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Our group studies excited state processes in molecules1–5 and materials6–9. In recent times, we have begun collaborative exploratory studies in biological systems.10,11 In this talk, we will focus on fluorogenic molecules are inherently nonfluorescent or weakly fluorescent, but become strongly emissive as a result of chemical reaction or solidification. They find potential application in the fields of fluorescence sensing and organic electronics. Salophen is a fluorogenic Schiff base (Figure 1). Its fluorescence quantum yield and lifetime increase upon complexation. The extent of enhancement is significantly more in its monomeric Al3+ complex than in its dimeric Zn2+ complex (SalZn). The increase in fluorescence upon complexation is ascribed to the blocking of nonradiative channels associated with flexing motion of the molecule. This can be achieved in uncomplexed salophen in another way, by taking it in solid form or incorporating it in a solid matrix like polymethylmethacrylate (PMMA).4 Such enhancement of emission is also observed in the case of SalZn. Interestingly, a quenching of fluorescence is observed in the crystals of SalAl+. These apparently conflicting trends have been rationalized in the light of the molecular arrangement of salophen and its complexes in a solid matrix and in the pure solid forms. In the case of SalAl+, X-ray crystal structure indicates the possibility of - interaction between the planar ligands of two neighboring complex molecules, which could lead to aggregation-caused quenching (ACQ). This provides a justification for the lower emissivity of dimeric SalZn, as compared to monomeric SalAl+ in solution. In case of salophen, solidification is not associated with - interaction and so, aggregation induced enhancement of emission (AIEE) is observed. Thus, the emissivity of salophen and its complexes is found to be governed by interplay of torsional motion and intermolecular interaction. Manipulation of these factors has been attempted by synthetic modification.12 Introduction of alkoxy groups in salophen ring is found to alter not only the photophysics, but also molecular packing of the molecules in their crystals, leading to a higher emissivity of the Al3+ complexes in their crystalline form.
References
1 D. Panda and A. Datta, J. Chem. Phys., 2006, 125.
2 S. Chatterjee, P. Karuso, A. Boulangé, X. Franck and A. Datta, J. Phys. Chem. B, 2015, 119.
3 A. K. Singh, S. Ghosh, R. Kancherla and A. Datta, J. Phys. Chem. B, 2016, 120, 12920–12927.
4 T. Khan and A. Datta, J. Phys. Chem. C, 2017, 121, 2410–2417.
5 V. Khorwal, U. Nudurupati, S. I. Mondal and A. Datta, J. Phys. Chem. C, 2017, 121.
6 S. Banerjee and A. Datta, Langmuir, 2010, 26, 1172–6.
7 D. De and A. Datta, Langmuir, 2013, 29, 7709–7714.
8 A. Dhir and A. Datta, J. Phys. Chem. C, 2016, 120, 20125–20131.
9 S. Banerjee, A. Gupta, R. Srivastava and A. Datta, Phys. Chem. Chem. Phys., 2018, 20, 4200–4207.
10 S. Arya, A. K. Singh, T. Khan, M. Bhattacharya, A. Datta and S. Mukhopadhyay, J. Phys. Chem. Lett., 2016, 7, 4105–4110.
11 A. Mukhopadhyay, T. Das, A. Datta and K. P. Sharma, Biomacromolecules, 2018, 19, 943–950.
12 T. Khan and A. Datta, Phys. Chem. Chem. Phys., 2017.
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