Indian Journa l of Chemistry Vol. 42A, March 2003 , pp. 484-492

Mixed ligand binuclear copper(l) complexes containing CUI2NS chromophores. Observation of emissinn(' from MLCT excited

states involving two diffc.LvIH ligands

Derek A Tocher

Department of Chemistry , University College Londo n, 20 Gordon Street, London WCIH OAJ , UIK

and

Michael G B Drew

Department of Chemistry, University of Reading, Whitekni ghts, Reading RG6 6A D, UK

and

Shubhamoy Chowdhury, Pradipta Purkayastha &. Dipankar Datta*

Department of Inorganic Chem istry, Indian Association for the Culti vation of Science, Calcutta 700 032. Indi a

Received 20 November 2002

Using the 1:2 condensate (L) of diethylenetri amine and benzaldehyde as the main ligand, binuclear copper( l) complexes [Cu2L2(4,4'-bipyridine)](CI04h.0.5H20 (la) and [Cu2L2(1,2-bis(4-pyridyl)ethane)](CI04l2 (lb) are synthesised. The two metal ions in la are bridged by 4,4'-bipyridine and those in Ib by l,2 ·bis(4-pyridyl)ethane. From the X-ray crystal structure of la, each meta l ion is found to be bound to three N atoms of L and one of the two N atoms of the bridg ing li gand in a di storted tetrahedra l fashion. The Cu(l)-N bond lengths in la lie in the range of 1.998(5)-2.229(6) A. Electrochem ical studi es in dichloromethane (DCM) show that the CU '2NS moieties in la and Ib are composed of two e~se nti a ll y non­interacting Cu'N4 cores wi th Cu'u, potential of 0.44 V vs. SCE. While la di splays metal induced quenching of the inherent em ission of 4 ,4'-bipyridine in DCM solution, Ib exhibi ts two weak emission bands in DCM solution at 425 and 477 nm (tota l quantum yield = 3.59 x 10.5) origi nat ing from MLCT exc ited states. With the he lp of Ex tended HUckel calcu lati ons it is establi shed that the higher energy emi ssion in Ib is from Cu(l) ~ bridg ing- ligand charge transfer exc ited state and the lower energy one in 1 b from Cu( l) ~ L charge transfer excited state.

Photo luminescent homoleptic mononuclear copper(l) complexes are known mainly with the derivatives of 1,1O-phenanthroline t-5. Few other N-donor ligands are capable of renderi ng such copper(l) complexes6

-9

.

However, photoluminescent binuclear copper(J) complexes of N-donor ligands are rare. So far, there are only three examples-Cu2(Ph-NNN-Phh containing a Cut2N4 core 10, a CU l

2N6 chromophore assembled with 2,2'-bis[( 4R)-phenyl] -1 ,3-oxazoline" and a CUt2NS chromophore isolated using bi s(pyri­dinal )ethylenedi imine I2. These three complexes are homolepti c. Herein we describe a binuclear copper(l) complex of mixed N-donor ligands containing a photoluminescent CU l

2Ns chromophore. To be specific, in the present complex, a bidentate pyridyl ligand connects the two copper(I) centers, each of which is bound to a tridentate N-donor ligand. This allows us to observe emiss ions from MLCT (metal-to­ligand charge transfer) excited states involving two differen t li gands.

Materials and Methods Deuterated dimethylsulphoxide (DMSO-d6) (99.9 atom % D) was purchased fro m Aldrich. [Cu(CH3CN)4]CI04 was prepared by a reponed procedure I3

. C, Hand N analyses were performed using a Perkin-Elmer 240011 analyse r. Copper was estimated gravimetrically as CuSC:-J. FTIR spectra (KBr di sc; 4000-400 cm-I) were recorded on a Nicolet Magna-IR spectrophotometer (se r· es II ), UV -vis spectra on a Shimadzu UV - 160A spectrophotometer and NMR spectra (in DMSO-d6 ; reference, TMS) by Brucker DPX300 spectrometer. Solu tion conductivity was measured by a Systronics (India) direct read ing conductivity meter (model 304). Cyclic vo ltammetry was performed using EG&G PARC electrochem ical analysis system (model 250/5/0) under dry nitroge n atmosphere in conventional three electrode configurations in puri fied dichloromethane (DeM ) with tetrabutylammonium perchlorate (TBAP) as the supporting electro lyte. A planar EG&G PARe G0229

