Photoinduced electron transfer in rigidly linked dimethoxynapthalene-N-methylpyridinium donor-acceptor molecules

Volume 195, number 2,3

CHEMICAL PHYSICS LETTERS

17 July 1992

Photoinduced electron transfer in rigidly linked dimethoxynaphthalene-N-methylpyridinium donor-acceptor molecules Andrew H.A. Clayton, Kenneth P. Ghiggino, Gerard J. Wilson Photophysics Laboratory, School of Chemistry, Universityof Melbourne, Parkville 3052, Australia

Peter J. Keyte and Michael N. Paddon-Row Department of Organic Chemistry, The Universityof New South Wales, P.O. Box 1, Kensington 2033, Australia

Received 10 March 1992

Photoinduced electron transfer (ET) is studied in a series of novel molecules containing a dimethoxynaphthalene (DMN) donor and either a pyridine (P) or N-methylpyridinium (P-Me + ) acceptor covalently linked via a rigid norbornalogous bridge (n sigma bonds in length). ET rates of the order of l0 ~° s -~ were measured for the DMN-n-P-Me+ series (n=4, 6), while no appreciable ET was observed for the DMN-n-P compound. Electronic and nuclear factors are discussed and the results rationalized in terms of Marcus-Hush and non-adiabatic ET theories.

I. Introduction

Electron transfer (ET) processes are amongst the most important of all chemical reactions. In the photosynthetic reaction centre following the absorption o f light, transfer of an electron from a special pair of chlorophyll molecules to bacteriopheophytin followed by a secondary transfer to a quinone acceptor is the first step in the conversion of light energy into chemical potential [ 1 ]. ET reactions also play a pivotal role in a variety of other chemical processes ranging from photochemically induced polymerizations to solar energy conversion and include the expanding field of molecules electronics [2]. Major progress has been made over the past few years in both theoretical and experimental approaches for probing the mechanisms o f photoinduced ET reactions and has resulted in several excellent recent reviews o f the field [2 ]. Correspondence to: K.P. Ghiggino, Photophysics Laboratory, School of Chemistry, University of Melbourne, Parkville 3052, Australia and M.N. Paddon-Row, Department of Organic Chemistry, The University of New South Wales, P.O. Box 1, Kensington 2033, Australia.

Much of the impetus for ET research has come from a need to understand the factors affecting ET processes in biological systems as well as to engineer artificial molecular devices. Considerable insights into the effect of distance, orientation and driving force for electron transfer rates have been recently obtained by studying long-range intramolecular electron transfer processes in rigidly linked d o n o r bridge-acceptor ( D - B r - A ) systems, in which the bridge length is variable. A variety of hydrocarbon bridges, covalently linked to the donor and acceptor groups have been used in such studies, including polynorbornyl (norbornylogous)bridges [ 3 ], cyclohexane, decalin, and steroid systems [4], bicyclo [ 2,2,2 ] octane [ 5 ], triptycene [ 6 ], and polyspirocyclobutanes [ 7 ]. One series o f norbornylogous systems, comprising of dimethoxynaphthalene ( D M N ) donor-{polynorbornyl bridge}-dicyanovinyl acceptor ( D C V ) , have provided information on the influence of D - A distance [ 3 ], orientation [ 8 ], electronic and vibronic coupling [ 9 ], and solvent effects [ 3,10 ] on the rate of ET, and these data have inspired several theoretical studies [ 11 ]. In the present work, photoinduced intramolecular

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ET in a series of novel molecules consisting of a photoexcitable dimethoxynaphthalene donor ( D M N ) and pyridine acceptor (Py) linked via a norbornalogous (fig. 1 ) bridge was investigated. The pyridine acceptor is amenable to synthetic variation by protonation or N-alkylation thus allowing a certain tunability of the driving force for ET in the donor-acceptor couple. The completely rigid norbornalogous bridge can also be varied in length allowing the distance dependence of ET rate to be examined. Results of fluorescence measurements are presented for DMN-6-Py, DMN-4-Py-Me + and DMN6-Py-Me + compounds. The approximate centre to centre D - A separations for the 4-bond and 6-bond compounds are 7 and 9 A, respectively, based on crystallographic data from the DCV analogues [ 12 ].

