Porphyrins are the most widely studied tetrapyrrole-macrocycles because of their diverse structures with unique properties and wide distribution in nature.Variations of the peripheral substituents on the porphyrin ring and the insertion/change of metal atoms into the macrocycle usually change the visible absorption spectrum. For this reason, in recent years,(metallo)porphyrins have become of major interest for applications in opto-electronics, data storage, solar cells and photomedicine. Metallo-tetraphenyltetrabenzoporphyrins (MTPTBPs) are different from other porphyrins because of the combination of electronic and structural factors. Recent development in photothermal therapy (PTT) has drawn extra attention to this type of molecules. For a compound to be an effective photothermal agent, it should possess high photostability, high molar absorption coefficient in the red spectral region and fast radiationless decay of the excited states, converting the photon energy into the thermal event.
Within this context, this dissertation has focused on tetrabenzoporphyrins coordinated with different metals to investigate central metal effects on the photophysical properties. Special attention has given to first row transition metals Cr(III), Mn(III), Co(II), Cu(II) and Zn(II) while PtII) analogue has been studied for comparison purposes. Only the theoretical investigations of Ni(II) and Pd(II) analogues have performed to give a full picture about the nature of the deactivation mechanism.
Zn(II)TPTBP was showed that the singlet state decayed through the ground state via fluorescence and the intersystem crossing to the triplet state. The produced singlet state was vibrationally hot and after cooling it decayed to the triplet state having ca.340 ps lifetime in pyridine.Triplet state showed a longer lifetime.
Compared with Zn(II)TPTBP, Pt(II)TPTBP showed a fast intersystem crossing (ca.500 fs lifetime) from singlet state to tripletstate. Low fluorescence quantum yield (0.0003) was observed compare to Zn analogue and it showed a high yield of phosphorescence.
Co(II), Cu(II), Mn(III), Cr(III) has introduced different features to the picture of the excited state deactivation mechanism having initiated by the unpaired metal electron. Co(II)TPTBP, excited singlet has converted to the triplet state within the instrument response time and then it was converted to hot d,d state, wherein intramolecular cooling has occurred and completed within 3 ps. After cooling the d,d state decayed into the ground state in an exponential manner having 17 ps lifetime in hexane solution. Cu(II)TPTBP ground state repopulation was occurred through the set of trip-doublet and trip-quartet states which is in a equilibrium via a lower lying LMCT states. The repopulation was completed within 500 ps in toluene solution.
After 640 nm excitation in Cr(III)TPTBPCl, the singlet state underwent fast intersystem crossing within a very short period of time to triplet state. The triplet state of Cr(III)TPTBPCl in toluene deactivated with a lifetime of 224 ps. In Mn(III)TPTBPCl,the excited singquintet deactivated to the tripquintet within the instrument response time. After short time period,it generated the hot d,d state wherein cooling had occurred within 4 ps and cooled d,d state repopulated the ground state having 120 ps lifetime in toluene.
Transient absorption spectrometry with femtosecond and nanosecond time resolution has been employed along with DFT/TDDFT theoretical examinations to investigate the sequence of events that follow Q band photo-excitation. Overall, the results of the present investigation reported a complete picture of the nature and energies of all electronic states induced by the different metals on the photophysical properties tetraphenytetrabenzoporphyrins.