic non-exponentiality to the solvation process. A major component of the relaxation is due to the librational motion of the molecules in the first solvation shell, At these short distances from the probe molecule dipole, the electric field should be very large (10’ V/cm) and non-linear responses should be expected. In addition to the influence of the solvent on both intramolecular electron transfer processes and timedependent fluorescence shifts, the effect of solute structure can be determined, in particular, whether or not relaxation times shorter than zL are observable. We have reported [ 1 ] that some TNSDMA derivatives exhibit z,, less than sL, and that the larger the dipole moment of the probe the faster the dynamics of salvation. Simon and Su [9] measured the rate of intramolecular electron transfer after excitation 4,4’-bis(dimethylaminophenyl)sulfone in propanol. The relaxation time of the initially locally excited state (LE) to a charge-transfer state (CT) was found to be smaller than sL, a result explained in terms of the theory of Sumi and Marcus [lo]. The LE and CT states correspond to the first and second DMABN states except for greater delocalization of the negative charge in the CT state of the sulfone. Barbara et al. [ 111 measured the time correlation function C( t ) for coumarins 102 and 3 11 in various aprotic dipolar solvents (nitriles and acetates). The solvation time, zs, is also dependent on the probe in a particular solvent. In addition, Barbara et al. also compared the solvation time, 7s, obtained by following shifts in fluorescence maximum with the time constants for the electron-transfer reaction, 7cetrfor the conversion of the first excited state of 9,9’-bianthryl (BA) to a charge transfer (CT) state. For nitriles and other aprotic unassociated solvents, both 7, and TV,, values are very similar and are slightly longer than zL, i.e. 1 ps or less. In the present study, we continue our efforts to understand solvent effects on the mechanism of intramolecular electron transfer. To probe the relative merits of the viscosity and relaxation times in correlating fast IET, we have utilized the fact that in 350

16September I988

diols, the two parameters do not scale linearly with the corresponding values for the monoalkanols. Three isomeric pentanediols were selected as solvents. The dielectric relaxations of pentanediols have been reported by Davidson [ 141. The pentanediols ( x2.5 f 0.2 D) have larger dipole moments than propanol ( 1.7 D), and larger viscosities than mono01s (ethanediol, q- 19 cP; ethanol, q= 1.2 cP). In contrast, the dielectric relaxation times are longer approximately by about a factor of 2 than closely related mono-ols (1,5-pentanediol, zo= 1530 ps; pentanol, 7,=920 ps). In the study of IET processes, monoalkanols have been frequently used to evaluate the solvent dependence of IET dynamics. The relaxation times of monoalkanols scale almost linearly with viscosity and the relative utility of the two parameters in evaluating salvation dynamics was not easily clarified [ 3 1. Non-exponential solvation dynamics [ 141, even in unassociated polar liquids, are an important concern. Associated polar liquids exhibit complex dielectric relaxations. Monoalkanols show three different relaxation times of which the slowest is the most distinctive. In contrast, dielectric dispersion for the isomeric pentanediols (except 1,5_pentanediol) is characterized by a Cole-Davidson distribution of relaxation times. Non-exponential solvation dynamics can arise from a multiplicity of relaxation times [ 15 1. Thus, comparing the solvation dynamics in isomeric pentanediols (distribution of relaxation times) with the solvation dynamics in monoalkanols or 1$pentanediol (single relaxation times) should provide pertinent information on the origin of the non-exponential nature of the solvation dynamics.

2. Experimental Two systems were used to follow the spectroscopic changes in solutions of TNSDMA derivatives after pulsed excitation. Both picosecond and subnanosecond systems have been described previously [ 16 1, Steady-state fluorescence spectra were measured using a Shimadzu spectrofluorimeter, RF-540, absorption spectra with a Varian Cat-y 219 spectrophotometer. The TNSDMA derivatives were synthesized and purified as described in ref. [ 17 1.

Volume 150, number

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CHEMICAL

PHYSICS LETTERS

Pentanediols (Aldrich, E. Merck) were redistilled under vacuum before use.

3. Results The steady-state fluorescence spectra of TNSDMA derivatives 1, 2 and 3 in pentanediols are similar to those obtained in alkanols [ 11. In 1,tpentanediol at room temperature, fluorescence maxima are found at 450 (l), 470 (2) and 495 nm (3).

