www.ijcrt.org © 2018 IJCRT | Volume 6, Issue 1 January 2018 | ISSN: 2320-2882 IJCRT1801338 International Journal of Creative Research Thoughts (IJCRT) www.ijcrt.org 370 SENSING MECHANISMS - CHEMOSENSORS Dr. Kuljit Kaur Assistant Professor, Post graduate Department of Chemistry Khalsa College, Amritsar, India-143001 ________________________________________________________________________________________________________ Abstract: The present research article elaborates different sensing mechanisms of chemosensors based on photophysical processes, such as, photoinduced electron transfer (PET), charge transfer (CT) etc. A brief introduction about types of chemosensors is followed by detailed discussion of various mechanisms. IndexTerms- Chemosensor, mechanisms, colorimetric, fluorescent. _______________________________________________________________________________________________________ 1. Introduction Development of optical chemosensors is ever growing area of research for last four decades. The concept of various optical sensors originated from molecular recognition which employed different approaches. All these approaches involve the presence of a signaling moiety (indicator) which can be a chromophore or a fluorophore resulting in chromogenic or fluorogenic sensors, respectively. Colorimetric/chromogenic sensors are especially attractive because the guest determination can be carried out by the naked eye, without the use of expensive equipment and they also find direct applications in the development of optodes and disposable dip- stick arrays based on absorption changes. In colorimetric sensors, a bathochromic or hypsochromic shift of absorption band or visual color change is affected by the respective increase or decrease in electron densities on the chromophore moiety which is more effectively carried by the association of a charged analyte i.e. cation or anion than a neutral molecule. The interaction of guest / analyte species with the receptor occurs only in its ground state. In the case of fluorescent chemosensors, the observed change in the fluorescence emission could be due to interaction of the analyte with the chemosensor in the ground state which results in formation of new chemical ensemble with different emission properties. Alternately, the analyte may preferentially interact with the excited state of the chemosensor resulting in either change in life time of the excited state resulting in change in the emission intensity or leading to new excited state with different emission properties. The excitation of the chemosensor also results in change in the electron distribution within the chemosensor and thus may enhance / lower the interaction of the chemosensor with the analyte. Thus, the possibility of variation in the excited state of the chemosensor both due to (i) interaction of the guest / analyte with the chemosensor in the ground state or in the excited state (PET) (ii) reorganization of the excited state (ESIPT, TICT, FRET etc.) provide significantly more possibilities for interaction of an analyte with the chemosensor in excited state than that observed in the ground state. In a normal fluorescence process, an electron is excited from the HOMO to the LUMO of the molecule to achieve singlet excited state. The vibrational relaxation of the excited state prior to emission of a photon from the excited state leads to decrease in energy of the emitted radiation. This decreased energy of the emitted radiation from the absorbed radiation is manifested as red shift of the emission spectrum – termed as “Stokes shift”. Sousa reported that naphthalene compounds 1 and 2 were perturbed by various alkali metal chloride salts causing increase and decrease in fluorescent quantum yield, phosphorescence quantum yield, and phosphorescence lifetime. 1 This is the first study of perturbation of excited state using a non-covalently held species. Several research groups have employed the fluorophore-spacer-receptor 2 design for fluorescent chemosensor including Czarnik 3 , Fabbrizzi 4 , Tsien 5 , Shinkai 6 , de Silva and many others. 2 Based on different photophysical processes, different sensing mechanisms viz. photoinduced electron transfer (PET), charge transfer (CT), energy transfer (ET), excimer / exciplex formation have been elaborated. 2. Sensing Mechanisms 2.1 Photoinduced Electron transfer (PET) Electron transfer (ET) is one of the most important chemical process in nature and it plays a central role in many biological, physical and chemical (both organic and inorganic) systems. In nature, ET occurs in photosynthetic reaction center where transfer of electrons is used to create charge imbalance across a membrane, originating a proton pumping mechanism to produce ATP. In chemical systems, ET at the metal surface with oxygen is responsible for the corrosion. Solid state electronics depends on the O O O O O O O O O O O O 2 1