Indian Journal of ChemistryVol. 29A, December 1990, pp. 1153-1164

Absorption and fluorescence characteristics of some 2-alkyl- and2-aryl-benzoxazoles in different solvents and at various acid concentrations

Joy Krishna Dey & Sneh K Dogra*

Department of Chemistry, Indian Institute of Technology, Kanpur 208 016

Received 11 January 1990; revised and accepted 11 June 1990

Absorption and fluorescence spectral characteristics of some 2-alkyl- and 2-aryl-benzoxazoleshave been studied in different solvents and at various acid concentrations. Dual fluorescence obser­ved in the monocations of 2-alkylbenzoxazoles is due to :n;* -+:n; and charge transfer (cr) transitions,indicating the near degeneracy of excited singlet states. 2-Arylbenzoxazoles and the correspondingmonocations exhibit only a single fluorescence band' due to :n;* -+:n; transition. pK. values for themonocation-neutral equilibria in both So and S] states of the molecules have been determined.Pariser-Parr-Pople method for all the molecules and CNDO/S method for 2-methyl-, and 2-phenyl­benzoxazoles have been employed to calculate transition energies, transition polarization and chargedensities at different atoms in the So and S] states to substantiate experimental data and, hence, toverify the applicability of the methods for heterocycles containing two hetero atoms.

The importance of benzoxazole derivatives due totheir use as chelating agents!, pesticides2 and lasermaterials3 in UV and visible regions as well astheir therapeutic properties has led to extensivestudies on these molecules. With the recent ad­vent of studies on the excited state intramolecular

proton transfer reactions 2-(2'-hydroxyphenyl)­benzoxazole has been investigated extensively5.But a systematic study on the spectral characteris­tics of 2-substituted benzoxazoles has not recei­

ved much attention except for a few sporadic at­tempts. Passerini6 and Reiser et al.7 have- sugges­ted that the long wavelength absorption band in2-alkylbenzoxazoles is of 31: -+ 31:* type and is local­ized on the benzene ring, whereas in 2-phenyl-'benzoxazole, this 31:-+ 31:* band is localized on thesubstituent phenyl ring6-9• Earlier studies on ben­zimidazoles have also led to the same observ­ationslO•l1. Moreover, monocations of 2-alkyl-ben­zimidazoles exhibit dual fluorescence of which thelongest wavelength band is ascribed to chargetransfer (CT) transition and the short wavelengthband to the normal 31:*-+31: transitionlO-!2. Fluor­

escence spectrum of 2-phenylbenzimidazole andquantum yield indicate that the phenyl ring is co­planar with the imidazole ring both in the So andS] states 13. Benzoxazole being similar to benzi­midazole (differing only in one of the hetero at­oms), the same sort of spectral characteristics areexpected for the former also.

SCF-LCAO-MO and Pariser-Parr!4-PopleI5(PPP), and CNDO/S!6,!7 methods have been quite

successful in calculating electronic transition ener­gies in many heterocycles containing one or twohetero atoms as well as benzoxazole and benzi­sooxazole!8. However, no attempt has been madeto verify the applicability of PPP and CNDO/Smethods for bigger molecules like 2-alkyl- and2-aryl-benzoxazoles.

The present study on 2-alkyl- and 2-aryl­benzoxazoles has been carried out to investigate:(i) the nature of transitions and their polarizationsinvolved in the neutral and monocation species,(ii) the geometry of the aryl group relative to thebenzoxazole moiety in the So and SI states, (Oi)the variation in basicity of the nitrogen centre ingoing from 2-alkylbenzoxazole to 2-arylbenzoxa­zole and also with excitation, and (iv) the trans­ition energies and charge densities at differentatoms.

Materials and MethodsBenzoxazole (BO), 2-methylbenzoxazole (MBO),

2,5-dimetftylbenzoxazole (DMBO), 2-methyl-5­phenylbenzoxazole (MPBO) and 2-(4'-biphenyl)­6-phenylbenzoxazole (BPPBO) were procuredfrom Aldrich Chemical Company (UK). 2-Phenyl­benzoxazole (PBO), 2-(2'-methylphenyl)benzoxa­zole (OMPBO), 2-(3'-methylphenyl)benzoxazole(MMPBO) and 2-(4' -methylphenyI)benzoxazole(PMPBO) were prepared by the procedure sug­gested in the literature 17.BO, MBO and DMBOwere used as' such. The other compounds, exceptBPPBO which was recrystallized from dlloro-

1153

INDIAN J CHEM, SEC A, DECEMBER 1990

form-methanol mixture, were purified by r peatedcrystallizations from ethanol-water mixture Purityof the compounds was checked on the asis oftheir melting points, absorption and ex itationspectra and fluorescence spectra at differe t exci­tation wavelengths. Analytical reagent gra e cyc­lohexane (SDS), dioxane (E. Merck), acet nitrile(E. Merck) and methanol (BDH) were pu fied asdescribed in literature2o. Triply distilled wa er wasused for preparing aqueous solutions. A ueoussolutions having pHs in range 4-11 were p eparedby mixing appropriate amounts of orth -phos­phoric acid and sodium hydroxide so utions.Modified Hammett's acidity scale21 for 2S04­water mixtures and Yagil's basicity scal 22 forNaOH-water mixtures were used for the s lutionswith pH < I and pH > 13 respectively.

