Photophysics of 6,11-dihydroxynaphthacene-5,12-dione in di†erentorganic solvents

Madhab C. Rath and Tulsi Mukherjee*Chemistry Division, Bhabha Atomic Research Centre, T rombay, Mumbai 400 085, India

The absorption and steady-state and time-resolved emission characteristics of 6,11-dihydroxynaphthacenedione (6,11-DHNpQ)have been investigated in di†erent organic solvents. A red-shifted absorption band has been observed at 555 nm in protic solventswhich are either hydrogen-bond donating or accepting in nature, e.g. alcohols and aprotic solvents such as DMSO and DMF.This band is absent in all other solvents. This observation is explained on the basis of intermolecular hydrogen-bonding com-plexes between the solute and the solvent molecules. An unusual emission characteristic has been observed in methanol, attrib-uted to the same hydrogen-bonding complexes. In benzene, DMSO, DMF and acetone the Ñuorescence quantum yield of thequinone is quite low owing to exciplex formation and/or subsequent charge separation depending on the solvent polarity.

The radiation chemical and photophysical properties ofhydroxyquinones, used as disperse dyes,1 in mimicking manybiological systems,2 and in medicine as anticancer agents,3,4are important in understanding the nature and the role of dif-ferent intermediates of these quinones involved in di†erentchemical and biochemical systems. One-electron reactions of5,8-dihydroxynaphthoquinone (naphthazarin)5,6 and 1,4-dihy-droxyanthraquinone (quinizarin)7 in aqueous media havebeen studied in detail. Photophysical properties of naphtha-zarin, quinizarin and some other quinones (e.g. carminic acid8)have been studied extensively in di†erent organic solvents9h12and the role and importance of intramolecular hydrogenbonding in those molecules has been elucidated. Rotationalanisotropy study of quinizarin by Palit et al.13 suggested thatstrong intramolecular hydrogen bonding plays a substantialrole in the attainment of an absolutely Ñat geometry for themolecule even in solution.

Here we report on the absorption and emission character-istics in di†erent organic solvents of 6,11-dihydroxynaphtha-cenedione (6,11-DHNpQ), a higher analogue of quinizarin,with an extra benzene ring and forming a centrosymmetricstructure of point group Quenching of Ñuorescence, par-D2h .ticularly in solvents like benzene, DMSO and DMF isexplained on the basis of exciplex formation between thequinone and the solvent molecules.

Experimental6,11-DHNpQ was obtained from Aldrich ([98% purity). Sol-vents were of spectroscopic grade from Fluka, BDH orSpectrochem India Ltd. A Shimadzu model UV-160A spectro-photometer was used for the ground-state absorption studies.A Hitachi model F-4010 spectroÑuorimeter was used for thesteady-state Ñuorescence studies. Fluorescence lifetime mea-surements were carried out in a Ñuorescence spectrometermodel 199 from Edinburgh Instruments which works on theprinciple of time-correlated single photon counting. A coaxialÑash lamp of ca. 1.0 ns pulse duration and 30 kHz pulse ratewas the excitation source for the lifetime measurements. Thedecay traces were collected and analysed by the Gem programcoupled with a Norland 5000 MCA card in a PC. All experi-ments were conducted at room temperature (298 K).

Results(a) Absorption studies

A red-shifted absorption band at ca. 555 nm is observed (Fig.1) in hydrogen-bond donating solvents such as methanol

(strongest band), ethanol, etc. and also in the hydrogen-bondaccepting solvents DMSO and DMF (not shown), but absentin the non-polar solvents such as cyclohexane and 1,4-dioxaneand also in other polar aprotic solvents such as acetonitrile.The 555 nm band starts rising upon addition of methanol toacetonitrile (Fig. 2), indicating that this band may be due to ahydrogen-bonded complex between the solute and the solventmolecules in their ground state. Such absorption character-istics are not observed for naphthazarin and quinizarin. Thereason for this may be the long aromatic chain of 6,11-DHNpQ with two OwHÉ É ÉH bridges on either side of thering forming a symmetrical shape of point group sym-D2hmetry. Spectroscopic parameters are listed in Table 1.