I6 September

1988

the first excited state (S,,,,) of derivative 1 in all three pentanediols is shown in fig. 2. The parameter /3 increases as the temperature decreases, approaching single-exponential behavior at the lowest temperature, - 5 “C. The values of /I fluctuate for derivatives 2 and 3 and the temperature dependence could not be ascertained. The values of p at various temperatures and solvents are given in tables 1-3 along with the fluorescence decay times, T,, as obtained from the Williams-Watts analysis.

4. Discussion 4.1. Non-exponential decay dynamics

The time-resolved fluorescence decay of the initially prepared electronic state (S,,,,) and the rise and decay of the charge transfer state (S,.,,) were measured over the wavelength interval 380-550 nm. The decay of the S,,np state at wavelengths between 380 and 470 nm was found to be non-exponential, in accord with our previous results for TNSDMA derivatives in alkanol solutions [ 11. The decay of the initial state was examined over a substantial temperature range (65 to - 5 “C) for derivatives 1, 2 and 3 in three pentanediols. The relaxation times are plotted as a function of 1/Tin fig. 1, together with the longitudinal relaxation times for the solvents. Good fits of the data on fluorescence decays are obtained using the Williams-Watts stretched exponent method [ 1 ]

z(t) =exp[ - (tl~)~l ,

(4)

Z(t) is the time-dependent fluorescence intensity, p is the Williams-Watts exponent parameter. The temperature dependence of the WilliamsWatts fl parameter for the correlation of the decay of

The molecular processes occurring in the excited state of TNSDMA derivatives are shown in scheme 1sThe great sensitivity of the CT emission to the nature of the substituent on the N-aryl ring has been explained in detail elsewhere [ 181. The dynamics of IET for three derivatives of the TNSDMA class in 1,5-, 1,2- and 2,4-pentanediols are better described by a complex solvent parameter rather than a single exponential, as found previously for the same compounds in monoalkanol solutions. Davidson [ 141 described various properties of live isomeric pentanediols, including the dielectric relaxation times as well as the infrared spectra, the dipole moments and the static dielectric constants. The data have been explained in terms of an increasing degree of internal hydrogen bonding with increasing proximity of the OH groups. The dielectric dispersion (Cole-Cole plot) loci are skewed arcs except for 1,5pentanediol. Using the Cole-Davidson relaxation distribution function to fit the dielectric dispersion, the P parameter decreases from z 1 to 0.55 in the series 1,5-, 1,4-, 1,2-, 2,4- and 2,3_pentanediol, in order of increasing relaxation time. In contrast to the dispersion behavior of pentanediols, mono-ols exhibit a principal dispersion region of the Debye type (semicircle, /3= 1). Davidson and others related the qualitative difference in the behavior of mono-ols and diols to the absence of regularity in intermolecular hydrogen bonding in the latter. Straightforward data analysis of the non-exponential fluorescence decay of TNSDMA derivatives in pentanediols should have been via the Cole351

16 September I988

CHEMICAL PHYSICS LETTERS

Volume 150, number 3,4

* NCH,KH,I

‘O

I 30

1

I

I

32

I

I

34

I

‘O

1

3.6

3.0

I

I

I

I

3.0

/

1,

3.2

I

3.4

I/T( 103)

I

IO(--LI-II/T( 103)

I

3.6

3.8

I/T(103)

Fig. I. A comparison of the temperature dependence of the solvent properties ( TD,A; rL, 0) with the relaxation times, T, for the first excited state, S,,,,, undergoing electron transfer to form the second excited state, S,,c,, for three different TNSDMA derivatives: T(NH(CH~)) 0; r(NCH,(H) n ; r(NCH,(CHj)) A (a) in 1,5-pentanediol, (b) in 1,2-pentanediol, (c) in2,4-pentanediol.

..*. I 11

(

II

NH-W3

I

0.8 0.6

. . .

.

.

.

.

. .

.

. :

0.0

t-;o

i -

’

3.5

llT(103)

’

’

‘1t

I I

3.0

2.4-

pentonedlol

l.O-

I I I

3.5

I/T1

-

0.0-

_

0.6-•,

-

0.4-

_

0.2-

l***

.

. l

.

:

. .

