An Efficient Indirect Mechanism for the Ultrafast Intersystem Crossing in Copper Porphyrins

Abstract

The ultrafast dynamics of copper tetraphenylporphyrin (CuTPP), copper octaethylporphyrin (CuOEP), and of the free base tetraphenylporphyrin (H2TPP), excited in the S2 state have been investigated in the gas phase by femtosecond pump/probe experiments. The porphyrins were excited in the Soret band at 400 nm. Strikingly, the S2-S1 internal conversion in H2TPP is very rapid (110 fs), as compared to that of ZnTPP (600 fs), previously observed. In turn, CuTPP and CuOEP, excited in S2, follow an efficient and different relaxation pathway from that of other open-shell metalloporphyrins. These two molecules exhibit a sequential four-step decay ending on a slow evolution in the nanosecond range (2)S2 → (2)CT → (2)T → (2)Ground State. This latter evolution is linked to the formation of the (2)T, tripdoublet state in CuTPP, observed in the condensed phase. It is shown that an intermediate charge transfer state plays a crucial role in linking the porphyrin centered (1)ππ* and (3)ππ* configurations. A simple model is presented that allows a rapid evolution between these two configurations, via coupling of the porphyrin π system with the free d electron on the copper. The mechanism obviates the need for the spin orbit coupling within the porphyrin. The result is that these copper porphyrins can exhibit an ultrafast apparent intersystem crossing, unprecedented for organic molecules.

Femtosecond to nanosecond excited state dynamics of vapor deposited copper phthalocyanine thin films

“…13 The ultrafast intersystem crossing has been rationalized by noting that the paramagnetic Cu atom may couple to the ligand based exciton and facilitate the intersystem crossing despite not having a large heavy atom induced spin orbit coupling. 22 Given that the internal conversion 5 Decay dynamics of the CuPc excited state absorption feature at 470 nm measured using either a 510 nm or 820 nm pump pulse on a 255 nm thick film. The instrument response function is shown and is ca.…”

Electronic state dependence of heterogeneous electron transfer: injection from the S1 and S2 state of phlorin into TiO2

“…The measured lifetime agrees with previously reported results for free base porphyrins in solution, 9,24 as well as gas phase measurements. 29,30 The ultrafast relaxation from S 2 to S 1 indicates relaxation through a conical intersection. 29 …”

Electrochemistry of nonplanar copper(ii) tetrabutano- and tetrabenzotetraarylporphyrins in nonaqueous media

“…1,2 Additional metal-centered reactions may also be observed for porphyrins with transition metal ions, the most studied of which have been the M II /M III processes involving porphyrins with Fe, Co, Ni or Mn centers. 1,2,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Our recent report of a Cu II /Cu III porphyrin oxidation involved highly substituted nonplanar compounds 12 and we wished to know how changing the planarity or p-ring system of the porphyrin macrocycle by addition of four b,b 0fused butano or benzo groups might affect the redox potentials and electron transfer mechanisms of these copper(II) porphyrins. 1,2 A number of meso and/or b-substituted copper porphyrins have been synthesized and characterized as to their spectroscopic properties and electrochemistry 1,2,12,13 and these synthetic copper porphyrins have been the subject of numerous investigations.…”