Are the orientation and bond strength of the RCO₂⁻⋯M link key factors for ultrafast electron transfers?

Abstract

The photo-induced electron transfers in the "straight up" ionic assemblies [Pd3(2+)]···MCP and [Pd3(2+)]···DCP···[Pd3(2+)] ([Pd3(2+)]* → MCP or DCP) are ultrafast (<85 fs) indicating that it is not necessary to have a strong coordination bond or a bent geometry to obtain fast electron injection in porphyrin-containing DSSCs.

Ultrafast Electron Transfers in Organometallic Supramolecular Assemblies Built with a NIR-Fluorescent Tetrabenzoporphyrine Dye and the Unsaturated Cluster Pd3(dppm)3(CO)2+

“…The LUMO is composed of the in-plane dσ*­(Pd–Pd) MO centered on the [Pd 3 2+ ] cluster. This computational result is consistent with previous DFT calculations on the free cluster and the cluster assembled with a dye . This MO “insertion” of a cluster-localized MO within the four π and π* levels of the porphyrin dye is relevant, as it adds the possibility of electronic transitions between the filled frontier MOs of the dyes and this LUMO, although these are expected to be forbidden due to poor MO overlaps.…”

“…These data indicate that oxidation quenching of the dye in the S 1 state is far more favorable than for the reduction. For comparison purposes, the reported Δ E values for donor *··· [Pd 3 2+ ] → donor +• ··· [Pd 3 •+ ] (+0.67; MCP , +0.72 V vs SCE; DCP ) are similar to those for TCPEBP and TCPEP (i.e., respectively +0.72 and +0.58 V vs SCE). This observation is expectedly consistent with the fact that the extension of the π-system decreases both the singlet-state energy and oxidation potential almost proportionally.…”

“…In the context of this work, we recently reported assemblies of the type dye•••[Pd 3 2+ ] x (dye = MCP, 5-(4-carboxylphenyl)-10,15,20-tristolyl(porphyrinato)zinc(II), DCP, 5,15-bis(4-carboxylphenyl)-15,20-bistolyl(porphyrinato)zinc(II)), which exhibit smaller binding constants (K 1x = 19 300 (MCP) and 22 000 M − 1 (DCP, x = 1, 2)). 8 The choice of Pd 3 (dppm) 3 (CO) 2+ ([Pd 3 2+ ], dppm = Ph 2 PCH 2 PPh 2 as a PF 6…”

“…This computational result is consistent with previous DFT calculations on the free cluster 22 and the cluster assembled with a dye. 8 This MO "insertion" of a cluster-localized MO within the four π and π* levels of the porphyrin dye is relevant, as it adds the possibility of electronic transitions between the filled frontier MOs of the dyes and this LUMO, although these are expected to be forbidden due to poor MO overlaps. As expected, the two lowest energy electronic transitions involve this dσ*(Pd−Pd) MO, and two π(dye*) → dσ*(Pd−Pd) transitions are computed (Table 7).…”

“…In the context of this work, we recently reported assemblies of the type dye··· [Pd 3 2+ ] x (dye = MCP , 5-(4-carboxylphenyl)-10,15,20-tristolyl­(porphyrinato)­zinc­(II), DCP , 5,15-bis­(4-carboxylphenyl)-15,20-bistolyl­(porphyrinato)­zinc­(II)), which exhibit smaller binding constants ( K 1 x = 19 300 ( MCP ) and 22 000 M –1 ( DCP , x = 1, 2)) . The choice of Pd 3 (dppm) 3 (CO) 2+ ( [Pd 3 2+ ] , dppm = Ph 2 PCH 2 PPh 2 as a PF 6 – salt) as the electron acceptor is based on the low reduction potentials and its well-known host guest behavior to bind carboxylates in a purely ionic manner. , Noteworthy, the photoinduced oxidative electron transfer occurs in a time scale < 85 fs in these two cases.…”