Photophysics of Soret-Excited Tetrapyrroles in Solution. I. Metalloporphyrins: MgTPP, ZnTPP, and CdTPP

Abstract

The photophysical behavior of three Soret-excited diamagnetic meso-substituted tetraphenylmetalloporphyrins, MgTPP, ZnTPP, and CdTPP, have been examined in a wide variety of solvents using both steady-state and femtosecond fluorescence upconversion methods. The S 2 population of MgTPP decays to S 1 on the time scale of a few picoseconds with unit S 2-S 1 internal conversion efficiency, and the decay rates conform to the weak coupling case of radiationless transition theory. The energy gap law parameters characterizing the coupling of the S 2 and S 1 states of MgTPP have been obtained. The most important accepting vibrational modes in the S 1 state are multiple in-plane C-C and C-N stretches in the 1200-1500 cm (-1) range. Net S 2-S 1 decay is the dominant decay path for ZnTPP and CdTPP as well, but the process occurs at rates that exceed (in the case of CdTPP, they vastly exceed) those predicted by weak interstate coupling. Alternate mechanisms for the radiationless decay of the S 2 states of ZnTPP and CdTPP have been explored. Large spin-orbit coupling constants and the presence of multiple, near-equiergic triplet states suggest that S 2-T n intersystem crossing might occur at rates competitive with internal conversion. However, the measured efficiencies of S 2-S 1 internal conversion show that, at most, only a few percent of the S 2 population of ZnTPP and no more than about 30% of the S 2 population of CdTPP can decay by a "dark" path such as intersystem crossing.

A cascade through spin states in the ultrafast haem relaxation of met-myoglobin

“…Soret emission is known to undergo a small Stokes shift and is thus limited to λ < 470 nm. 13,22 The broadband centred at about 580 nm appears as a mirror image of the Q band absorption which peaks at 510 nm and has a shoulder at 540 nm (see red dashed line, the absorption features at lower wavelengths were assigned to charge transfer bands 23 ). It is therefore, attributed to emission of the S 1 state.…”

Nonlinear Absorption Dynamics Using Field‐Induced Surface Hopping: Zinc Porphyrin in Water

“…The presence of such a state has also been proposed experimentally. [26,29] The detailed composition and the character of the low-lying electronic states of Zn porphyrin, as well as a comparison with results obtained by using the time-dependent density functional theory are presented in the Supporting Information.…”

“…[22,23] A considerable number of time-resolved studies of transient absorption, fluorescence, and photoionization have been conducted on ZnTPP both in ultracold supersonic jets [24,25] as well as in various organic solvents. [26] Most investigations focused on the dynamics induced by B band absorption and strived for revealing the various radiative and nonradiative relaxation processes, such as internal conversion from B to Q band, fluorescence from both bands, [27,28] as well as intersystem crossing to a manifold of triplet states, [26,29] eventually leading to phosphorescence. It has been found that the precise choice of the excitation wavelength can have a profound influence on the induced dynamics.…”

Transient Anisotropy in Degenerate Systems: Experimental Observation in a Cd-porphyrin

“…The time dependence of the anisotropy on the fs-timescale is governed by rearrangements in the excited state which can have drastic variations given how small geometry changes can interchange between GSB, SE and ESA as observed in the wavelength dependence of the zero time anisotropies. Moreover, the long-time constant about 65 ps show that about 30% excited molecules can undergo direct intersystem crossing to triplet manifold in CdTPP and suggest an additional relaxation pathway competing with ground state bleach [5,7]. Another possible or additional reason for the slow decay of the transient anisotropy could be attributed to the specific structure of CdTPP which is more rigid.…”