Indian Journal o f Chemistry Vo l. 42A, September 2003. pp. 2198-2204

Free-base, copper(II) and zinc(II) derivatives of an azomethine bridged 'porphyrin-pyrene' dyad: Synthesis, spectroscopy and photochemistry

Prashanth Kumar Poddutoori & Bhaskar G Maiya* School of Chemistry, Uni versity of Hyderabad, Hyderabad 500 046

Received 27 December 2002

Free-base, copper(lI) and zinc(lI ) derivatives of a porphyrin-pyrene conjugate having an azomethine group separating the two photoactive subunits have been synthesized and fully characterized by mass (FAB), fR, UV/visible, IH NMR and ESR spectroscopies and also by the cyclic- and differential pulse voltammetric methods. An analysis of the data reveals that the spectral and electrochemical properties of the individual chromophoric entit ies are retained and that there exists no specific 11:-11: interacti on between the porphyrin and pyrene subunits in these dyads. Excitation of CH2CI2 solutions of the free-base and zinc(Il ) derivatives at 550 nm results in no quenching of fluorescence due to the tetrapyrrolic moiety, but that at 290 nm results in a quenching of fluorescence due to the appended pyrene moiety. Excitation spectral data indicates that photons collected by the hydrocarbon subunit are effectively tran sferred to the porphyrin in both these donor-acceptor systems.

In view of their importance in both biological and abiological applications, such as biomimetic photosynthesis, molecular electronics etc., studies on excitation energy transfer (EET) reactions occurring in porphyrin-based donor-acceptor (O-A) systems are increasingly being pursued in recent years I. Intramolecular EET has been elucidated in a great variety of porphyrin-based systems in which a porphyrin or a metalloporphyrin is covalently/non­covalently linked to either an energy donor or an acceptor2. We have been interested in the design, synthesis and mechanistic elucidation of the photoinduced electron transfer (PET) and EET reactions of intramolecular hybrid porphyrin systems in which a tetraaryl porphyrin is linked to either nitroaromatic or anthracene subunit/s3

. Of specific relevance to the present paper are our recent reports on the intramolecular EET and PET reactions of a pentad system in which a porphyrin is endowed with four anthracene donor subunits3c. a series of covalently linked isomeric porphyrin-anthracene (P­A) dyads3d

, a non-covalent P-A system assembled via complementary nucleic acid base-pairing3e and a recently reported tin(IV) porphyrin that is appended with anthracene donor subunits at both the axial and peripheral positions3r

. Besides these, there are a number of reports from other workers on EET occurring in P-A systems2b,2g

-i. EET reactions of P- A systems are attractive largely due to the facts that (i) anthracene is a good energy donor with its singlet

exci ted state lying above that of the porphyrin and (ii) it has well-defined spectral and electrochemical signatures4

. Pyrene, the next hydrocarbon in the homologous series, also possesses photophysical and spectral qualities similar to those of anthracene. However, photochemical studies on porphyrin-pyrene conjugates are scarces. With the above considerations in mind, we have recently initiated studies on bichromophoric species having porphyrin acceptor and pyrene donor subunits . In the present paper, we report the design, synthesis and intramolecular EET reactions of an azomethine bridged porphyrin - pyrene dyad (H2L2) and its copper(lI) (CUL2) and zinc(lI) (ZnL2) derivatives, the structures of which are shown in Fig. 1.

Materials and Methods The common chemicals and solvents utilized in this

study were purchased from B 0 H (Mumbai, India). Most of the fine chemicals were purchased from either Aldrich Chemical Co. (USA) or Lancaster Chemical Co. (UK). The solvents utilized for the spectroscopic and electrochemical experiments were further purified using standard procedures6

.

Meso-5-(4-aminophenyl)-1O, 15, 20-tri(4-methyl­phenyl) porphyrin (H2L I), the precursor porphyrin used for the synthesis of the present dyads, was prepared by a reported m'ethod7

. Copper(1I) and zinc(IJ) derivatives CULl and ZnLI were prepared and purified according to the standard proceduresB

•

PODDUTOORI el 01.: FREE BASE, Cu(lI) & Zn(ll) DERIVATIVES OF PORPHYRIN-PYRENE DYAD 2199

R

R

M = 1-11, H2Ll

= Cu(TI), CuLl

= Zn(II) . ZnLl

(i) --...... R

R = P - lolyl

R

M = H2, I-I1L2

= Cu(TI), CuLl

= Zn(II), ZnL2

(i) Py-CHO, toluene (dry), 4Aomolecular sieves, 12-18 h ., reflux, N2.

