6 Biomolecules, Photostability and 1 πσ ∗ States
133
multiphoton signal observed at 2.5 ps [26, 65]), and is attributed to the onset of
statistical H-atom elimination from hot S 0 species. This statistical feature appears
with a time-constant of > 270 ps, shown by the transient in Fig. 6.7(c) (open circles). Selective deuteration of the N–H bond (N–D) also terminates any H + signal,
revealing that H elimination in imidazole is localized to the N–H bond coordinate,
even after coupling back onto S 0 [26]. In stark contrast, pyrazole exhibits a sizable
gain in statistical H + counts by 600 ps, relative to imidazole (see Figs. 6.7(a) and
(c)), and appears with a time-constant of τ = 165 ± 30 ps. Further measurements on
deuterated isotopomers of pyrazole (either exclusive deuteration of all C–H bonds
(C–D) or only the N–H bond (N–D)) reveals that there is change to ∼ 1:1 ratio for
statistical H loss from C–H:N–H coordinates in pyrazole [26] (unlike imidazole). It
has been conjectured that irreversible photodamage of pyrazole through C–H bond
fission and the generation of toxic free radicals may have played a role in the structural selectivity of imidazole over pyrazole in photostable biomolecules [26].
6.4.3 Excited State H-Atom Tunneling Dynamics
Over the years quantum tunneling of protons/hydrogen atoms has been implicated
in a vast array of chemical [67] and biological [68] processes, such as photoacidity behavior [69, 70], alcohol dehydrogenase enzymes [71] and around the oxygen
evolving complex site in photosystem II [72]. With respect to the latter, a series of
proton-coupled electron-transfer reactions [73] are proposed to occur, one of which
involves de-protonation of an O–H bond in a tyrosine residue via proton tunneling
to a neighboring histidine acceptor site [72]. The residue of the amino acid tyrosine is the UV chromophore phenol, shown inset in Fig. 6.8. In recent years there
has been a growing body of evidence to suggest that, over a specific UV excitation
range (275–248 nm), photoexcited phenol itself exhibits a propensity for H-atom
tunneling dynamics along the O–H coordinate under a 1 ππ ∗ / 1 πσ ∗ CI [27–29, 74].
It is this aspect of phenol’s UV induced photochemistry which forms the topic of
discussion for this section.
Calculated potential energy cuts along phenol’s O–H bond coordinate by Ashfold and co-workers [28], shown in Fig. 6.8, serve to indicate how, after excitation
to the zero point energy (ZPE) of the 1 ππ ∗ state (275 nm), population may tunnel
under the barrier formed by the 1 ππ ∗ / 1 πσ ∗ CI onto the 1 πσ ∗ state at elongated
O–H bond lengths. This process subsequently gives rise to the elimination of high
KE H-atoms, in coincidence with ground state phenoxyl radicals, C 6 H 5 O( ˜
X). Such
a mechanism was first speculated in the theoretical work of Sobolewski et al. [14].
Later, experimental work by Pino et al. added further credence to this proposed tunneling mechanism [27]. This was achieved through TR-MS measurements, which
compared the lifetimes extracted from parent cation signal decays following excitation to the 1 ππ ∗ ZPE in a wide range of chemically substituted phenols and their hydrogen bonded complexes with NH 3 . With the aid of complementary theory calculations, it was highlighted that phenol derivatives with a smaller vertical energy gap
133
multiphoton signal observed at 2.5 ps [26, 65]), and is attributed to the onset of
statistical H-atom elimination from hot S 0 species. This statistical feature appears
with a time-constant of > 270 ps, shown by the transient in Fig. 6.7(c) (open circles). Selective deuteration of the N–H bond (N–D) also terminates any H + signal,
revealing that H elimination in imidazole is localized to the N–H bond coordinate,
even after coupling back onto S 0 [26]. In stark contrast, pyrazole exhibits a sizable
gain in statistical H + counts by 600 ps, relative to imidazole (see Figs. 6.7(a) and
(c)), and appears with a time-constant of τ = 165 ± 30 ps. Further measurements on
deuterated isotopomers of pyrazole (either exclusive deuteration of all C–H bonds
(C–D) or only the N–H bond (N–D)) reveals that there is change to ∼ 1:1 ratio for
statistical H loss from C–H:N–H coordinates in pyrazole [26] (unlike imidazole). It
has been conjectured that irreversible photodamage of pyrazole through C–H bond
fission and the generation of toxic free radicals may have played a role in the structural selectivity of imidazole over pyrazole in photostable biomolecules [26].
6.4.3 Excited State H-Atom Tunneling Dynamics
Over the years quantum tunneling of protons/hydrogen atoms has been implicated
in a vast array of chemical [67] and biological [68] processes, such as photoacidity behavior [69, 70], alcohol dehydrogenase enzymes [71] and around the oxygen
evolving complex site in photosystem II [72]. With respect to the latter, a series of
proton-coupled electron-transfer reactions [73] are proposed to occur, one of which
involves de-protonation of an O–H bond in a tyrosine residue via proton tunneling
to a neighboring histidine acceptor site [72]. The residue of the amino acid tyrosine is the UV chromophore phenol, shown inset in Fig. 6.8. In recent years there
has been a growing body of evidence to suggest that, over a specific UV excitation
range (275–248 nm), photoexcited phenol itself exhibits a propensity for H-atom
tunneling dynamics along the O–H coordinate under a 1 ππ ∗ / 1 πσ ∗ CI [27–29, 74].
It is this aspect of phenol’s UV induced photochemistry which forms the topic of
discussion for this section.
Calculated potential energy cuts along phenol’s O–H bond coordinate by Ashfold and co-workers [28], shown in Fig. 6.8, serve to indicate how, after excitation
to the zero point energy (ZPE) of the 1 ππ ∗ state (275 nm), population may tunnel
under the barrier formed by the 1 ππ ∗ / 1 πσ ∗ CI onto the 1 πσ ∗ state at elongated
O–H bond lengths. This process subsequently gives rise to the elimination of high
KE H-atoms, in coincidence with ground state phenoxyl radicals, C 6 H 5 O( ˜
X). Such
a mechanism was first speculated in the theoretical work of Sobolewski et al. [14].
Later, experimental work by Pino et al. added further credence to this proposed tunneling mechanism [27]. This was achieved through TR-MS measurements, which
compared the lifetimes extracted from parent cation signal decays following excitation to the 1 ππ ∗ ZPE in a wide range of chemically substituted phenols and their hydrogen bonded complexes with NH 3 . With the aid of complementary theory calculations, it was highlighted that phenol derivatives with a smaller vertical energy gap