| 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, such as extension of the pi-system, meso -substitution and highly distorted macrocycle. 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.;Metallotetrapyrroles with first row transition metal centers have been shown to undergo fast radiationless deactivation of their excited states since the metal incorporated into the macrocycle cavity introduces a manifold of electronic states with metal-centered and/or metal+pi--system character, some of which can be situated at lower energy than the pure pi,pi* states that are populated by a photo-excitation process. These low-lying states provide rapid non-radiative channels for the excited deactivation, which is an optimum situation for PTT.;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 3rd row Pt (II), 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. Previously, Ni(II)TPTBP has studied in this laboratory in both experimental and theoretical points of view.;Zn(II)TPTBP was investigated in order to provide experimental evidence for the spectral properties of the pi-localized singlet and triplet states of the tetrapyrrole macrocycle. It showed that the formation of the singlet (pi,pi*) state decayed through fluorescence to the ground state and the intersystem crossing to the triplet state (pi,pi*). 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 of Zn(II)TPTBP deactivated to the ground state with ca. 236 mus lifetime in pyridine.;Compared with Zn(II)TPTBP, Pt(II)TPTBP showed that pi localized S 1 state undergoes fast intersystem crossing (ca. 500 fs lifetime) to triplet state. Low fluorescence quantum yield (0.0003) was observed compare to Zn analogue and it showed a high yield of phosphorescence. The triplet state lifetime found to be ca. 41 mus and it was shorter than Zn(II) triplet state showing that faster intersystem crossing initiated from the spin orbit interaction introduced by heavy Pt atom in the third row transition series.;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 (pi,pi*) has converted to the pi localized 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. The TDDFT results indicated that the lowest d,d transition arises from a fully occupied metal dpi orbital to a partially occupied metal dz2 orbital and could be responsible for the observed d,d state. Cu(II)TPTBP ground state repopulation was occurred through the set of trip-doublet 2T1 and trip-quartet 4T1 states which is in a equilibrium via a lower lying LMCT states. The dependence of the observed lifetime on solvent polarity confirmed the participation of the LMCT states in the overall deactivation process. The repopulation was completed within 500 ps in toluene solution.;After 640 nm excitation in Cr(III)TPTBPCl, the S1 state underwent fast intersystem crossing (4S1→4T 1) within a very short period of time (ca. 0.05 ps lifetime) to 4T1 state. The 4T 1 state of Cr(III)TPTBPCl in toluene deactivated with a lifetime of 224 ps, resulting the 4T1 ↔ 6T 1 equilibrium.;In Mn(III)TPTBPCl, the excited singquintet, 5S1(pi,pi*) state deactivated to the tripquintet, 5T1(pi,pi*) 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 of tetraphenytetrabenzoporphyrins. |