TOCHER el al.: EMISSIONS FROM MLCT EXCITED STATES 485

glassy carbon milli electrode was used as the working electrode in cyclic voltammetry. Under the experimental conditions employed here, the ferrocene-ferrocenium couple appears at 0.47 V vs. SCE (saturated calomel electrode) with a peak-to­peak separation of 90 mY at scan rate v = 50 mY s -, . All photoluminescence studies were carried out in air using a Spex Fluorolog spectrofluorimeter. The quantum yields (<I» of the emissions of various compounds involved in the present study have been determined against quinine sulphate'4 in 0 .1 N H2S04. AM I and Extended Hi.ickel calculations were performed by using HyperChem package purchased from Hypercube Inc., Canada.

Syntheses L-Benzaldehyde (5. 1 cm3

, 50 mmol) and freshly distilled diethylenetriamine (dien) (2.7 cm3

, 25 mmol) were taken in 40 cm3 of anhydrous methanol and refluxed for 6 h. Then the reaction mixture was evaporated on a water bath to 10 cm3 to obtain a yellowish white viscous liquid. Then it was extracted with 125 cm3 of hot petroleum ether (40-60°C). The colourless extract was evaporated at room temperature under reduced pressure to obtain a white viscous mass which was kept in vacuo over fused CaCh for two days; yield: 5.53 g (79%). Elemental analyses were consistent with the stoichiometry C, sH2,N3 [Found: C, 76.31 ; H, 8.16; N, 14.87. Calc: C, 77.37; H, 7.58 ; N, 15.04%], FfIR data (cm-'): 1645(vs) (C=N). UV/vis (OCM) : }Jnm (Eldm3 mol" cm") : 245 (22 100).

[Cu2L2(4,4'-bpy)](CL04hO.5H20 (la)-L (0 . .56 g, 2 mmol) and 4,4'-bipyridine (4,4'-bpy) (0.16 g, 1 mmol ) were dissolved in 20 cm3 of anhydrous degassed methanol. To this solution, freshly prepared [Cu(CH3CN)4]Cl04 (0.66 g, 2 mmol) was added under dry N2 atmosphere and stirred for 15 min . The reddish brown compound precipitated was filtered, washed with 10 cm3 of diethylether and stored in vacuo over fused CaCho It was recrystallised from a 1:3 mi xture of OCM and petroleum ether (40-60°C); yield: 0.50 g (48 %). Elemental analyses were consistent with the stoichiometry C46Hs,Cl2Cu2NsOs.s [Found: C, 52.68; H, 4.95; N, 10.60 ; Cu, 12.08. Calc: C, 52.60; H, 4.90; N, 10.67 ; Cu, 12.11 %]. FfIR data (cm-') : 1629 (s), 1605(m) (C=N); lO92(vs), 627(m) (Cl04). AM/mho cm2 mol" (CH30H): 185 (1 :2 electrolyte) . UV/vis (OCM): }Jnm (Eldm3 mol" cm-') : 245 (65 700), 366sh (6400).

[Cu2L2( J,2-bpye)](CL04h (Jb)--L (0.28 g, 1 mmol) and 1,2-bis(4-pyridyl)ethane (l,2-bpye) (0.09 g, 0.5 mmol) were dissolved in 15 cm3 of anhydrous degassed methanol. To this solution, 0.33 g ( I mmol) of freshly prepared [Cu(CH3CN)4]CI04 was added under dry N2 atmosphere and stirred for 15 min . The deep yellow compound precipitated was filtered, washed with 10 cm3 of diethylether and stored ill vacuo over fused CaCho It was recrystalli sed from a 1:3 mixture of OCM and petroleum ether (40-60°C); yield: 0.28 g (52%). Elemental analyses were consistent with the stoichiometry C4sHs4ChCU2NsOs [Found: C, 53 .02; H, 5.14; N, 10.49; Cu, 11.85. Calc: C, 53.91 ; H, 5.09; N, 10.48 ; Cu, 11.89%]. FfIR data (cm"): 1630(m), 1605(m) (C=N); 1145(m), 111 5(s), 1087(vs), 630(m) (CI04). AM/mho cm2 mol" (CH30H): 203 (1:2 electrolyte). UV /vis (OCM ): }Jnm

(Eldm3 mol" cm-'): 247 (41 100), 306sh (7 400), 350sh (5 000).

