Ultrafast Dynamics of meso‐Tetraphenylmetalloporphyrins: The Role of Dark States

Abstract

Studying the relaxation pathways of porphyrins and related structures upon light absorption is crucial to understand the fundamental processes of light harvesting in biosystems and many applications. Herein, we show by means of transient absorption studies, following Q‐ and Soret‐band excitation, and ab initio calculations on meso‐tetraphenylporphyrinato magnesium(II) (MgTPP) and meso‐tetraphenylporphyrinato cadmium(II) (CdTPP) that electronic relaxation following Soret‐band excitation of porphyrins with a heavy central atom is mediated by a hitherto disregarded dark state. This accounts for an increased rate of internal conversion. The dark state originates from an orbital localized at the central nitrogen atoms and its energy continuously decreases along the series from magnesium to zinc to cadmium to below 2.75 eV for CdTPP dissolved in tetrahydrofuran. Furthermore, we are able to directly trace fast intersystem crossing in the cadmium derivative, which takes place within (110±20) ps.

On the origin of high transient anisotropies: An exemplification in a Cd-porphyrin

“…The first time constant (τ 1 ) was interpreted as structural rearrangement in the excited state which also leads to a temporal spectral shift of the minima caused by GSB in transient spectra. 27 The second time constant (τ 2 ) can be interpreted as ISC which agrees with the value of Harriman 30 of ∼65 ps but is a factor of 2 shorter as compared to more recent investigations 6,27 which is discussed in the supplementary material. 31 …”

“…32 Additional information can be obtained from Figure 5 in Ref. 27: Here, it can be seen that the transient absorption spectrum has a similar shape as the scaled sum of the absorption and fluorescence spectrum in Figure 1 (absorption and fluorescence give rise to a negative signal in the transient spectrum through GSB and SE). When adding the scaled absorption spectrum to the transient spectrum, the remaining positive residual can be attributed to ESA.…”

“…The fast anisotropy decay could be caused by small relaxations within the molecule, as revealed by shifts of the bands in transient absorption of MgTPP in THF upon excitation at 620 nm (see supporting information of Ref. 27), which induces changes in ESA, GSB, and SE that might be large enough to cause a fast anisotropy decay.…”

“…10,[16][17][18][19][20]25 In their original article, Galli et al 6 investigated 5,10, 15,20-tetraphenyl-porphyrinato magnesium (II) (MgTPP) dissolved in tetrahydrofuran (THF) at pump and probe wavelengths of 620 nm 6 and detected a time zero anisotropy of r(t 0 ) = 0.65. At this wavelength, MgTPP is excited to its lowest lying singlet state, the Q(0,0)-band, 26 which lives for 7-8 ns before intersystem crossing (ISC) takes place 14,27 (however, reorientations on the ps-timescale were found to cause small band shifts 27 ). The anisotropy decayed biexponentially with time constants of 210 fs and 1.6 ps.…”

“…6 However, recent time resolved experiments showed that the transient absorption signals in the 620 nm region of MgTPP and CdTPP are actually similar and result from a superposition of GSB, SE, and ESA. 27 This means (i) that SE is not the dominant effect and (ii) that the difference between the anisotropy signals of MgTPP and CdTPP cannot be caused by a dramatic effect. Along this line, we became interested in the differences of MgTPP and CdTPP and the origin of the high anisotropy, especially as we saw no reason for similar values to be absent in CdTPP.…”