the temperature to 100 K, in near-perfect agree-
ment with the behavior of [Fe(btz) 3 ]3+.Fitting
the temperature-dependent lifetimes by an
Arrhenius model retrieves an activation barrier
of 3 kJ mol−^1 and preexponential factor of 1 ×
109 s−^1 , suggesting that for the decay channels
that dominate this temperature dependence, the
energy barriers for deactivation of [Fe(phtmeimb) 2 ]+
and [Fe(btz) 3 ]3+are almost identical but that the
preexponential factor for [Fe(phtmeimb) 2 ]+is
lower. It is thus the decreased preexponential
factor for the transition that results in an increased
lifetime, and we tentatively ascribe this to an effec-
tive reduction of the crossing frequency from the(^2) LMCT state to the (^4) MC state, owing to the com-
bined effect of several structural factors, including
the higher symmetry and tighter spatial confine-
ment of the [Fe(phtmeimb) 2 ]+ligand system.
With excited-state redox potentials ofEo(III/
II) = 1.0 V andEo(IV/III) =−1.9 V versus Fc [1.6 V
and−1.3 V versus NHE (normal hydrogen elec-
trode)], the^2 LMCT state should be potent as both
a photo-oxidant and photoreductant; furthermore,
its nanosecond lifetime should enable efficient
Kjæret al.,Science 363 , 249–253 (2019) 18 January 2019 3of5
Fig. 3. Excited-state dynamics and computational analysis of
[Fe(phtmeimb) 2 ]+photophysics.(A) Time-correlated single-photon
counting data (black, leftyaxis), transient absorption data at 390 nm
(green circles, rightyaxis), and monoexponential fit of 1.96 ± 0.04 ns (red).
a.u., arbitrary units; O.D., optical density. (B) Potential energies and Fe–C
equilibrium bond lengths (Q) for relevant electronic states of [Fe(phtmeimb) 2 ]+
(triangles) and potential surfaces (lines) drawn through the energy of
each state at the geometry of the two other states (circles). The^2 LMCT
surface was extrapolated from the ground-state (GS) shape and experi-
mental energy as described in the supplementary materials section.
Table 1. Photophysical parameters of [Fe(phtmeimb) 2 ]+,[Fe(btz) 3 ]3+, and [Ru(bpy) 3 ]2+in air-saturated acetonitrile at room temperature.[Fe(btz) 3 ]3+data
are from ( 19 ); [Ru(bpy) 3 ]2+data are from ( 28 , 34 ).lmax,abs, maximum absorption wavelength;lmax,emiss, maximum emission wavelength;t, CTexcited-state lifetime.
Complex lmax,abs(emax) [nm (10^3 ·M−^1 cm−^1 )] lmax,emiss[nm] A[10^7 ·M−^1 cm−^2 ] kr[10^6 ·s−^1 ] knr[10^7 ·s−^1 ] t[ns] Fe[%]
............................................................................................................................................................................................................................................................................................................................................
[Fe(phtmeimb)............................................................................................................................................................................................................................................................................................................................................ 2 ]+ 502 (3.0) 655 2.5 15 49 2 2.1
............................................................................................................................................................................................................................................................................................................................................
[Fe(btz)............................................................................................................................................................................................................................................................................................................................................ 3 ]3+ 558 (1.2) 600 0.47 4 990 0.1 0.03
............................................................................................................................................................................................................................................................................................................................................
[Ru(bpy)............................................................................................................................................................................................................................................................................................................................................ 3 ]2+ 452 (14.6) 625 7.5 0.1 0.56 | 0.10 180 | 930 1.8 | 9.5
Parameters for oxygen-free acetonitrile solution of [Ru(bpy) 3 ]2+.
Fig. 2. Electrochemistry and spectroscopy of [Fe(phtmeimb) 2 ]+in dry, air-saturated acetonitrile at room temperature.(A) Cyclic and differential pulse
voltammetry. (B) Optical absorption (left black curve), normalized PL (right black curve), and normalized excitation spectra (red circles). (C) Visible
orange PL of 50mM[Fe(phtmeimb) 2 ]+in dry, air-saturated acetonitrile upon 532-nm excitation.
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