Effect of substituents on the T1 energy of trans-stilbenes

Volume 64, number 3

CHEMICAL PHYSICS LETTERS

IS July 1979

EFFECT OF SUBSTITUENTS ON THE T 1 ENERGY OF TRANS-STILBENES L. A L D E R , D. G L O Y N A , W. W E G E N E R , F . P R A G S T and H.-G. H E N N I N G

Sektion Chemic, Wissenschaflsbereich Organische Gtemie, Humboldt-Universit~t Berlin, DDR-104 Berlin. German Democratic Republic Received 14 February 1979; in final form 4 April 1979

Oxygen-induced singlet--triplet absorption and electrochemiluminescence quenching experiments x~ith substituted stilbenes indicate a small influence of monosubstitution or donor--acceptor disubstitution on the triplet energy. This implies a decrease o f the S l--Tt energy difference particularly in the ease of donor--aeceptor substituted stilbenes.

T 1 level. As a rule b o t h spectra overlap in t h e region o f the 0 - 0 band. A n o t h e r m e t h o d f o r indirect deterruination o f t h e triplet energy is energy transfer e i t h e r b y sensitization o r e l e c t r o l u m i n e s c e n c e experiments_ In tile case o f t h e u n su b st i t u t ed trans-stilbene, w h i c h does n o t p h o s p h o r e s c e [ 5 - - 7 ] , t h e T 1 energy was determined b y means o f s i n # e t - - t r i p l e t ab so r p t i o n , in h e a v y - a t o m solvents [8,9] o r u n d e r o x y g e n pressure

1. Introduction The e f f e c t o f s u b s t i t u t i o n on th e S 0 - S 1 e x c i t a t i o n energy o f trans-stilbenes is well k n o w n . E l e c t r o n don o r o r a c c e p t o r substituents in th e p - p o s i t i o n o f the phenyl groups r e d u c e the e n e r g y o f the S t level c o m pared to u n s u b s t i t u t e d stflbene [ 1 , 2 ] . On the o t h e r hand substituents in t h e o r t h o and in the a - p o s i t i o n as a rule give rise t o a h y p s o c h r o m i c shift o f t h e longest wave a b s o r p t i o n band d u e t o deviations f r o m planarity [ 3 , 4 ] . T h e i n f l u e n c e o f substituents o n t he triplet level o f s t f l b e n e s has n o t b e e n investigated, alt h o u g h t h e energy gap b e t w e e n t h e S I and T I state /s i m p o r t a n t f o r t h e rate o f i n t e r s y s t e m crossing and f o r the p r o b l e m o f p a r t ic ip a t io n o f T 1 in th e n o n sensitized p h o t o i s o m e r i z a t i o n o f s t f l b e n e s . By means o f p h o s p h o r e s c e n c e o r singJet-tripIet a b s o r p t i o n spectra o n e can d e t e r m i n e the relative energy o f the

[io,lH. We present s o m e results o f 0 2-induced T 1 + - S o absorption and e l e c t r o l u m i n e s c e n c e q u e n c h i n g experiments, and discuss t h e m in t er m s o f t h e T I ener~ e s o f s o m e substituted trans-stilbenes 1 (table 1).

2. E x p e r i m e n t a l Th e O2-induced T 1 ~ S O a b s o r p t i o n spectra were

Table I

RI- - ~ ~,~

Comp.

