Electronic absorption spectra of some azo compounds with condensed ring systems in non-aqueous and mixed buffer solvents

Specmchmca ha, Vol 42A Pnnted I” Great Bntam

No

5 pp 631435.

1986 0

0584-8539/86 S3 00 + 0 00 1986 Pergamon Press Ltd

Electronic absorption spectra of some azo compounds with condensed ring systems in non-aqueous and mixed buffer solvents H A DESSOUKI,*$ H M KILLAT and A ZAGHLoULt Science, Benha Umverslty, Benha, Egypt and tZagazlg Unlverslty, %=g, Egypt

*ChemistryDepartment, Faculty of

(Recewed 22 July 1985, tn jinal form 25 October 1985, accepted 28 October 1985) Abstract-The electromc absorption spectra of aght substituted phenylazo denvatlves of a- and /?-naphthol, 9-phenanthrol and 5- and 6-chrysenol were studled m ddferent orgamc solvents The assignment of the spectral bands obtamed and also the effect of orgamc solvents on the colour of the compounds was Investigated The spectra m buffer solution over the pH range 2-14 were recorded and explamed,also the pK, values were determmed

INTRODUCTION

Little 1s reported m the literature about the solvent influence on the spectra of azo compounds containing condensed rmg systems Thus It IS necessary to carry out more detaded mvestlgatlons on the spectral behavlour of azo compounds with condensed rmg structures under solvent actlon to draw some general conclusions concerning the interaction of these compounds with the solvent BRODE [l] found wtth azobenzene no extensive shft of the band on changmg the typlcal solvent Some studies of the effect of the nature of the organic solvent on the spectra of azodyes were performed [2-6], and mvestlgatlons of the spectral behavlour of azodyes m acid and alkaline media as well as the determination of the pK, values were also performed [7-91 The present mvestlgatlon was almed to assign the absorption bands due to electronic transitions from studies m cyclohexane, to discuss the spectral behavlour m organic solvents of different polarities and

tn aqueous buffer solutions contammg ratios of etha-

nol to determine the pK, values of the compounds under study EXPERIMENTAL Materrals and solutions The azocompounds were prepared as described by KAMEL and AMIN[~~] Thetr punty was checked by meltmg point and the results ofelemental analysts The compounds have the general structure

N=N-Y

where X = OH(I), COOH(II), 0CH3 (III) and Y =

OH OH

a

HO

OH

e

d

The solvents utlhzed were obtamed from commerctal sources and punfied according to recommended procedures [ 1I] The buffers used for pH control were members

$Author to whom correspondence should be addressed 631

H A DJZSOUKI et al

632

Table 1 Assignment of different bands of compounds and then 1,

Compound Ia

Ila Ib IIb IIC IIIC IId IIIe

Solvent Ethanol Cyclobexane Ethanol Cyclohexane Ethanol Cyclohexane Ethanol Cyclohexane Ethanol Cyclohexane Ethanol Ether Ethanol Ether Ethanol Cyclohexane

lL, + 1A L,, emax

lBa + 1A lL, + 1A &liU %lUX ,Gn.¶x smax

205 212 207 210 215 210 210 212 210 210 220 227 225 217 213

230 230 240

204 088 10 271 3 18 44 8; 5s93 46 07 045 045 s

2; 232 230 260 255 257 257 272 270 257 250

sh 04 263 443 15 745 5so3 603 176 09 264 s

307 307 290 265 270 310 268 290 290 322 320 270 -

22 067 147 sh sh 127 7 75 2 56 2 53 242 006 sh -

and E_ m orgamc solvents

lB,+ 1A

ImaX smax 350 350 365

175 055 1 13

415 415 420 390 400 400 400 373 370 400 395

056 sh S

sh S

sh sh 095 064 087 s

CT Hydrazone &a, %Ux

-

-

505 460 482 475 -

1 32 13 275 s -

-

-

500 500

3s s

CT Hydroxyazo 1 mox &ma, 535 480 500 460 530 502 500 490 480 500 487 520 490 525 525

199 06 047 0 50 185 267 5; 3s5 393 045 002 06 s

sh = shoulder, s = saturated

of the umversal senes of BRITTON [12] lo-’ M solutions of

the dyes were prepared by dlssolvmg an accurate we@ of recrystalbzed product III the appropnate volume of pure solvent Solutions for spectral measurements were obtamed by accurate Qluhon of the stock ones The rn& m buffers usually contamed 25 and 40 y0 ethanol Apparatus The absorption spectra at 25°C were recorded m the wave length range 200-650 nm on a Beckman dIetal spectrophotometer (model 25) usmg lcm matched s111cacells The pH measurements were camed out on an Onon research pHmeter model 6OlA/D@al Ionolyser