Caution--Though we have not met with any incident during our studies, care should be taken in handling the metal complexes as perchlorate salts are potentially explosive. These should not be . prepared and stored in larger amounts .

X-ray crystallography Reddish orange single crystals of 1a were grown by

the direct diffusion of petroleum ether (40-60°C) into a dilute OCM solution of the complex. Oiffraction data were collected at 293(2) K with Mo-Ka radiati on using the MARresearch Image Plate System. The crystals were positioned at 70 mm from the Image Plate. 100 frames were measured at 2° intervals with a counting time of 2 mins. Oata analysis was carried out with the XOS program'S to provide 6526 independent reflections [ROnt) = 0 .0435], The structure was solved using direct methods with the SHELXS-86 program. ' 6

Non-hydrogen atoms were' refined with ani sotropic thermal parameters. The ;lydrogen atoms bonded to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached . An empirical absorption correction was carried out using DIFABS.17 The structure was refined on F2 usi ng SHELXL-93' s to Rl = 0 .0716 and wR2 = 0.2264 for 2607 reflections with J > 2a(/) . The positional parameters along with equivalent isotropic thermal parameters for the non-hydrogen atoms in 1a are given in Table I .

486 INDIAN J CHEM, SEC A, MARCH 2003

Table I-Final coord inates (in A) and equivalent isotropic disp lacement parameters (i n A3) for the non-hydrogen atoms in

la"

Atom

Cu i

Nt

N4

N7

N2 1

C2

C3

C5

C6

CI I

CI2

CI3

CI4

CI5

CI6

CI7

C22

C23

C24

C25

C26

C71

C72

C73

C74

C75

C76

C77

*CII

*0 11

*0 12

*0 13

*0 14

*C12

*0 15

*0 16

*0 17

x 0.24726(4)

0.3164(3)

0.2265(3)

0.2997(3)

0.1576(3)

0.2908(4)

0.2825(4)

0.2460(5)

0.3 147(4)

0.3680(3)

0.3996(3)

0.3882(4)

0.4196(4)

0.4639(4)

0.4753(4)

0.4439(4)

0.1687(4)

0.1091(3)

0.0328(3)

0.0214(4)

0.0847(3)

0.3313(4)

0.3237(4)

0.3866(5)

0.3809(6)

0.3 132(6)

0.2493(6)

0.2542(4)

0.02524(13)

-0.0428(7)

-0.0148(8)

0.0846(9)

0.0444(9)

O.02524( 13)

0.0818(8)

0.0754(10)

0.0037(10)

*0 18 -0.0416(8)

*0100 1/2

Y 0.97401(4)

1.0777(3)

1.0436(3)

0.8886(3)

0.9671(2)

1.1451 (4)

1.1142(4)

0.9815(5)

0.9277(4)

1.0948(3)

1.0368( 4)

0.9515(4)

0.900 1(4)

0.9327(5)

1.0168(5)

1.0687(4)

0.9362(4)

0.9346(3)

0.9645(3)

0.9960(4)

0.9962(4)

0.8177(4)

0.7690(3)

0.7184(4)

0.6754(5)

0.6770(5)

0.7270(5)

0.7724(4)

O. 79449( I I)

0.7842(8)

0.8465(9)

0.8428( 1 0)

0.7125(7)

0.79449( 11 )

0.7804(9)

0.8303(10)

0.7097(7)

0.8401(9)

1.257(2)

z

0.09334(4)

0.1334(3)

-0.0173(3)

0.0359(2)

0.1 488(3)

0.0783(4)

-0.0022(3)

-0.07 18(4)

-0.0344(3)

0.1945(3)

0.2575(3)

0.2502(3)

0.3115(4)

0.3799(4)

0.3870(4)

0.3262(3)

0.2205(3)

0.262 1 (3)

0.2288(3)

0.1545(3)

0.1173(3)

0.0526(3)

0.1205(3)

0.1561 (4)

0.2203(5)

0.2488(5)

0.2136(5)

0.1489(4)

0.47092(14)

0.3984(7)

0.5177(8)

0.4530(9)

0.4935(8)

0.47092( 14)

0.5531 (7)

.4280(8)

0.4536(9)

0.4822(9)

1/4

Ucq

0.0538(3)

0.0539( 17)

0.0620( 17)

