PHOTOPROTOLYTIC ACRIDINE A.

REACTIONS

IN NONAQUEOUS B.

Demyashkevieh

OF

SOLUTIONS and

B.

M.

UDC 541.143

Uzhinov

In a study of the r e a c t i o n of p h o t o t r a n s f e r of a proton in the s y s t e m 2 - n a p h t h o l - b a s e , it was shown [1] that such adiabatic r e a c t i o n s a r e a c c o m p a n i e d by e m i s s i o n l e s s d e g r a d a t i o n of the e n e r g y of the excited p r o t o n donor, induced by the b a s e . The p r o c e s s of induced e m i s s i o n l e s s deactivation c o m p e t e s with the r e a c t i o n of p h o t o t r a n s f e r of a proton and depends on the nature of the r e a g e n t s and solvent. It is of i n t e r e s t to d e t e r m i n e w h e t h e r this p r o c e s s is g e n e r a l for adiabatic r e a c t i o n s of proton p h o t o t r a n s f e r . In this w o r k we studied the r e a c t i o n of p r o t o n p h o t o t r a n s f e r in the s y s t e m of excited a c r i d i n e - p r o t o n donor in alcohol solutions. Special attention was paid to the e l e m e n t a r y p r o c e s s e s of proton t r a n s f e r in a complex with a hydrogen bond of a c r i d i n e to the solvent. The s u b s t a n c e s and solvents u s e d w e r e purified as follows: acridine was r e c r y s t a l l i z e d f r o m ethanol; a m m o n i u m n i t r a t e (analytical grade) w a s r e c r y s t a l l i z e d f r o m w a t e r ; m e t h a n o l , ethanol, and n - p r o p a n o l w e r e purified a c c o r d i n g to the well known p r o c e d u r e s [2]. Solutions of a m m o n i u m n i t r a t e in alcohols w e r e p r e p a r e d by a g r a v i m e t r i c method. The concentration of a c r i d i n e in the alcohols was m o n i t o r e d s p e c t r o p h o t o m e t r i c a l l y , kept constant in all the e x p e r i m e n t s , and was equal to 10 -5 M. The a b s o r p t i o n s p e c t r a of the solutions w e r e m e a s u r e d on a Unicam SP 8000 t w o - b e a m a u t o m a t i c s p e c t r o p h o t o m e t e r at r o o m t e m p e r a t u r e . The f l u o r e s c e n c e s p e c t r a w e r e m e a s u r e d in t h e r m o s t a t i c a l l y controlled cuvettes on a Jobin and Ivon s p e c t r o f l u o r i m e t e r at 25~ The f l u o r e s c e n c e of acridine and its ion was excited by light with a

J,t

. I

/,0

0,/

.

2

" i

f

0,1

8,2 "

O,gCuH,~so ~, M

Fig. i . Change in the quantum yields of f l u o r e s c e n c e in the s y s t e m a c r i d i n e - a m m o n i u m ion in ethanol at 25~ Quantum yields of the f l u o r e s c e n c e of the n e u t r a l (~o0, ~o) and protonated ( ~ , ~o') f o r m s of a c r i d i n e in the a b s e n c e (~o0) and in the p r e s e n c e of an e x c e s s (~0) of p e r c h l o r i c acid and in a definite c o n c e n t r a tion of a m m o n i u m n i t r a t e (~o, ~o'), r e s p e c t i v e l y . ~o0/~o (1) ~o

/~0 + ~'/~'o (2). Fig. 2. Dependence of ~ ~ 0 / ~ o ~ on the NH4NO 3 concentration, for the s y s t e m a c r i d i n e - a m m o n i u m ion in ethanol at 25~ T r a n s l a t e d f r o m Zhurnal P r i l d a d n o i Spektroskopii, Vol. 21, No. 3, pp. 496-500, S e p t e m b e r , 1974." Original a r t i c l e s u b m i t t e d July 18, 1973.

