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somewhat similar to HBDI) all undergo fast isomerisation in the excited state
[ 66 - 69 ]. The mechanism proposed involves a decreased bond order for the bridg-
ing double bond(s) in the excited state allowing nearly free rotation. The increase
in the ground state energy during this excited state rotation causes ground and
excited states to approach in energy or to cross at a conical intersection. At or near
this point, rapid IC to the ground state occurs. Evidence supporting an isomerisa-
tion mechanism has been presented on the basis of stationary photochemical
experiments, where the formation of a new ground state isomer under irradiation
was observed [ 70 - 74 ]. The cross-section was dependent on the solvent as is the
rate of the reverse isomerisation in the ground state [ 71 , 73 ]. Interestingly, the
solvent-dependent reverse process in the ground state (which is calculated to have
a high energy barrier) may proceed by an addition elimination reaction [ 71 , 75 ].
Excited state isomerisation involves large-scale structural reorganisation (e.g.,
a cis - trans isomerisation) on the upper potential energy surface, which may be
opposed by solvent friction. Thus, measurements of the excited state lifetime as a
function of solvent viscosity yield information about the nature of the coordinate
promoting IC. The mean excited state lifetime for anionic HBDI increases by only a
factor of 3 when the solvent is changed from methanol to glycerol, a 40-fold
increase in viscosity [ 63 ]. This weak viscosity dependence suggests that the coor-
dinate promoting IC in HBDI is not very sensitive to solvent friction (or is
energetically sufficiently strongly downhill to provide a strong driving force to
overcome the solvent friction). Thus, it seems unlikely that the coordinate promot-
ing IC involves a large-scale structural change, such as a complete rotation about
either of the exocyclic double bonds, as such a motion would have to displace a
large volume of solvent. The weak viscosity dependence is consistent with a
volume conserving motion involving a rearrangement localised on the bridging
bonds - a hypothesis supported by studies of analogues of HBDI synthesised to lock
the chromophore in the planar structure [ 76 ].
Further information can be obtained from the temperature dependence, which
can also reveal the existence of barriers in the radiationless relaxation coordi-
nate. Huppert and co-workers measured the fluorescence decay of HBDI in
glycerol-water over a wide temperature range [ 59 , 77 ]. They were able to
model their data with a two-dimensional model involving phenyl ring torsion
and a swinging motion in the bridging bonds [ 59 ]. Such an internal reorganisa-
tion could be volume conserving as suggested by the viscosity measurements.
Litvinenko et al. presented an isoviscosity analysis of the fluorescence yield and
ground state recovery time of HBDI in three charge states [ 61 ].Thedatawere
similar in all charge states suggesting a common radiationless coordinate, and
suggested a zero or negligible activation barrier along that coordinate. Mandal
et al. proposed that the wavelength-independent non-exponential kinetics they
observed in time-resolved fluorescence could be analysed in terms of a two-
dimensional coordinate [ 62 ].
Further analysis of the coordinate leading to ultrafast IC is possible through
quantum chemical calculation. Weber et al. considered the energetics of an excited
state isomerisation reaction involving the bridging bonds of HBDI via a 90 rotation
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