In methanol, the peak ratio of the absorbancesdecreases with increase in the concentration ofA555 nm/A515 nmthe quinone. Similar observation is made on addition of water

to a dilute solution of the quinone (data not shown). The red-shifted absorption band is progressively reduced upon addi-tion of H` ions whereas it becomes more prominent onaddition of OH~ ions (Fig. 3).

(b) Fluorescence studies

The steady-state Ñuorescence characteristics of 6,11-DHNpQ(Table 2) shows no signiÐcant spectral di†erences in polar

Fig. 1 Absorption spectra of 6,11-DHNpQ in di†erent organic sol-vents

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Table 1 Absorption characteristics of 6,11-DHNpQ in di†erent organic solventsa

solvent jmax/nm (e/dm3 mol~1 cm~1)

MeOH 265(59 115) 451(4800) 481(9050) 515(12 350) 555(9200)EtOH 263(79 200) 451(9710) 480(15 260) 515(14 720) 554(2200)TFE 264(183 050) 450(5620) 480(9020) 514(8550) 560(710)PriOH 263(36 990) 453(4080) 481(6330) 515(6210) 554(450)acetone È 453(14 425) 480(21 970) 514(19 050) 550(600)benzene È 459(46 880) 487(73 350) 521(65 120) ÈDMF È 455(12 300) 483(18 810) 517(16 950) 565(2300)DMSO È 456(12 100) 484(18 025) 519(15 100) 560(300)chloroform 263(114 250) 456(20 150) 485(32 670) 520(30 640) È1,4-dioxane 263(40 870) 455(7000) 484(10 380) 517(8650) Ècyclohexane 262(70 300) 455(9200) 483(15 050) 518(16 560) Èacetonitrile È 454(11 500) 481(17 130) 514(14 540) È

a TFE\ tetraÑuoroethanol, DMF \ dimethylformamide, DMSO\ dimethyl sulfoxide.

Fig. 2 Absorption spectra of 6,11-DHNpQ in acetonitrile with dif-ferent methanol concentrations : (1) 0.0 M, (2) 3.2 M, (3) 4.1 M, (4) 4.9 M,(5) 5.7 M, (6) 6.4 M, (7) 7.7 M, (8) 9.0 M, (9) 10.6 M, (10) 12.4 M

Fig. 3 Absorption and Ñuorescence spectra of 6,11-DHNpQ inmethanol and methanol in presence of H` and OH~ ions ; (ÈÈ)MeOH (È È È), MeOHÈH` (È È È) and MeOHÈOH~

Table 2 Emission characteristics of 6,11-DHNpQ in di†erentorganic solventsa

solvent / qb/ns jmax/nm

MeOH 0.54 5.00 527 565 600EtOH 0.44 5.81 528 564 605TFE 0.41 6.27 528 564 605PriOH 0.58 5.32 528 565 605acetone 0.37 5.93 528 565 605benzene 0.29 5.18 540 570 ÈDMF 0.22 6.00 538 570 616DMSO 0.10 5.52 545 572 620chloroform 0.49 6.36 529 567 6061,4-dioxane 0.32 5.57 525 565 605cyclohexane 0.53 5.96 526 566 605acetonitrile 0.42 5.34 530 568 606

nm except for DMSO, DMF, benzene, acetonitrile anda jex \ 270acetone where nm, nm in each case. b Single-jex\ 450 jem \ 525exponential decay giving the same q value for nm in alljem \ 560solvents apart than methanol.

and non-polar solvents except for methanol, benzene, DMSOand DMF. Fig. 4 shows the Ñuorescence spectra of thequinone in methanol, acetonitrile and benzene. There is a redshift in benzene, DMSO and DMF and a concentration-dependent emission pattern in methanol. Quantum yield mea-surements in these solvents (Table 2) show that theÑuorescence is largely quenched in these solvents. In meth-anol, the ratio of the Ñuorescence peak intensities,

increases non-linearly from less than to greaterI530 nm/I570 nm ,than unity as the concentration of the quinone is increased(Fig. 5). The ratio remains constant in all other solvents, indi-cating that the photophysical properties of 6,11-DHNpQ inmethanol are di†erent from those in the other solvents. Theaddition of a small amount of water (up to ca. 25% byvolume) to a very dilute solution of the quinone in methanolleads to an increase in the Ñuorescence yield as well as anincrease in this peak ratio, similar to the e†ect observed onincreasing the concentration of the quinone in methanol. Sincein waterÈmethanol the solubility of the quinone is very muchreduced, the dilute solution behaves like a methanol solutionwith higher quinone concentration. Addition of water to aconcentrated solution of the quinone in methanol does nothave any e†ect on the peak ratio A similarI530 nm/I570 nm .observation is made in a dilute methanolic solution of thequinone by adding a little H`, upon which the peak ratio

increases. On addition of a small amount ofI530 nm/I570 nmOH~ to this solution a reverse trend is observed and peakratio is reduced (Fig. 3).