0.0513( 17)

0.0493(17)

0.068(2)

0.072(3)

0.077(3)

0.067(2)

0.0536( 19)

0.0540( 19)

0.059(2)

0.074(2)

0.076(3)

0.072(3)

0.061(2)

0.057(2)

0.0526( 19)

0.0468( 17)

0.0522( 17)

0.0523( 19)

0.058(2)

0.059(2)

0.076(3)

0.095(4)

0.097(4)

0.091(3)

0.069(2)

0.0950(9)

0.1347( 17)

0.1347(17)

0.1347(17)

0.1347( 17)

0.0950(9)

0.1347( 17)

0.1347(17)

0. 1347(17)

0.1347(17)

0. 188( II )

" UC(I is 113 of the trace of the orthogonali sed Uij tensor. Starred atom sites ha ve a S.O.F less than 1.0. For atom labelling scheme, see Fi g. 3.

Crystal data The cell constants and crystallographic data are: Mol. Formula C46HsICl2Cu2NgOss; mol. wt. 1049.92; monoclinic; space group C2/c; a = 17.06(3) A; b = 16.08(3) A. ; c = 17.84(3) A.; f3 = 102.64(1)°; V = 4775(14) Al; Z = 4 ; F(OOO) = 2172; Dc= 1.461 g cm·) ;

J..l = 1.065 mm· l.

Results and Discussion The tridentate N-donor ligand used in the presen t work is the 1:2 condensate (L) of dien and benzaldehyde. Its I H NMR spectrum is found to be very rich. Though we have not been able to decipher it, it is evidlent that it does not correspond to the structure I shown in Chart 1. However, its 13C NM R spectrum and the DEPT (dynamic enhancement by polarisation transfer) 135 spectrum (Fig. 1) are found to be consistent with the structure II in Chart I. The l3C NMR spectra clearly show presence of two methine carbon atoms of considerably different types and four methylene carbon atoms i L. Of the two methine carbon atoms, one is identified as an imino carbon (C6) from its 8 value of 162 .05 ppm l 9

. The other methine carbon atom which appears at a much higher field of 83.81 ppm is assigned as C I . For comparison, we mention that the alkyl quaternary carbon atom in 8,8a-diphenyl-l ,2,3,5,6,8a-hexahydro­imidazo[ 1 ,2-a]pyrazine, which has a chemi ca l environment very similar to that of C 1, resonates at 82.08 ppm in CDCI3 (ref. 19). Si nce the chemical environments of C6 and C I are quite different, the two quaternary aromatic C atoms appear separate ly at 137.02 and 142.23 ppm. Thus we ass ign structure II (Chart 1) to L. It should be noted that two closely spaced resonances are observed for each of C4, C5 and C6 in Fig. 1. This is because two isomers are possible for II depending on the orientation of the phenyl group on C6 relative to the two H atoms on CS. From our AM 1 calculations2o, the energy difference between these two isomers (Fig. 2) in the gas phase is found to be only 1.4 kcal mol" I.

Reaction of L with [Cu(CH3CN)4]CIO.. In . anhydrous methanol in 1: 1 molar proportion under N 2 atmosphere leads to a yellow reaction mixture from which no definite compound can be isolated . However, when the reaction is carried out in presence of 4,4'-bpy 111 O.S molar proportion, a reddish brown precipitate IS obtained. Recrystallisation of this precipitate from a 1:3 mi xture of DCM and petroleum ether (40-60°C) yields [CulL2( 4 ,4'­bpy)](Cl04h .0.SH20 (la).

Ph r-\ r-\ Ph )==N N N==(

H H H

C hart 1- Two possible structures of L IOgether with the numbering scheme for some of the C atoms in structure II

TOCHER et al. : EMISSIONS FROM MLCT EXCITED STATES 487

(a)

C6 CI C5 C4

(b)

ii' f' I Ii i I I ' , , ' l ' Ii i I , ' I " Iii i I I ti" f I, i ' I i • • ....,....,--r -'. 'I' " II I I f 'I" I I

pp. 160 140 120 100 80 60 40

Fig. I-(a) Normal DC NMR spectrum (300 MHz) of Lin DMSO-d6 (reference, TMS) with some possible assignments; the resonances around 40 ppm are due to the solvent. (b) DEPT 135 spectrum of L in the same solvent highlighting the phase differences between the methylene C atoms and the methine ones. It should be noted that the aromatic quaternary C atoms resonating at 137.02 and 142.23 ppm in the normal "c NMR spectrum [(all do not appear in the DEPT spectrum [(b)]. For the identifications of the various C atoms. see structure II of Lin Chart I.

