We have prepared a green chromophore, 1,7-bis(pyrrolidin-1-yl)-3,4:9,10-perylene-bis(dicarboximide) (5PDI),
that exhibits photophysical and redox properties similar to those of chlorophyll a (Chl a), yet unlike Chl a,
it can be easily functionalized and incorporated into a wide variety of biomimetic electron donor−acceptor
systems. The N,N
‘-dicyclohexyl derivative (5PDI) absorbs strongly (ε = 46 000 M-1 cm-1) at 686 nm in
toluene and fluoresces at 721 nm with a 35% quantum yield. Additionally, 5PDI is both oxidized and reduced
in CH2Cl2 at 0.57 V and −0.76 V vs SCE, respectively, making it a facile electron donor or acceptor. Rodlike
covalent electron donor−acceptor molecules 5PDI−PI, 5PDI−NI, and 5PDI−PDI were prepared by linking
the imide group of the 5PDI donor to pyromellitimide (PI), 1,8:4,5-naphthalenebis(dicarboximide) (NI), and
1,7-bis(3,5-di-tert-butylphenoxy)-3,4:9,10-perylene-bis(dicarboximide) (PDI) acceptors via an N−N bond.
Following femtosecond laser excitation of 5PDI, 5PDI−PI, 5PDI−NI, and 5PDI−PDI in both toluene and
2-methyltetrahydrofuran, the formation and decay of their excited and radical ion pair states were monitored
directly by transient absorption spectroscopy. We also report steady state emission and spectro-electrochemistry
data for these molecules, which aid in elucidation of the transient spectra and the mechanisms of photoinduced
charge separation. In toluene, charge separation occurs with high yield only in 5PDI−NI and 5PDI−PDI,
whereas for 5PDI−PI charge separation is slow relative to excited-state decay of 1*5PDI−PI indicating that
ΔG
CS ≅ 0. This fact provides a means of estimating the ionic radii of the photogenerated ions, which for
perylene chromophores 5PDI and PDI are 7.6 ± 0.5 Å, whereas those of the PI and NI electron acceptors are
5.6 ± 0.5 Å. These ionic radii are used in turn to determine the free energies of reaction of the remaining
molecules with the series. Electroabsorption measurements are used to show that the change in dipole moment,
Δμ that occurs upon formation of 1*5PDI is 3.5 D. The rates of charge separation in 5PDI−NI, 5PDI−PI,
and 5PDI−PDI are compared to those of related donor−acceptor molecules having a 9-(pyrrolidin-1-yl)-perylene-3,4-dicarboximide (5PMI) donor. The 5PMI donor with Δμ = 15.4 D has a lowest excited singlet
state with significantly higher charge-transfer character than does 5PDI, and has greater electron density near
the imide group to which the acceptor is attached. The rate constants for charge separation from 1*5PMI are
greater than those from 1*5PDI, which suggests that the rates of electron transfer from donors with CT excited
states to an attached acceptor depend on the charge distribution in the CT excited state.