Absorption spectra were recorded on a Shi­madzu 190 UV spectrophotometer, e ippedwith a U135 chart recorder. Fluorescenc mea­surements were carried out with the s anningspectrofluorimeter, fabricated in our lab ratory,the details of which are reported elsewhere 3. Flu­orescence spectra were corrected accordin to theprocedure of Parker24 and the quantum yieldswere measured for the solutions having bsorb­ances less than 0.1 using quinine sulph te25 in0.lNH2S04 as standard (~=0.55). pHs of the so­lutions were measured by a Toshniwal p metermodel CL44A. Concentrations of the s lutionswere of the order - 10 - 5M, the methanol ontentwas not more than 0.6% (v/v), except for PPBOwhere it was - 10 - 6M, dioxane content w s 0.8'10(v/v). Even at such low concentration, PPBOwas insoluble in 20% ethanol-water mixtu e (v/v)and thus it was impossible to find the pKa or themonocation-neutral equilibrium either in ) or inSI state. To avoid hydrolysis26, the solutio s wereprepared just before taking measureme ts. Nochanges in the absorption spectra were 0 servedat the end of the experiment. For fluorimet 'c titr­ations the solutions were excited at the wave­

lengths of isosbestic points which are 27 , 278,280, 292, 285 and 290 nm for MBO, MBO,MPBO, PBO, OMPBO, MMPBO and PBOrespectively. For quantum yield calculatio s, thewavelengths of excitation used were taken as theAmaxof the long wavelength absorption band.

The program used for Pariser-Par -Pople(PPP)I4.15calculations was obtained from CPE,Indiana University, Bloomington. Due to ack ofsufficient experimental data, molecular geo etriesof all these molecules were approximated s a re­gular hexagon bonded to a regular pentag n with

1154

a bond length of 1.395 A. The parameters re­quired for the calculations were taken from litera­tureI6.17.27a. The coordinate system chosen is asshown belo~ in structure {\. The relevant dataare compiled in Tables 1 and 2.

y

• X

(A)

The closed shell spectroscopic CNDO/S molec­ular program of DelBene and Jaffel6, as deve­loped by Ellis et al27b, was used for the study.The excited states were generated from the oc­cupied ground state and virtual orbitals throughthe application of configuration interaction proce­dure among the twenty lowest singly excitedstates. The excited state electron densities werecalculated using the equation,

where p ~ is the ground state electron densIty onatom A and Cjm is the coefficient of the mth con­figuration contributing to the jth state; C!!jand Cl1iare the orbital coefficients of the final and initialMOs involved in the mth configuration. CNDO/Scalculations have been carried out only on twomolecules, i.e., MBO and PBO as representativesof the class of molecules to which they belong.The relevant data have been compiled in Table 3.

Results and Discussion

Effect of solventsThe absorption and fluorescence spectral data

collected in Table 4 show that the spectra of BOand MBO are nearly similar, whereas the spectraof DMBO are red-shifted compared to that ofBO. The absorption as well as the fluorescencespectra of these molecules remain unaffected withthe change in polarity and proton donor ability ofthe solvents except the loss of vibrational struc­ture as observed in cyclohexane. The fluorescencequantum yields for BO, MBO, DMBO andMPBO are very small. Though the fluorescenceintensity decreases with increase in solvent polar­ity, the fluorescent quantum yield could not becalculated accurately because of the limitation of

"

DEY et aL: ABSORPTION & FWORESCENCE SPECfRA OF 2-ALKYL~ & 2-ARYL-BENZOXAZOLES

-Table 1 - Electronic transitions, ground and excited state charge densities at nitrogen and oxygen atoms of BO, MBO, DMBO, MPBOand BI

Compound

Transitions (nm)a(O)tElectron density

Obs.tt

Calc. NN*00*

BO

2772862191.1851.1921.8171.804270 263

254343

MHO

2772861811.2241.2111.8261.812271 264

259346

DMBO

2842902081.2251.2251.8261.817

278 258.6346

MPBO

2902771631.~41.2041.8251.830

279

269336255

260173254

345

HI

278278 1.4971.312271

243

PBI

317293.53501.3311.351

308

269.9241302.5

252.5278

295,291

t a(O)is the angle (in degree) of transition moment with the pOl'itive direction of X-axis.

tt In cyclohexane.

the instrument. The absorption (Table 5) and flu­orescence (Table 6) characteristics of OMPBO,MMPBO and PMPBO resemble those of PBO. In

PBO, the long wavelength absorption and fluor­escence bands are structured and more red-shift­ed than those in 2-alkylbenzoxazoles. On theother hand, the absorption and fluorescencespectra of BPPBO (Tables 5 and 6) are quite dif­ferent from those of PBO in the sense that the

long wavelength absorption band is highly red­shifted. All these arylbenzoxazoles exhibit a blueshift in the long wavelength absorption band withthe increase in polarity and hydrogen bond form­ing tendency of the solvents. Unlike absorptionspectra, the fluorescence spectra of 2-arylbenzox­azoles show a red shift with the increase in sol­

vent polarity. The fluorescence quantum yield of2-arylbenzoxazoles is very high (- 0.8) and it isstill higher in BPPBO ( - 1.0).