0.2

a

NH-CH,

1.2.Pentonedlol

:

.**

0.4

. .

P

1 II

103)

bI I

40

0.0

l

3:o

.

I 1 3.5’ a I I

C L

l/T(103)

Fig. 2. The temperature dependence of the Williams-Watts ,@parameter as a function of the reciprocal temperature for the TNSDMA derivative NH(CH3) (a) in l$pentanediol, (b) in 1,2-pentanediol, (c) in 2,4-pentanediol.

352

16 September 1988

CHEMICAL PHYSICS LETTERS

Volume 150, number 3,4 Table 1

The relaxation times and /? values a1of TNSDMA derivatives in I,%pentanediol as a function of temperature NH(CHj)

NCH,(H)

b, a (PS)

T(K)

B

333 328 323 318

35 40 50 65

308 298 288 283 278 273

100 155 140 175 210 240

0.48 0.48 0.50 0.55 0.65 0.70 0.65 0.9 0.85 0.85

268

330

0.85

NCHj(CHj)

c,

T(K)

t (PS)

B

303 298 293 288 283 278 273

90 110 150 180 210 215 270

0.75 6.73 0.80 0.8 0.7 0.7 0.65

d,

T(K)

7 (PS)

P

291

105

0.70

284 218

150 180

0.75 0.73

273 268

220 265

0.75 0.75

a) Williams-Watts parameter. b, 6-(4-methylphenylamino)-2-naphthalenesulfon-N,N-dimethylamide. Cl N-methyl-6-(phenylamino)-2-naphthalenesulfon-N,N-dimethylamide. d, N-methyl-6-( 4-methylphenylamino)-2-naphthalenesulfon-~,N-dimethyl~ide.

Davidson

relaxation

dielectric

relaxations

related in this way Davidson

distribution

tanediol

[ 191, since

function,

of all TNSDMA

since

the fluores-

derivatives

in, 1,5-pen-

(single

exponential.

are well cor-

behavior

[ 141. We have not used the Cole-

distribution

cence decays

function

of pentanediols

ation The

Debye

Hence

relaxation)

the reason

are

also

is not solely due to the macroscopic

pattern

non-

for non-exponential relax-

of the solvent.

non-exponential

character

of the

solvation

Table 2 The relaxation times and j? values ” of TNSDMA derivatives in 1,f-pentanediol as a function of temperature NH(CH3) b,

NCH,(H) <)

NCH,(CHj) d,

T(K)

5 (PS)

B

T(K)

7

333 328 323 318 313 307 303 298 293 288 283 278 268

37 50 74 86 98 126 180 197 210 240 215 326 533

0.50 0.55 0.60 0.65 0.65 0.70 0.75 0.75 0.80 0.85 0.85 0.90 0.90

303 298 293 288 283 278 273

120 135 157 190 200 326 397

(PS)

B

T(K)

7

0.80 0.85 0.80 0.8 0.8 0.7 0.75

303 298 293 288 283 218 273

64 72 105 120 147 160 183

(PS)

B

0.90 0.90 0.80 0.75 0.72 0.72 0.62

a) Williams-Watts parameter. bJ 6-( 4-methylphenylamino)-2-naphthalenesulfon-N,N-dimethylamide. ‘) N-methyl-6-(phenylamino)-2-naphthalenesulfon-N,N-dimethylamide. d, N-methyl-6-(4-methylphenylamino)-2-naphthalenesulfon-N,N-dimethylamide. 353

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CHEMICAL PHYSICS LETTERS

Volume 150, number 3,4

Table 3 The relaxation times and fi values a) of TNSDMA derivatives in 2,4-pentanediol as a function of temperature NCH,(CHI) d,

NCH,(H) ”

NH(CHs) b1 T(R)

r (ps)

P

T(R)

= (PS)

B

7. (R)

r (PS)