Fig. I- Scheme leading to the synthesis of new 'porphyrin - pyrene' dyads investigated in thi s study.

Synthesis of H2L2 H2LI (0. os g, 0.074 mmol) and I-pyrenecarboxal­

dehyde, Py-CHO, (0.085 g, 0.37 mmol) were dissolved in 40 ml of dry toluene containing 4 A molecular sieves. The resulting solution was refluxed under the nitrogen atmosphere for 12 h. The solvent was removed and the crude product was dissolved in CH2CI2 and filtered. Evaporation of the solvent gave a purple solid, which was recrystallized twice from CH2CIz-CH3CN to give H2L2 in pure form. Yield = 0.055 g (83%). FAB-MS (mJz): 884 (M+Ht, IR (KBr pellets): 1581 cm- I (C= - stretch)

Synthesis of CuL2 and ZnL2

These compounds were prepared, starting with CuLl or ZnLI (0.025 g, - 0.034 mmol) and Py-CHO (0.04 g, O. 17 mmol) , in a manner analogous to that described above for H2L2. Yields: CuLc: 0.028 g (86%) ; ZnL2 : 0 .025 g (78%). IR (KBr pellets): 1585 cm·1 (C=N stretch)

Physica!lIIeasurell1ents The UV/vis spectra were recorded with a Shimadzu

Model UV -310 I PC UV /vis spectrophotometer for l x 10-6 M (porphyrin Soret band) -5x I0-5 M (Q-bands and pyrene bands) solutions. Steady-state fluorescence spectra (uncon'ected) were recorded usi ng a Spex model Fluoromax-3 spectrofluorimeter for sol utions having optical density at the wavelength of excitation (ll.cxc) < 0.2 . The fluorescence quantum yields ($) were estimated by integrating the fluorescence bands and by using either 5, 10, IS , 20-tetraphenylporphyrin (H2TPP) ($ = 0.13 in CH 2C1 2),

5, 10, IS , 20-tetraphenylporphyrinato zinc(ll) {[(TPP)Znll]} ($ = 0.036 in CH2CIz) or pyrene as the

standards9.5a

.4 . Dilute solutions (- 10-7 M) of the f1uorophores were utilized for the excitation spectral measurements. Corrections to the instrument response and procedures for the spectral normalization employed during the estimations of the excitation energy transfer efficiencies are essentially similar to those described In our previous studies3c

-h

.

Fluorescence lifetimes were measured by the time correlated single photon counting (TCSPC) method at the National Centre for Ultrafast Processes (NCUFP, Chennai) . The IH NMR spectra were recorded with either a Bruker NR-200 AF-IT NMR or a 400 MHz leol lNM-LA 400 IT NMR spectrometer using CDCb as the solvent and tetramethylsilane (TMS) as an internal standard. ESR spectra for the copper(II) systems were run on a JEOL lES-FA200 X-band ESR spectrometer in toluene at 100±3 K. Diphenyl picryl hydrazide (DPPH) was used as the g-marker. The FAB mass spect ra were recorded with a JEOL SX 102/DA-6000 mass spectrometer/data system. Cyclic­and differential-pul se voltammetric experiments (C H2CIz, 0 .1 M tetrabutylammon ium perchlorate, TBAP) were performed on a CH Instruments model CHI 620A electrochemical analyzer as detailed in our previous studies (working- and auxi liary electrodes: Pt; reference electrode: Ag)3.1O. Fc+/Fc (Fc = fen'ocene) couple was used to calibrate the redox potential values, which are reported in V vs . SCE (E1/2 (Fc+/Fc) = 0.48 V vs. SCE in CH2C12, 0.1 M TBAP under our experimental conditions).

Care was taken to avoid the entry of direct, ambient li ght into the samples in all the spectroscopic and e lectrochemical experiments . Unless otherwise specified, all the experiments were carried out at 293± 3 K.