The molecular structure of benzoxazoles indi­

cates the possibility of three types of transitions,

viz., (i) n- Jt*, (ii) Jt - Jt* and (ill) charge transfer(CT). Although the fluorescence quantum yield of2-alkylbenzoxazoles is quite low, the large molarextinction coefficient (E), high fluorescence quan­tum yield of arylbenzoxazoles, small Stokes shift,and the less dependence of absorption and fluor­escence spectra on the solvent polarity clearlysuggest that the long wavelength transition in ben­zoxazoles is of Jt - Jt* type. Data in Tables 1-3 al­so clearly indicate that all the transitions involvedin the molecules studied are of Jt - Jt*.

It can be seen from the data of Table 3 that al­

though all the HOMOs involved in the longestwavelength transition are not rigorously localisedon the benzene ring, most of them are almost en­tirely confined to it. Similar is the case of LUMOinvolved in this transition. This also agrees withthe earlier results on BO which show that the

long wavelength transition is localised primarilyon benzene ring27,28. The only major differencebetween the PPP and CNDO/S calculations is the

1155

INDIAN JCEM, SEC A, DECEMBER 1990

Table 2 - Electronic transitions, ground and excite

state charge densities at nitrogen and oxygen atoms of PBO, OMPBO,MMPBO, PMPBO '~Compound

Transitions (nm)a(O)tElectron density

Obs.

Calc. NN*00*

PBO

313302.73531.1901.1901.8221.86

299

262.8285

291.5

259.3256

286.5OMPBO

3133073561.1991.1951.8221.86

299.5

268119

292.5258253

288.0MMPBO

3143023531.191.1921.8221.86

299.5

264.8333292.5

258.9264287.5

PMPBO

315306.83531.1951.2041.8231.856

301

261260294

24388289

BPPBO

326347.83540.8690.9871.8221.856

287272.9244

245.9146

Table 3 - Electronic transitions, ground and excited

tate charge densities of nitrogen and oxygen atoms of MBO and PBOc

culated by CNDO/S

Molecule

OrbitalAa(O) Charge densityIndex of atoms on which MO's

promotion(nm) are localized

SostateSI state

Initial MOFinaiMO

0,N301N}

:n-:n*

288344-0.16-0.233 -0.130-0.1971,4,5,7,82,3,4,6,7

MBO

j[ -:n*258352 2,3,4,6,8,92,3,4,6,7

Jt -n*223310 2,3,4,6,8,94,5,7,8

1t -n*

320355-0.114-0.222-0.079- 0.2632,3,4,6,8,92,3,10,11,13,15

]t- :n*

283340 II ,12,14, 152,3,10,11,13,15PBO 1t -n*

27596 1,3,4,5,6,7,82,3,10,11,13,15

1t-1t*

24273.4 2,3,4,6,8,9delocalized

direction of transition moments in 2-alkylb nzox­azoles. In the former case it is towards th ben­zene ring, whereas in the latter case it is t wardsthe oxazole ring. The difference could be b causeof the inclusion of sigma orbitals in CNDO S cal­culations; otherwise, this difference is diffi ult toexplain. The similar spectral behaviour of Band

1156

MBO and the red shift observed for DMBO andMPBO compared to BO also suggest that thistransition is localized on the benzene ring. Thisview is further supported by the vibrational struc­ture observed in the vapour phase fluorescencespectra of BO, benzimidazole (BI) and benzothia­zole (BTJ28. A similar structure is also observed

"

DEY et af.: ABSORPTION & FLUORESCENCE SPECTRA OF 2-ALKYL- & 2-ARYL-BENZOXAZOLES

Table 4 - Absorption maxima [A./nm (log Ea]and fluorescence maxima [A/nm (+1)]of benzoxazoles at 298

Solvent

BOMBODMBOMPBO

Aa(lOgE)

A~+I)Aa(logE)A~+I)Aa(logE)A~+I)Aa(logE)A~+I)

Cyclohexane

277 (3.49)295277 (3.93)295284 (3.63)300290 320- -270 (3.50)

287271 (3.92)2R7278 (3.61)293279 (3.60)310-263 (3.33)

(0.001)26.4sh(0.001)274(0.004)255(0.065)

230 (3.84)

232 (4.30)234 (3.98)228 (4.48)

Dioxane

276.5 (3.19)295277 (3.89)290284 (3.63)304290 320-270 (3.24)