CH :CH--~ ~

la

lb

R2

1

lc

ld

le o

Rt R2

H H

OCH3 H

NMe2 p-OCH3

Ph2P H

If

Ig

lh

0

o

0

Ph2P p-OCH 3

Ph2P m-OCH3

Ph2P o-OCHa

li o

Ph2P /~NMe2

0 Ph2P m-NMe2 503

Volt:me 64, number 3

CHi~MICAL PHYSICS LETTE_RS

recorded o n a Specord UV/VIS C Zeiss J e n a spect r o m e t e r using a 5 cm Beckman high-pressure cell and chromatographically purified CHCI 3 [12] as a solvent. A f t e r saturation with 0 2 b y shaking for 5--6 rain at 140 a t m the s o l u t i o n exhibited a very small absorption i n the 1 3 0 0 0 - - 2 6 0 0 0 c m - 1 region which could b e neglected. I n the electrochemical experiments dimethylformamide (DMF), purified b y a k n o w n m e t h o d [ 1 3 ] , was used. T h e preparation o f the d i p h e n y l p h o s p h i n y l stilbenes l d - - l i will b e described [ I 4 ] , that o f I b and l e is k n o w n [ 1 5 ] . Trans-stilbene l a was from Merck "'for scintillation". All c o m p o u n d s were recrystallized several times. TILe emission spectra were recorded o n a Hitachi MPF-2A fluorescence spectrometer_ T h e electrogenerated chemiluminescence (ECL) experiments were performed in 0_ ! M [N(C 2 I-I5)4 ] CIO4 in DMF at Pt electrodes using 50 Hz square-wave voltage excitation, as described elsewhere [ 1 6 - - 1 8 ] .

15 July 1979

400

500

6oo (o)

s •

Jl lJ

J

Is

s

tJ

~.~., t,l_.la 3.~-r",

06

O.2

• is t i#

) tb.l

1.0

,1 0.6

L |l

j

3. Results a n d conclusions 25

The phosphorescence properties o f the hitherto u n k n o w n c o m p o u n d s I d - - l i were tested in m e t h y l cyclohexane + t o l u e n e (1 : I ) . Besides fluorescence n o o t h e r emission could b e detected. T h e application o f a glassy solvent with heavy atoms (C2H5I + diethylether + toluene, I : I : I), which is k n o w n to increase the phosphorescence q u a n t u m yield o f other c o m p o u n d s [ 1 9 ] , yielded the same negative result. In the T t ~ S O absorption spectra (fig_ I) the longwave absorption tail exhibits a b a t h o c h r o m i c shift due to charge-transfer interaction [20,21 ] between the stilbenes I and 0 2. Moreover in the ease o f I a-- l g additional structured a b s o r p t i o n b a n d s appear in the 17000--22000 em - I region. The origin o f these uniformly shaped b a n d s is the O2-induced T I ~ S O transition as in the unsubstituted trans-stilbene l a [8,•0, I 1 ] _ TILe spectral position o f these bands does n o t depend on the 0 2 pressure within the accuracy o f recordirlg, -+ 200 crn - I . l h exhibits a long-wave CT-absorption, so that one c a n n o t discriminate a n y T 1 *-- S o band. No absorption is observed between 13000 and 17000 cm - I ; it follows that the T I energy o f l h must be higher than 17000 c m - 1 . l i does n o t show a n y T I "~-S0 absorption up to 2 0 0 0 0 cm - I . T h e fluorescence behav504

20

15

Fig. 1. Absorption spectra of stilbenes 1 in C H C l 3 in the presence of air (dotted line) and at 140 atm oxygen pressure (solid line); concentrations: Ia: 1.0 M; l b : 0.8 M; ld: 0.3 M; l e : 0 . 8 M; If= 0 . 4 M; lg: 0.6 M.

iour and the usual absorption spectrum [22] differ from that o f o t h e r stflbenes due to an i n t m m o l e c u l a r charge transfer in l i . The T! energy is nearly unaffected b y s u b s t i t u t i o n (table 2). The triplet energies o f l a - - l g lie in the region 1 7 1 5 0 - - 1 7 5 5 0 cm - 1 (205-210 kJ m o l - l ) . In order to c o n f i r m these results b y art i n d e p e n d e n t m e t h o d , triplet q u e n c h i n g experiments were carried out using electrogenerated chemiluminescence (ECL). The principle o f the ECL triplet mechanism and o f its use for estimating triplet energies has been described elsewhere [ 16-- 18]. The triplet states generated b y electron transfer (3) can bring a b o u t emission via triplet--triplet annihilan o n (4). By addition o f one o f the stilbenes 1 this emission will be q u e n c h e d , i f its triplet energy is equal to or smaller than that o f the primarily generated triplet state:

Volume 64, n u m b e r 3

15 July 1979

CHEMICAL PHYSICS LETTERS

Table 2 The O2-induced T ! ~ So absorption in CHCI 3 a t 293 K, SI energies and Sl --Tl energy gaps in cm -1 TI

la

~Soabsorpt~n ~

17400 17750 d) 17550 17500 17150 17500 17400

lb ld le If lg

19050 19100 d) 19000 18800 18650 18850 18400

20600 20600 d) 20500 20300 20150

e,

21250 21900 21000

20550 19950

(I)

D -- e --->-D~,

(2)

- A e' + D ° --->3 A * + D,

(3)

23A*-+ 1 A * +

(4)

A,

1A* ~ A + hu,

(5)

3A*+ 1 -+A + 3(1)*,

f6)

3( 1 ) * -> a~ 1 ) + ( 1 - - a ) ( e i s - - 1).

(7)

-A ° + R - - ~ ( C H 3 ) 2 ~ 3 A * + R - N ( C H 3 ) 2

(8)

i f E T ( A ) <~ E T ( R - - N ( C H 3 ) 2 ) , w h e l e R = p - m e t h o x y stilbenyl. With rubicene, perylene, 9,10-diphenylanthracene, 7,14-diphenylacenaphth(1,2-k)-fluora n t h e n e a n d p y r e n e ( E T ~_ 2.1 e V ) t h e E C L o f t h e h y d r o c a r b o n s c o u l d b e d e t e c t e d . U s i n g fluoranthene,

29800

12400

28100 28300 26800 27900 26600

10550 10800 9650 10400 9200

c) _+300 cm - t _

7 , 8 , 1 0 - t r i p h e n y l f l u o r a n t h e n e a n d e h r y s e n e ( E T 1> 2.3 eV) only a very weak emission was measured, the o r i g i n o f w h i c h is n o t y e t c l e a r . T h e r e f o r e , t h e dimethylamino group does not alter the triplet energy o f t h e s t i l b e n e b y m o r e t h a n + 0.1 e V ( 8 1 0 c m - l ) . 12"

-'3

I:--,

-4

16 Er 10, c m 18 "

20

L~ I~. -O.. . . . . . .

I n s t e a d o f 3 A * also 3 D ~ m a y b e f o r m e d . I n fig. 2 t h e ratio 1Q/I 0 of the ECL intensities in the presence and a b s e n c e o f c o m p o u n d I b ( I 0 - 3 M) is s h o w n as a function of the T 1 energies, ET, for ten ECL systems; the T 1 energy of lb can be estimated to be between 2.1 a n d 2 . 3 e V ( 1 6 9 4 0 - - 1 8 5 6 0 c r n - l ) . A l m o s t t h e s a m e I Q / I 0 v a l u e s w e r e fotrtid f o r t h e u n s u b s t i t u t e d stilbene la and for the derivative If suggesting that t h e r e is n o e s s e n t i a l c h a n g e i n t h e E T v a l u e s d u e t o substituents_ S i n c e t h e d i m e t h y l a m i n o c o m p o u n d l e is o x i d i z e d at a more negative potential than most of the compounds D, it could not be investigated in this way. T h e r e f o r e l e w a s u s e d as c a t i o n r a d i c a l p r e c u r s o r D in a series of ECL experiments.

A(S~--TI)

21900 22000 d)

a) _+200 em - l . b) Voo in methylcyelohexane + toluene (1 : I ) at 77 K, ± I 0 0 cm -1 . d) Determined by Evans [I01 .

A+e-+A

Fluorescence b)

10

I

-3 4 0~__0_. 2

. . .

o5

"',.