2200 INDIAN J CHEM, SEC. A, SEPTEMBER 2003

Results and Discussion The scheme leading the synthesis of H2L2

, CuL 2

and ZnL2 is illustrated in Fig. I . Synthesis of all the three new compounds has been accompli shed here, in good-to-moderate yields, by a simple Schiff base condensation between the amino porphyrin (L') derivatives and Py-CHO. These compounds have been fully characterized by FAB-MS, IR, UV/vi s, '1-1 NMR and ESR spectroscopies and also by the cyclic­and differential pulse voltammetric techniques as described below.

Grollnd slate properties The mass (FAB) spectrum of 1-12L2 showed a peak

ascribable to the (M+Ht ion. '1-1 NMR spectral data of H2L" and ZnL'2 along with their individual constituents (i.e. I-hL ', ZnL' and Py-CHO) are summarized in Table I and the spectra of H2L 2 and ZnL2 are displayed in Fig. 2. A comparison of these spectra with those of H2L'/ZnL' and Py-CHO reveals that -the resonance positions of the various protons present either on the porphyrin macrocycle or the pyrene subunit are not shifted considerably upon linking the two chromophores via the azomethine spacer. Thi s is reasonable if one considers that the two aromatic rings are at a 'frans' configuration with respect to the C=N spacer, avoiding the steri c

interaction. The structure of H2L2 derived from the molecular mechanics (PCM) calculations indeed suggests the same. ]n addition, pin Hamiltonian parameters derived from the ESR spectrum of Cue (tol uene, 100±3 K) are also quite close to those of

H2L2

~JJ~~. ! .Jl..._),-,"i-- -

* Z1lL 2

~Uj '~~

6 4 I

'. 10 8 2

Pig. 2-'H NMR spectr:! of H2L2 and ZnL" (CDCl), TMS)

Table I- 'H NMR Data a

Compd.

ZnL '

Py­CliO

ZIlL'

~-pyrrole

8.92 (d. 2H) 15.21 8.85 (d. 61 1) [4.0]

8.94 (111, 8H)

lUHi (111 . SH)

X.% ('11 . 811)

O. ppm Il, Hzl Porphyrin

Meso- tolyl

8.1 0 (d, 6H). S.OI (d,2H) [H] 18.61

7.55 (d. 6H). 7.07 (d. 21-1 ) [7 .81 18.61

R. IO ( m. 6H), 7.87 (d . 2H) [7.S J

7.54 ( m. 61-1 ), 6.62 (d. 21-1 ) 17 .81

lU3 (m. 21 1). 8.13 (m,61-1) 7.79 (d. 2H ), 7.57 (d. 6H)

[6.81 [(J.S]

105 (m 2H). 8.1~ ( Ill. 611). 7.80 (d. 2tl), 7.:j7 (d. o Il )

[7.9J [7.51

-NH

2.71 (s,9H) - 2.71 (s.2H)

2.70 (5. 9H) -

2.72 (s, 9H) -2.7 1 (52 H)

2.73 (s. 9H)

-CH=N-

9.87 (s, I H)

9.R7 Is, I H)

Pyrene

10.78 (5 . I H). 9.41 (d, I H), [9.31

8.31 (d. 11-1),8.26 (m, 5H), [93 1

8.1 1 (01 , 2H) 9.25 (d, I H), 8.95 (m, : 1-1),

[S.81 8.32 (m, 51-1). S.21 (01, I H), 8.13 (m, IH)

9.24 (e1. I H), 9.06 (m. 11-1), [8.SJ

g.28 (m. 5H). 8.18 (11l, IH). 8.05 (m. IH)

PODDUTOO RI el ai.: FREE BASE, Cu(ll ) & Zn(l I) DERIVATIVES OF PORPH YRIN-PYRE E DYAD 2201

Tahlt: 2-Redox potel1!ial data ill CH 2C11• 0.1 M TSAP a

Compound

Py-CHO H2TIP b

CuTIp b

ZnTTP b

HzLz CuL2

ZnL2

___ -'Potenti al (V'-' v-'S"-' =-SC-=-E=-)'--__ _ Oxidation Reducti on

1.51 0.88, 1.1 2 0.93, 1.25 0.79, 1.08 0.98, 1.20, IA3 0.93. 1.23, lAO 0.73, 1.04, 1.38

- 1.41 - 1.23, - !.57 - 1.30, - l. 72

-1.31 , - 1.55 -LIS, - I A7 - 1.29. - 1.52 -1.33. - IA8, - 1.57

a. Error limits: E ln , ± 0.03 V b. H2LI alld its metal(ll ) derivatives gave ill -def~ned voltammogrnms. Hence, TIP (lelra-p-lolyl porphyrin) derivatives are employed as reference compounds.