287270 (3.89) 278 (3.62)293279 (3.80)310-263 (3.09)

(0.001)265sh(0.001)-- (0.003)255(0.064)

231 (3.58)

232 (4.29)234 (4.04)229 (4.59)

Acetonitrile

275.5 (329}295276 (3.90)290284 (3.62)293290 320-269 (3.32)

287270 (3.89) 277 (3.62)(0.002)279 (3.71)310

263 (3.17)

(0.001)264sh(o.om)- 255(0.063)

I229 (3.71)231 (429)234 (3.99)228 (4.50)

\

Methanol275.5 (3.46)295215 (3.97)295284 (3.64)300290 320

269 (3.49)

287269 (4.00)(0.001)277 (3.64)(0.001)279 (3.69)310

262.5 (3.34)

(0.001)263sh -- 255(0;064)

230 (3.86)

229 (4.20)233 (3.99)227 (4.46)

Water(~6)

215 (3.38)275295283 305290 320- --268.5 (3.45)

-269 276(0.0002)279(0.029)

262 (3.31)

--255

230 (3.83)

230233221

Monocation ~)-3 280

420274 (3.60)410276 (3.67)420290 (3.85)470- ---232

295267 (3.69)300- -330-237

sh 240sb 308236 (4.41)

216

225 (3.70)

H~-lOA

- 268410271 (3.68)420283 (4.15)410

228

228 (3.69)

242 (4.41)

1157,

--'

'I

DEY et aL: ABSORPTION & FWORESCENCE SPECfRA OF 2-ALKYL- & 2-ARYL-BENZOXAZOLES.

Table 6 - Fluorescence nlaxima (~, nrn) and quantum yields (+1) of 2-arylbenzoxazoles in different solvents and at various acid

concentrationsSolvent/Species

PBOOMPBOMMPBOPMPBOBFPBO

~

+1~+1~+1~+1~+1

Cyc10hexane

3170.813170.603180.903170.903600.98

333

333332337380- ---348

347350348400

366

360365364420

Dioxane

3200.803250.613200.913220.953700.74

335

337337337385-350

350350350400

365

365430

Acetonitrile

3200.793250.583200.873200.903900.80

335

340335338- --350

350350350

Methanol

3220.783400.603200.883200.953900.79

335

337337- -350

350350-365

Water

Monocation

Ho-1O.4

360

385

390

0.78

1.00

365

390

390

0.54

1.00

360

390

390

0.86

0.76

345

360

390

390

0.92

0.54

385

410

425

460

430

in the spectra of 2-alkylbenzoxazoles in nonpolarsoivents. The shorter wavelength band in alkyl­benzoxazoles is also of 1[ -+ 1[* type bu~it is local­ized on the oxazole ring. The spectral data and al­so our theoretical calculations (Table 1) indicatethat the. direction of polarization of the longwavelength band is towards the benzene ringwhereas that of shorter wavelength band is tow­ards the oxazole ring. The large Stokes shift(2220 cm- 1) observed in MPBO compared tothat in MHO and DMBO could be due to the in-

creased conjugation in the excited state (51) as aresult of rotation of the phenyl ring at 5~position.A similar behaviour is observed in biphenyF9 and2-phenylnaphthalene3o also.

The absorption spectral characteristics of PBOindicate that the long wavelength transition is of1[ -+ 1[* type. The results of Brocklehurst31 andNurmulkchametov et al.8d on 2-biphenylbenzoxa­zole and other compounds confirm this explana­tion. Similar to those of alkylbenzoxazoles, all thetransition have 1[ -+ 1[* character (Tables 2 and 3).

1159

I

~

INDIAN J CIIIEM, SEC A, DECEMBER 1990

II

<?NLf\ /\4---0/~- - - -@

Effect of acid concentrations: Ground stateThe absorption spectra of all··the molecules re­

main unchanged in the pH/H_ range of 2 to 16.At higher acid concentrations, a smaIl blue shift(of the order of 2-3 nm) is observed in the longwavelength absorption band but a major change isnoticed in the 232 nm band of MBO and DMBO.

plains partly the above explanation. The followingcanonical structure (II) can also be obtained andthe presence of structures (II, III) adds to the ri­gidity of molecule in the 51 state.

The blue shift dbserved in the long wavelengthbands of absorption spectra of 2~arylbenzoxazolesin hydrogen bond forming solvents may be due tothe partial loss of conjugation of phenyl or biphe­nyl ring with the BO moiety as a result of rotation.of the phenyl or biphenyl group either at the ni­trogen or oxygen centre of the oxazole ring, in­duced by the interaction with polar solvent mole­cules. But in the excited state, the charge migra­tion is completely delocalized over the wholemolecule. This will increase the bond order of theinterannular bond and thus reduce the rotation of

the biphenyl or phenyl rings or increase the con­jugation in 51 state. The interaction with the sol­vents is not enough to rotate the phenyl ring. Thisexplains the red shift in the fluorescence spectraof these molecules in polar and hydrogen bondforming solvents. The greater sensitivity of fluor­escence spectrum towards red shift in BPPBO indifferent polar solvents could be due to the ca­nonical structure (II or III) in which the dipolemoment is increased in the excited state .. The

larger charge displacement (transition dipole mo­ment) in absorption and emission incr~ases thetransition probability which in turn decreases theradiative life time of the excited state. This is the

reason for the high fluorescence quantum yieldsin 2-arylbenzoxazoles.

