Direct observation of geminate recombination within photochemically produced ion pairs in polar solutions

Volume 26, number 3

CHEMICAL PHYSICS LETTERS

1 June 1974

DIRECT OBSERVATION OF GEMINATE RECOMBINATION WITHIN PHOTOCHEMICALLY PRODUCED ION PAIRS IN POLAR SOLUTIONS D.M. GOODALL*,

N. ORBACH and M. O’ITOLENGHI

Department of Physical Chemistry, The Hebrew University.3endem.

Israel

Received 15 March 1974 Revised manuscript received 26 March 1974

Pulsed laser excitation of pyrene @‘j/N, N-diethylaniline (DEA) solutions in methanol at low temperatures leads to the generation of the P- and DEA+ radical ions. A fast initial decay of the ions (T = 37 nsec at -4O’C) is observed and is attributed to geminate recombination within the initially formed (2P-__‘3 DEk+) ion pair. The quench-

ing of tP* by DEA is also associated with substantial intersystem crossing, yieldingJP*. The data rule out both gemiate and non-geminate (homogeneous) ionic recombinations as sources for the initial yield of 3p*. in keeping with a prompt triplet formation mechanism.

The non-homogeneous initial distribution of primary irradiation products in photochemistry and radiation chemistry (leading correspondingly to the well-known “cage” and “spur” effects) has been the subject of considerable experimental and theoretical study. Geminate recombination of the primary products may be monitored indirectly via the dependence of the free ion or radical yields on the concentration of added scavengers. A more powerful experimental technique involves the application of pulsed excitation methods, and this has made possible the direct observation of fast, initial (non-homogeneous) decay processes. Most work of this type relates to spur reactions in pulse radiolysis [I]. Recently, Eisenthal has observed the geminate recombination of iodine atoms generated photochemically by a picosecond pulse from a mode-locked laser [2]. No data appear, however, to be available for corresponding geminate recombinations of charged particles produced photochemically. The neutralization of an isolated ion pair within a spur is believed to proceed from a thermalization distance of the order of 102.A, and approximate solutions to the diffusion equation for this pro* On leave from the Chemistry Department, York University. York, UK

cess have been derived and evaluated by Mozumder [3]. An ion pair generated photochemically will have a much smaller initial separation, and the diffusion time in the geminate recombination will in general be considerably shorter than that seen in pulse radiolysis experiments. In a particular solvent the recombination time is expected to lengthen as the temperature is lowered and the viscosity increased. In the present communication we report laserphotolysis experiments in a low-temperature chargetransfer system which Pave led to the direct observation of geminate recombi;%tion within an isolated ion pair t . We have previously reported [S--7] photochemical experiments in which pyrene (P), excited at room temperature in polar solvents by means of 3371 A (15 nsec) N, laser pulses, was subsequently quenched by N,N-ciiethylani&e (DEA). The quench.TVery recently Irie, Masuhara, Hayashi and Mataga [4]. studying the photoinduced ionic polymerization of Qmethylstyrene by exciting its charge-transfercomplex with tetracyanobenzene. have observed a fast decay of ions formed from.ke singlet state which they attributed to geminate recombination within ion pairs Their observations and conclusions are in general agreement with those of the present work

:

..

365. .

:.

1 :.. .,. -_-.

-_ : -.;: -...:;; ,;:..; .’ : ...:,_ ..-.:.‘. j .‘.-;

.‘.. 1

._. _.-’ : I-

.-.

Volume 26, number 3

CHEhlICAL PHYSICS LETTERS

ing process leads to ionic dissociation ‘P’

+DEAt

+DEA+P-

to:

,

by the homogeneous,

followed

according

1 June 1974

(1)

t

diffusion-contro!led,

I

,

20 TV

I

r

J

I

I I 20 nsec

I

ionic recombinations 3P* + DEA (where

3’

denotes

the

pyrene triplet state) and (pre-

sumably) k2b

+DEA++

P-

P+DEA.