B

338 328 318 315 308 303 298 293 288 283

70 75 100 135 200 210 275 295 355 390

0.58 0.6 0.6 0.7 0.77 0.8 0.8 0.81 0.82 0.85

335 328 318 314 323 313 308 304 299 293 296 288 282

29 36 55 73 50 80 124 160 212 245 220 360 485

0.75 0.7 0.75 0.8 0.77 0.75 0.8 0.8 0.8 0.75 0.75 0.75 0.8

333 323 318 312 305 303 298 293 288 283 277

26 31 38 60 79 100 125 196 240 392 555

0.85 0.78 0.75 0.8 0.8 0.75 0.75 0.8 0.85 0.7 0.8

a) Williams-Watts parameter. b, 6-(4-methylphenylamino)-2-naphthalenesulfon-N,N-dimethylamide. c) N-methyl-6-(phenylamino)-2-naphthalenesulfon-N,N-dimethylamide. d, N-methyl-6-(4-methylphenylamino)-2-naphthalenesulfon-N,N-dimethylamide.

is characterized by a Williams-Watts parameter 0.8 + 0.05 for derivatives 2 and 3 in all three pentanediols at room temperature, as previously found for the same compounds in various alkanols ill. The non-exponential behavior of the IET dynamits may arise from the nature of the microscopic

dynamics

solvation dynamics, not directly reflected in macroscopic dielectric relaxation measurements. As described in section 1, recent experimental and theoretical studies have shown that the non-exponential solvation dynamics can be described by microscopic solvation models. Maroncelli and Fleming [ 41 noted for cu 153 that the average solvation time

Scheme 1. Photophysical conversions and molecular charge distributions for TNSDMA (1).

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constant is in general greater than ~~and approaches rb in highly polar solvents with very large dielectric constants, in accord with mean spherical approximation calculations [ 5,6]. The MSA approach leads to the conclusion that solvation dynamics proceeds on multiple time scales. The short-time scale corresponds to the bulk relaxation time rL, while the longtime scale is associated with rearrangement of the first solvation layers, rb. The SOhatiOn time constants for the three derivatives of TNSDMA are slightly shorter than fL and maintain the order NH(CH3)>NCH3(H)>NCH3(CH3) for all solvents examined [ 11. 4.2. Temperature dependence The second important feature of the present study is the finding that the temperature dependence of the solvation dynamics for the three derivatives in 1,5and 1,2-pentanediols is slightly less than that of the longitudinal relaxation time. The solvation times for all TNSDMA derivatives are shorter than rL at temperatures below room temperature. A low temperature dependence of r,, was found previously for derivative 1 in propanol and hexanol [20] and all three derivatives in methanol and ethanol [ I]. The temperature dependence of the solvation dynamics for 1, 2 and 3 in 2,4-pentanediol is similar to that of distinguishing this solvent from alkanols and 1,2- and 1,5-pentane dials. Three relaxation ranges are found [ 2 1 ] in associated liquids like monoalkanols, among which the lowest frequency relaxation is dominant. The long relaxation time has been attributed to rotation of individual molecules, accompanied by the rupture of hydrogen bonds in molecular aggregates. The intermediate relaxation time is ascribed to rotation of free, monomeric molecules and the short relaxation time is for the relaxation of the hydroxyl group by rotation around its C-O bond. A plausible explanation for the temperature dependence of the microscopic solvation dynamics might be given as follows. At close proximity to the solute molecule (first solvation layer), the concentration of monomeric solvent molecules is much higher than in the bulk. The monomers are “extracted” from the aggregates by the intervention of the solute and are thus held more strongly by the sol-

16 September 1988

ute than by other solvent molecules. The relaxation of the first solvation shell should be different from that of the bulk and thus will affect the dynamics of solvation. Solvent-controlled intramolecular electron transfer rates will be affected by the presence of solvent components with fast relaxation times shorter than those of the bulk, the net result being non-exponential behavior of the fluorescence decay which reflects the IET. At temperatures low enough to make 7L comparable to the radiative lifetime or even longer, the relative contribution of the first-shell solvent molecule behavior to the relaxation process would be increased, the relaxation time of the first shell being shorter than the radiative lifetime. Thus, the temperature dependence of the IET is smaller than that of tL. 4.3. The Williams- Watts /I parameter The temperature dependence of p for derivatives 2 in ethanol [ 1 ] can be explained in the same way. At low temperatures, /3approaches a maximum of 1. A monotonic increase in the relative importance of monomeric species reorientation in the total reorganization process eventually results in a “homogeneous” relaxation in the solvent around the soluble. The strong temperature dependence of the stretched exponent parameter /3 for derivative 1 in all three pentanediols studied can be explained as follows: The NH group of derivative 1 is capable of forming a hydrogen bond with hydroxyl groups of the pentanediols. The hydrogen bond affects the firstshell solvent reorientation time. Evidence for substantial interaction of the NH with solvent molecules is based on the nature of the emitting states, which in turn are firmly based on substituent effects [ 181. At low temperatures, the contribution of the bulk solvent to the reorganization energy relative to the contribution of the monomeric species increases. In derivative 1, the phenyl ring is conjugated to the nitrogen in the emitting charge transfer state, Sl,cttCj, yet the nitrogen was conjugated to the naphthalene in the initial state. The initially formed charge-transfer state, Sct(ol, must have the phenyl ring unconjugated to the nitrogen. The formation of the emitting SW(C) state must proceed via proton dissociation1 and

