Indian Journal of Chemistry Vol. 4OB, September 2001 , pp. 850-853

Note

Naphthalene derivatives as fluorescent probe

Tarasankar Pal* , Tarun K Ghosh & Anjali Palt

Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India

E-mail: tpal @chem.iitkgp.ernet.in

Received J 8 January 2000; accepted (revised) J 6 August 2000

The electronic and fluorescence spectral properties of several synthesized naphthalene derivatives have been examined in acetone and in an anionic micelle. The change in fluorescence emission process of several naphthalene compounds have been compared with their parent moiety after hydroxylation, acetylation and with the increase in their skeletal rigidity. This report supplements the first hand information to choose naphthalene compounds as probe molecules in organic solvents as well as organized medium.

Fluorescence probe (FP) techniques are becoming increasingly popular for studying the micro­environmental properties 1 . Sometimes dye molecules permit their use at low concentration as probe2 in micellar media due to their high molar extinction coefficient and the absorption study becomes useful in this regard. Perturbation of micellar structure by the dye molecules is well documented in the literature3• Already we have ascertained incorporation of several hydroxylated compounds in micelle2 and noticed the shift of acid base equilibria from the absorption studies. Such observation has become useful in studying micro-environmental properties of micelles and bio-membranes. Since micelles are known to be the membrane mimetic agents, these hydroxylated compounds, as well, can act as useful probes in biological systems. Three different types of micelles were chosen for fluorescence studies. Also while studying the suitability of several naphthalene derivatives as their possible use as fluorescence probe4, a simple cationic surfactant, CTAB (cetyltrimethyl ammonium bromide) was observed to quench the intrinsic fluorescence due to heavy atom effect5.6• And nonionic surfactant TX- IOO (poly oxyethylene isooctyl phenyl ether) was not used for the possible incorporation of water molecules in the micellar core7•8 which caused fluorescence quenching. Moreover, the compounds having naphthalene moiety

tDepartment of Civil Engineering

find application in a new field to design molecular devices4• So it would be important to report a few smaller naphthalene derivatives with intnnslC fluorescence for their use as fluorescence probe in micellar environment. The possibility of any or little perturbation of the micellar structure would not be there due to the incorporation of the small molecules in micelle3• As fluorescence is more sensitive than absorption phenomenon, we tried to study the fluorescence characteristic of several naphthalene derivatives with inherent fluorescence in suitable micelle.

Experimental Section Compounds were prepared (2-4, 6 and 7)5/ obtained

(1 and 5, BDH, India) and purified from methanol. Their purity was checked from mp, co-TLC, IR and Rf values before use. Concentrations of all naphthalene derivatives used were in the range of 1 0-4_ 1 0-5 mol dm-3• Sodium dodecyl sulfate (SOS) and the AR grade solvents (methanol, ethanol and acetone etc.) were purchased from Aldrich Chemical Co. and Hayman Ltd. England respectively. Double distilled water was used throughout. Fluorescence spectral measurements were done with a luminescence spectrofluorimeter (Perkin-Elmer LS 50B). All absorption spectra were measured with a Shimadzu UV- I 60 digital spectrophotometer with a quartz cell having I -cm path length.

Results and Discussion

With a view to finding a few suitable fluorescence probes, we have selected and synthesized several phenolic compounds, their coupled products and the corresponding acetylated derivatives5• Finally, we have examined their fluorescence characteristics both in organic solvents and in micellar environment. Anionic micelles have already shown promise in this regard2

• Fluorescence emissions of some naphthalene derivatives were found to increase through hydroxylation, acetylation and even by increasing rigidity of the molecules by increasing ring numbers. It is interesting to note that further enhancement of fluorescence intensity occurs sometimes in micellar medium for the phenolic compounds. In the present study, seven closely related naphthalene derivatives

NOTES 85 1

Table I-Spectral characteristics of the compounds

Compd

2-Naphthol 1

2-Naphthyl-acetate 2 1 , 1 '-Bis(2-hydroxynaphthalene) 3

1 , 1'-Bis(2-acetoxy naphthalene) 4 2,7-Dihydroxy naphthalene 5 2,7-Diacetoxy naphthalene 6 1 ,6,7, 12-Perylenetetraacetate 7

Values in ( ) indicate fluorescence intensity

A..:x in acetone (nm)

320 340 440 460

345 400 394

have been chosen for the fluorescence studies in acetone and in aqueous anionic micelle (Table I).