II

Fig. 2-Line drawings of the stereo views of the two isomers corresponding to structure II of L (Chart I ) as optimised by AM I method. Isomer A is calculated to be more stable than B in the gas phase by 1.4 kcal mOrl.

From X-ray crysta llography, the structure of 1a is found to consist of di screte [Cu2L2(4,4'-bpy)f+ cat ions and two CI04- anions together with a water molecule on a two-fold axis w ith 50% occupancy_ The cation , which contai ns a crystallographic two-

fold axis, is shown in Fig. 3. Interesting ly, coordinated L has the s tructure I of Chart I . Evidently, upon coordination the five membered ring in L opens up to assume structure I. Each meta l center in the cation has a very distorted tetrahedral N.j

coordination sphere. 4,4' -bpy serves as a bridge between the two copper atoms, each of whi ch is bonded to three nitrogen atoms of an L fragment. With the amino N, the metal forms a bond [2.229(6) A] much longer than those with the imino ones [2 .077(5) and 2.034(5) A] or that with the pyridyl one [1 .998(5) A]. The angle between the two pyridine

rings in the coordinated 4,4'-bpy unit is 49 .6°.

When L, [Cu(CH3CN)4]CIO.j and 1,2-bpye are reacted in I : 1 :0 .5 molar proportion in a manner similar to that adopted to synthes ise la, deep yel low [Cu2Lz( I ,2-bpye)](C104h (lb) is obtained. So far. we have not been able to grow single crystals o f 1 b

488 [NOlAN J CHEM, SEC A, MARCH 2003

Fig. 3- The structure of the cation [Cu2L2(4,4'-bpy)f+ in la with ellipsoids at I S% probability show ing the numbering scheme for the non-hydrogen atoms. Selected bond di stances (A) and angles (") : Cu l-N I 2.077(6). Cul -N2 1 1.998(6), Cul-N4 2.229(7), Cul­N7 2.034(6), N I-C II 1.273(7), N7-C71 1.268(8), N J-Cu I-N4 82.S(2). N4-Cul-N7 84.00(2), NI-Cul-N7 IIS.9(2 ), N4-Cul-N21 11 8.4(2), NI-Cul-N21 108.2(2), N7-Cul-N21 132.9(2).

8H 811

411

J 8H

suitable for X-ray crystallography. However, by comparing its IH NMR spectrum with that of 1a (Fig. 4), we conclude that 1b has a structure essentially similar to that of la, i.e. we have a CU l

2Ns chromophore in 1b also with L assuming structure I and 1,2-bpye acting as a bridge between the two copper atoms by coordinating through the pyridyl N's .

The two complexes 1a and 1b are stable towards aerial oxidation in the solid state for about a month . However, their solution stability in air depends on the solvent. In OCM, 1a is stable for about 20 min while 1b is so for only 10 min. These are less stable in polar solvents like methanol , OMSO etc:

The electrochemical behaviour of 1a and 1b has been studied by cyclic voltammetry at a g lassy carbon electrode in purified OeM under dry N2 atmosphere. Complex 1b displays a quasireversible voltammogram with a half-wave potent ial of 0.44 V vs. SCE; the peak-to-peak separation is 120 m V at v = 50 mV S·I (Fig. 5) . Comparison of the observed peak currents with those of the ferrocene-ferrocenium couple at the same scan rates shows that in 1b we have two one-step one-electron transfers, not a one­step two-electron transfer. This indicates that the two copper centers behave independent of each other, i.e . there is no interaction between them even in the oxidised state?1 This was, however, an ticipated in

s

4H

811

s

411

8H 4H

411 I 4H I

~~LJ,---~U lJ '-L. _ _ ~_

Fig. 4-300 MHz IH NMR spectra of la (lower trace) and lb (upper trace) in DMSO-d6 (reference, TMS). The peaks marked as S are due to the solvent.