Comparable results are obtained in the lifetime measure-ments. Single-exponential decays are obtained in these sol-vents for the 530 nm emission peak (highest concentration ofquinone in methanol was used in this study) as well as the

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Fig. 4 Fluorescence spectra of 6,11-DHNpQ in methanol (ÈÈ),benzene (É É É) and acetonitrile (È È È)

emission at 560 nm except for methanol (Table 2). In meth-anol a concentration-dependent biexponential decay pattern isfound when monitoring at 560 nm. The contribution of theshorter lifetime component increases whereas that of thelonger lifetime component decreases with increase in concen-

Fig. 5 Fluorescence spectra of varying concentration of 6,11-DHNpQ in methanol ; (1) 2.4 lM, (2) 6.0 lM, (3) 8.0 lM, (4) 25.0 lM, (5)45.0 lM

Table 3 Fluorescence lifetime data of 6,11-DHNpQ in methanol atdi†erent concentrationsa

q1/ns q2/nsconcentration/lM (rel. %) (rel. %) s2

8.80 4.18(24) 7.35(76) 1.37114.43 4.38(18) 7.24(72) 1.10821.74 5.11(60) 7.52(40) 1.075

nm, nm (second emission peak).a jex \ 265 jem\ 560

tration of the quinone (Table 3). The shorter lifetime is com-parable with that in other solvents. There is no change in thelifetime with concentration of the quinone in all other sol-vents.

The above observations in methanol are summarized inScheme 1, where the reactions (a), (b) and (c) yield the specieshaving similar structures although there is a di†erent formobtained in reaction (d). However, the complex and the speciesfrom the reaction (d) are structurally similar, as observed fromtheir spectral characteristics.

DiscussionThe quinone 6,11-DHNpQ can exist in two di†erent forms insolution (Scheme 2) depending on the nature of the solvent.9A weak dimer is also expected in some of the solvents, e.g.methanol, at higher concentration of the quinone. At lowerconcentration of the quinone in methanol molecular structureII (Scheme 2) could be the likely structure since methanol is astrong hydrogen-bond donating as well as accepting solvent.However, as the concentration of the quinone increases, twoquinone molecules may come closer and form a weak dimervia intermolecular hydrogen bonding. In this process the indi-vidual molecule may acquire a structure similar to I (Scheme2), which is similar to the form existing in non-polar solventssuch as cyclohexane and also in polar but non-hydrogenbonding solvents such as acetonitrile.

An emission pattern similar to those in non-polar solventsand also in polar but non-hydrogen-bonding solvents isexpected in methanol at higher concentrations of the quinone.This suggests that the total number of hydrogen bondsbetween the solute and the solvent molecules is reduced as theconcentration of the solute increases. Hence the 555 nmabsorption band, assigned to the hydrogen-bonded complex(Scheme 2) is expected to be reduced as the concentration ofthe quinone in methanol is increased. This is the same trendas we experimentally observed. The equilibrium constant (KD)

Scheme 1

Scheme 2

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for the dimer formation was determined as follows :

Q ] Q A8B

KDQ2 ; KD \ [Q2]/[Q]2 (1)

where Q and are the monomer and dimer respectively. TheQ2total concentration of the quinone, under any given condi-CTtion will be :

CT \ [Q] ] 2[Q2] (2)

[Q2]\ (CT [ [Q])/2 (3)

