Diaza-18-crown-6 based chromophores for modulation of two-photon absorption cross-section by metal ions
Graphical abstract
Two novel aza-crown ether based chromophores have been synthesized in good yields which show large two-photon absorption cross-sections in presence of metal ions. Theoretical calculations also support the experimental findings.
Introduction
Molecules that exhibit optical nonlinearity are presently in demand as they are potentially useful in opto-electronics as well as all-optical data processing technologies. Third-order optical nonlinearity of molecules measured in terms of two-photon absorption cross-section (σ2) is particularly important in bio-photonics and materials science such as photodynamic therapy [1], optical power limiting [2], three-dimensional optical data storage [3], two-photon up-conversion lasing [4], and so on. Besides, modulation of TPA activity of molecules in the presence of certain ions are important from the perspectives of detection and determination of static concentration of these ions in bio-systems which is important in understanding of biological processes. An important class of compounds capable of exhibiting large TPA cross-section are organic molecules with symmetrical charge transfer possibilities. When a metal ion influences this charge transfer, modulation of the TPA activity can be achieved. It should be noted, however, that TPA enhancement rather than its lowering in the presence of a metal ion will be better understood. With this design principle in mind, we have attached 4-(dimethylamino)benzene or ferrocene group to diaza-18-crown-6 to have two D–π–D′ moieties symmetrically disposed (Scheme 1).
Crown ethers can bind biologically relevant alkali and alkaline earth metal ions while mixed aza-oxa crown ethers can complex other types of metal ions as well. Diaza-18-crown-6 is a good receptor for Zn(II), Cd(II) and Mg(II) ions in addition to alkali metal ions such as Na(I) and K(I). Zinc is an essential nutrient required in normal growth and development [5] and for cellular processes such as DNA repair [6] and apoptosis [7]. This metal plays a key role in the synthesis of insulin and the pathological state of diabetes [8]. On the other hand, many enzymatic reactions are mediated by Mg(II) while Ca(II) acts as a universal second messenger in cells [9].
Metal ions are excellent 3D templates and can assemble simple organic NLO-phores around with concomitant tuning of the molecular nonlinear optical properties by virtue of inducing a strong intra-ligand charge transfer (ILCT) transition. There are very few examples on the metal ion induced TPA cross-section reported earlier in the literature [10]. Herein, the NLO-phores L3 and L4 are designed such that in absence of a metal ion, the two D–π–D′ units present in each compound, are electronically independent. When a metal ion occupies the cavity, it can electronically connect the two D–π–D′ moieties into D–π–A–π–D leading to increased symmetry of charge transfer and hence larger TPA cross-section. Theoretical calculations were carried out to support the experimental findings.
Section snippets
Materials
Reagent grade 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane, 1-fluoro-4-nitrobenzene, 4-(dimethylamino)benzaldehyde, ferrocene carboxaldehyde, 10% Pd in activated charcoal, Rhodamine-6G and all the metal perchlorate salts were acquired from Aldrich (USA) and used as received. Reagent grade hydrazine hydrate, Na2CO3 and the solvents were purchased from SD Fine Chemicals (India). The solvents were purified before use following established methods. All the reactions were carried out under
One-photon absorption spectra of the chromophores and their corresponding metal complexes
One-photon absorption spectra for L3, L4 and their different metal complexes, taken in acetonitrile (10−5 M) are collected in Fig. 1. The ligand L3 exhibits two charge transfer bands, one at 334 nm and the other at 546 nm consistent with other ferrocenyl chromophores [17]. The 334 nm band is due to ligand-centered π–π∗ electronic transition. This band makes a slight red-shift (2–15 nm) and becomes more intense in the presence of a metal ion. The lower energy band at 546 nm does not shift to any
Conclusion
In this paper, the linear and nonlinear properties of two new dipolar chromophoric systems were studied in detail. The TPA properties have been also investigated using femto-second laser. The results reveal that there is an increasing order of TPA cross-section value from alkali metal ions to transition metal ions followed by alkaline earth metal ions depending upon the extent of binding with aza crown ether receptor which not only depends on the conjugation length, but also on the strength of
Acknowledgements
Financial support received from CSIR, New Delhi, India to P.K.B. is gratefully acknowledged. D.G. thanks DST, MCIT (India), and International SRF Program of Welcome Trust (U.K.) for the financial grant. A.J. and A.K.D. thank CSIR for their fellowship. We also thank Dr. Kousik Giri, School of Chemistry, University of Birmingham for his help regarding theoretical calculations.
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da18C6 bond dissociation energies (BDEs) after properly accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and the lifetimes for dissociation. Density functional theory calculations at the B3LYP/def2-TZVPPD and B3LYP/6-31+G* levels of theory are used to determine the structures of da18C6 and the M+(da18C6) complexes and provide molecular constants necessary for the thermodynamic analysis of the experimental data. Theoretical BDEs are determined from single point energy calculations at the B3LYP and MP2(full) levels of theory using the def2-TZVPPD and 6-311+G(2d,2p) basis sets using the B3LYP/def2-TZVPPD and B3LYP/6-31+G* optimized geometries. The agreement between B3LYP/def2-TZVPPD theory and experiment is excellent for all four M+(da18C6) complexes. The M+