TOCHER ef al .: EMISSIONS FROM MLCT EXCITED STATES 489

. t

0.9

T 2)J..A i

I I

I /

V

I I

/ I

I I

-------~ 7

E/V vs seE

Fig. 5-Cyclic voltammograms of la (----; concentration c = 0.97 ml110l dm·J ) and Ib (- ; c = 0.43 mmol dm·3) in DCM (0.1 mol dm') in TSAP) at a glassy carbon electrode at v = 50 mY 5".

view of the presence of the intervening -CH2CHr group in the bridging ligand. Thus the observed couple in lb can be represented by Eq. (1) which means that we actually have two CU IllI couples in lb

. .. (I)

with same redox potentials. However, in the case of la, while a single anodic wave is observed, the cathodic response is composed of two waves of unequal current heights (Fig. 5). The nature of the cathodic response suggests that for la the oxidised species generated in the electrode process is not at all stable . Consideration of the anodic peak currents indicates that in la also couple (I) is operative, i.e. the two metal ions are non-interacting in la also irrespective of the oxidation state. In binuclear copper( lI ) complexes, 4,4'-bpy as a bridge is known to bring about only very weak interactions between the two metal centers. For example, the intramolecular exchange coupling constant In

[Cu2(d ienh( 4,4'-bpy)(Cl04h ](Cl04) 2, where 4,4'-bpy connects the two copper(II) centers22

, is - -1 cm" . As such, the crystallographically observed non­cop lanari ty of the two pyridine rings in the 4,4'-bpy frag ment precl udes the possiblity of any signifi cant interaction between the two copper atoms in la.

Our electrochemical results show that the complexes la and 1b are composed of essen tially two non-communicating CulN4 chromophores . Such a chromophore, at least in lb, has a CU IlIl potential of 0.44 V liS. SCE which is quite high when one compares it with that (0.06 V vs. SCE in aqueous

medium)23 in [CU(pY)4r (py == pyridi ne) . It is now

well understood that CU Il/ I potential of a Cu'N4 core increases with the 1t-acidity of the N-donor ligand involved and the extent of tetrahedral distorti on in the corresponding Cu ll N4 moiety . In our complexes, apart from the py fragments, the imino function s are

capable of acting as 1t-acids. There is ev idence for appreciable 1t back bonding between a C=N fragment and copper(l) in the IR spectra; the C=N stretch ing frequency of 1645 cm'l in the free ligand is lowered to - 1630 cm,l in 1a and lb.

In the electronic spectra in OCM, the ligand L displays a relatively narrow band at 245 nm with a high intensity (E) of 22 100 dm3 mor' cm" due to 1t ~ 1t* transition localised on the imino fragment. The intraligand charge transfer band in 4,4' -bpy appears at 238 nm in OCM with an E of 12 800 dm3 mor ' cm" . However, such a band in 1 ,2-bpye is found to be much weaker (E = 3 000 dm3 mor' cm" ) with the

position shifted towards longer wavelength (',1'111ax = 256 nm). These ligand centered transi tions are found to occur in the copper(I) complexes la and lb as an intense and somewhat broad band around 246 nm . The two metal complexes show additional charge transfer band(s) around 350 nm as shoulder(s) which can be assigned as MLCT one(s).

As a part of our on-going program on the search for the N-donor ligands other than the phenanthro lines that can yield photoluminescent copper(\) complexes8.9., 1.12, we have studied the emiss ion properties of la and lb. Our ligand L does not emit light in OCM at room temperature upon excitation at 350 nm. One of the lessons from the studies on the photophysics of the copper(I)-bisphenanth ro lines is that the copper(l!) center generated in the photoexcited state has to be stable enough. An idea about this aspect can be had beforehand from electrochemical measurements. For example, since in cyclic voltammetry we have found that [Cu2L2( 4.4' ­bpy)t+ generated in the electrode process ( I ) is unstable, we do not expect to observe an y photoluminescence from an MLCT exci ted state in the case of la. This is indeed found to be true. Our studies show that upon excitation at 350 nm (within the MLCT envelope) in OCM at room temperature. la displays severe quenching of the inherent emis. ion of the 4,4'-bpy fragment (Fig. 6) ; <1> in 4,4'-bpy is 1.83 (emission maxi ma Acm = 4 13 nm) and that in la 1. 73x 10'5 (Acm = 478 nm). The quenching is metal induced. It should be noted that quenchin g. 0 1" fluorescence of an organic fluorophore by a metal ion

490 INDIAN J CHEM, SEC A, MARCH 2003

400 450 500 550 600 650 700 Wavelength I nm

Fig. 6---Emission spectra of 4,4' -bpy (-) and la (----) in DCM at room temperature. Excitation wavelength, 350 nm. Absorbance at 350 nm: for 4,4"-bpy, 0.04; for la, 2.35.

