FURTHER STUDIES ON PHOTOINDUCED INTRAMOLECULAR ELECTRON TRANSFER OF SUBSTITUTED BENZOATE ESTERS OF 9-ANTHRACENE METHANOL

The role of'ihe solvent and the substituents on the chromophore arc important in the efficiencies of photoinduced intramolecular electron transfer (PIET) processes. In tlus study estcrs of 9-antlxncene methanol wew syntl~esized and characterized. The fluorescence quantum yields of these esters were measured and electron transfer r ates estimntecl. The estimated electron transfer rate constants were correlated linearly wit11 a values. The fluorescence quantum yields were also measured in two different micellar solutions in order to investigate tile behavior of PIET in organized mcdia. The values of the reaction constant, p, for tl~e photoinduced electron transfer were calculated as 3.28 (r = 0.98), 3.42 (r = 0.96) and 2.41 (r = 0.99) in methanol, sodium dodecyl sulphate (SDS) and Triton X-100 (TX-100j respectively. This variation is explained by considering tlle microviscositj~ and micropolarity of the interior of the micelle systems. micellar eRccd.


INTRODUCTION
Photoinduced intramolecular electron-transfer (PIET) is a complex phenomenon.It is strongly influenced by the molecular structure, nature and length of the chain of the molecules, solvent polarity, viscosity and temperature1.Several valence-linked bichromophoric systems have previously been reported to exhibit fluorescence quenching.The studies done by Pjncoclc and De Costa%ave shown t h a t two chromophores connected by -CH,-0-(C=O)-group shows lower fluorescence quantum yields.Recently we r e p o r t e d q h e fluorescence quenching of fjve substituted 9-anthracene methyl benzoate esters, 2a-e, (Fig~zre 1).PIET explains the quenching process from the excited anthracene chromophore to the benzoate chromophore.This explanation was supported by the estimated values of free energies and solvent effects of electron transfer processes.We observed that electron-withdrawing groups attached to the benzoate chromophore enhance the electron transfer rates giving less fluorescence.On the other hand electrondonating groups give the opposite effect.
In this paper, the results of an investigation of the intramolecular electron transfer of eight more compounds is included in addition to the excited state life time of some of the compound.^.The fluorescence quantum yields were also M.D.l? de Costa and K.A.D. Sriyani Mallika measured in two different micellar solutions in order to investigate the behavior of PIET in organized media.The impact of organized media on the kinetics of chemical reactions is widely recognized4.In particular, excited state behavior in a micellar environment has drawn much attention5.This is due to the compartmentalization of the excited state by the micelle in contrast to homogeneous solution.The confinement of fluorescent molecules and quenchers i n small discrete aggregates, as in micellar solutions, have two important effects on the overall fluorescence decay kinetics" The proximity effect makes t h e quenching rapid in micelles containing both an excited species and quenchers and a shielding effect hinders the quenching of excited molecules created in micelles without the quenchers.By using proper concentrations of the fluorescent probe and micelle, one can create a situation where each compartment of the micelle is singly occupied by the cl~romophore.This prevents the quenching process and enhances the fluorescence quantum yields.Many studies have employed fluorescence probes in order to estimate important physical parameters in mjcelle systems such as critical micelle concentration (cmc)', aggregation number (n)" roughness of the micelle surface-nd degree of water molecules t h a t penetrate into surfactant aggregates1".Fluorescence characteristics such as wave length for excitation and emission, fluorescence life times and polarization values and fluorescence quenching rates, etc. have been employed.However, as far a s we know, PIET processes have not been used to study characteristic properties of micellar systems.In this study we have investigated sodium dodecyl sulphate (SDS) and Triton X-100 (TX-100) mjcellar systems.

METHODS AND MATERIALS
Absorption spectra were obtained using a Jasco V-500 UV\Visible Spectrophotometer.Infrared spectra were recorded on a Jasco FTfIR -5300 and are given in wave numbers (cm-I), IH NMR spectra were obtained on a Bruker ACF 200 instrument at 200 MHz with chemical shifts relative to TMS.Fluorescence studies were done using a RF-5000 Shimadzu Spectrofluorometer a t 2 5 C, and spectra were corrected.Melting points were taken on a Stuart melting point apparatus.
All the chemicals except 9-anthracene aldehyde (from Ald.rich) were purchased from Fluka.All the solvents used in spectroscopic measureme~~ts are either of Analar grade or general purpose grade (purified by distil.lation).The surfactant sodium dodecyl sulfate, SDS (BDH) was purified before use and Triton X -100 (TX -100, Aldrich) was used as supplied.

Al~sorption spectra:
The absorption spectra of 1x10-" solutions of the esters 1 and 2a-rn in methanol were recorded at 25°C.Nitrogen gas was bubbled througl~ the solution for 20 min, prior to recording the spectra.
Fluorescence stu,dies: The emission spectra of esters in the above solutions were recorded.The samples were purged with nitrogen for 20 min before measurements were taken.
Fluorescence life time nteasuremerrts: Fluorescence life time was measured using a PRA single photon counting apparatus with a hydrogen flash lamp of pulse width about 1 ns.

Flu.orescence measurements i n nzicellar media:
The micellar solutions were prepared by injecting a methanol solution of the substrate into the micellar solution (Methanol concentration < 1%).All solutions were prepared using deoxygenated solvents.
General method for preparation of esters: In our previous w o r k we prepared t h e esters, 2a-e by the reaction between 9-anthracene methanol and the corresponding acid chloride in benzene i n t h e presence of pyridine.However, we found that the yields could be tremendously increased by the following method as shown in Figure 1.The esters 2f-m were prepared by this new method.