DMN

DMN-6-Py

+

+

2.1. Materials Synthesis of the compounds was carried out using a method described elsewhere [ 13 ]. Solutions were prepared in spectroscopic grade ethanol (Aldrich 99%+ ) to an optical density of 0.1-0.2 at the absorbance maximum of dimethoxynaphthalene (290 nm) and were free of fluorescent impurities in the spectral region of interest. Ethanol was chosen on the basis of its good glass forming properties. The solutions were thoroughly degassed by repeated freezepump-thaw cycles prior to use. 2.2. Methods Temperature dependent spectra in the range 77298 K were recorded using an Oxford Instruments DN704 liquid nitrogen cryostat with model DTC-2 digital temperature controller. Absorption spectra were collected using a Hitachi model 150-20 spectrophotometer/data processing system. Total luminescence spectra were recorded on a Hitachi model F-4010 fluorescence spectrophotometer with a WD315 cut-offfilter in the emission path to exclude second-order Rayleigh scattering. Fluorescence decay curves were collected using picosecond laser excitation/time-correlated single photon counting detection [ 14 ] and analyzed using iterative reconvolution routines. The experimental system has been described previously [15]. The total instrument response of the detection system was 80 ps fwhm.

3. Results

DMN-6-Py-Me+

Steady-state emission spectra for the DMN model donor and the linked DMN-6-Py and DMN-6-PyMe + compounds, obtained in ethanol solvent, are shown in fig. 2. The emission from the DMN model donor increases in intensity on cooling to 77 K. It also becomes more vibronically resolved and the intensity maximum is blue-shifted by approximately 2300 cm-~. In addition, structured luminescence is observed in the region 450-650 nm corresponding to phosphorescence from the DMN triplet state [ 16 ].

Me

Fig. 1. Spatiallyconstraineddonor-bridge-acceptormolecules. 250

2. Experimental

DMN-4-Py-Me+

Me

~

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17 July 1992

Table 1 Trends in Gibbs reaction energy and fluorescencequenching in DMN-n-Py-Me÷ and DMN-6-Py

(A)

DMNmodeldonor

Acceptor

n

AGO(eV)

¢,-A/0r,

ke, (s -l )

Py Py+-Me Py+-Me

6 4 6

-0.04 - 1.59 - 1.57

_ a) 0.0038 0.022

_ 3.7 X 10l° 6.4× l0 9

~>The fluorescencelifetimesof the modelDMN donor and DMN6-P are 7.6 and 7.4 ns, respectively, indicating no quenching of DMN fluorescencein the latter.

(B)

DMN-6-Py DMN-6-Py-Me+~ i ~ l , , (c)

DMN-6-Py°Me+~ l T /

78K 100K 150K

/

/

/ f-

~

A I

\ ~

\

\ 320

400 Wavelength

3000

400.0

500.0

600.0

Wavelength (nm)

Fig. 2. Temperaturedependent luminescencespectra of (A) DMN donor (B) DMN-6-P and (C) DMN-6-P-Me÷. All spectra measured in ethanol. Very similar, qualitative behaviour is observed for the 6-bond compound, DMN-6-Py, fig. 2b and there is no noticeable quenching of d o n o r fluorescence. The effect of N-methylation of the pyridine acceptor is depicted in fig. 2c. At room temperature there is a reduction in fluorescence intensity of the D M N d o n o r chromophore. As the temperature is lowered, in addition to the phosphorescence and the structured, blue-shifted donor fluorescence, DMN-6Py-Me ÷ exhibits a new broad, structureless b a n d at approximately 4 0 0 - 6 0 0 nm. This new b a n d dominates the emission a n d undergoes a slight blue shift as the temperature is lowered from 120 to 80 K. Similar spectral behaviour is observed for the shorter DMN-4-Py-Me ÷ homologue but with additional quenching of the d o n o r fluorescence (cf. table 1 ). The time-evolution of the emission from DMN-6Py-Me ÷ at 77 K is shown in fig. 3. As time evolves,

(nm)

480

560

Fig. 3. Time-resolved fluorescence spectra of DMN-6-P-Me+ measured at 100 K in ethanol. Delay times: (a) 0.3 ns; (b) 126

as.

the emission intensity in the 340 n m region decreases with a concomitant increase in the intensity of emission at 460 nm. Inadvertant excitation of two ground state species can be ruled out from a careful comparison of the fluorescence excitation spectra monitored at 340 a n d 460 n m which reveal that both emissions are due to excitation of the same ground state species.

