Interpretation of Photoinduced Electron and Energy Transfer Processes in Some Mono and Dinuclear Iridium(III) and Rhodium(III) Complexes

Seth, Sourav Kanti (2020) Interpretation of Photoinduced Electron and Energy Transfer Processes in Some Mono and Dinuclear Iridium(III) and Rhodium(III) Complexes. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (Phd thesis of Sourav Kanti Seth (12RS021)) 12RS021.pdf - Submitted Version Restricted to Repository staff only Download (6MB)

Abstract

Light or energy harvesting, of any kind, especially solar light harvesting, is absolutely indispensable in today’s world to cope with the limited fossil fuels. Since long, numerous molecular systems have contributed to achieve improved solar light harvesting efficiency. Luminescent cyclometalated Ir(III) complexes have shown promising prospects in this regard because of their superior and rich photophysical properties. However, among all the advantages, low molar absorption coefficient values of cyclometalated Ir(III) complexes in the visible spectral region, somewhat, limit their uses. The Ir(III) complexes which have been mostly studied are the mononuclear ones (molecule consisting of one Ir centre). Recently, the dinuclear or polynuclear Ir(III) complexes (molecule consisting of two or more Ir centres) have grown interests because some of them have been found to be superior to their mononuclear analogues in some aspect and also they have greater molar absorption coefficients in the visible region than the corresponding mononuclear ones. Photoinduced electron transfer (PET) and energy transfer processes play key roles in determining light harvesting efficiency. At this stage, it is worth making a comparative study of PET and energy transfer efficiencies between the mononuclear and dinuclear complexes. midazolyl modified phenanthroline ancillary ligands were used to synthesize a few mononuclear and dinuclear Ir(III) and Rh(III) complexes. Though Rh(III) complexes are not much explored and not known to display photophysical properties as rich as the Ir(III) ones, but being the element of the same group and red-ox active nature, they are compared with the Ir(III) analogues. Three dinuclear complexes (one hetero- and two homo-complexes) were synthesized and their photophysical aspects were explored. The studies revealed interesting facts about the synthesized complexes, based on which the PET efficiency was compared between the mono and dinuclear Ir(III) complexes. These complexes mostly emit from the excited triplet states in microsecond lifetime-scale. Triplet state quencher, such as molecular oxygen (³O₂) thus affects their emission significantly and reactive oxygen species (ROS), especially singlet oxygen (¹O₂), is produced through excited triplet state energy transfer. Inclusion of pyrene moiety to the Ir(III) complex, significantly alters the triplet energy levels and the triplet species lifetimes, thus producing significant amount of ¹O₂. The experimental results and the nature of the electronic transitions in the ligands and complexes are supported by theoretical calculations using DFT-TDDFT methods. The results are intriguing and demand further research.

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