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Inorganic Chemistry Communications 10 (2007) 593–595

Nonlinear optical properties in tetrametallic Fe3Mn complexeswith pseudo-C3 symmetry

In Su Lee a,*, Young Keun Chung b,*

a College of Environment and Applied Chemistry, Kyung Hee University, Gyeonggi-do 449-701, Republic of Koreab Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-742, Republic of Korea

Received 13 December 2006; accepted 12 February 2007Available online 20 February 2007

Abstract

New type of 3D chromophores [{CpFeC5H4ACR@CHA4-py}3Mn(CO)3]BF4 (1) with weakly interaction subchromophore were syn-thesized and found to display improved nonlinearity compared with their 1D reference systems [(CpFeC5H4ACR@CHA4-py)Mn(CO)5]BF4 (2).� 2007 Elsevier B.V. All rights reserved.

Keywords: Nonlinear optics; Organometallic chromophore; Hyperpolarizability; Manganese carbonyl complex; Ferrocene

There has been considerable interest in the nonlinear opti-cal (NLO) properties of molecules that possess a higher sym-metry [1]. For chromophores well described by 1D push/pullmodel, the long wavelength absorption and loss of transpar-ency in visible region is an unavoidable consequence of anyincrease in nonlinearity [2]. Thus, preparing transparentNLO chromophores without sacrificing the molecular non-linearity has been the subject of intense investigation [3].Recently, some progress has been made in designing mole-cules with good transparency and high nonlinearity usingtransition metal complexes [4,5]. A metal may act as eithera strong donor or acceptor of electrons and, at the same time,as a 3D template for the symmetric arrangement of ligands,that may not be easily accessible in organic systems.

As a result of its significant electron donating strength,the ferrocenyl moiety is regarded as a good donor part ofa donor-p-acceptor system for the design of chromophoreswith high nonlinear optical response [6]. In previous study,we have reported the NLO behavior of ferrocenyl bimetal-lic compounds with a series of metal carbonyl moieties and

1387-7003/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2007.02.008

* Corresponding authors. Tel.: +82 31 201 3823; fax: +82 31 202 7337(I.S. Lee); tel.: +82 2 880 6662; fax: +82 2 889 0310 (Y.K. Chung).

E-mail addresses: insulee97@khu.ac.kr (I.S. Lee), ykchung@plaza.snu.ac.kr (Y.K. Chung).

remarkable abilities of MnðCOÞþ5 and ReðCOÞþ5 to makeup effective push/pull chromophores with a ferrocenyl frag-ment [7].

In this context, this work has focused on finding newtype of 3D chromophores with high molecular nonlinearityby simply controlling the number of ligands around a man-ganese carbonyl moiety. Herein we report the synthesis andmolecular quadratic optical nonlinearities of 3D organotet-rametallic systems [{CpFeC5H4ACR@CHA4-py}3Mn-(CO)3]BF4 (1a, R = H; 1b, R = Me; 1c, R = Ph) withpseudo-C3 symmetry. These new 3-D chromophores showthe improved nonlinearities with little change of absorptionband as compared to their corresponding 1D analogues.

Compounds [{CpFeC5H4ACR@CHA4-py}3Mn(CO)3]-BF4 (1) have been prepared by the reaction of [CpFe-C5H4ACR@CHA4-py] with [(naphthalene)Mn(CO)3]BF4

in CH2Cl2.

N

R

Fe Mn+

(CO)3

BF4-

CH2Cl2, reflux, 3hr+

N

Fc

MnCO

Fc

N

NFc

CO

OC

RR

R BF4-

1a (R=H)1b (R=Me)1c (R=Ph)

ð1Þ

594 I.S. Lee, Y.K. Chung / Inorganic Chemistry Communications 10 (2007) 593–595

For comparison, 1D analogues [(CpFeC5H4ACR-@CHA4-py)Mn(CO)5]BF4(2) (2b, R = Me; 2c, R = Ph)also have been prepared by the same method as the synthe-sis of 2a (R = H) (Eq. (2)) [7].

BF4-

+Mn (CO)5N

R

Fe N

R

Fe Mn(CO)5BF4

CH2Cl2, r.t., 4hr

2a (R=H), 2b (R=Me), 2c (R=Ph)

ð2ÞCompounds 1 and 2 are based on the easily available

ferrocenyl pyridine and manganese carbonyl cation andtheir preparation is quite straightforward [8].