9 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10'011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15. 00.

1217

TABLE System

i. Rate Constants Acridine-Ammonium

Solvent

of Phototransfer of a Proton Ion in Alcohols at 25~

z~ nsec

k~~10-9 mole/liter, sec

o,s_+o,t o,s•

Methanol Ethanol n-propanol

in the

I

[

0.9+0,1

I

o,79§ 0 , ~" 5 _-~ 0,10

0,55+0.10

wavelength 360 rim. To isolate the f l u o r e s c e n c e of the n e u t r a l and p r o t o n a t e d f o r m s of acridine we used P S - 1 3 and 0 8 - 1 1 g l a s s f i l t e r s , r e s p e c t i v e l y . The shape of the f l u o r e s c e n c e s p e c t r a of neutral and p r o tonated f o r m s of a c r i d i n e was unchanged by the addition of a m m o n i u m n i t r a t e ; t h e r e f o r e , the r a t i o of the quantum yields of the f l u o r e s c e n c e of both f o r m s w a s r e p l a c e d by the r a t i o of the intensities at the m a x i m a of the e x p e r i r a e n t a l l y m e a s u r e d f l u o r e s c e n c e s p e c t r a . T h e kinetic c u r v e s of t h e quenching of the f l u o r e s c e n c e of a c r i d i n e w e r e m e a s u r e d with the aid of a n a n o s e c o n d s p e c t r o m e t e r f r o m ORTEC Company. The duration of the flash of the pulsed l a m p was equal to a p p r o x i m a t e l y 2 n s e c . Since the l i f e t i m e of the excited acridine m o l e c u l e in alcohol solutions is s h o r t e r than the duration of the flash of the p u l s e d l a m p , ~che e x p e r i m e n t a l l y obtained quenching c u r v e was analyzed a c c o r d i n g to the method d e s c r i b e d in [3]. The a c r i d i n e m o l e c u l e in the excited s t a t e is a s t r o n g e r b a s e (pK a = 10.65) than in the e l e c t r o n i c ground s t a t e (PKa = 5.45) [4]. Such a s h a r p change in the a c i d - b a s e p r o p e r t i e s of a c r i d i n e during e x c i t a tion p e r m i t s the s e l e c t i o n of a p r o t o n donor that p r o t o n a t e s a c r i d i n e only in the excited state, and not in the ground state. In this w o r k , the a m m o n i u m ion, which was produced as a r e s u l t of the d i s s o c i a t i o n of a m m o n i u m n i t r a t e , $ w a s u s e d as the p r o t o n donor. The a b s o r p t i o n s p e c t r a of a c r i d i n e in methanol, ethanol, and n - p r o p a n o l a r e unchanged when the a m m o n i u m ion c o n c e n t r a t i o n is i n c r e a s e d , which indicates the a b s e n c e of a r e a c t i o n of a c r i d i n e with the a m m o n i u m ion in the e l e c t r o n i c ground state. In this c a s e the f l u o r e s c e n c e s p e c t r a of solutions of a c r i d i n e undergo a s h a r p change, which is e x p r e s s e d in a d e c r e a s e in the intensity of the s h o r t - w a v e f l u o r e s c e n c e band (425 nm) and the a p p e a r a n c e of a f l u o r e s c e n c e band in the l o n g - w a v e r e g i o n (400 nm). The l a t t e r coincides with the f l u o r e s c e n c e band of the p r o t o n a t e d f o r m of a c r i d i n e , obtained by the addition of p e r c h l o r i c acid to an alcohol solution of a c r i d i n e . Quenching of the f l u o r e s c e n c e of a c r i d i n e by the a m m o n i u m ion gives a l i n e a r r e l a t i o n s h i p in the S t e r n - V o l m e r c o o r d i n a t e s (Fig. 1). T h i s is evidence, in the f i r s t p l a c e , of the a b s e n c e of a r e a c t i o n between a c r i d i n e and the a m m o n i u m ion in the e l e c t r o n i c ground s t a t e , and in the second p l a c e , of the total d i s s o c i a t i o n of a m m o n i u m n i t r a t e in alcohol solutions within the c o n c e n t r a t i o n r a n g e used. T h e s c h e m e of the p r o c e s s e s o c c u r r i n g in the s y s t e m excited a c r i d i n e - a m m o n i u m ion in alcohols can be r e p r e s e n t e d a s follows: J