Qb)

r

I

I I I I I I I I

m

The second-order nature of the above process was quantitatively established by Weller and co-workers ISIWhen such experiments were extended to lower temperatures, we observed transient absorbance phenomena

as shown

in figs. 1 and 2. In striking

to the room-temperature ions, followed

behavior\

at the characteristic

the decay absorption

1 psec’

contrast of the maxima

I

of

P- (495 nm) and DEA+ (465 nm), exhibit two distinct kinetic stages. In a 2 X IV4 M pyrene/O. I5 M DEA methanol solution at -4O”C, a fast initial decay (7 112 = 37 nsec) is followed by a much slower one in the microsecond range (or,* =3 25 psec). A gradual variation of the temperature between 20°C down to -40°C clearly indicated that the slow decay (lower oscillogram in fig_ 1) is that corresponding to the bimolecular process previously observed at room temperature (where 71,2 = OS wet). Fig. 2 shows that in the 440-520 nm range of the absorption due to the ions, the initial spectrum (recorded 20 nsec after pulsing) is, within the Iimits of experimental accuracy, identical to that recorded at 100 nsec, after completion of the first stage of the decay. This establishes the spectral, and thus the physica1, identity of the species undergoing the two distinct decay stagesi. The above observations are readily rationalized in terms of the initial formation of a solvated ion pair

I

I

I

I

Fig. 1. Characteristic oscillograms in the pulsed nitrogentaser photolysis of 2 X lo4 pyrene/O.lS M DEA solutions in methanol or -40°C. The upper oscittogram, recorded at 502 nm, demonstrates the fast initiat component of the ionic decay attributed to geminate recombination. The lower one (A = 497 nm) corresponds to the homogeneous charse annihilation process (2). Upper and louver traces in each oscillogram are recorded in the absence and in the presence of the monitoring beam, respectively.

(2P- ...2DEA*) undergoing subsequent neutralization (k,), in competition with dissociation (kd) to the homogeneously distributed ions. Thus, at relatively low temperatures, the solvent viscosity is sufficiently high so as to bring the geminate recombination process into the time resolution range of our nanosecond laser apparatus. The scheme ‘p* +&A

2 (2p-..?DEA+)

kd + +-

f 2DEA+

* An attempt to attribute the fitst initia1 decay to a shortlived CT e?rciplex formed between pyrene and DEA can be reacfdilyruled out by. the lack of any exciplex emission in a poker solvent such as methanol. Moreover, although exhib iting bands characteristic of ?!-nnd DEA+. the nbs&pfion spectrum of the exciplex contains additional transitions .. [9] which are not observed in the present transient _

./ -, .:

spectra. ,-

is thus consjstent with the second-order nature [8] of the slow recombination [(reaction (2)] and with ?he observed ir&nsitivity of the fast initial decay tim? to the laser pulse intensityAs shown in fig. 2; the 100 nsec spedtrum, recorded after the disapp&irance of (ZP- ..?pEA+),

: ._. . ._.,._ : -. _,. __’ ..-:.. _.- :;-. .‘-_: :. : :,_:.:..:y .,‘.:. .. ... I .:: ..-. ‘. :.xll.,:_ I. ._:‘- . ..‘... :‘-__.:z.f. __: .:-,. I: -,..-,::: _;.‘..2_1._’.... _ -...:- .:.;: _.___ --._ ‘__ ..-_ ‘:.-.‘..-:.I ..,, .---.-:...I .-.. . . .I . . ._,, -;. . ,‘,. ,, . . ,: :. . _ (.. __,., __ _;;:-::-: :.. ‘:,Y’_.._ .:I_.-. ,. ._. .:.:_r .,_.,, :... ‘;..;_:: -2 ; :...-,,.: ;:.;\’ ._‘:.;._ _::.__ \....,,,..,:-:.. . ._-:.-‘ ; 1.:_ ) -::_:.:=.‘-I-.: :,. ._;: -:..._:;... ._.y_ I, _.:. ;.._>,;; .-.,-.,‘.: .:3-‘“:._, ,.. :-‘ ._ . 1,:I:-: z.::.. ._:.i_:. _.-.\.....,.:. ir:‘.-.--‘y ,L ;_.:-,.~,: ___.;_:-: __-: ‘,_ ,_,l..i.‘_._:.;r:j~ :._., _I’ ,396.

-::-

Volume 26. number 3

CHEMICAL PHYSICS LETTERS

‘P’c OEA in methonol at-4WC

0.01

*

I

400

450

I

500

X.nm

Fig. 2. Transient spectra in the laser photolysis of pyrene/DEA

solutions in methanol at -4O”C, recorded 20 nsec and 100 nsec after triggering. Conditions are the same as described in f-l-l. 1.

