CIRCULARLY POLARIZED LUMINESCENCE (CPL)
Circularly polarized light is a type of electromagnetic wave having constant magnitude electric field vector which rotates in a plane perpendicular to the direction of propagation. It can be produced from plane polarized light followed by the use of a quarter wave plate. Circular dichroism (CD) is a ground state phenomenon which demonstrates the difference in molar absorption coefficient of chiral luminophore for left and right circularly polarized light. However, in circularly polarized luminescence (CPL), difference between left and right circularly polarized radiation emitted by the chiral luminophore is measured which unfolds the excited state characteristics. Pictorial demonstration of CD and CPL is shown in below figure. The degree of polarization of emission can be quantified by dissymmetry factor (glum) which follows the below equation.
glum= (IL-IR) / [1/2(IL IR)] = ΔI / I
where IL and IR are luminescence intensity of left- and right-CPL respectively.
Circular polarization offers better resolution compare to conventional photoluminescence which allows to develop smarter optoelectronic devices like OLED and 3D displays, optical data storage, spintronics device, sensing and biomedical imaging. Owing to their magnetic dipole-allowed transitions, lanthanide complexes usually possess large dissymmetry values. However, low luminescence efficiency and chemical instability restricts them for optoelectronic applications and hence it demands other alternatives. Chiral transition metal complexes despite of having low glumvalues, its ability to harvest both singlet and triplet excited states helps in increase in luminescence efficiency. In contrast, chiral organic luminophores have quite low dissymmetric factor due to electric dipole-allowed transitions. However, many advantages like low cost, facile synthesis and structural modifications, tunable chiroptical characteristics make organic molecules as a promising candidate for CPL applications. In this regard, very few reports are present in literature highlighting the chiral organic molecules with quite high gfactor which can compete with lanthanide complexes. Also, CPL spectrometer is commercially available only from JASCO company in Japan, which explains the lack of improvement in this area of research. We are interested in design, synthesis and tunable chiroptical properties of small organic molecule based chiral luminophores which can address the pressing issues. Also, we are involved in development of phosphorescent chiral organic molecules which can harvest the triplet states and consequently result in improved CPL efficiency.