'e,7 i

oi

05 t I I I I

00~15

2~0

~--

910

25

Er . eV

Fig. 2. Relative ECL intensities, IQtIo, o f some mixed triplet systems A/D (concentrations 10 -3 M) after addition o f 10 -a M compound l b in 0.I M [N(CaHs)4]CIO4. in DMF, as a function o f ET, and the calculated curve [18] f o r e T = 2.12 . eV and for thermodynamically controlled energy transfer;

1 = perylene*/N,N,N',N'-tetramethyl-p-phenylenediamine O'MPD); 2 = N-p-tolylphthalimide (TPI){1-N-p-anisyl-3-styryl5-phen.~ l-_-pyrazol,ne ; ~ = 9,10-diphenylanthracene*f/3lPD; 4 -- TPIll-p-anisyl-3-p-biphen~ lyl-5-phenyl~-pyrazoline*; 5 = TPl[l-p-anisyl-3,5-diphenyl-2-pyrazoline*; 6 = TPI/1-p-tolyl3-p-anisyl-5-phenyl-2-pyrazoline*; 7 = 7,14-diphenyt-acenaphtho( 1,2-k)-fluoranthene*/N,N-dimethylaminobiphenyle (DMAB); 8 = pyrene*lTMPD; 9 = 7,8, I 0-triphenyl-fluoranthene*/D_MAB; I 0 = fluoranthene*/DMAB; (* = primarily excited compound)_ 505

VoIume 64, n u m b e r 3

CHEMICAL PHYSICS LETTERS

Contrary to the triplet, the S t energy strongly depends on substituents (table 2). Therefore the S 1T 1 energy gap differs significantly for the stilbenes I , e.g., t h e S t - - T 1 t e r m s p l i t t i n g is d i m i n i s h e d b y a donor--acceptor substituent pair from 12400 cm -1 in the case of la to 9200 cm -1 for lg. It follows from the different substituent effect on S 1 and T 1 t h a t t h e s t a t e s d i f f e r e s s e n t i a l l y i n t h e i r e l e c t r o n distribution.

References

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506

15 July 1979

[7 ! A.A. Lamola, G.S. Hammond and F.B. Mallory, Photochem. PhotobioL 4 (1965) 259. [8] R.H. Dyck a n d D.S. McClure, J. Chem. Phys. 36 (1962) 2326. [9] H. Stegemeyer, D/ssertat/on, Hannover (1961). [10] D.F. Evans, J. Chem. Soc. (1957) 1351. [ 11] A. Bylina and Z.R. Grabowski, Trans. Faraday Soc. 65 ( I 9 6 9 ) 458. [12] G_ WohlIehen, Angew. Chem. 67 (1955) 74 I. [13] L R . Faulkner and A.J. Bard, J. Am. Chem. Soc. 90 (1968) 6284. [14] D. Gloyna, L. Aider, H. K6ppel and H.-G. Henning, in preparation. [15] G.A.R. Kon and R.G.W. Spickett, J. Chem. Soc. (1949) 2724. [16] D.F. Freed and L.R. Fau "Lkner, J. Am. Chem. Soe. 93 (1971) 2097. [ 17] F. Pragst, C. Gr~nberg, W. Lamm, H.-P. Boomgaarden and W. Jugelt, Z. Physik. Chem. (Leipzig) 255 (1974) 683. [18] R, Ziebig and F. Prag~t, Z. Physik. Chem. (Leipzig), to b e published. [ I 9 ] M. Zander, Z. Naturforsch. 28a (1973) 1869; 32a (1977) 339_ [20l D.F. Evans, J. Chem. Soe_ (1953) 345; (1961) 1987. [2I ] H. Tsubomura and R.S. Mulliken, J. Am. Chore- Soc. 82 (1960) 5966. [22] L Alder, M.V. Koz'menko, N.A. Sadovskij, M.G. Ku~'mLn, D. Gloyna and H.43. Henning, Opti "ks i Spektroskopiya 46 0 9 7 9 ) 76.