1 June 1974

coefficients of 3P* and p- 4 DEAf around 4 IS nm vary significantly with temperature, the lack of such a growing-in (see f@. 2), rules out the geminate annihilation as an effective ISC’ path*. It therefore appears that the spin-lattice relaxation time in methanol at -40°C is longer than the time (E=4 X 1O-8 set) required for geminate ionic recombination. (The ion pair Formed from the singlet IP* + DEA system has a total spin quantum number of 1, so that charge neu.tralization wilI lead to the ground state P + DEA rather than to 3P* + DEA.) It may be concluded that ISC precedes the formation of the initial solvent-shared ion pair, in agreement with our previous predictions [S]. We are currently carrying out detailed kinetic and theoretical studies of the process of geminate i‘on-pair annihilation, investigating its sensitivity to changes in the environmental variables of temperature, solvent polarity, and viscosity. * Actually, ifthe initial fast decay is not accompanied by the generation of 3P*, one should observe a small decay at 415 nm. (The extinction coefficient of the pair P-+ DEA+ at 415 nm is = 20% of that at 495 nm.) Since this is not observed, the possibility that a small fraction of the recombining ion pairs will actually lead to 3P* cannot be definitely ruled out. A clear answer will be provided by improving the resolution of the faser experiments and by obtaining accurate ionic and triplet spectra in methanol at -40°C.

shows a very high contribution around 415 nm (not belonging to either P- or DEA+) which may be assigned without ambiguity to the pyrene triplet. This intense triplet band cannot arise from the homogeneous reaction (2), since the half life of this reaction is = 2 X 10s6 set (fig. 1, Iower oscillogram) and less than 5% of the ions will have homogeneously reacted within the first 100 nsec. A prompt triplet generation path must precede the homogeneous charge annihilation. We had previously deduced this from roomtemperature data (where the triplet contribution is much smaller than at -4O”C), so that our present results refute recent objections to our conclusions [8]. The question obviously arises as to the nature of the intersystem crossing mechanism leading to the triplet absorption observed after the fast initial decay of the ions. One posslMity is that 3P* is a product of the geminate neutralization of the ioh pair. In such a case one wouId expect to observe (exactly as for the homgeneous recombination at room-temperature [7, 81) a growizig-in at 415 nm kineti@iy matching the decay of the ions around 495 or 465 nm. Since there is no reason to believe that the relative extinction

References [I J J.K. Thomas and R.V. Bensasson, J. Chem. Phys 46 (1967) 4147; G.B. Buxton, Proc. Roy. Sot. A328 (1972) 9; F-S. Dainton and C. Gopinathan, Tranr Faraday Sot. 6.5 (1969) 151; G.B. Buxton, F.C.R. Cattell and F.S. Daintoq Trans. Faraday Sot. 67 (197 1) 687; I.A. Taub and H.A. CiIIis, J. Am. Chem. Sot. 91 (1969) 6507; J.B. Willard and M. Shirom. J. Phys. Chem. 72 (1968) 1702; J.A. Leone and W.&L Hamill, J. Chem. Phyr 49 (1968) 5294; J.K. Thomas, K. Johnson, T. Klippert and R. Lowers, J. C&em. Phys. 48 (1968) 1608; S.K. Ho. S. Siegel and H.A_ Schwartz, J. Phys Chem. 49 Cl9681 5294; A. Barkatt. M. Ottolen&i and J. Rabani, J. Phys Chem.

77x1973) 2857;

.:

:_ _. ‘.

367

Volume

26, number

3

CHEMICAL

CD. Jona, E.J. Hart and MS. Matheson, J. Phys. Chem. 77 (1973) 1838. [2] K. Eisenthal, presented at the VII International Conference on Photochemistry, Jerusalem, September (1973). [3] A_ Mozumder, J. Chem: Phys. 48 (1968) 1659; 55 (1971) 3026; G.C. AbeU and A. Mozumder, J. Chem. Phys. 56 (1972) 4079. [4] N. Maraga. private communication.

PHYSICS

LETTERS

1 June 1974

[5j N. Orbach, R. Potashnik and M. Ottolenghi. J. Phys. Chem. 75 (1971) 1025. 161 N. Orbach. Y. Novros and M. Ottolenghi. J. Phys. Chem. 77 (197312831. , -_-__ [7] M. Ottolenghi, Accounts Chem. Res. 6 (1973) 6. 18 J H. Schombnrg, H_ Staerk and A. Weller. Chem. Phys. Letters 21 (1973) 433; 22 (1973) 1. [9] R. Potashnik. CR. Goldschmidt. M. Ottolenghi and A. Weller. J. Chem. Phys 55 (197 1) 5344.