Py ·CHO

1.5 1.0 · 1.2 · 1.6

POIential(V vs SCE)

Fig. 3--Diffe renli al pul se voltammograrns of H2L2 H2TIP and Py-CHO in CH2C1 2• 0.1 M TBAP (scan rate = 10 mV S·I) .

CULl (gn. g.l and A.cu, A/", At and A.l N (x 104 em- I) values are: CUL2: 2.176, 2.035, 204, 33.0, 14.6 and 16.5; CuLl: 2. 160, 2.028, 202, 33 .0, 14.2 and 16.5).

Table 2 summarizes redox potential data (CH2Cb, 0.1 M TBAP) of the O-A systems investigated in thi s study along wi th that of the corresponding reference compounds. Figure 3 illustrates the differential pulse voltam mograms of H2e, H2TIP [5, 10, IS, 20-tetra (p-tolyl) porphyrin] and Py-C HO. As seen in Fig. 3 and also data given in Table 2, each investigated new

compound shows up to three reduction- and oxidation peaks under the experimental conditions employed in this study. Wave analys is suggested that wh il e the first two oxidati on and the first reduction processes represent reversibl e (i pJ ipa=0.9-1.0) and diffusion controlled [i pc/'O"2=constant in the scan rate ('0) range 50-500 m V Is] one-electron transfer (i'1Ep=60-70 m V; i'1Ep = 65±3 mV for ferrocene+/ferrocene couple) reactions, the remaining electrode processes are either quasi-reversible (ipJi pa = 0.6-0.8 and i'1Ep = 80-150 mY) or ilTeversible under similar experimental conditions ". While the peaks occurring at potentials that are less anodic than 1.25 V are ascribed to successive one-electron oxidations of the porphyrin parts of H2L2, ZnL2 and CuL2, the peak occurri ng at more anod ic potential represents the oxidation of the appended pyrene moiety. Similarly , while the first reduction peak corresponds to the reduction of the porphyrin ring, the remaining extensively overlapping peak/s are assigned to electron additi on to the porphyrin/pyrene moiety in each case. This analysis, which is based on a comparison between the voltammograms of the dyads (which are Schiff-bases) and those of the reference compounds (H2TIP, CuTTP or ZnTIP and Py-CHO), reveals that the redox potentials of the two subunits in the dyads are close to those of the corresponding reference compounds.

The wavelengths of max imum absorbance (Amax)

and molar extinction coefficient (log £) values of the three new dyads and those of their constituent individual components (i.e. H2Ll, CULl and ZnLl and Py-CHO), as obtained from the UY/visible studies, are summari zed in Table 3. UV/vi sible spectrum of H2L2 is compared with the spectra of H2Ll and Py­CHO in Fig. 4. Comparison of these spectra and the data given in Table 3 suggests that the appended pyrene chromophore in H2L

2, CUL2 or ZnL2 dominantly absorbs between ca. 230-350 nm, a region in which porphyrin part of each molecule shows its so-called Nand L bands. Nonetheless, the peak at -290 nm for each dyad corresponds largely to the absorbance of the pyrene moiety because the Nand L bands are quite weak compared to the pyrene absorbance in thi s wavelength region. On the other hand, porphyrin parts of H2L

2, CuL2 and ZnL2 show four/two Q-bands in the wavelength region (500-700 nm) where the pyrene part of each molecule does not absorb. Both porphyrin and pyrene chromophores of these compou nds are seen to absorb between ca. 350-

2202 INDI AN J CHEM, SEC. A, SEPTEMBER 2003

Table 3----UV/visible data"

Compd Soret

Py­CHO

ZnL"

band

421 (5.44)

418 (5.47 )

422 (5.51)

422 (5 .52 )

419 (5.50)

423 (5.58)

Anw' nm (log E )

Q-bands

650 (3.65), 594 (3.62), 555 (3.94) . 518 (4. 12) 578 (3.5 1), 54 1 (4.22) , 506 (3.48) 590 (3 .71). 550 (4 .20). 5 14 (3.43)

649 (3 .67), 593 (3.66) , 554 (3.99), 518 (4. 17) 577 (3.50) , 541 (4.22),502 (3.49) 589 (3.78), 550 (4.30 ),5 12 (3.5 I )