-

The major contributions of HOMO and UMOinvolved in the longest wavelength transit on areonly one each. Unlike MBO, the localiz ion ofthe HOMO and LUMO at various atomic entresare distributed, but still the HOMO is p 'marilylocalised on BO moiety whereas the L MO islocalised on the phenyl ring. The polariz tion ofthe first transition, calculated by PP andCNDO/S methods is also in the same di ection.This indicates that in 2-aryl-substituted b nzoxa­zoles, unlike that in 2-alkylbenzoxazoles, t e longwavelength transition is localised on t e arylgroup and is perturbed by the BO ring. It is alsobelieved that the Jt -+ Jt* transition which i local­

ized on the benzene ring is hidden un er thebroad band of 2-arylbenzoxazoles. The bsorp­tion and fluorescence spectra of BPPBO r semblethose of 2-biphenylbenzoxazoles (BPB )8d. Inother words, no effect of phenyl group t posi­tion-6 of benzene ring is observed on th spec­trum of BPBO.

However, in the case of MPBO the phenylgroup does influence the spectral charac eristicsof MBO. This is because in MBO the Ion wave­length transition is localized on the benze e ringwhereas in BPBO the long wavelength and islocalized on the biphenyl moiety. Furth r, thestructured absorption and fluorescence b nd ofBPO indicates the phenyl ring to be coplan r withthe oxazole moiety in both 50 and 51 stat s. Ourtheoretical calculation (Table 2) also supp rts theabove explanation. A similar kind of obse ationis noticed in the cases of 1-fluoro-2-phen lnaph­thalene32, 2-phenylbenzimidazoleI3, 2-ben imida­zole carboxylic acid33 and bibenzimidazole 4a also.The same explanation can be offered in t e case

. of BPPBO, i.e., the phenyl ring at 6-positi nandthe biphenyl ring at 2-position attain cop aritywith the BO system in the 51 state. The reaterStokes shift (2700 em -1) observed for BPP 0 ex-

M ) < )G)

III

1160

" 11'111' 1,IIIIUt II. 1~;1 II

DEY et al.: ABSORPTION & FLUORESCENCE SPECTRA OF 2~ALKYL- & 2-ARYL-BENZOXAZOLES

abs. flu.

2.39

1.17

0.78

3.30 3.90

3.78

3.80 4.5

4.40 4.6

6.85"

Compound pK"

MBO

0.5

DMBO

0.5

MPBO

-1.2

PBO

0.0

OMPBO

0.1

MMPBO

0.0

PMPBO

0.1

BPPBO

abs. and flu. = Forster cycle method using absorption and flu­

orescence data respectively.

a = l1pK" = pK" * - pK".

Table 7 - pK" values for the monocation-neutral equilibrium ofvarious benzoxazoles in 5" and SI states

pKa*

drawing group decreases pKa. Further, the pKa'sof 2-arylbenzoxazoles are much lower than that of2-phenylbenzimidazole. The charge densities(Tables 1, 2 and 3) clearly indicate that the chargedensity on the tertiary N-atom in PBO is less thanthat in 2-alkylbenzoxazoles and 2-phenyl-benzi­midazole.

The spectral changes observed in the monoc­ations of the above mentioned molecules are due

to the protonation at the tertiary nitrogen atom.The absorption characteristics of the monocationsof alkylbenzoxazoles confirm our earlier assign­ment of spectral bands, i.e., the long wavelengthband in MBO, DMBO and MPBO is localised onthe benzene-ring and is long axis-polarized where-

The 232 nm band is replaced by two bands; oneat - 239 nm and the other at - 222 nm. The for­

mer appears as a shoulder to the latter. The be­haviour of MPBO is different from that of MBOand DMBO at low pH region. At pH - 3, the 255nm shoulder develops into a broad band and theband system at 290 nm merges with it. At Ho - 0,the 255 nm band disappears and again two bandsdevelop at 236 and 290 nm. No further change isobserved in the absorption spectra of these alkyl­benzoxazoles except in the case of MPBO wherethe long wavelength band is blue shifted at Ho­- 9.0. The absorption spectra of the monocationsof PBO, OMPBO, MMPBO and PMPBO are closelysimilar and exhibit a red shift in the long wave­length band alongwith a new band system at 248nm. Since BO gets hydrolysed and BPPBO is in­soluble in aqueous media, the monocation-neutralequilibria could not be studied in these cases. Butat Ho = - 9, BPPBO becomes soluble· and thelong wavelength absorption band is blue shifted.The absorption spectral characteristics of neutraland monocations of BO, MHO, DMBO andMPBO are listed in Table 4 whereas those ofPBO, OMPBO, MMPBO, PMPBO and BPPBOare compiled in Table 5. The absorption spectraof various proto tropic species of BPPBO areshown in Fig. 1.

