Mechanistic study of the photochemical hydroxide ion release from 9-hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine

Article abstract of DOI:10.1021/ja3031888

The excited-state behavior of 9-hydroxy-10-methyl-9-phenyl-9,10- dihydroacridine and its derivative, 9-methoxy-10-methyl-9-phenyl-9,10- dihydroacridine (AcrOR, R = H, Me), was studied via femtosecond and nanosecond UV-vis transient absorption spectroscopy. The solvent effects on C-O bond cleavage were clearly identified: a fast heterolytic cleavage (τ = 108 ps) was observed in protic solvents, while intersystem crossing was observed in aprotic solvents. Fast heterolysis generates 10-methyl-9-phenylacridinium (Acr⁺) and ^-OH, which have a long recombination lifetime (no signal decay was observed within 100 μs). AcrOH exhibits the characteristic behavior needed for its utilization as a chromophore in the pOH jump experiment.

Full text of DOI:10.1021/ja3031888

Communication
pubs.acs.org/JACS
Mechanistic Study of the Photochemical Hydroxide Ion Release from
9-Hydroxy-10-methyl-9-phenyl-9,10-dihydroacridine
Dapeng Zhou,† Renat Khatmullin,† Janitha Walpita,† Nicholas A. Miller,† Hoi Ling Luk,‡ Shubham Vyas,‡
Christopher M. Hadad,‡ and Ksenija D. Glusac*,†
†Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403, United States
‡Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
S
* Supporting Information
investigation of the excited-state dynamics of AcrOR via
femtosecond (fs) and nanosecond (ns) transient absorption
(TA) spectroscopy (Scheme 1), including the varying solvent-
ABSTRACT: The excited-state behavior of 9-hydroxy-10-
methyl-9-phenyl-9,10-dihydroacridine and its derivative, 9-
methoxy-10-methyl-9-phenyl-9,10-dihydroacridine
(AcrOR, R = H, Me), was studied via femtosecond and
nanosecond UV−vis transient absorption spectroscopy.
The solvent effects on C−O bond cleavage were clearly
identified: a fast heterolytic cleavage (τ = 108 ps) was
observed in protic solvents, while intersystem crossing was
observed in aprotic solvents. Fast heterolysis generates 10-
methyl-9-phenylacridinium (Acr+) and −OH, which have a
long recombination lifetime (no signal decay was observed
within 100 μs). AcrOH exhibits the characteristic behavior
needed for its utilization as a chromophore in the pOH
jump experiment.
Scheme 1. Excited-State Behavior of AcrOH in Different
a
Solvents: MeOH (blue) and ACN or Benzene (red)
hotohydroxide emitters are a class of compounds that
P
release hydroxide anions (−OH) to induce a pOH jump
upon UV−vis irradiation. This process is similar to the pH
jump methods achieved using various photoacids.1−4 The pOH
jump is generally achieved through heterolytic cleavage of the
C−O bond, as is exhibited by compounds such as 9-
fluorenol,5,6 9-phenylxanthen-9-ol,7 and triphenylmethane
hydroxide,8,9 whose resonance structures stabilize the resulting
carbocations. The application of these compounds in driving a
photochemical pOH jump is mostly limited by the fast
recombination of hydroxide and the carbocation.5−7 Addition-
ally, homolytic C−O bond cleavage can complicate the
chemistry with the formation of hydroxyl radical (•OH).5,7,10
Malachite green carbinol base (MGCB) has been suitable for
photochemical pOH jump experiments9 because the lifetime of
−OH, which is generated in 300 ps,8 is several minutes, and
a
For AcrOMe, τhetero = 83 ps and τISC = ∼550 ps.
facilitated pathways of Acr+ generation. Generation of Acr+ is
obvious with protic solvation (methanol, MeOH), while an
intersystem crossing (ISC) process is observed with aprotic
solvation (ACN and benzene).
To evaluate the driving force for the excited-state heterolytic
and homolytic C−O bond cleavage in AcrOH, a Forster energy
̈
diagram15 was constructed from pH-dependent absorption and
emission measurements along with the previously known
reduction and oxidation potentials of Acr+ and −OH (Figure S1
in the Supporting Information). From pH-dependent absorp-
tion measurements, the ground-state pseudobase pKa value of
AcrOH was determined to be 11.11, which is consistent with
•OH is not formed. Upon excitation, MGCB was found to
induce a jump from pH 6 to 11, making it a possible agent for
studies of conformational changes in DNA/RNA11,12 and also
for the release of drugs from host molecules.13
the literature.16 On the basis of the Forster energy diagram, the
̈
driving forces for the excited-state hetero- and homolytic C−O
́
Recently, Frechet and co-workers discovered that 9-hydroxy/
bond cleavages were evaluated as Δ1G*hetero = −1.048 eV,
methoxy-10-methyl-9-phenyl-9,10-dihydroacridine (AcrOR, R
= H, Me) exhibits photochemical behavior somewhat similar to
that of MGCB.14 Upon irradiation in H2O/acetonitrile (ACN)
mixtures, AcrOH releases 10-methyl-9-phenylacridinium (Acr+)
Δ3G*hetero = −0.263 eV, Δ1G*homo = −1.513, and Δ3G*homo
=
−0.728 eV, indicating that both heterolytic and homolytic
cleavage are possible in the excited state.