6

RESULTSANDDISCUSSION Spectra tn cyclohexane The A,, and E,, values of the different bands obtamed from the spectra of compounds Ia-b, IIa-b, IIIe m the non-polar solvent cyclohexane are collected m Table 1 (Fig 1) The spectra compnse a band m the regon 212-215 nm which 1sdue to the transition (1 L, + 1A) of the phenyl rmg and a second m the range 230-255 nm which 1s due to the transition (18, + 1A) of the naphthalene, phenanthrene or chrysene system These two bands are msensltlve to variation of solvent polarity, being due to the localized electronic transitions There are two other bands m the range 27&307nm and at 361_415nm, m some cases the latter bands appear as a shoulder These two bands show a slight irregular shift m I,, denoting that they are to be attributed to the transition m the benzenold system Thus the third band 1s due to the (lLb + 1A) state of the phenyl rmg and the fourth 1s due to the transltlon (lB, + 1A) of the naphthalene (a, b), phenanthrene (c)and chrysene systems (d, e) Generally, the bands due to the condensed system d, e are red shifted compared to those of c or a, b The band lying within the range 460-535 nm 1s highly influenced by the type of substltuent (X), the number of condensed rings and

zoo

300

400

500

600

h(nm)

Rg 1 Electromc spectra of some azo compounds m cyclohexane at 25°C

the nature of the organic solvent This band can be attnbuted to a transltlon within the N=N group influenced by an mtramolecular charge transfer through the whole molecule The followmg mesomenc shifts for 111~and IIIe for example, indicate that I, of the CT band of IIIe must be red shifted compared to 111~ This charge transfer seems to ongmate from the OH, COOH or OCHS groups as source to a phenyl rmg of the condensed system as smk The CT nature of the band can be shown by studying the spectral behavlour m buffer media of varying pHs (Fig 5) Thus, the band IS red shlfted wrth increasing pH of the medium as well as m the presence of ethanol This behavlour can be discussed on the basis that the lomzatlon of the OH group leads to destruction of the

Azo compounds wth condensed ring systems

633

Ilk

-

intramolecular hydrogen bond and so faclhtates the transltlon of charge Hrlthm the whole molecule The CT band ofcompounds Ib, IIb and IIIeexhlhts sphttmg due to the presence of the hydroxyazo-hydrazone tautomenc eqmhbnum The first spht band 1s due to the structure of hydrazoqumone and the second one to the hydroxyazo form

This can be substantiated by increasing the absorbance of the second split band as well as rts red shift m 1, by increasing the pH of the medium whereas the first IS still unaffected The tautomenc equlhbrmm has been discussed m detail Cl33 This behavlour 1s comparable to that observed with some annulated azo compounds[14] The other compounds (IIc, IIId and 111~)show a composite band in which for 111~a clear sphttmg m buffer medium with shift m 1, ISexhibited with increasing pH SOLVENTEFFECTSON THE ELECTRONIC SPECTRA The 1, and E, values of spectral bands for the transitions to different energy levels are gven m Table 1 The states (lLa + 1A) and (1~5, + 1.4) of the phenyl nng are solvent independent or of no regular vanatlon m the band position with changng the organic solvent However, the 1, values of the bands of the states (l& c 1A) and ( lBI + 1A) of the condensed nng systems vary shghtly with the solvent This may beattnbuted to the influence of the polanty of the solvent on the delocahzatlon of electrons Hrlthm the condensed system or a changed solvatton effect The longer broad

band shows a wide vanatron m its posltlon (Fig 2) with different solvents supportmg the CT nature of the band The band dsplacement can be discussed m terms of macroscopic solvent polanty via dlelectnc constant and also refractive mdex According to the GATISZALAYequation [lS] a plot of AV (cm- ‘) vs (D - l)/ (D+l) or (n2- 1)/(2n2 + 1) (F(n)) gives a non-linear relation In addition, according to SUPPAN[ 163 plots of 1,, of the CT band vs F(D) = (2(0 - 1)/(2D + 1)) or 4(D) = (D - l)/(D + 2) are not linear, mdlcatmg that neither the dlelectnc constant nor the refractive index of the solvents are the mam factor causing the shift of the band Plots of L, vs the nucroscoplc solvent polanty parameters Z and E, (Fig 3) gve straght lines, except for I, which 1s curved, mdlcatmg that the solvatlon properties or the solute-solvent interaction can be considered as the mam factor affecting the CT band posltlon Plots of the other mlcroscoplc parameters (a) and (B) which are the parameters ofthe TAFT-KAMLET equation [ 171 and indicate the acldlty and baslclty of the protlc and aprotlc solvents, respectively, @ve straight lines for all solvents except ether (Fig 4) This denotes the occurrence of a specific assoclatlon through proton donor-acceptor systems Accordmgly, it can be concluded that spectral displacement of the CT band with changed solvent IS the result of the dlelectnc properties of the medium and changed solvatlon of the ground and excited states as well as mtermolecular hydrogen bonding between solute and solvent molecules One can notice that the band mtensltles are mfluenced by the nature of the solvent They are increased with solvents of low polanty and that make a solute-solvent interaction with the azomolecules The size of the solute and solvent molecules may have a role as well as their geometrical shapes, but no such factors are quantitatively considered