From the absorption, Ñuorescence and Ñuorescence excita-tion spectra it is assumed that the red-shifted absorption bandwith maximum at 555 nm arises only from the monomerhydrogen-bonded complex with the methanol molecules,whereas both the monomer as well as the dimer have di†erentcontributions in the absorption at \555 nm. Similarly, theemission maximum at 530 nm only arises from the dimer forwhich the closed structure I exists. The molar absorption coef-Ðcient, e, of the monomer has been determined by taking aknown weight of the quinone in a Ðxed volume of methanolon the assumption that at very low concentration of thequinone only monomers will exist and with increase in con-centration the fraction of dimer in the solution increases(Table 1). The total concentration of quinone was determinedby diluting the parent solution by a known amount and thencalculating its concentration from the absorbance value. Theconcentration of the monomer in the concentrated solutionwas calculated from the absorbance value at 555 nm. The con-centration of the dimer was calculated using eqn. (3). Theequilibrium constant, was calculated from solutions of dif-KDferent total concentration of quinone and was found to be2.1] 104 dm3 mol~1. The molar absorption coefficient of thedimer was calculated as follows :

Aj\ ejQ[Q] ] ejQ2[Q2] (4)

where is the absorbance at wavelength j, is the molarAj ejQabsorption coefficient of the monomer molecule and is theejQ2molar absorption coefficient of the dimer molecule at thatwavelength ; thus calculated was found to be 12 800 dm3ejQ2mol~1 cm~1 at 480 nm and 10 900 dm3 mol~1 cm~1 at 515nm.

The molecule 6,11-DHNpQ forms a hydrogen-bondedcomplex ca. 555 nm) with methanol molecules, estab-(jmaxabslished by successively adding methanol to an acetonitrile solu-tion (Fig. 2). The closed structure I in Scheme 2 is theprobable structure in acetonitrile where there is no suchhydrogen bond between the solute and the solvent. Two meth-anol molecules probably bind to the quinone and the equi-librium can be written as follows :

Q ] 2MeOH A8B

K21 : 2 complex (5)

The equilibrium constant is obtained from the equation :14K21

[MeOH]2\

K2[Q]0(e2[ e)A2

[ K2 (6)

where is the initial concentration of the quinone ; e and[Q]0are the absorption coefficients of the quinone alone and thee2apparent absorption coefficient of the 1 : 2 complex respec-tively and is the absorbance of the complex. From theA2linear plot of [MeOH]~2 vs. in the region where e \ 0,A2~1

can be found from its intercept and can be found fromK2 e2the slope. From the measurements at 555 nm, was esti-K2mated to be 0.043 dm6 mol~2 (Fig. 6). The absorption coeffi-cient for the complex was calculated from the slope usinge2the value of and matches with the values obtained for theK2 ,monomer (Table 1).

From the decrease of the ratio in methanolA555 nm/A515 nmeither by increasing the concentration of the quinone or by

Fig. 6 [MeOH]~2 vs. (absorbance)~1 plot at 555 nm in acetonitrile

addition of water to a dilute solution of the quinone in meth-anol, it is thus expected that there is a reduction in thenumber of intermolecular hydrogen bonds between thequinone and the methanol molecules in both cases. At lowerconcentrations of the quinone, the absorbance at 555 nm iscomparable to that at 515 nm. At higher concentrations of thequinone the absorbance at 555 nm becomes less than at 515nm. This is explained in that these two absorption bands arisefrom two di†erent species. Similarly, the origin of the 530 nmemission band, the intensity of which increases on increasingthe quinone concentration or by adding water to methanol,could be di†erent from the other two emission bands at 570nm and 605 nm. It is inferred that the 530 nm emission bandarises mostly from the closed structure I in Scheme 2 which isa dimer in a concentrated methanol solution. From the emis-sion patterns observed in waterÈmethanol it is evident thatthere is a structural similarity of the quinone in dilute solutionin waterÈmethanol and a concentrated solution of the quinonein methanol alone. These observations could be explained intwo ways. First, water molecules take part in stronger inter-molecular hydrogen bonding with methanol molecules soreducing the hydrogen bonding between the quinone andmethanol. Secondly, owing to the insolubility of the quinonein its neutral form in water, the presence of water could reducethe solubility of the quinone so resembling a concentratedsolution of the quinone.

The addition of a little acid (H`) to very dilute solutions ofthe quinone in methanol could result in a molecular structureIII (Scheme 3) which is similar to structure I in Scheme 2.Addition of H` reduces the contribution of the hydrogen-bonded structure between the quinone and methanol,resulting in a change of the emission pattern resembling thosein other non-polar or polar aprotic solvents and even in meth-anol at higher concentration of the quinone.