−
and OH, resulting in a pOH jump that lasts longer than that
of MGCB. However, the photochemical mechanism of AcrOH
has not been studied until now. This study presents a detailed
Received: April 2, 2012
Published: July 5, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
•OR). The absorption spectrum of Acr• was obtained by a
one-electron reduction of Acr+ (see the cyclic voltammogram in
Figure 1d). The UV−vis spectroelectrochemical results shown
in Figure 1c demonstrate that Acr• absorbs at 500 nm, which
matches well with the previously reported Acr• spectrum
obtained using chemical reduction.19 Given the large mismatch
between the 550 nm band obtained in the TA spectra and the
Acr• signature band, we conclude that homolytic C−O bond
cleavage from AcrOH* does not occur. The experiments
performed on the ns time scale show that the 550 nm transient
is long-lived (τ550 nm = 277 ns; Figure S5) and readily quenched
by molecular oxygen. Thus, this intermediate is assigned to the
triplet excited (T1) state of AcrOH. These results suggest that
even though the homolytic bond cleavage is thermodynamically
favorable (Figure S1) and previous reports on similar systems7
demonstrate the formation of the homolytic products, the S1
state of AcrOH does not undergo homolytic C−O bond
breaking. This is possibly due to a competing deactivation
channel of the S1 state via ISC to form the T1 state.
Additionally, formation of the Acr• absorbance at ∼500 nm
was not observed in the ns TA measurements, showing that
homolytic C−O bond cleavage on the T1 state surface does not
The geometry of the AcrOH ground state was optimized and
the characters of the AcrOH excited states were evaluated using
density functional theory (DFT) and time-dependent DFT
calculations, respectively (Table S1 and Figure S2). The
difference density plots17,18 shown in Figure S2 indicate that
the S0−S1 transition exhibits π−π* character within the acridine
framework, while the S0−S2 transition generates a π−π* state in
which an intramolecular electron transfer from the acridine
framework to the phenyl moiety occurs. The laser measure-
ments shown in this work were performed using a 310 nm
excitation pulse; when compared to the calculated vertical
excitations (Figure S2), this laser excitation should generate
only the singlet excited (S1) state with π−π* character.
The photochemical behavior of AcrOH in different solvents
(ACN, benzene, and MeOH) was investigated using the fs TA
technique. In ACN, the t = 0 TA spectrum of AcrOH exhibits a
broad absorbance from 350 to 800 nm with λmax at ∼780 nm
that decays with a lifetime of 1.4 ns (Figure 1). This component
•
lead to separated Acr• and OH radicals, even though the
evaluated AcrOH T1 energy is higher than the energy needed
for the C−O bond homolysis (Figure S1). A possible scenario
that can explain this absence of observable homolytic products
is one in which a conical intersection between the T1 and S0
states exists along the C−O bond coordinate. Since the T1
lifetime of AcrOH (277 ns; Figure S5e) is shorter than the T1
lifetimes of most organic compounds (usually in the μs to ms
range20), it is possible that such a mechanism operates in this
case. Indeed, DFT calculations suggested that homolytic C−O
bond cleavage in the T1 state occurs at a C−O bond distance of
1.8 Å. This triplet transition state would lead to a pair of
separated radicals at a distance of 3.0 Å. However, along the C−
O distance, the ground-state singlet surface and the triplet
surface cross each other at a separation of 2.8 Å. Thus, we
suggest that homolytic C−O bond cleavage in the triplet state
leads to a conical intersection between the S0 and T1 states,
resulting in the recovery of AcrOH in the ground state (see
Table S2 and Figures S8−S10).
On the other hand, the behavior of AcrOH in protic solvents
is different (Figure 2). The initially formed S1 state decays
within 108 ps to generate a product that absorbs at 425 nm.
