states and intermediates involved in the reaction pathway of
the DMB cyclization process. We have used a similar
approach to obtain insight into the photodeprotection mech-
anism of p-hydroxyphenacyl-caged phototrigger compounds
and also predicted a new type of photochemical reaction of
a fast photodecarboxylation coupled by ultrafast excited-state
intramolecular proton transfer (ESIPT) for some aromatic
carbonyl compounds using the high-level ab inito methods
Scheme 1
1
3,14
of CASSCF and CASSCF/CASPT2.
By using the same
3
increases substantially to become ∼0.64 compared to only
0.26 for the benzoin acetate phototrigger that does not
ab initio methods for the excited state computations, we focus
here on the particular steps related to the photocyclization
of 3′,5′-dimethoxybenzoin acetate (DMBA) and explicitly
describe how the photocyclization happens and clarify the
uncertainty surrounding the mechanisms of DMB photot-
rigger compounds.
∼
1
b
contain any m-methoxybenzylic substitution. A concerted
cyclization-deprotection mechanism mediated by a benzoyl-
localized nπ* triplet state was proposed for the parent
1
b,4
benzoin-based PRPG.
In contrast to this, several pieces
of experimental evidence from triplet quenching studies and
spectral characterization of some intermediates suggest that
the deprotection of DMB probably proceeds via a singlet
pathway and a number of different intermediates (including
a cyclic cation, a carbocation, an intramolecular charge
transfer (CT) exciplex, and a diradical species) have been
The ab initio calculations were done at the CASSCF level
of theory with a total of 10 electrons in 8 active space orbitals
(
10e/8o) and the 6-31G*, 6-31G basis set. The minimum energy
1
profiles (MEPs) in the nπ* singly excited state (SNP( nπ*)) and
the short distance CT excited state (SSCT( ππ*)) as well as the
long distance CT excited state (SLCT( ππ*)) were mapped by
1
1
5
-10
proposed for involvement in the DMB deprotection.
Recent studies employing femtosecond transient absorption
using two roots state-average multistep optimizations along the
fixed O1-C2 distance. To consider the dynamical electron
correlation effects for these points, the refined single-point
energy was recalculated at the multiconfiguration second-order
perturbation theory level (CASPT2) using a four roots state
averaged CASSCF (10e/8o)/6-31G zeroth-order wave function.
The vertical excitation energies, the corresponding oscillator
strengths, and the transition dipole moments for the lowest three
excited states of DMBA were found by using the ground state
1
0,11
(
fs-TA) spectroscopy
and nanosecond time-resolved
3
11
resonance Raman (ns-TR ) spectroscopy concluded that
an electronic interaction between the benzoyl (Bz) and
dimethoxybenzylic (DMBn) chromophores plays a primary
9
-1
role in initiating a rapid (∼1 × 10 s rate) stepwise DMB
cyclization-deprotection. It was observed experimentally
that the DMB cyclization is a very fast process occurring
on the tens of picosecond time scale to produce a biradical
species as the transient precursor to the subsequent depro-
tection step of the reaction. The fast cyclization was proposed
to arise from a CT exciplex that is produced on an even
faster time scale of several picoseconds. The formation of
this CT exciplex appears to compete efficiently with the
inherently rapid intersystem crossing observed previously in
the parent benzoin system and has been considered as the
key factor to account for the singlet nature of the reaction
mechansim and the very high yield of the DMB PRPG.
The unusually rapid cyclization of DMB has little prece-
dent in the literature. To gain more insight into how this
fundamentally important reaction takes place and also to
assess the long discussed essential role of a CT species in
govening the excited state reaction pathway of aromatic
0
(S ) CASSCF/6-31G optimizations followed by four roots state
average CASPT2 and CASSCF state interaction (CASSI)
computations. All of the calculations here were performed by
15
16
using the Gaussian 03 and Molcas program packages. For
more computational details see the approriate section of the
Supporting Information.
0
As shown in Table 1, the lowest excited state of the S f
1
S
NP( nπ*) transition originates from the promotion of the one
electron of the lone pair carbonyl O1 to a π* orbital that is
delocalized along the whole Bz chromophore. The S
0
f
1
S
NP( nπ*) transition for DMBA shows a modest dipole
-5
moment (0.763 D) and oscillator strength (3.2 × 10 ). This
suggests a “dark” spectroscopic state and a lower charge
transfer (CT) character for the n f π* transition, which is
a common feature for compounds containing the Bz chro-
12
carbonyls, we used quantitative electronic structure calcula-
tions here to explicitly describe the nature of the excited
13
mophore. Unlike the n f π* transition, two other excited
(
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