Excited state dynamics of tetrapyrrole macrocycles: The dependence on structural and electronic factors

Metallo tetrapyrrole systems have been widely investigated for many decades due to the wide variety of applications. Recent developments in photothermal therapy and photodynamic therapy have drawn extra attention to these compounds. The nature of the central metal ion plays a decisive role in changing the electronic structure and hence the excited state dynamics of the compounds. The influence of transition metals with or without vacant d-orbitals on the excited state properties of the tetrapyrrole macrocycles has been studied but a quantitative understanding is lacking.; In this work, metallo-octabutoxyphthalocyanine and metallo-octabutoxynaphthalocyanine with Ni group and Fe group metals ions were studied to gain an idea about the change in excited state dynamics with structural and electronic factors. Further, excited state dynamics of Ni(OBu)₈Pc and Ni(OBu) ₈Nc in coordinating solvent were also studied. Ultrafast and nanosecond transient absorption spectrometry were used as the main research tool along with other spectroscopic and electrochemical methods.; Compounds containing Ni(II) and Fe(II) as central metal ion in a low spin complex show very rapid, non-radiative deactivation which take only a few nanosecond to complete. In the case of NiPc(OBu)₈, evidence of the involvement of charge transfer states in the relaxation pathway has been established. The initially formed S₁(π*) state populates the d-d state, which subsequently populates 1,3LMCT states and the ground state repopulation can be seen from all three excited states. Compounds with Ni(II) as the central metal ion in piperidine (high spin complexes) show significantly different excited state dynamics with long lived (microseconds) triplet state, which formed with the decay of 3LMCT state.; The complexes with Ru(II), Os(II) and Pd(II) as central metal ions show the formation of long lived triplet state and the formation of singlet oxygen.; The SiPc μ-oxo oligomeric compounds were studied to understand the excitonic interactions and the excited state dynamics upon addition of monomer units. It has been demonstrated that the tetramer is approaching the polymeric behavior and the further addition of monomer units does not change the photophysical properties dramatically.