This product is assigned to Acr+ by comparison to the ground-
state absorption spectrum (Figure 2a inset). The protic
solvation clearly has a strong effect on the energetics of the
heterolysis, as schematized in Figure S1. As previously
discussed by Steen,21 the protic solvation stabilizes −OH by
promoting the migration of the negative charge along the
hydrogen-bonded solvent network. The formation of Acr+
occurs without any observable intermediate steps. To be
specific, the formation of the intermediate Acr+* via adiabatic
heterolytic bond breaking along the S1 surface of AcrOH was
not observed in the TA spectra. The steady-state emission
measurements also confirmed that Acr+* is not formed as an
intermediate during heterolytic bond cleavage (Figures S2 and
S6), since no Acr+* emission (450 to 650 nm) was observed in
the emission measurements of AcrOH or AcrOMe in ACN or
water.
Figure 1. (a) Femtosecond TA spectra at time delays of −4.5 ps
(black), 0 ps (red), 11 ps (blue), 310 ps (green), 570 ps (gray), 1150
ps (aqua), and 1574 ps (orange). (b) Kinetics of 1 mM AcrOH in
ACN (λEX = 310 nm). (c) UV−vis spectroelectrochemical spectra of 1
mM Acr+ in ACN at −0.56 V vs SHE at time delays of 0 s (red), 7 s
(blue), 14 s (light blue), 21 s (magenta), and 49 s (olive). The black
curve is the UV−vis absorption spectrum of Acr+. (d) Cyclic
voltammogram of 3 mM Acr+ in anaerobic ACN containing 0.1 M
tetrabutylammonium perchlorate. Ered = −0.3 V vs SHE.
is assigned to the S1 state of AcrOH. The decay of the S1 state is
accompanied by the growth of a new band at ∼550 nm that
does not decay within 1.5 ns (the time limit of fs TA
detection). The excited-state behavior of AcrOH and AcrOMe
in other aprotic solvents, such as benzene (Figure S3 and Table
S3), is similar to that shown in Figure 1. If the excited-state
heterolytic C−O bond cleavage occurred, the 550 nm signal
could be assigned to the formation of either Acr+ (nonadiabatic
process) or Acr+* (adiabatic process). However, comparison of
the absorption spectra of Acr+ (λmax = 425 nm; Figure 1c) and
Acr+* (λmax = 390 and 470 nm; Figure S4) to this 550 nm band
shows that the heterolytic C−O bond cleavage does not occur
from the S1 state of AcrOH. On the basis of previous
literature,7 this ∼550 nm peak could be postulated to arise from
the acridine radical (Acr•), which would be formed by
The TA experiments on the ns time scale showed that the
Acr+ signal at 425 nm is long-lived, and no signal decay was
observed within 100 μs (Figure 2d). The long-lived charge
homolytic C−O bond cleavage (AcrOR + hv → Acr•
+
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Journal of the American Chemical Society
Communication
AUTHOR INFORMATION
Corresponding Author
kglusac@bgsu.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by BGSU, the National Science
Foundation (CHE-1055397 CAREER Award to K.D.G. and
CHE-0743258 to C.M.H.), the Ohio Supercomputer Center,
and Delta Electronics Foundation (Delta Fellowship to D.Z.).
We thank Felix N. Castellano (BGSU) for help with
spectroelectrochemistry measurements.
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behavior was observed for AcrOMe; however, some differences
are also worth mentioning. In the fs TA spectra of AcrOMe in
MeOH (Figure S7), most of the AcrOMe was converted to
Acr+ after the laser flash, though there was still some generation
of the T1 state of AcrOMe (540 nm). These results reflect the
fact that ISC is faster for AcrOMe. Comparison of the lifetime
of the C−OH bond cleavage (108 ps) and the C−OMe bond
cleavage (83 ps) shows that the C−OMe bond is weaker than
the C−OH bond in MeOH. This conclusion is further
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PhCH2−OMe (78 and 68−70 kcal/mol, respectively).8 The
lower bond energy of C−OMe also reflects the different
reactivities of AcrOH and AcrOMe in aprotic solvents: while
AcrOH does not generate Acr+ in aprotic solvents, the TA
spectra of AcrOMe in ACN (Figures S3 and S5) show the
formation of Acr+ from the singlet excited state.
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In conclusion, photochemical excitation of AcrOH has been
shown to result solely in heterolytic cleavage in protic solvents.
Since the heterolysis is fast (108 ps) and followed by slow
recombination of the ions (time scale of hours14), the acridine
derivatives such as AcrOH show promise for use in producing
fast, long-lived pOH jumps.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis, TA laser setup, computational methods, and
additional fs and ns TA results. This material is available free
of charge via the Internet at http://pubs.acs.org.
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