Polychlorotriphenylmethyl Radicals: Towards Multifunctional Molecular Materials - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 54

Locally Excited State
[A B + ]* −

E op

Excited State
[A + − B]

2 H AB

E th

D G

E op

D G °

Bridge

A +

B

Ground State
[A − B + ]

A

Bridge

B +

Figure 2.18 Potential energy curve of a Class II system where the optically (E op ) and thermally (E th ) induced
IETprocessesareshown.DifferentenergetictermsinvolvedinIETprocessesarealsodepicted:Energydifference
between the ground and the first excited state (

G ◦ ), the thermal energy barrier ( G ± ) and the coupling

parameterH AB .

excited [A + - B] state in a M-V compound or through a direct transition from the ground state [A - B + ]to
a locally excited state [A - B + ] ∗ in a D-A dyad. In both mechanisms neither the solvent nor the internal
geometry of the molecular wire are allowed to relax during the IET. As a consequence, an IVT band
corresponding to the vertical excitation is observed in the optical absorption spectrum of these compounds.
From the position, intensity and width of such an IVT band, the IET can be easily characterized. In most
cases, E op is the energy required for the optically-induced electron transfer, G ± is the activation barrier

for the thermally-induced electron transfer, H AB is the electronic coupling matrix between the diabatic
potential energy surfaces of the two states and G ◦ is the free energy difference between both states; λ is

the reorganization energy value, which is composed of two terms, one inner term, λ i , corresponding to the

reorganization inside the molecule and an outer term, λ o , that depends on the surrounding media. According

to Marcus - Hush theory, the parameters controlling the optical and thermal induced IET processes are
closely interrelated by Equations 2.2 and 2.3:

G ◦ )

2

(λ +

G = =

(2.2)

4 λ

G ◦

E op = λ + (2.3)

The effective electronic coupling H AB (expressedincm − 1

between the two states can be determined
based on Equation 2.4, where r is the effective separation of the two electroactive sites (in A),

)

ε max is

the maximum extinction coefficient (in M − 1 cm − 1 · ) of the IVT band and ν max is the transition energy and

ν 1 / 2 is the full width at half height (both in cm − 1

) of the IVT band.

ε max ν 1 / 2
ν max

1 / 2

ν max
r

10 − 2 H AB = ( 2 . 05 · ) (2.4)