400 450 500

Wavelength I nm

550 600

Fig. 7-Emission spectrum of Ib in DCM at room temperature. The curve drawn is not least-squares fitted. Excitation wavelength, 350 nm. Absorbance at 350 nm, 1.56.

is a very common phenomenon; only a very few cases are known24 where some metal ions enhance the fluorescence of the organic fluorophore. However, the situation in Ib is quite different where the corresponding copper(II) species is found to be stable in cyclic voltammetry. Moreover, 1,2-bpye does not

fluoresce like 4,4' -bpy upon excitation at 350 nm. Thus upon excitation at 350 nm (wi thin the MLCT envelope) in DCM at room temperature, complex Ib shows two weak and somewhat well-resolved emission bands with maxima at 425 and 477 nm (total <1> = 3.59xlO·5; Fig. 7). Owing to poor <1>, we have not

TOCHER et al.: EMISSIONS FROM MLCT EXCITED STATES 49 1

(e)

(b) J. II}

/' \\ . ". ' ) '.. /

I I I

~-:;.=:=:~:-----

I

Fi g. 8-Calculated contour diagrams of (a) HOMO, (b) LUMO, (c) LUMO+I and (d) LUMO+2 in [CuL(pyW fragment. LUMO is locali sed on C II-N I moiety and LUMO+2 on C71-N7 moiety. See Fig. 3 to identify CII, N I, C71 and N7. Relati ve energies (in eV) : (a) set to 0, (b) 1.13, (c) 1.82 and (d) 2.06.

been able to determine the lifetimes of these emissions.

Earlier in connection with the electrochemical studies, we have indicated that our complexes la and lb can be considered as composed of two isolated Cu'N4 chromo-phores. In the photophysics of Cu'N4 chromophores, observation of more than one emission band is rare4.9. Two emission bands are observed in some copper(l)-bi sphenan-throlines at lower temperatues4. In these cases, the two bands are thought to arise from a higher lying 'MLCT and a lower lying 3MLCT states ; the energy separation between these two states are expected to lie within 1000-2000 cm-14.9.25.26. The energy separation between the two emission bands in lb is found to be 2565 cm-' (from Fig. 7) which seems to be rather large. Moreover, in photoluminescent Cu'N4 chromophores, the emission from the 3MLCT state is thought to be much weaker than that from 'MLCr·9. But in lb the lower energy emission is much stronger than the higher energy one (see Fig. 7) . Thus the two emission bands in lb do not seem to arise from the triplet and

singlet configurations of the same MLCT excited state. In order to understand the origin of these two bands in lb, we have performed Extended HUckel calculations on the [CuL(py)t fragment of the cati on [Cu2L2( 4,4' _bpy)]2+ at the crystallographically determined geometry. The calculated contour diagrams of the HOMO (highest occupied molecul ar orbital), LUMO (lowest unoccupied mol ecul ar orbital) and two next energetically higher MO 's, LUMO+ 1 and LUMO+2, are shown in Fig. 8. As expected, the HOMO has a considerable meta l character. The other three subsequent higher MO's are ligand centered. The LUMO is locali sed on the imino fragment , the N atom of which forms a longer bond [2.077(6) A] with copper(I) and LUMO+2 on the other imino fragment of L. LUMO+ 1 is essenti al ly localised on the pyridine moiety. Thus electronic transition from HOMO to any of the three hi gher MO's can result in MLCT states. We reckon that the 477 nm emission band in lb arises from the MLCT excited state involving HOMO and LUMO, and 425 nm one from the MLCT excited state in volving

492 INDIAN J CHEM, SEC A, MARCH 2003

HOMO and LUMO+ 1. Another MLCT excited state involving HOMO and LUMO+2 is conceivable, the emission from which is expected to occur at a wavelength shorter than 425 nm. Since in the present case the excitation wavelength is 350 nm, we could not observe it. From the pictorial representations of the various MO's in Fig. 8, we can say that the higher energy emission band in Ib is from Cu(l) --t pyridine charge transfer excited state and the lower energy one from Cu(l) --t an imine-fragment (of L) charge transfer excited state. In view of the relative <I>'s of the two emission bands in Ib (Fig. 7), it can be concluded that an imino N is more effective than a pyridyl one in designing new photoluminescent Cu'N4 chromo-phores. This seems reasonable as [CU(PY)4t is not known to display emission(s). Further, even 2,2' -bipyridine, which is a close kin of 1,10-phenanthroline, has not been able to afford photoluminescent Cu'N4 chromophores . On the other halld, in recent times we have been able to provide new examples of photoluminescent homoleptic coppere/) complexes with ligands having unconjugated imino N's (refs 8,9,11).