The pKa values of monocation-neutral equilib­ria of these molecules determined spectrophotom­etrically34b are listed in Table 7. The pKa of MBOagrees with the literature value. If the change ob­served in the absorption spectrum of MPBO atpH - 3 is due to prototropic reaction, the firstpKa is at 2.0 and the second one is at - 1.2. ThepKa values of 2-arylbenzoxazoles are consistentwith the fact that the presence of electron with-

0·3

. 0·2'"~~jc( 0.1

225 275 325A(nml

241

560 600

Fig. 1 - Absorption and fluorescence spectra of various pmtotropic species of BFPBO at 298 K [1, neutral pH 6.1; 2, mono­cation Ho-3; 3, ~)-10.4

1161

INDIAN J Cl-lEM, SEC A, DECEMBER 1990

as the short wavelength band is localised on theoxazole ring and it is short axis-polaris d, Thelone pair orbital of the nitrogen atom, eing inthe plane of ring, cannot have resonance nterac­tion with the Jt cloud of the molecule but an pt::r­turb weakly the transition localised on t e ben­zene ring through inductive effect. Conse uently,the long wavelength band is expected to b affect­ed slightly when tertiary nitrogen atom i proto­nated. This explains the blue shift in t e longwavelength band of alkylbenzoxazoles. n theother hand, the short wavelength band will belargely affected by protonation of the ter iary ni­trogen. Similar behaviour has been obs rved inbenzimidazolesl1 and phenanthro[9,1 ]imida­zoles3:i also. On the other hand, if the Ion wave­length band in PBO or BPPBO had bee local­ized on the benzene ring, one would have expect­ed a blue shift, as is normally observed in 2-alkyl­benzoxazoles. The opposite trend observ d con­firms our earlier assignment of transitions in aryl­benzoxazoles. The small blue shift in t e longwavelength band of BPPBO may be du to theformation of dication through protonatio of thephenyl ring at 6"position which prevents onjuga­tion with the benzoxazole moiety.

The absorption spectrum of MPBO is verycomplicated and it may contain the feat res ofboth BO and the phenyl ring. It can be sp culatedthat the long wavelength doublet (290 , 279nm) and the 227 nm band may be due to the 130band system and the shoulder at 255 nm m(\y bedue to the phenyl group at 5-position. The in­crease in intensity of 255 nm band may e due tothe intensity borrowing from the strong nd sys­tems of BO. The' first change. observed a pH- 2

in the spectrum ofMPBO cannot be as 'gned toany prototropic. reactions. This may b due tosome complex formation between the s lute andsolvent, whereas the second change may e tue toprotonation at the tertiary nitrogen at m. Thisconclusion is based on the following onsider­ations. (i) The pKa for the protonation 0 BO andMBO is - 0.1 and 0.5 respectively. The presenceof electron withdrawing phenyl group sh uld dec­rease the pKa' If the first change is due 0,. proto­tropic equilibrium we should have obs rVedqnopposite trend. (ii) pKa ( -1.2) for sec6nchangeagrees with the pKa of MBO. (iii) Like BO artd.DMBO, dual fluorescence (see later) is observedat Ho - - 1 and similar explanation can be givenas indicated earlier. (iv) Nearly ground tate pKais observed from fluorimetric titration a noticedfor MBO and DMBO. (v) A similar kin of com-

1162

Table 8 - Fluorescence maxima (Vnm) of the monocation ofMPBO

Solvent Am •• (nm)

CT

:rt* ...• :rt

Cyclohexane

420325Acetonitrile

450328Methanol

450330Water

470330

plex formation was reported in the case of diary­loxazoles36 in the acidic solutions.

Excited singlet stateThe prototropic reactions taking place in the

excited singlet state of MBO, DMBO and MPBOare the same as observed in 50 state. But a dualfluorescence appears at pH - 1 of which the lon­gest wavelength band is very broad. The excita­tion spectra recorded at both the fluorescencebands resemble the absorption spectrum of themonocation. The fluorescence intensity of MPBOis quenched in the pH range 4 to 2 without ap­pearance of a new band. Dual fluorescence is ob­served at Ho - 0 in this case. At very high acidconcentration, a blue shift is noticed in the fluor­escence spectrum of MPBO monocation and thenormal Stokes shifted band. disappears.

Unlike alkyl BOs, monocations of 2-arylben­zoxazoles exhibit only one fluorescence band. Nochange observed in the fluorescence spectra ofmonocations at higher acid concentrations is con7sistent with the ground state behaviour except inthe spectrum of BPPBO ..In the case of BPPBO asmall blue shift is noticed at Ho - lOA in the ab­sorption speetrum. The fluprescence spectral dataof BO, MBO, DMBO and MPBO are collected inTable 5, whereas those of PBO, OMPBO,MMPBO and ·PMPBO are listed in Table 6. Thefluorescence spectra of various species of BPPBOare depicted in Fig. 1.