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suggesting that the solvent may interact quite strongly with the NH.

recombination,

5. Conclusions The pattern which now emerges from fast intramolecular electron-transfer reactions (and charge rearrangements) is that the reaction kinetics are (a) often non-exponential, (b) have a temperature dependence smaller than that of TVand (c) exhibit Williams-Watts parameters, j_?,which approach 1 at low temperature.

Acknowledgement We are grateful to J. Jortner, J. Klafter and I. Rips for stimulating discussions.

References [ 1] D. Huppert, V. Ittab and E.M. Kosower, Chem. Phys. Letters 144 (1988) 15. [2] D. Huppert, S.D. Rand, P.M. Rentzepis, P.F. Barbara, W.S. Struve and Z.R. Grabowski, J. Chem. Pbys. 75 ( 1981) 57 14. [3] F. Heisel and J.A. MiehC, Chem. Phys. 98 (1985) 233,243; Chem. Phys. Letters 128 (1986) 323. [4] M. Maroncelli and G.R. Fleming, J. Chem. Phys. 86 (1987) 6221.

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[5]P.G. Wolynes, J.Chem. Phys.86 (1987) 5133. [ 61 I. Rips and J. Jortner, J. Chem. Phys. 88 ( 1988) 8 18; I. Rips, J. Klafter and J. Jortner, J. Chem. Phys. 88 (1988) 3246. [7] M. Maroncelli and G.R. Fleming, J. Chem. Phys., to be published. [8] M. Maroncelli, E.W. Castner Jr., B. Bagchi and G.R. Fleming, Faraday Discussions Chem. Sot. 85 ( 1988), to be published. [9] J.D. Simon and S.-G. Su, J. Chem. Phys. 87 (1987) 7016: J. Phys. Chem. 92 (1988) 2395; J.D. Simon, Accounts Chem. Res. 21 (1988) 128. [ lo] H. Sumi and R.A. Marcus, J. Chem. Phys. 84 ( 1986) 4894. [ 111 M.A. Kahlov, J. Kang and P.F. Barbara, J. Phys. Chem. 91 (1987) 6452; P.F. Barbara, Accounts Chem. Res. 2 1 ( 1988) 195. [ 121 M. Sparpaglione and S. Mukamel, J. Phys. Chem. 91 ( 1987) 3938. [ 131 M.D. Newton and H.L. Friedman, J. Chem. Phys. 88 ( 1988) 4460. [ 141 D.W. Davidson, Can. J. Chem. 39 (1961) 2139. [ 151 I. Ripsand J. Jortner, Chem. Phys. Letters 133 (1987) 411. [ 161 D. Huppert and E. Kolodney, Cbem. Phys. 63 (1981) 401. [ 171 H. Dodiuk and E.M. Kosower, J. Phys. Chem. 81 ( 1977) 50. [ 181 E.M. Kosower and H. Dodiuk, J. Am. Chem. Sot. 100 (1978) 4173; E.M. Kosower and H. Kanety, J. Mol. Struct. 84 (1982) 259. [ 191 C.J.F. BGttcher and P. Bordewijk, Theory of electric polarization, Vol. 2 (Elsevier, Amsterdam, 1978) chs. 7,9. [20] D. Huppcrt, H. Kanety and E.M. Kosower, Faraday Discussions Chem. Sot. 74 ( 1982) 161. [ 211 S.K. Gargand C.P. Smith, J. Phys. Chem. 69 ( 1965) 1294.