Pyrene

398 (4.34), 372 (4.39),290 (4 .55)

378 (4.39), 367 (4.38), 292 (4.36)

364 (4 .27), 292 (4.45)

379 (4.34). 362 (4.37),292 (4.48)

a. Spectra were measured in CH2C1 2. Error limits: Ama .. ± I nm, log E, ± 7%

420 nm resulting in the broadening of the peaks as we ll as appearance of pyrene bands in the porphyrin Soret band region. Spectral data given in the Table 3

further reveal that , in general , both Amax and log E

values of each dyad are within the same range as those of the constituent individual components. Finally , it was observed that the Am,. and spectral shapes of the bands of H2L2/CuL2/ZnL2 are more or less similar to those of solutions conta ining one mole equivalent each of H2L '/CuL'/ZnL' and Py-C HO.

The spectroscopic and electrochemical features described above suggest that electronic communication between the porphyrin and pyrene chromophores is quite neglig ible in these new conjugates. More importantly, the UV /vis ible data sugges ts that it is possible to individually address photochemi stry of the porphyrin and the pyrene subunits in these bichromophoric sys te ms. T hi s situation is unlike the case with the previous ly repo rted porphyrin-pyrene compounds in which the hydrocarbon subunitls is directly attached to the porphytin n-framcwork5a

.b

, but is quite sim il ar to the phosphorus(V) porphyrin-pyrene sys tems wherein the two chromophores are linked, via, alkyloxy spacers at

0,4

\" 1

0.2 Vl!,,\ Py -CH O

-J~ -

G l.O

;;: )~ i 2.0 Xl0

l.O

~ I.!'

~\ XIO

lOO 400 500 600 700 ;.(11 III)

the axial sites5c. While it is no longer correct to

describe the system in terms of separate subuni ts and the whole entity functions as a modified porphyrin in the fo rmer systems, the spectroscopic signatures of the po rphyrin and pyrene subunits are eas ily identifi able in the latter ones. Despite the presence of a short C=N bond separating the two subunits, the series of porphyrin-pyrene conj ugates investigated in the present study seem to belong to the latter class of compounds as sugges ted by the spectroscopic data presented above. It may be noted here that the C=N bridge is located at the para position of one of the aryl rings o f the porphyrin in these dyads and that the aryl rings are not in plane with the macrocycl ic ring . Armed with this information in hand, we set out to probe the photochemical properties of the new conjugates as described below.

Singlet stale properties While CuL2 was found to be totally non-emISSIve

probably due the paramagnetic nature of this compl ex, both H2L2 and ZnL2 were fo und to be photoche mically act ive species. In Fig. 5, the steady­state flu orescence spectra of these compounds measured in CH2CIz are compared with spectra of the correspond ing indi vidual components constituting them. Spectral shapes and wavelengths of maximum

PODDUTOORI et al.: FREE BASE, Cu(JI) & Zn(/l) DERIY ATIYES OF PORPHYRIN-PYRENE DYAD 2203

. :\. = 550 11111 ,_ . !iI

n I\f\ } -V \

-~ -' a,>--

Ir\,~onm (,) \

... . .. .. ~~

420 600 iOO 800 \Va ~ I englhlnn l)

Fig. 5--Fluorescence spectra (C H2Ci2. O.D. = 0.17 at each ACXl') of (a) Aexc = 290 nm: Py-CHO (--) and H2L2 ( . .. ... .. ); Acxc = 550 nm: H2LI (--) and H2L2 ( ..... ... ) (b) Aexc = 290 nm: Py-C HO (--) and ZnL2 ( . .... ... ); Aexc = 550 nm: ZnL' (--) and ZnL2 ( .... .... ).

emission (Aem) of these D-A systems, when they were irradiated at the porphyrin absorption band (Aexc=550 nm), are seen to be similar to those in the spectrum of either H2LI or ZnLI . However, excitation at 290 nm (dominant pyrene absorption) resulted in broadening and marginal red shifts of the fluorescence bands in both the compounds in comparison with the fluorescence due to Py-CHO. From an overlay of the absorption and fluorescence spectra of these O-A systems, the 0-0 spectroscopic transition energies (Eo.o) of the pyrene (3.56±0.05 eV) and the porphyrin moieties (1.92±0.04 eV for H2L2 and 2.09±0.04 eV for ZnL2) were obtained, and these values are found to be close to those of pyrene and H2 TIP/ZnTTP, respecti vely.