Three types of surfactants above their critical micellar concentrations (cmc) were considered for fluorescence studies. The suitability of all seven naphthalene derivatives as their possible use as fluorescence probe were studied in the presence of simple cationic surfactant CTAB (cetyltrimethyl ammonium bromide) and CT AB was observed to quench the intrinsic fluorescence due to heavy atom (i.e., Bf) effectS, and hence not used. Nonionic surfactant TX- lOO (polyoxyethylene isooctyl phenyl ether) was not used as it caused fluorescence quenching presumably due to the incorporation of water in the micellar cores. Moreover, deep penetration of all these compounds within the anionic surfactant core is more probablel •9 that caused the shift of the absorption maxima to a higher wavelength regionlO to a significant extent. Information from the earlier report2• 10 helped us to use SDS (sodium dodecyl sulfate) as the anionic surfactant for all purposes and observed the red shift of the emission maximum with the enhancement in the fluorescence intensity. All compounds under consideration could exhibit 1t-1t* and n- 1t* transitions. Polar solvents can bring about a change in the order of the energy levels, making 1t-1t* state as the lowest excited level with the subsequent appearance of fluorescence. In polar solvents, non-bonding electrons are stabilized in comparison to less polar solvents and hence the absorption band is blue shifted for n-1t* transition. The effect of polar solvent is not that pronounced for the 1t-1t* transition as the 1t* level is stabilized in this case and a red shift is observed. This is due to the anti bonding character of 1t* orbital. This fact was evidenced after the incorporation of the compounds in anionic micelle. Anionic micelles can cause solubilization of the compounds in the micellar Stem

A..:m in acetone A..:x in O.OIM SDS A..:m in O.OIM SDS (nm) (nm) (nm)

365 (0.9 ) 340 450(65 ) 370(245 ) 295 340(230 ) 480 (28 ) 320 365( 150)

5 17 ( 1 5 ) 280 350 (6 15 )

385 ( 30 ) 335 360 (30) 453 (90 ) 280 342(420) 452 ( 17 ) 394 445 ( 880)

oorOH . o 0 Acetylation •

1 AcjJ/PY

oorOAC

2 l Ag gelatin complex

Acetylation • Ac lOOloH

OOOH AcfJlPy

�o

O OOAC

3 4 Scheme I-Coupling of 2-naphthol

H[OOr0H H[OOr0AC o 0

Acetylation • 0 0

5 AcP/Py 6

1 HgO

H Acetylation Ac • ACP/Py A

Scheme lI-Coupling of 2,7-dihydroxy naphthalene

layer and work as a true substitute to polar solvents. The enhancement of the fluorescence intensity was observed keeping the same excitation energy and concentration. So the question of concentration effect does not arise. Phenol coupling (formation of 'C-C' bond) results in an increase in the number of rings (Schemes I and II) which can cause a change in Aex energy. This is sometimes due to the increase in the extension of the 1t conjugation or increase in the number of electron donating groups like -OH, -OAc etc. in the molecule. Hydroxylation of the naphthalene

852 INDIAN J CHEM. SEC. B. SEPTEMBER 2001

10

2 '""" tJ '2 ::I

� :0 � 0 ';;; c B ,: 5 � u c � u '" � 0 ::l

u:

Wavelength (nm)

Figure I-Fluorescence spectra in acetone

1 0 7

'""" tJ '2 ::I

� � 0 ';;; c B 5 ,: � u c � u '" f! 0 ::I u:

Wavelength (nm)

Figure 2-Fluorescence spectra in SDS micelle

NOTES 853

moiety caused a red shift in the absorption profile. In all these cases, acetylation of the parent compound leads to a red shift to an extent by 20-50 nm when luminescence was examined in ethanol. Acetylation affects both the ground as well as the excited states of the molecules I I as shown in Figures 1 and 2.

In anionic micelle, blue shift has been observed which is associated with n- n* transition and spectral intensity increased by 4-25 times for all the compounds indicating hydrophobicity related environmental effect I 2. The shift in the Aem in SDS micelle with enhanced fluorescence intensity suggests that these molecules reside in the palisade region. The anionic micelle solubilizes the compounds in the core without any change of micellar structure as the salt concentration is negligible. But for 7, no shift in excitation energy even after the incorporation in SDS indicated ineffective interaction with SDS micelle possibly due to its inherited skeletal rigidity. However, the molecule becomes highly fluorescent after its incorporation in micelle, presumably due to solubilization of the compound 7 in SDS 13. This study reveals that the most rigid molecule 7 that might be useful to be used as a probe in a suitable organic solvent. Other three molecules 2, 5 and 6 can well serve the same purpose with less promise. But molecules 1 , 2 and 4 would be very promising fluorescent probes in anionic micellar medium.

Conclusion The fluorescence intensity (FI) of the phenolic

compounds increases while incorporated in anionic

micelle if they are excited with the excitation energy of the phenolic compounds dissolved in acetone. But the fluorescence intensity increases further when phenolic compounds receive their proper excitation energy after their incorporation in micelle. However, the acetates show a decrease in fluorescence intensity in the micelle while excited in the excitation energy of the acetate dissolved in acetone. With the proper excitation energy the micelle bound acetate showed FI enhancement. Hence in all the cases different excitation wavelength was used for significant fluorescence signal.

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1 27.