Indian Journal of Chemistry Vol. 50A, March-April 2011, pp. 511-518 A comparative study of third-order optical nonlinearity of symmetrical dipolar chromogenic probes and their enhancement by different metal ions Atanu Jana a , Jong Min Lim b , Sun Woo Park b , Dongho Kim b, * & Parimal K Bharadwaj a, * a Department of Chemistry, Indian Institute of Technology Kanpur, 208 016, India Email: pkb@iitk.ac.in b Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Republic of Korea Received 3 December 2010; accepted 9 January 2011 We have investigated the two-photon absorption properties of three symmetrical NLO-phores, L 1 , L 2 and L 3 , having different conjugation lengths as well as different binding sites like cryptand, crown ether and bipyridyl moiety. The TPA properties have been measured by a femtosecond laser at 800 nm, which is in the window of ‘in vivo’ imaging. Upon metal ion binding, the TPA cross-section values are enhanced to different extents depending on the nature of metal ion and the overall architecture of the molecular system as well. The unique CT transition process after metal ion incorporation, along with various conjugation lengths of the NLO-phores, fully elucidates the structure-property relationship of the photophysical properties. Keywords: Bioinorganic chemistry, Coordination chemistry, Optical nonlinearity, NLO-phores, Two photon absorption, Chromogenic probes Molecules exhibiting optical nonlinearity are presently in great demand for various applications. Third-order optical nonlinearity can be related to two-photon absorption (TPA) that involves excitation of molecular species from the singlet ground state to the singlet excited state by simultaneous absorption of two photons. The efficiency of this process is determined by measuring the two-photon absorption cross-section (σ (2) ), value which is related to the imaginary part of the second hyperpolarizability. Molecules with large TPA cross-section are potentially useful in several areas of bio-photonics and materials science 1 . Specifically, when the TPA cross-section of a molecule changes significantly in presence of a particular metal ion, it can be very useful for the detection and determination of static concentration of that ion in bio-systems for understanding various biological processes 2 . Important classes of compounds capable of exhibiting large TPA cross- section values are organic molecules with symmetrical charge transfer possibilities. When a metal ion influences this charge transfer, modulation of the TPA activity can be achieved. Since intramolecular charge transfer (ICT) contributes strongly to the TPA capability, many two-photon chromophores have been reported in literature 3 . These systems are built-up from electron donating and/or withdrawing groups bridged by π-conjugated systems. The present investigation deals with the design and synthesis of a versatile derivatizable set of TPA synthons and their development towards TPA chromophores with large TPA cross-section (σ (2) ) values. Zinc is an essential nutrient required in normal growth and development 4 and for cellular processes such as DNA repair 5 and apoptosis 6 . This metal plays a key role in the synthesis of insulin and the pathological state of diabetes 7 . On the other hand, many enzymatic reactions are mediated by Mg(II), while Ca(II) acts as an universal second messenger in cells 8 . Therefore, trapping these metal ions by a suitable receptor for reporting their static concentration for various neurophysiological studies is an important research area of current interest. Strategies have been evolved to append different D-π-A to suitable skeletons to have multi-polar systems. The present investigation deals with three conjugated dipolar chromogenic sensors, L 1 , L 2 and L 3 , where the cryptand or crown ether receptors are exposed at the end of dipoles to have overall