Supplementary material Tables containing listings of positional parameters,

thermal parameters, all distances and angles for la are available from the authors.

Acknowledgement MGBD thanks EPSRC and the University of

Reading for funds for the Image Plate system and DD the Department of Science and Technology, New Delhi for financial support.

References Buckner M T & McMillin D R, J chem Soc, Chem COIllIll/lII, ( 1978) 759.

2 Cunningham C T , Moore J J, Cunningham K L H, Fanwick P E & McMillin D R, Inorg Chem, 39 (2000) 3638.

3 Miller M T & Karpishin T B, Illorg Clwl/, 38 (1999) 5246 . 4 Felder D, Nierengarten J -F, Barigelletti F & Armaroli N. J

Am chem Soc, 123 (2001) 6291. 5 Armaroli N, Chem Soc Rev, 30 (2001) 113. 6 Ichinaga A K, Kirchhoff J R, McMillin D R. Di etri ch- ·

Bucheker CO, Marnot P A & Sauvage J -P, II/org Chem . 26 (1987) 4290.

7 Riesgo E C, Hu Y -Z, Thummel R P, Scaltrito D V & Meyer G J, Illorg Chem , 40 (2001) 3413.

8 Chowdhury S, Patra G K, Drew M G B, Chattopadhyay N & Datta D, J chem Soc, Dallon TrailS. , (2000) 235.

9 Panja S, Chowdhury S, Drew M G B & Datta D, II/org Chem COllllllun,5 (2002) 304.

10 Harvey P D, Inorg Chelll, 34 (1995) 2019. II Patra G K, Goldberg I, Chowdhury S K. Maiti B C. Sarkar

A, Bangal P R, Chakravorti S, Chattopadhyay N, Tocher D A, Drew M G B, Mostafa G, Chowdhury S & Datta D. Nell' J Chem, 25 (2001) 1371.

12 Pal P K, Chowdhury S, Purkayastha P. Tocher D A & Dalla D, II/org Chelll COIllIllUII, 3 (2000) 585.

13 Hemmerich P & Sigwart C, Experielllia , 19 (1963) 488. 14 Demas J N & Crosby G A, J phys Chelll, 75 ( 1971 ) 991. 15 Kabsch W, J appl C,yslal/ogr, 21 ( 1988) 916. 16 Sheldrick G M, SHELXS-86, Program for crystal strllcllIre

solution, ACla C'y slal/ogr, A46 (1990) 467. 17 Walker N & Stuart D, DIFABS, Au{{ c,ysral/ogr. A39

(1983) 158. 18 Sheldrick G M, SHELXL-93, Program for crystal strllc lLire

refinement, University of Gottingen, (1993). 19 Bangal P R, Patra G K & Datta D, New J Chem. 24 (2000)

719. 20 Dewar M J S, Zoebisch E G, Healy E F & Stewart J J P. J

Am chelll Soc, 107 (1985) 3902. 21 Hasty E H, Wilson L J & Hendrickson D l'\. II/org Chcm. 17

(1978) 1834. 22 Julve M, Verdaguer M, Faus J , Tinti F, Moratal J, Monge A

& Gutierrez-Puebla E, II/org Chem , 26 (1 987) 3520. 23 James B R & Williams R J P, J chem Soc , ( 1961 ) 2007. 24 Purkayastha P, Chattopadhyay N. Patm G K & Dalla D.

Indian .I Chem, A39 (2000) 375 and refs therein. 25 Kirchoff J, Gamache R E, Jr., Blask ie M W, de l Paggio A A.

Lengel R K & McMillin DR , II/org Chem , 22 ( 1983) 2380. 26 Demas J N & Crosby G A, J phys Chem. 93 ( 1989) 5692.