DCC, DMAP, CH2CL2
Figure 1 : Scheme for preparation of esters 2a-m The method of preparation of esters 2a-e was given in the previous papera

Determination of Q u a n t u m Yields
In our previous report: the quantum yield of fluorescence for the esters 2a-e were estimated by taking the anthracene as the reference compound.This method does not give the accurate absolute values due to the long excited state lifetime of anthracene.In this report we have used quinine sulphate in 1M H,SO, as the reference compound (0, = 1.0).The fluorescence quantum yields of esters 1 and 2a-m in methanol are given in Table 1.Fluorescence quantum yields of 1 were determined in various concentrations of aqueous SDS and TX -100 solutions and tlie data presented in Table 2.The fluorescence quantum yields of 1 and 2 were measured in 20 mM of SDS and 5 mM of TX-100 and the data presented in Tables 3  and 4.

Determiilatio~l of Electron transfer r a t e s
Reaction Scheme wllicll was proposed in our previous reportn can be used to explain tlie variation of fluorescence quantum yields with substituent for the esters 2f-m.The following relationship which we derived in the earlier report has been used: ( 0; and.
are quantum yields of fluorescence for benzoate, 2 and acetate, 1 respectively.kP, and <be,, are respectively the rate constant and quantum yield of electron transfer process.z is the life time of the acetate, 1 ester.
The percentage of fluorescence quenching was plotted against the substituent constant 0-as given in Figure 1 for methanol, SDS and TX-100 respectively.in the text Values taken fi-om reference ( 14)  in the text.

(b)
Estimated values using the equation (3) in the text.

DISCUSSION
The points corresponding to 2c, 2d and 2m (X = 4-NO, and 4-Br, 3-NO,, respectively) have been excluded in the plot between log(ke:/kep) and a-, (Figure 2).The unusual singlet excited state behavior of chromophores has been reported when they are substituted with nitro and bromo groupsl1.Therefore 2c, 2d and 2in could have different behavior compared to the rest of the compounds.The linear plot is strong evidence for the validity of the proposed mechanism for the'PIET.
As mentioned in the introduction, fluorescence is normally enhanced in organized media.The increase in fluorescence quantum yields with increasing concentration of micelle can be explained by the lower penetration of water molecules to the interior of the micelle.As the concentration of surfactant increases the polarity of the interior of the micelle decreases.Many researchers have shown12 that fluorescence quantum yields of poly aromatic compounds are significantly enhanced in micelles.The main cause of the enhancement is its isolation from quenchers as well as increased microviscosity of the environment1" The higher fluorescence quantum yield of 1 in TX-100 compared to that of SDS can be attributed to the higher microviscosity of the TX-100 micelle.The higher microviscosity restricts the mobility of the enclosed molecule in the micelle, thus enhancing the fluorescence.
Using the cmc, aggregate number, n, and the substrate concentration ( 1 0 -W ) the minimum concentration of t h e micellar solutions required to achieve the mono occupancy can be estimated as 8.008 and 0.26 mmol d m -5 n SDS and TX-100, respectively.In our study, the fluorescence quantum yields of 1 and 2a-k were measured in solutions with concentrations of 20 mmol dm-" for SDS and 5 mmol dm-"or TX-100.Use of these concentrations allows us to make t h e reasonable assumption t h a t each substrate molecule is singly occupied in the micelle system.The micelle interior is a liquid phase less fluid than hydrocarbon solvents of similar chain length.There is some porosity near the micelle-water interface where water molecules can penetrate1" deeply.Therefore both microviscosity and micropolarity of the environment of a molecule associated with a micelle may differ substantially from those of the bulk aqueous phase.These factors are responsible for the common properties of micellar systems.The higher microviscosity of the interior of the micelle restrict the dynamic behavior of the solubilized molecules and enhance the fluorescence quantum yields.This explains the higher fluorescence quantum yield of the substrate in micelle compared to that of the same molecule in methanol, i.e., the fluorescence quantum yield of 1 in methanol is 0.195 compared to 0.318 in SDS and 0.349 in TX-100.Dielectric constant and E,,,,, values are commonly used in quantification of polarity of the solvents.Higher values for dielectric constant and ET(,,, have been observed for more polar solvents whereas non polar solvents give lower values.The dielectric constant for methanol is 32.7 whereas for micelle-water interface of SDS and TX-100 are 45 and 15 respectively.The E,,,,,, values for methanol, SDS and TX-100 are 55.0,56.7 and 46.9, respectively.The magnitude of E, :, , in micellar systems suggest t h a t the polarity of the solubilization site of t h e micelle is mainly determined by the amount of water molecules that penetrate into the interior.This is greater in SDS than TX-100 and therefore gives a higher E, : , , , value for SDS.The order of the E,,:!,, values is SDS > methanol > TX-100.This indicates t h a t the polarity of the solubilized site of SDS is higher than that of methariol while polarity of the solubilized site of TX-100 is lower than that of methanol.
The values of the reaction constant for SDS, methanol and TX-100 are 3.40, 3.28 and 2.41 respectively.This variation is parallel to the variation of E,p(,,,.The plot between reaction constant, p and Em,, is given in Figure 6.Although this plot has only three points, it is in agreement with our explanation of the medium effect on the electron transfer process.Both these effects namely the substituent and the solvent effect agreed with our explanation for t h e quenching of fluorescence by electron transfer.
In conclusion, the substituent on the benzoate chromophore with electron accepting capabilities enhance the electron transfer rate by making the process thermodynamically more favorable.On the other hand medium with lower polarity reduces the electron transfer rates.Therefore both substituent and solvent effects play a significant role in the photoinduced intramolecular electron transfer processes.

Figure 2 :Figure 3 :
Figure 2 : Relationship between percentage quenching and cr i n methanol, SDS and TX -100.