Whereas the fluorescence quantum yield (<I» for excitation into the porphyrin part of each dyad was similar to that of the unlinked porphyrin (i.e. H2L2/Zne, <I> = 0.14±0.01 and 0.048±0.005, respec­tively) , fluorescence from the pyrene part was found to be strongly quenched, in comparison with the fluorescence of free Py-CHO (or pyrene), Fig. 5. The quenching effici ency values (Q),

Q = <I>(Py - CHO) - <I>(H 2L 2 /Znr3)

<I>(Py -CHO) .00 (1)

[where <I>(Py-CHO) and <I>(H2e/ZnL2)] refer to the fluorescence quantum yields for Py-CHO and the D-A systems H2e/ZnL2, respectively; Aex=290 nm) have been calculated for these systems in CH2Cl2. These values are 98±1O% for both H2L2 and ZnL2. As expected, the fluorescence life times ('t) of the porphyrin parts of both H2L2 and Zne (8.09 and 1.72 ns , respectively; Aexc=41I nm) remained quite similar to those of H2TIP and ZnTIP (8.43 and l.60 ns , respectively) but, those of their pyrene subunits (Aexc = 290 nm) were found to be considerably shorter (-0.1 ns) in comparison with the lifetime of pyrene4

.

The observation that there exists a considerable overlap between the emission of pyrene and the absorption of porphyrin in these D-A systems (compare the corresponding spectra in Figs 4 and 5) suggests that quenching of the pyrene fluorescence observed for H2L 2 and ZnL 2 may be due to intramolecular excitation energy transfer from the singlet pyrene to the porphyrin . Indeed, excitation of ca. 10-7 M solution of these two O-A systems at 290 nm resulted in an appearance of well -defined porphyrin emission bands in CH2Ch. Conclusive evidence for intramolecular EET comes from the excitation spectral measurements. When emission was recorded at the porphyrin emission maximum (650 nm for both H2L2 and ZnL\ the excitation spectra of these D-A systems showed bands characteristic of pyrene absorption. The corrected and normalized excitation spectra of these dyads were overlayed with the corresponding absorption spectra in CH2Ch. A comparison of these spectra in the 40.0xI03-30.7x103

cm- I region gave almost 100% energy transfer efficiencies (%T) in each case.

A comparison of the photophysical properties of H2L2 and Zne with those of the previously reported porphyrin-pyrene conjugates reveals that energy transfer is nearly quantitative in those porphyrin­pyrene systems wherein the pyrene subunit is either directly linked to the porphyrin5a

•b or is separated from

the porphyrin by a short C=N bridge (this work). On the other hand, energy transfer has been shown to be on ly a minor process between the excited pyrene and the porphyrin in a series of phosphorus(V) porphyrin­(pyreneh triads wherein the two pyrene subunits are linked to the porphyrin at the axial sites of the

2204 INOlAN J CHEM, SEC. A, SEPTEMBER 2003

phosphorus(Y) iOIl via alkyloxy bridgesc . Thi s analysis is consistent with our recent report of ' unidi rectional' energy transfer in a tin(IY) porphyrin -(anthracene)) tetrad in which two anthracene subunits are linked at the axial sites and one at the peripheral si te3f.h. Overall , it appears that energy transfer from a donor subuni t located at the axial site of a porphyri n is inefficient while that from a donor located at the peripheral si te is efficient. As discussed in our prev ious work, origin of this 'u nidirectionality' of the EET event lies in the mutual orientations of the transition dipoles of the porphyrin and the aromati c donor subunits in thi s class of D-A compounds 3r.h. It is also possible that the bridging azomethi ne group may have a role to play in the efficient EET observed in the present set of 'porphyrin-pyrene' dyads, but it is not apparent from the res u Its obtained in this study . Studies addressing thi s issue are currentl y in progress.

Acknowledgement Financi al support fo r this work from the BRNS

(Mumbai) is gratefull y acknowledged. PPK thanks the UGC fo r the award of a SRF. We thank Prof. T. K. Chandras hekar (lIT, Kanpur), NCU FP (Madras University, Chennai) and CDRl (Lucknow) fo r 400 MHz NMR, TCSPC and FAB-MS data, respecti vely. We also thank the UPE program of the UGC for some of the instrumentation fac iliti es.

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