Synthesis and Charge-Transfer Dynamics in a Ferrocene-Containing Organoboryl aza-BODIPY Donor–Acceptor Triad with Boron as the Hub

Abstract

A N,N'-bis(ferroceneacetylene)boryl complex of 3,3'-diphenylazadiisoindolylmethene was synthesized by the reaction of an N,N'-difluoroboryl complex of 3,3'-diphenylazadiisoindolylmethene and ferroceneacetylene magnesium bromide. The novel diiron complex was characterized by a variety of spectroscopic techniques, electrochemistry, and ultrafast time-resolved methods. Spectroscopy and redox behavior was correlated with the density functional theory (DFT) and time-dependent DFT calculations. An unexpected degree of coupling between the two Fc ligands was observed. Despite a lack of conjugation between the donor and acceptor, the complex undergoes very rapid (τ = 1.7 ± 0.1 ps) photoinduced intramolecular charge separation followed by subpicosecond charge recombination to form a triplet state with a lifetime of 4.8 ± 0.1 μs.

Photophysics, Redox Processes, and Electronic Structures of Ferrocenyl-Containing BODIPYs, aza-BODIPYs, BOPHYs, Transition-Metal Dipyrromethenes and aza-Dipyrromethenes

“…Oxidation of the ferrocene substituent(s) in BODIPYs and their analogues under chemical or spectroelectrochemical oxidation conditions reduces the effectiveness of such electron-transfer pathway and results in partial restoration of the fluorescence, which can be again suppressed by the reduction of ferricinium group to ferrocene (Figure 10). [7,10,11,16,17,25,29,33] Similar partial restoration of the steady-state fluorescence was also reported for ferrocene-containing BODIPY system in viscous solvents [32] and upon addition of water [28] to a solution of some ferrocene-containing BODIPYs. It was speculated that the dihedral angle (and thus degree of conjugation between ferrocene group and chromophore's π-system) is responsible for the fluorescence increase in the former case, while partial degradation of the organometallic fragment is responsible for fluorescence increase in the latter.…”

“…In the majority of cases, such charge-separated states undergo direct recombination to the ground state (Table 1). Ferrocene-containing aza-BODIPY dyad 55, [17] however, behaves quite differently. Indeed, upon excitation, this system rapidly forms a charge-separated state followed by subpicosecond charge recombination to form a long-lived (4.8±0.1 ms) triplet state (Figure 12).…”

“…[7,12,14,17,[19][20][21]23] In these cases, the ferrocene fragments are electronically decoupled from the BODIPY's or aza-BODIPY's π-system, and to a large extent have independent behavior. For all cases, the DFT-predicted HOMOs are entirely or almost entirely localized at the ferrocene fragments, while the LUMOs are pure BODIPY's or aza-BODIPY's centered orbitals.…”

“…Transient absorption data and time-dependent decay profiles for aza-BODIPY 55. Modified from reference [17] .…”

“…[14,23] Such behavior is not surprising as the efficient electron-transfer pathway between two organometallic sites cannot be established in this case. It is interesting that a ~100 mV separation between the first and the second oxidation waves is observed in aza-BODIPY 55 [17] (DCM/0.05M TBA[B(C 6 F 5 ) 4 ] electrolyte system). Spectroelectrochemical data, however, did not confirm electronic coupling between two ferrocene fragments and thus such separation in oxidation potentials was attributed to the ion-pairing effect.…”