(1) CI3 Hg.~

C13H9 NH*

w h e r e ClzH~N and CI~HgN a r e m o l e c u l e s of a c r i d i n e in the excited and ground e l e c t r o n i c s t a t e s , r e s p e c t i v e ly; k 0 and k~ a r e the r a t e c o n s t a n t s of e m i s s i o n l e s s deactivation of the n e u t r a l and protonated f o r m s , r e ! s p e e t i v e l y ; ks and k s a r e the r a t e constants of e m i s s i o n of the neutral and protonated f o r m s , r e s p e c t i v e l y ; ki is the r a t e constant of the phototransfer, of a proton; k 2 is the r a t e constant of induced e m i s s i o n l e s s d e activation. Since the s u m r 1 6 2 + ~ ' / r r e m a i n s constant and equal to one when the a m m o n i u m ion c o n c e n t r a t i o n is i n c r e a s e d f o r solutions of a c r i d i n e and all t h r e e alcohols (Fig. 1), f r o m this it follows that the r a t e c o n s t a n t of induced e m i s s i o n l e s s d e a c t i v a t i o n of a c r i d i n e under the action of the a m m o n i u m ion is negligible in c o m p a r i s o n with the r a t e constant of p h o t o t r a n s f e r of a proton, and the f i r s t p r o c e s s can be neglected. T h e n f r o m s c h e m e (1), neglecting the r e v e r s e r e a c t i o n , we obtain ~'% : k1% [NH4+],

(2)

w h e r e 70 is the l i f e t i m e of the excited a c r i d i n e m o l e c u l e in the a b s e n c e of a proton donor. t A m m o n i u m n i t r a t e in alcohol solutions at the c o n c e n t r a t i o n s u s e d is p r a c t i c a l l y e n t i r e l y d i s s o c i a t e d into the c o r r e s p o n d i n g ions [5]. 1218

TABLE Proton

2. Probabilities of the Reaction in the System Acridine-Ammonium

!~ Solvent

of Phototransfer of a Ion in Alcohols at 25~

5lDZS s~ ~.i0~ NH4+.IO,I / 2 _ ^-t '.see-' /em-~e~ t

.

an9 ~

I

1,05 0,54

Methanol Ethanol n-Propanol

0,30

1,55 0,52

0,29

~aRa, nm

va

0,040 • 0,004 0,094• 0,123+0,012 !

0,040 0,094 0,123

R e p r e s e n t i n g the e x p e r i m e n t a l r e s u l t s in a plot of ~ ' ( P 0 / ~ v e r s u s [NH~] (Fig. 2), we find k i t 0 and, knowing r 0, we d e t e r m i n e d k 1 (Table 1). A c o n s i d e r a t i o n of the nonequilibrinm c h a r a c t e r in the initial p e r i o d of the r e a c t i o n (according to the W e l l e r m e t h o d [6]) leads to a c o r r e c t i o n that is l e s s than 1~ of the value of the r a t i o of the quantum yields of f l u o r e s c e n c e . As is well known [7], t r a n s f e r of a proton to solvents of the type of alcohols o c c u r s along the chain of h y d r o g e n bonds f o r m e d by the solvent. T h i s p r o c e s s is v e r y rapid; t h e r e f o r e protolytic r e a c t i o n s in such s o l v e n t s , as a r u l e , a r e controlled by diffusion, and the r a t e constants of these r e a c t i o n s can be cal= culated with the aid of the Debye theory [6. 8]. A c c o r d i n g to this theory, for the r a t e constant of the b i m o l e c u l a r r e a c t i o n , controlled by diffusion in the a b s e n c e of eoulombic interaction, the following e x p r e s sion is c o r r e c t [6]:

k~ --- 4nY~ (D~ -7 D~) y~ R,, 1000 " "