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Scheme 3

Addition of more H` ions leads to a complete disap-pearance of the 555 nm absorption band and a change in itsemission (Fig. 3) but in a concentrated solution of quinone itdoes not have any e†ect on its emission pattern. The pinkcolour of the solution due to the hydrogen-bonded complex inmethanol changes to yellow in presence of H`, where no suchcomplex is present. On addition of a little hydroxide (OH~) toa very dilute solution of quinone a molecular structure of typeIV (Scheme 3) which is similar to structure II (Scheme 2) isexpected to predominate, where the intermolecular hydrogenbonding between the quinone and its solvent sphere increasessubstantially, causing an increase in the 555 nm absorptionband and a reduction in the peak ratio (Fig. 3).I530 nm/I570 nmThus the complex and the species obtained in the reaction (d)(Scheme 1) must be of similar structure. The equilibrium con-stant in Scheme 1 and the equilibrium constant in eqn.Kc K2(6) are di†erent as they are studied in di†erent environments.The complex obtained in Scheme 1 corresponds to the openstructure II in Scheme 2 whereas, the complex in eqn. (6) cor-responds to the closed structure I. We could not determine theequilibrium constant in Scheme 1, but it is evident thatKcn [ 2 where n is the number of methanol molecules bonded toit ; four methanol molecules are assumed in structure II inScheme 2 indicating a complete hydrogen-bonding pattern.

It is found from quantum yield measurements that the Ñuo-rescence is quenched in solvents such as benzene, DMF,DMSO, 1,4-dioxane and acetone and the quantum yield / islowest in DMSO. From a comparison of the Ñuorescencespectra of the quinone in cyclohexane, cyclohexaneÈbenzeneand benzene it is found that a weak exciplex emission overlapswith the pure quinone emission in the latter two solventsystems. An exciplex formation or sometimes even strongerinteraction such as charge- or electron-transfer is thusexpected to be responsible for the above quenching and thusthe low Ñuorescence quantum yields of the quinone in thesesolvents. Similar observations were made for quinizarin andother hydroxy-substituted anthraquinones.9

Steady-state quenching of the quinone Ñuorescence incyclohexane solution by benzene follows the usual SternÈVolmer (SV) linear relationship. The bimolecular quenchingconstant obtained from the slope of the SV plot and thekqlifetime value in cyclohexane is of the order of 8.18] 107 dm3mol~1 s~1. The dynamic quenching (i.e. the reduction of theÑuorescence lifetime) does not follow a SV relationship. Theplot of vs. benzene concentration in cyclohexane (Fig. 7),q0/qwhere and q are the Ñuorescence lifetimes in the absenceq0and in the presence of the quencher, respectively tends to alimiting value at higher concentrations of benzene. A plot of

vs. [benzene] plot in cyclohexaneFig. 7 q0/q

vs. [benzene]~1 yields a straight line, also indi-(q~1[ q0~1)~1cating that the quenching is due to exciplex formation.15h17The quenching by DMSO in cyclohexane could not be mea-sured owing to immiscibility of DMSO in cyclohexane.However, the quenching by DMSO in acetonitrile solutionhas been investigated following steady-state Ñuorescencequenching. The quenching rate constant calculated fromkqthe SV plot is 2.2 ] 107 dm3 mol~1 s~1. For this quenchingthere is no change in the Ñuorescence spectra of the quinoneexcept a reduction in the Ñuorescence intensity as the DMSOconcentration is increased. This indicates that the intermediateexciplex (AÉ É ÉQ)* is unstable in polar solvents such as aceton-itrile (and also in pure DMSO) and is quickly charge-separated to give ion-pairs (A~] Q`).

Exciplex formation in the present systems can be expressedby Scheme 4 where A is the Ñuorophore (quinone) and Q isthe quencher.