634

H A

DESSOUKI et al

16

10

01

02

oc 200

250

300

350

400

450

500

550

600

h(nm) Fig 2 The electromc spectra of 4 x 10m5M of Ib In different orgatuc solvents, 1 ether, 2 cyclohexane, 3 CHCI,, 4 Ccl,, 5 acetone, 6 dloxane, 7 ethanol, 8 rsopropanol, 9 n-propanol

P ‘”

Id

‘a

m

2

E r<

520

53c1: 5X I-

500

51c)*

490

50CIL9C). L80 L,.

Fig 3 I,,

*

60

70

80

30

40

50

SO 60

2

abc

ET

db‘,c

Z plot for IP (a), Ib (b) and 111~(c)and I,,,,,+ plot for In (a’), lb (b’) and IIIc (c’)

470.

02

.

04

06 .

aa,b

08

B c.d,e B,f

Fig 4 I,,-a plot for Ib (a)and IIb (b), L,,-/l Ib (d), IIb (e) and 111~(f)

plot for In (c),

SPECTRA IN BUFFER SOLUTIONS

The spectra of the compounds m aqueous buffer soluttons contammg 25 and 40% ethanol (by volume) show variable behavlour The N=N CT band shows a regular mcrease m absorbance with increase of the pH of the medmm mdlcatmg the development of some new absorption bands at this repon m which the changes also depend on the percentage of ethanol

(1) In some cases the composite band of WC and the slightly split one of IIb or IIIe show two apparent bands m aqueous buffer denotmg the appearance of a new ionized form (n) Clear lsosbestlc pomts are observed for Ib and III denoting an eqmhbnum of acid-base type resulting from the lomzatlon of the OH and COOH groups m an alkaline medium (Fig 5)

Azo compounds wtth condensedrmg systems

635

Table 2 Values of pK, and AG* obtamd m buffer solutions at 25°C

25%

la IIP Ib IIb

107 102 94 10 17 546 671 3 77 999 908 766 64

IIC

400

500

6bo h(nm)

760

P&

Compound

IIIC IIe III IIIe

-AG* (kcal M-l) 25% 40%

40% 1138 11 76 1011 98 61 85 71 70 55

12 88 1395 1463 1389 747 515 1366 1242 1039 875

13 81 1608 1545 134 83 1162 971 9 57 792

Group OH OH OH OH COOH OH COOH OH OH OH OH

Fig 5 Electronic spectra m buffer solutions contammg 25 % EtOH for Ib (5 x lo-’ M)

(m) In the case of In, one band at 535 nm appearmg m a solution of 25 % ethanol increases m absorbance with increasing pH whereas m a solution of 40% EtOH another broad shoulder ISobserved at 610nm Starting at pH 7 14, the shoulder appears as a band which increases m absorbance with increasing pH value This band 1s attributed to the formation of lomzed species This may be explamed on the basis of a decrease m the strength of the mtramolecular hydrogen bond m the presence of higher percentages of ethanol This lomzatlon may be represented as follows 0..

7

/NN’

..OH C2H5

+ OH@

The pK, values of the compounds (Table 2) are obtained by making use of the vanatlon of absorbance with pH and applying the different spectrophotometnc methods previously described,, namely, the half height [is], modified hmltmg absorbance [19] and COLLETER methods [203 For the same compound the change of the substltuent (X) from the electron donor OH or 0CH3 to the electron acceptor COOH leads to a decrease of the pK, value smce the acceptor group decrease the mtramolecular hydrogen bondmg of the o-OH group The number of the phenyl rings mfluences the value of pK,, it decreases m the order IIb > IIc > IId so annulatlon increases the lomzatlon of the OH or COOH group due to the increase of the interaction of the mtramolecular charge transfer transition for such groups with the increased delocahzatlon of the aromatic moelty It 1sworth noting that the pK, values decrease with an increasing percentage of EtOH

and also with the free energy of lonrzatlon (Table 2)

AC*

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--c

+ C,H,OH+H*O

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