(3)

where N0is Avogadro's number; D A and D B are the diffusion coefficients of the particles A and B, entering into the reaction; Ra is the average distance between the centers of the molecules A and B in the collision complex; Ta is the probability of the reaction in the collision complex. Knowing the corresponding diffusion coefficients and the experimentally determined rate constants of the reactions, we can determine TaRa. The distance R a represents the sum of the radii of the particles A and B and the thickness of the layer of solvent molecules in the collision complex. Knowing these quantities we can determine Ta, which gives information on the reaction of phototransfer of a proton. The results are cited in Table 2.* From Table 2 it is evident that Ta increases in the series methanol-ethanol-n-propanol. This fact can apparently be explained from the standpoint of the mechanism of proton transfer along the chain of hydrogen bonds of the alcohol molecules. Proton transfer necessitates that a hydrogen bond be effected in the collision complex between the alcohol molecules from the solvate shells of the donor and aceeptor, specifically solvating the latter, i.e., the structure of the collision complex in the state most promoting proton transfer should take the form of a complex with a hydrogen bond: H+ , I CIaHgN. 9 9H--O.- 9H--O. 9 9H'--N--H,

R

R

(4)

H

where ROH is an alcohol molecule (it is assumed that two alcohol molecules lie between the proton donor and aeeeptor, which corresponds to monomolecular solvating layers). Evidently in view of the increase in steric hindrances, the probability of reactions in the collision complex Ta should decrease in the series methanol-ethanol-n-propanol. An increase in Ta, however, permits us to assume that a greater influence is exerted by processes occurring in the complex with a hydrogen bond itself (4). For example, if the structure

(4) is followed

by the structure H •

CIaH9N ....I-I--O. 9 9H--O--HI I R R

I

9 .N--H, ! H

(5)

9 The diffusion coefficients for acridine were determined from [9], for the ammonium ion from the limiting equivalent electric conductivity. The distance R a was considered the same for all the alcohols and equal to into. In the calculation it was assumed thatRN...HO = 0.28nm, RO ..HO = 0.27nm, R O...HN

= 0.30 rim. 1219

then an increase in the basicity of the alcohols in the s e r i e s m e t h a n o l - e t h a n o l - n - p r o p a n o l should promote an increase in the probability Ta. Fur t her t r a n s f e r of a proton along the chain of hydrogen bonds leads to the appearance of a protonated form of acridine, which is expressed in the summary reaction (!), LITERATURE 1. 2. 3j 4, 5. 6.

7. 8, 9,

1220

CITED

B. M. Uzhinov, M. A. Pospelova, I. Yu. Martynov, and M. G. Kuz'min, Zh. Prild, Spectroskopii, 17, No. 5, 911 (1972). A. Weisberger, E. P r o s k a u e r , J. Riddik, and E. Toops, Organic Solvents [Russian translationf, [L, Moscow (1958). J. N. Demas andA . W. Adamson, J. Phys. Chem,, 75, 2463 (1971). A. Weller, Zs. fiir Elektroehem. B e t , Buns. Phys. Chem., 61, 956 (1957). The Chemist's Handbook [in Russian], Vol. 3, Khimiya, Moscow-Leningrad (1964). A. Weller, Zs. Phys. Chem. N. F., 13, 335 (1957). New P r o b l e m s of Physical Organic Chemistry [Russian translation], Mir, Moscow (1969), p. 207. i~. Debye, T r a ns . Am. Electrochem. Soc., 82, 265 (1942). D. F. Othmer and M. S. Thakar, Ind. Engng. Chem., 45, 589 (1953).