The pKa values calculated using Forster cyclemythod37 as well as those determined by fluori­metric titrations arc collected in T~ble 7. The flu­orimetric ti1ratioIlS have given o~y ground statevalucs indicating that the prototrOpic equilibriumis not established .in the 51 s~aie. This indicatesthat the radiative, decay' rate$of these moleculesare faster than the rate ofprotbnation, whereas thespectral data clearly iI)dicate that the tertiarynitrogen atom becomes stronger base in the 51state. Further, the agreement between the pKa*values obtained by absorption and. fluorescencedata is not bad. This is because the geometries

'IIi I I~I "11'1111

DEY et aZ.: ABSORPTION & FWORESCENCE SPECTRA OF 2-ALKYlr & 2-ARYL-BENZOXAZOLES

of the molecules are not different in the 50 and 51

states and have nearly the same solvent relax­ations in these states for the conjugate acid-basepair.

Dual fluorescence band systems are assigned tothe formation of monocations. The broad largeStokes-shifted band is assigned to the chargetransfer (CT) transition and the normal Stokesshifted band to the 3t* -- 3t transition. The broad­ness of the long wavelength band is. due to theloss of vibrational quantization as a result of nuc­lear adjustment in the CT state. The main reasonfor the stabilization of this CT transition is themigration of charge from the homocyclic ring tothe heterocyclic ring when the tertiary nitrogen at­om is protonated. This is further supported by thefact that the large Stokes shifted band in m0noca- .tion of MPBO is stabilized in more polar solvents(Table 8), while the short wavelength fluorescenceband is insensitive to the solvent polarity. The ex­istence of multiplicity of first excited singlet state(51) has been proposed earlier to explain the flu­orescence properties of benzimidazole monoc­ations10•11• The absence of dual fluorescence in

PBO and other 2-arylbenzoxazoles is due tothe fact that the positive charge on the protonatednitrogen atom of the corresponding monocationsis not localised on the nitrogen atom alone but isdelocalized over the whole molecule. Similar resu­lts were obtained for 2-arylbenzimidazoleslo.13.38.39also.

ConclusionsThe following conclusions can be drawn from

the above studies:

(i) The long wavelength transition in neutralBOs is of 3t -- 3t* type. It is localized on the ben­zene-ring in case of 2-alkylbenzoxazoles and onthe phenyVbiphenyl ring in case of 2-arylbenzoxa­zoles. In all these molecules the long wavelengthtransition is polarized along the long axis of themolecule, whereas the short wavelength band inalkylbenzoxazoles is short axis-polarized and lo­calized on the oxazole ring.

(il) Methyl group at the 2-position.of BO has noeffect on its electronic absorption spectrum (longwavelength band), but a red shift is observedwhen the substituent is in the benzene ring.

(ill) Monocations formed by the protonation oftertiary nitrogen atom of alkylbenzoxazoles exhib­it dual fluorescence of which the long wavelengthband is due to CT transition and the short wave­

length band is the normal 3t* -- 3t transition.

(iv) Phenyl group present either at 2-positionor at 5- or 6-position of the benzene ring reducesthe pKa value by extending the conjugation.

(v) The geometry of the 2-arylbenzoxazoles inthe 51 state is not much different from that in 50state.

(vi) A good correlation between the change inpKa values with change in the substituent, with ex­citation and the charge density at the nitrogen at­om has been observed. Also, good agreement ofcalculated transition energies with the observedvalues proves (a) validity of assumptions madeand (b) suitability of the parameters used in PPPcalculation for benzoxazoles.

(vii) The spectral behaviour of benzoxazolesand benzimidazoles is more or less similar.

References1 Lang T J, Sam C S C & Kandathil A J, J Arn chern Soc,

82 (1960) 4462.2 Domino E F, lIOlla & Kerwin K, JPharmacoZ exptZ The,..

ap,105(1952)486.3 (a) Dunwell D W & Evans D, J rnednZ Chern, 20 (1977)

169.(b) Dunwell D W, Evans D & Hicks T A, J rnednZ Chern,18 (1975) 52.(c) Evans D, Smith C E & Williamson W R N, J rnednZChern, 20 (1977) 169.(d) McNeil F C, Smith R T, Kron K M, Peak W P & Her­man 1 F, J Arn rned Assoc, 160 (1965) 40 and referenceslisted therein.

4 (a) Rulliene C & Joussat-Dublen J, OpZt Cornrn, 24(1978)38.(b) Bazyl 0 K, Gruzinskie V W, Danilova V T, HopylovaT N & Marer G V, OpZt Spectroso, 48 (1980) 147.

