Simultaneous Generation of Different Types of Ion Pairs upon Charge-Transfer Excitation of a Donor−Acceptor Complex Revealed by Ultrafast Transient Absorption Spectroscopy

Abstract

The excited-state dynamics of the methylperylene/tetracyanoethylene (MPe/TCNE) donor-acceptor complex has been investigated in various solvents using femtosecond transient absorption spectroscopy. The transient spectra reveal the formation of two types of ion pairs: The first (IP1), constituting the major fraction of the total ion-pair population, is characterized by a broad and red-shifted absorption spectrum compared to that of the free MPe cation and by a subpicosecond lifetime, whereas the second (IP2) has a spectrum closer to that of MPe cation and a lifetime of a few picoseconds. A substantial polarization anisotropy was observed with IP1 but not with IP2, indicating a relatively well-defined structure for the former. The reaction scheme that best accounts for the observed dynamics and its solvent dependence involves the simultaneous excitation of complexes that differ by their electronic coupling. The more coupled complexes have a high absorption coefficient and thus yield IP1, which undergoes ultrafast charge recombination, whereas the less coupled complexes have a lower probability to be excited and lead to the longer-lived IP2.

Ultrafast excited-state dynamics of ferrocene-bridge-acceptor system

“…The observed solvatochromism is probably dominated by dispersion interactions and by dipole-induced-dipole interactions [15,16] for which the polarizability of the solvent determines the stabilization energy. A similar type of the solvatochromic shifts has already been suggested for other CT systems [17].…”

Overcoming the Cut‐Off Charge Transfer Bandgaps at the PbS Quantum Dot Interface

“…The dynamics were fit to double‐exponential functions with time constants of a few to tens of picoseconds, and tens to hundreds of picoseconds. The two observed components suggest the occurrence of two types of donor–acceptor ion pairs with different associated couplings, in accordance with previous reports on other donor–acceptor systems . The fittings for the bleach recoveries of PbS‐840, PbS‐1000, and PbS‐1300 indicate that not only is the percentage recovery greater for smaller PbS QDs, but the rate of CR is also enhanced with the smaller QD size.…”

Bimolecular Excited-State Electron Transfer with Surprisingly Long-Lived Radical Ions

“…[45][46] As can be clearly seen in Figures 2 and 3 The two observed components suggest the occurrence of two types of donor-acceptor ion pairs with different associated couplings, in accordance with previous reports on other donor-acceptor systems. [47][48][49][50] The geometry and electronic coupling between donor and acceptor molecules can greatly affect both charge separation and charge recombination rates. 49 Hence, it is expected that the strong electronically coupled ion pairs will undergo fast charge recombination either to the neutral molecule or to the less coupled ion pairs.…”