According to this exciplex formation and decay scheme wehave the relations,15h17

I0/I\ 1 ] KSV[Q]\ 1 ] kq q0[Q] (7)

q0/q\1 ] kq q0[Q]

1 ] K[Q](8)

[q~1[ q0~1]~1\ MK~1(kp [ q0~1)~1N[Q]~1

] (kp[ q0~1)~1 (9)

where K is the equilibrium constant for the exciplex(\k1/k2)formation, is the inverse of thekp\ (kf] knr ] kcs\ qex~1)exciplex lifetime. From the analysis of the vs.[q~1[ q0~1]~1[Q]~1 straight-line plot the exciplex formation constant (K)and the exciplex lifetime have been estimated for the(qex)quinoneÈbenzene system in cyclohexane as K \ 5.43 and

ns respectively.qex \ 5.15Fluorescence lifetimes of the quinone in di†erent solvents

are given in Table 2. Single-exponential Ñuorescence decaysare found for all cases where nm (highest energyjcm\ 530band). At nm (the middle band) similar lifetimejem \ 560values were obtained from the single-exponential decay curvesexcept for methanol where a double-exponential analysis gives

Scheme 4

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a better Ðt to the observed Ñuorescence decay. With increasein the concentration of the quinone in methanol the percent-age of the shorter lifetime component increases and that of thehigher lifetime component decreases (Table 3). The shorterlifetime component observed from 560 nm peak matches withthe single lifetime observed from the 530 nm peak at high con-centration of the quinone and was assigned to the dimer andthe higher lifetime was assigned to the monomer. This indi-cates that with increase in concentration of quinone the rela-tive percentage of the lifetime corresponding to the dimerincreases, but that corresponding to the monomer decreases.This clearly shows that the origin of 530 and 560 nm bandsare di†erent. This unusual characteristic of the Ñuorescencedecay of 6,11-DHNpQ in methanol is attributed to the forma-tion of a hydrogen-bonded quinoneÈsolvent complex in theground state. Our schemes on 6,11-DHNpQ explain manyphotophysical properties which were not known previouslyfor its lower analogues quinizarin and naphthazarin.

authors are grateful to Dr. D. K. Palit and Dr. H. Pal forThehelpful discussions.

References1 M. W. Rembold and H. E. A. Kramer, J. Soc. Dyers Colour.,

1980, 96, 122.2 A. F. Bordie, in Biochemistry of Quinones, ed. R. A. Morton, Aca-

demic Press, New York, 1965.

3 J. R. Brown, Prog. Med. Chem., 1978, 15, 125.4 B. Kalyanraman, E. Perez-Reyes and R. P. Mason, Biochim.

Biophys. Acta, 1980, 630, 119.5 E. J. Land, T. Mukherjee and A. J. Swallow, J. Chem. Soc.,

Faraday T rans. 1, 1983, 79, 391.6 T. Mukherjee, E. J. Land, A. J. Swallow, J. M. Bruce, P. C. Beau-

mont and B. J. Parsons, J. Chem. Soc., Faraday T rans. 1, 1988,84, 3423.

7 T. Mukherjee, A. J. Swallow, P. M. Guyan and J. M. Bruce, J.Chem. Soc., Faraday T rans. 1, 1990, 86, 1483.

8 J. P. Rasimas and G. J. Blanchard, J. Phys. Chem., 1995, 99,11333.

9 D. K. Palit, H. Pal, T. Mukherjee and J. P. Mittal, J. Chem. Soc.,Faraday T rans. 1, 1990, 86, 3861 and references therein.

10 F. Hibbert and K. J. Spiers, J. Chem. Soc., Perkin T rans. 2, 1987,1617.

11 H. Inoue, M. Hida, N. Nakashima and K. Yoshihara, J. Phys.Chem., 1982, 86, 3184.

12 S. R. Flom and P. F. Barbara, J. Phys. Chem., 1985, 89, 4489.13 D. K. Palit, H. Pal, T. Mukherjee and J. P. Mittal, Chem. Phys.,

1988, 126, 441.14 H. Pal, D. K. Palit, T. Mukherjee and J. P. Mittal, J. Chem. Soc.,

Faraday T rans. 1, 1993, 89, 683.15 Y. Tanimoto and M. Itoh, Chem. Phys. L ett., 1978, 57, 179.16 H. Mizukoshi and M. Itoh, Bull. Chem. Soc. Jpn., 1980, 53, 590.17 T. Nakagawa, S. Kohtani and M. Itoh, J. Am. Chem. Soc., 1995,

117, 7952.

Paper 6/07875C; Received 20th November, 1996

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