4. D i s c u s s i o n

The similarity between the fluorescence lifetimes of the model D M N d o n o r (7.6 ns) and the linked DMN-6-Py (7.4 ns) indicates little if any quenching of donor fluorescence in the latter. Quantitative data concerning the quenching observed for the other D M N - n - P y - M e + c o m p o u n d s together with the quenching rate constant and calculated driving force for ET are depicted in table 1. The absence of any overlap between the absorption and emission spec251

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tra of the fragment chromophores preclude a Forster-type energy transfer mechanism [ 17 ] while the observation of a broad long-wavelength luminescence is characteristic of emission from a chargetransfer (CT) state [ 2 ]. Furthermore, the trends in driving force and quenching ratio are consistent with an ET mechanism. For non-adiabatic ET the rate is given in the usual Golden Rule expression by, ket =2n/hH~[FCWD],

(1)

where Hrp is the electronic matrix element coupling the reactant and product states, and FCWD is the Franck-Condon weighted density of states [ 18,19 ]. The [FCWD] term contains the nuclear factors (temperature dependence, activation energy, and vibrational effects) responsible for ET, while Hw pertains to electronic factors (the degree of overlap of donor and acceptor wavefunctions, either throughspace or via through bond, i.e. superexchange). In the classical limit ie. ho9 << kT, where o9 is the characteristic frequency of medium modes, [FCWD] can be expressed as [19]

[FCWD]=(2~2kT)-t/2exp(-AG*/kT),

(2)

with the classical Marcus activation energy given by AG* = (AG°+2)z/42 [19]. The thermodynamic driving force for ET is AG O while 2 is the total reorganization energy which includes inner-sphere reorganization of the molecule (20 and outer-sphere reorganization (2o) of the solvent [ 19 ]. Of general interest is how the relevant electronic and nuclear factors affect the observed ET behaviour in these donor-acceptor systems and how these factors can be controlled and quantified. In the following discussion the trends in ET rates are explained within a classical Marcus framework and describe the distance dependence of electronic and nuclear

17 July 1992

factors for the DMN-n-P-Me + compounds. The driving force for ET (AG°) has been determined using literature values for the one-electron oxidation and reduction potentials of the relevant chromophores with corrections for solvent and Coulombic effects [3]. The reorganization energy ( 2 = 2 i + 2 o ) has been evaluated assuming an innersphere term of 0.6 eV based on data from the analogous DCV systems [ 3 ] while the outer-sphere term has been calculated using the usual spherical solute equation [ 20 ]:

2o=e2(1/r-1/Rc) (1/n2-1/~),

(3)

with Rc being the centre-to-centre D - A distance and r ( =4.5 ~,) the average ion radius assumed to be the same as for the analogous DCV systems [3 ]. The Marcus activation energy and total reorganization energy are displayed in table 2. Inspection reveals that the lack of quenching in DMN-6-Py is due to a large activation barrier to ET which is refleeted in the low (almost zero) thermodynamic driving force to ET, even in a polar solvent. The large ET rates observed for the DMN-n-Py-Me + compounds are seen to be due to both the low activation barriers and large driving force for ET. Interestingly, while the driving force for ET (table 1 ) increases in going from DMN-6-Py-Me + to DMN4-Py-Me + the activation barrier increases. This is a characteristic of ET reactions occurring in the Marcus inverted region (i.e. - A G O / 2 > 1 ). However, the -AGO/2 values for DMN-4-Py-Me + and DMN-6-PyMe + of 1.3 and 1.1, respectively, indicate that the ET reactions are only weakly in the inverted region and could also be considered to be in the barrierless regime (AGO/2~ 1 ). The distance dependence of the nuclear and electronic factors for the DMN-6-Py-Me + compounds are given in table 2. The ratio of nuclear factors has