5 (a) Durneis J, Karuas M & Mamuasek, Coli Czech ChernCornrnun. 38 (1973) 215.(b) Betin 0 I, Nurumukharnetov R N, Shigonon D N &Chenova Nt, DokZ Akad Nauk SSSR, 227 (1976) 126.(c) Williams D L & Heller A, J phys Chern, 74 (1970)4473.

(d) Mordzinski A & Grabowsha, Chern Phys Lett, 90(1982) 122.(e) Woolfe G J, Melzig M, Schaneider S & Dorr F, ChernPhys, 77 (1983) 213.

6 Passerini R, J chern Soc, (1954) 2256.7 Reiser A, Karvas M, Saunders D, Mijovic M J, Bright A

& Bogie J, J Arn chern Soc, 94 (1972) 2414.8 (a) Durmis G, Karvas M & Mansek Z, Coli Czech Chern

Cornrnun, 38 (.1973) 224.(b) DiLanardo G, Trombetti A & Zalini C, J chern Soc(B), (1968) 756.(c) RoussiheJ & Parllous N, J chirn Phys, 80 (1983) 595.(d) Nurmukhametov, Cobov G V & Physhikine L N, RussJ phys Chern, 41 (1967) 831.

9 Fery-Forgues S & Paillous N, J org Chern, 57 (1986) 672.10 Tway P C & Clinelove L J, J phys Chern, 86 (1982)

5523,5227.11 (a) Krishnamurthy M, Phaniraj P & Dogra S K, J chern

Sac Perkin Trans 2, (1986) 1917.(b) Sinha H K & Dogra S K, J chern Sac Perkin Trans 2,(1987) 1465 and references mentioned therein.

11'63

INDIAN J CHEM, SEC A, DECEMBER 1990

12 Konda M & Kuwano H. Bull chern Soc lpn, 4~ (1969)1433.

13 Mishra A K & Dogra S K, Spectrochim Acta, 39~ (1983)609.

14 Pariser R & Parr R G, 1 chern Phys, 21 ( 1968) 4050.15 Pople 1 A. Trans Faraday Soc, 49 (1953) 1315.16 Delbene J A & Jaffe H H, 1 chern Phys, 48 (196$) 1807,

4050.

17 Hinze 1 & 1affe H H. lAm chern Soc, 84 (1962) 540

18 Kamia M. Bull chern Soc lpn, 43 (1970) 3344.19 Hein D W, Alheim R 1 & Leavitt 11, 1 Am chern ISoc, 79

(1957)427.

20 Riddick M 1 & Bunger W, Organic solvents (Wild lnters­cience, New York) 1970, pp. 632. 695, 801.

21 Jorgenson M 1 & Hartter D R, 1 Am chern $oc, 85(1963)878.

22 Yagil G, 1 phys Chern. 71 (1967) 1034.

23 Swaminathan M & Dogra S K, Indian 1 Chefrt, 22A(1983) 853.

24 Parker C A, Photoluminescence of solutions with I applic­ations to photochemistry and analytical chemistry (Elsev­ier, Amsterdam) 1965, pp. 261-26S.

25 Meech S R & Phillips, 1 Photochem, 23 (1983) 183.26 Jackson P F, Morgan K J & Turner A M, 1 ch~m Sac

Perkin 2. (1972) 1582.27 (a) Matage W & Nishimoto, Zphys Chern, 13 (1957)1140.

1164

(b) Ellis R, Kuchnlenz G & Jaffe H H, Theoret Chim AC'­ta, 26 (1972) 1842.

28 Gorden R D & Yang R F, Can 1 Chern, 48 (1970) 1722.29 Lim E C & Li Y, 1 chern Phys, 52 (1970) 6416; 57

(1972) 5017.

30 Hughes E, Wharton J & Nauman R, 1 phys Chern, 75(1971)3097.

31 BrocklehuestP, Tetrahedron, 18(1962)299.

32 Hollaway E, Nauman R & Whorton J, 1 phys Chem, 72(1968) 4468.

33 Sinha H K & Dogra S K, Bull chern Soc lpn (in press).34 (a) Kumar Awadesh & Dogra S K, Can 1 Chem, 67

(1989) 1200.(b) Paul M A & Long F A, Chern Rev, 57 (1957) 1.

35 Swaminathan M & Dogra S K, 1. chern Soc Perkin Trans2,(1983) 1641.

36 Druzhinn S T, Kreshialchov S A, Troyanovasky J M &Uzhinov B M, Chern Phys, 116 (1987) 231.

37 Forster Th, Z Elektrochem, 54 (1950) 537.38 (a) Mishh A K & Dogra S K, 1 Photochem, 29 (1985)

435.

(b) Mishra A K & Dogra S K, Indian 1 Phys, 58B (1984)400; Indian 1 Chern, 24A (1985) 364; Bull chern Soc Jpn,5S (1985) 3387; Indian J Chern, 24A (1985) 615; JPhotochem, 31 (19S5) 333.

39 Sinha H K & Dogra S K, Chern Phys, 102 (1986) 337; 1Photochem, 36 (1986) 149.

"