Speetrochimica Acts, Vol. $1A, pp. 187 to 189. Pergamon Press 1975. printed in Northern Ireland

RESEARCH NOTE

Solvent effect on the absorption spectra of substituted azobenzenes

(Received 8 A ~ ! 1974)

A b ~ a c t - - T h e electronic spectra of some nitrobenzenes have been obtained in a variety of solvents and briefly discussed, lnformations about the excited state electronic distribution are obtained from solvent shift.

I t is well known tha t solvents have a considerable effect on the electronic absorption and emission spectra of organic compounds. I n particular ~* ~ - n absorption bands are charac- terized by solvent blue-shi~, while ~r* ~-~r bands are red-shifted. Moreover information con- cerning variations of electron distribution following electronic excitation can be obtained from solvent effect studies.

According to SuPPA_W [1] the shift in the electronic transitions AE is related to the dielectric constant funct ionf(D) by the relation:

AE pg

A/(D) -~ (p" -- p ' ) " ~"

2(D -- 1) where a is the cavity radius and.f (D) ---- 2 D + 1

Nitro-azobenzenes have high dipole moments in the ground state, pg (Table 1), so tha t an appreciable solvent effect is expected on the electronic transitions of these compounds.

Spectra were measured on a Pye Unieam spectrophotometer Mod.SP 1800; dipole moments were determined in benzene solution a t 25°C using a W.T.W. Dipolmeter Mod. DM01 by standard

Table 1

AE

Compound Z* max 8* max Af(D) a t (A) P8 (D) pe(D) (nm)

I 4-OH, 4t.NO2. __450 Azobenzene 368 26700 --2550 4.07 5.64

H 4-OCH a, 4'.NO 2. __450 Azobenzene 370 21800 --1004 4.14 5-63

H I 2-OH, 5-CHs, 4~-NOs- 430 9200 978 4.14 4.03 Azobenzene 338 23600 --193

IV 2-OCHs, 5-CHs, 460 4'-~Oz-Azobenzene 39--0 8300 --588 4.22 5.62

324 11000 - 6 2 2

8.15

0-60 4.71

7.18 7.27

* In eyolohexane; underlined values denote inflections. t Calculated as reported in Ref. 4.

[1] P, S~PP~r, J . Chem. ~o¢. A. 3125 (1968). 187

188 Re~earoh note

4 , ` 5

4,0

/ /

° • 3 , `51 " •

~o

250 300 3.50 4 0 0 450 `500

~., m/~

Fig. 1. Absorption spectra of ~ compound I ; . . . . . compound I I I ; . . . . compound IV.

. . . . compound I I ;

t l a

28,0

6 2~0~ L 3 4 o o o ~ / i i i

i I b 27,`5 2 o

c 5 6

i 0,4 0,6 0,8 0,4 0,6 0,8

f (O) f (D)

3o,o~ f ¢z 06

I -°~- - - - - - -~o-- 'o____5 ] 29~ I ~ 3 , o -

5o 1 ~" / I I t i

I~o °

t 3o.ot- ,

Fig. 2. Solvent shifts in azobenzenes, a: ~* *-~r transition of compound I ; b: ~r $ *-~r transition of compound I I ; c" ~* * - ~ transition of compound I I I ; cz: ~r $ ~- ~r transition of compound I I I ; d: first ~r* ~- ~r transition of compound IV; dz: second ~r* +-~r transition of compound IV; Solvent numbering, (1) n-pentane, (2) n-hexane, (3) diethyl ether, (4) ethyl acetate, (5)acetone, (6)

acetonltrile.

Research notes 189

procedure and following the HAT.V]~gSTAD-KuMLER method [2]. Compounds were prepared following literature indications.

The spectral eharacteristies of the compounds examined are reported in Table 1. I t can be seen a little shift at longer wavelengths of the K band, which in azobenzene is located a t 316 n m ( e - - 22,900), on passing from the compound I to the compound I I . The low intensi ty 9" *- n band, which in the azobenzene is located at 444 n m (e = 450), appears as a shoulder at about 450 nm in both compounds. I n the compound I to the =* ~-- n band should contribute also the ~r-electron system through the formation of the pseudo six-numbered ring v/a the intramolecular hydrogen bond. I ts intensi ty is in fact toO large (e = 9,150) for a pure ~r* ~--n band; on the other hand it shows the characteristic blue-shift of the =* ~-- n band (Fig. 2).

Methylation of I causes blue-shift and ipocromic effect in the K band (from 338 to 324 nm); this behaviour is a t t r ibuted by BURAWOY and CF[AM'R]~RT.AII~ (3) to a revels1 of the effect of hydrogen bond formation and to steric interaction between the - - O CH a group and the azo- group, probably determining a loss of planari ty in the molecule. The solvent effect on the K band is similar for both compounds. Moreover methylation of I causes the appearance, as a shoulder, of the ~r* ~-- n band and of the benzenic band at 390 nm.

The solvent shift for the ex~mlned compounds is correlated wi thf (D) in Fig. 9, where D are the usual dielectric constant values at 25°C. A good linear correlation is found for all compounds except for I probably owing to specific solute-solvent interactions through hydrogen bonds. These results allow to calculate approximate values of pe for the different electronic transitions, so tha t some picture of the electronic distribution in the excited states can be achieved.

Acl~nowZec~lem~ts---The partial financial support of the I ta l ian CIqR is acknowledged.

Institute of Physical Chemistry D. G ~ s s o University of Catan/a S. Mrr,r,~IO~Z Italy S. FASON~

[2] I . F. TTAT.VEBSTADT and W. D. K IITMT;I~., J. Am. 6f)~'/B. Soc. ~ , 2988 (194,2). [3] A. BURAWOY and J. T. CHAMB~.m~r~, J. Chem. Soc. 3734 (1952). [4] G. FAvI~I, A. GAMMA, D. G~SSO and S. ~[rr.r.EFiORi, TrG~s. of the _~offadc~y ~oe. 3139 (1971).