6 Biomolecules, Photostability and 1 πσ ∗ States
139
resolved TKER spectra indicate that a 1 πσ ∗
O–CH 3
feature appears by 1.2 ns (centered
at ∼ 7500 cm −1 ), although analysis of the energetics for O–CH 3 dissociation reveals
that, unlike O–H fission along 1 πσ ∗
O–H , CH 3 elimination occurs adiabatically with
respect to the 1 πσ ∗
O–CH 3
/S 0 CI (TKER max for adiabatic O–CH 3 fission is indicated
by the vertical blue arrow on the left panel of Fig. 6.11(c)). These adiabatic dynamics generate ground state CH 3 radicals in coincidence with para-hydroxyphenoxyl
radicals in their first electronically excited state, HOC 6 H 4 O ( ˜
A). This behavior has
been tentatively attributed to a geometric phase effect around the 1 πσ ∗
O–CH 3
/S 0 CI
[80]—[91] provides a broader discussion on such phenomena.
Between 280 > λ > 245 nm 1 πσ ∗
O–CH 3
driven O–CH 3 scission prevails over
any H-atom elimination processes. Unlike the 1 πσ ∗
O–H state, the potentials in
Fig. 6.10(b) show that 1 πσ ∗
O–CH 3
exhibits a quasi-bound well in the vertical FranckCondon region (∼ 0.6 eV deep), which arises from strong mixing between a 1 π 3s
Rydberg state (where the 3s orbital is associated with the O atom on the O–CH 3 coordinate) and the 1 πσ ∗
O–CH 3
valence state (reference [58] provides a general review
of Rydberg-valence mixing). As a result, excitation between 280 > λ > 245 nm induces population directly into the bound 3s Rydberg well of the 1 πσ ∗
O–CH 3
surface,
localizing electron density onto the O–CH 3 moiety, and ultimately causes O–CH 3
fission to dominate over H elimination pathways. At λ ≤ 245 nm, excitation to
2 1 ππ ∗ occurs and subsequent dynamics along both 1 πσ ∗
O–H and 1 πσ ∗
O–CH 3
surfaces
are active. However, the right panel in Fig. 6.11(c) highlights that O–CH 3 fission via
1 πσ ∗
O–CH 3
occurs on a timescale (τ = 2.5 ± 1.3 ps) an order of magnitude slower
than O–H fission along 1 πσ ∗
O–H at these excitation wavelengths, indicating that the
initial coupling processes onto 1 πσ ∗
O–CH 3
and 1 πσ ∗
O–H from 2 1 ππ ∗ must be kinetically competitive (both < 180 fs based on the timescale recorded for 1 πσ ∗
O–H driven
O–H fission). The timescale differences are thus attributed to the very different topographies of the two 1 πσ ∗ states; the purely dissociative profile of 1 πσ ∗
O–H enables
ultrafast elimination of H-atoms in a ballistic manner, whereas time is required for
population to evolve out of the quasi-bound well on 1 πσ ∗
O–CH 3
and adiabatically
advance towards electronically excited HOC 6 H 4 O ( ˜
A) radicals, impeding O–CH 3
fission dynamics.
These detailed TR-VMI studies of mequinol, together with previous studies
on hydroxyindoles [83, 84] and adenine [82], demonstrate the feasibility of untangling competing 1 πσ ∗ mediated dynamics in more complex heteroaromatic
biomolecules. Future prospects include unraveling the role of competing 1 πσ ∗
dynamics in dihydroxyindoles (subunits of photostable biological eumelanin copolymers [92–94]) and DNA nucleosides.
6.4.5 Outlook
Developing a firmer understanding of 1 πσ ∗ mediated dynamics in photoexcited
DNA bases, amino acids and their subunits provides the foundations for extending
139
resolved TKER spectra indicate that a 1 πσ ∗
O–CH 3
feature appears by 1.2 ns (centered
at ∼ 7500 cm −1 ), although analysis of the energetics for O–CH 3 dissociation reveals
that, unlike O–H fission along 1 πσ ∗
O–H , CH 3 elimination occurs adiabatically with
respect to the 1 πσ ∗
O–CH 3
/S 0 CI (TKER max for adiabatic O–CH 3 fission is indicated
by the vertical blue arrow on the left panel of Fig. 6.11(c)). These adiabatic dynamics generate ground state CH 3 radicals in coincidence with para-hydroxyphenoxyl
radicals in their first electronically excited state, HOC 6 H 4 O ( ˜
A). This behavior has
been tentatively attributed to a geometric phase effect around the 1 πσ ∗
O–CH 3
/S 0 CI
[80]—[91] provides a broader discussion on such phenomena.
Between 280 > λ > 245 nm 1 πσ ∗
O–CH 3
driven O–CH 3 scission prevails over
any H-atom elimination processes. Unlike the 1 πσ ∗
O–H state, the potentials in
Fig. 6.10(b) show that 1 πσ ∗
O–CH 3
exhibits a quasi-bound well in the vertical FranckCondon region (∼ 0.6 eV deep), which arises from strong mixing between a 1 π 3s
Rydberg state (where the 3s orbital is associated with the O atom on the O–CH 3 coordinate) and the 1 πσ ∗
O–CH 3
valence state (reference [58] provides a general review
of Rydberg-valence mixing). As a result, excitation between 280 > λ > 245 nm induces population directly into the bound 3s Rydberg well of the 1 πσ ∗
O–CH 3
surface,
localizing electron density onto the O–CH 3 moiety, and ultimately causes O–CH 3
fission to dominate over H elimination pathways. At λ ≤ 245 nm, excitation to
2 1 ππ ∗ occurs and subsequent dynamics along both 1 πσ ∗
O–H and 1 πσ ∗
O–CH 3
surfaces
are active. However, the right panel in Fig. 6.11(c) highlights that O–CH 3 fission via
1 πσ ∗
O–CH 3
occurs on a timescale (τ = 2.5 ± 1.3 ps) an order of magnitude slower
than O–H fission along 1 πσ ∗
O–H at these excitation wavelengths, indicating that the
initial coupling processes onto 1 πσ ∗
O–CH 3
and 1 πσ ∗
O–H from 2 1 ππ ∗ must be kinetically competitive (both < 180 fs based on the timescale recorded for 1 πσ ∗
O–H driven
O–H fission). The timescale differences are thus attributed to the very different topographies of the two 1 πσ ∗ states; the purely dissociative profile of 1 πσ ∗
O–H enables
ultrafast elimination of H-atoms in a ballistic manner, whereas time is required for
population to evolve out of the quasi-bound well on 1 πσ ∗
O–CH 3
and adiabatically
advance towards electronically excited HOC 6 H 4 O ( ˜
A) radicals, impeding O–CH 3
fission dynamics.
These detailed TR-VMI studies of mequinol, together with previous studies
on hydroxyindoles [83, 84] and adenine [82], demonstrate the feasibility of untangling competing 1 πσ ∗ mediated dynamics in more complex heteroaromatic
biomolecules. Future prospects include unraveling the role of competing 1 πσ ∗
dynamics in dihydroxyindoles (subunits of photostable biological eumelanin copolymers [92–94]) and DNA nucleosides.
6.4.5 Outlook
Developing a firmer understanding of 1 πσ ∗ mediated dynamics in photoexcited
DNA bases, amino acids and their subunits provides the foundations for extending