Table 2 Comparison of nuclear and electronic factors for ET in the DMN-n-Pyand DMN-n-Py-Me+ compounds Acceptor

n

AG* (eV)

2

x,,/x,,

2

2

H n , , / H q, 6

ot

(A -l) Py Py+-Me Py+-Me

252

6 4 6

0.3 0.03 0.005

1.4 1.2 1.4

0.98

5.47

0.77

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been o b t a i n e d by application o f eq. ( 2 ) , while the ratio o f electronic factors has been o b t a i n e d using the ratio o f the rate constants together with eqs. ( 1 ) and (2). Analysis o f the results indicates that decreasing the d o n o r - a c c e p t o r distance causes a slight decrease in the nuclear factors for ET. However, this is accomp a n i e d by a very large increase in the electronic factors for ET. A m o r e quantitative measure o f the distance d e p e n d e n c e o f electronic coupling is c~ which describes the a t t e n u a t i o n o f distance o f the electronic coupling element, H~o, i.e.: Hrp = H o e x p [ - ½o~(r-ro) ] .

(3)

Rearranging eq. 3 we have 2 2 H~,( 2 ) / Hrp( 1 ) = e x p [ --ot( r2 --r, ) ] ,

(4)

where r 2 - r ~ is the difference in c e n t r e - c e n t r e D - A separations between c o m p o u n d 2 and c o m p o u n d 1. Typically, a values in the range 2 - 5 A-~ have been reported for systems where the d o n o r and acceptors couple electronically through-space (or solvent) while values o f 0 . 2 - 2 A - l have been found to be consistent with a through-bond or superexchange m e c h a n i s m [2]. The a value o f 0.77 A - l in D M N n-Py-Me ÷ indicates that such a through-bond mechanism is occurring via the norbornalogous bridge. The results are consistent with other studies on analogous dicyanovinyl acceptor systems ( D C V ) [3 ] and other studies on related dienes using photoelectron spectroscopy [ 11 ], electronic transmission spectroscopy [ 12 ] and ab initio m e t h o d s [ 3 ]. The similarity in a values o f the current system to that o f the D C V systems ( a = 0 . 8 2 A - t ) indicates (as expected), that the norbornalogous bridge is the m a i n c o m p o n e n t m e d i a t i n g the distance d e p e n d e n c e o f electronic factors to ET. At this stage such an analysis is tentative as d a t a from higher homologues (n = 8, 10 etc. ) are required before a reliable estimate o f ot can be made. This study has shown that d r a m a t i c changes in ET quenching can be realized by derivitisation o f the nitrogen on the p y r i d i n e acceptor group a n d shows great potential in being able to tune the driving force for electron transfer. Further work is in progress to investigate the role o f the d o n o r - a c c e p t o r separation, m e d i u m , and

17 July 1992

counter-ion on ET in these c o m p o u n d s and other related derivatives.

Acknowledgement K P G and M N P R thank the Australian Research Council for support.

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[7] C.A. Stein and H. Taube, J. Am. Chem. Soc. 103 (1981) 693; C.A. Stein, N.A. Lewis and G. Seitz, J. Am. Chem. Soc. 104 (1982) 2596. [8]A.M. Oliver, M.N. Paddon-Row, J. Kroon and J.W. Verhoeven, Chem. Phys. Letters 191 (1992) 371. [ 10] J. Kroon, J.W. Verhoeven, M.N. Paddon-Row and A.M. Oliver, Angew. Chem. Intern. Ed. 30 ( 1991 ) 1358. [ 11 ] M.N. Paddon-Row and S.S. Wong, Chem. Phys. Letters 67 (1990) 432; M. Braga, A. Broo and S. Larsson, Chem. Phys. 156 ( 1991 ) 1; J.R. Reimers and N.S. Hush, Chem. Phys. 146 (1990) 105. [ 12 ] D.C. Craig and M.N. Paddon-Row, Australian, J. Chem. 40 (1987) 1951.

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