All the compounds were analyzed by HRS technique andtheir quadratic hyperpolarizabilities are listed in Table 1 [9].

The experimental results show that an improved nonlin-earity is satisfyingly reached for the 3D chromophores of 1.The hyperpolarizability of 1a is about 2.5 times larger thanthat of its 1D analogue 2a, while kmax values of 1a and 2a

are in the same region. The b value of 1b is also found toincrease compared to that of 2b. Interestingly, the kmax of1b does not shift to the longer wavelength region from thatof 2b, rather move to the 14 nm shorter wavelength region.In case of 1c, the hyperpolarizability is almost the same asthat of 2c and the kmax is found to be 41 nm blue-shiftedfrom that of 2c. Thus, compared to those of 2, the nonlin-earities of 1 are all substantially high and the red-shifts ofkmax associated with the increase b are not observed for 1.

The enhancement of molecular nonlinearities can beattributed to an increase in number of donor–acceptor unitand their symmetric arrangement in a molecule. It has beenproposed that in the multiple donor (or multiple acceptor)system with a higher symmetry, the less the interaction oftheir multiple donors, the larger the contribution made bytheir multiply excited states to the molecular hyperpolariz-ability [10]. H1 NMR spectra of 1 do not show any differenceamong three CpFeC5H4ACR@CHA4-py moiety, indicat-ing their identical chemical environment and symmetricmolecular structure. The two infrared carbonyl stretchingmodes (1a, 2035, 1925 cm�1; 1b, 2034, 1925 cm�1; 1c,2034, 1930 cm�1) of 1 imply that the molecular shape of 1

is close to pseudo-C3. The cyclic voltamograms of 1 exhibit

Table 1Physical properties for 1 and 2

kmax (nm)a Half wave redoxpotential of ferrocene (mV)b

b (10�30 esu)c

1a 501 (9100)d 99 15011b 479 (7900) 65 10681c 508 (7500) 96 9972a 502 (5200) 96 6022b 493 (4900) 90 8092c 549 (5300) 152 937

a Lowest CT band measured in nitromethane.b Measured in CH2Cl2.c Measured by HRS method in nitromethane at 1064 nm. The known b

value of CH3NO2 was used as an internal reference.d Extinction coefficient (cm�1 M�1).

a single reversible redox wave, suggesting that the threeferrocenyl moieties do not interact at all or interact veryweakly with each other. From these, 1 can be consideredto meet the proposed qualifications to be a multiple donorsystem having efficient nonlinear optical properties. TheMnðCOÞþ3 moiety in 1 is serving as an electron-acceptorand as a connector of subchromophores. Thus, theincreased b values of 1 are partly due to the multiple low-lying excited states and the weak interactions between theirmultiple donors.

Table 1 shows a good correlation between the kmax val-ues and the half-wave potentials. The blue shift of kmax val-ues of 1b and 1c compared to those of 2b and 2c may bedue to the decrease of the acceptability of manganese car-bonyl, e.g. the decrease of the number of carbonyls coordi-nated to the manganese metal center. The less anodic halfwave potentials of ferrocene moieties in 1b and 1c thanthose of 2b and 2c also support indirectly the decrease ofelectron-withdrawing ability of the manganese carbonylsin 1b and 1c. The decrease of the electron-accepting abilitywould be reflected to the relatively low increase of b valuesin 1b and 1c and the blue shift of MLCT bands of ferro-cene. In the same vein, similar phenomena may be expectedfor 1a and 2a. However, the situation is not so simple asexpected. The electronic and steric effect of the R groupmay be responsible for the different behaviors.

In conclusion, we have demonstrated that 3D organotet-rametallic compounds 1 with pseudo-C3 symmetry can beprepared and their NLO effects show enhancement of non-linearities, compared to the 1D analogues 2, without anycost of transparency. Our results confirm recent statementsmade on the effect of a higher molecular symmetry onhyperpolarizability–structure relationship [10].

Acknowledgement

We thank Dr. Nam Woong Song (KRISS) for HRSmeasurement. This research was supported by the‘‘GRRC’’ Project of Gyeonggi Provincial Government,Republic of Korea (I.S.L.).

Appendix A. Supplementary data

Supplementary data including experimental procedureand characterization of complexes can be found in theonline version, at doi:10.1016/j.inoche.2007.02.008.

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