A new visible light triggered Arrhenius photobase and its photo-induced reactions

Article abstract of DOI:10.1039/c9nj05404a

Arrhenius photobases are of potential use for excited state hydroxide ion dissociation (ESHID), photo-induced pOH jump experiments, and base-catalyzed reactions. However, previously studied Arrhenius photobases have to be excited by UV light and undergo ESHID reactions only in protic solvents. These characteristics have become a disadvantage to their application in many fields of research. In this work, we have designed and synthesized a new Arrhenius photobase (NO₂-Acr-OH), the ESHID reaction of which is readily triggered by visible light excitation. In contrast to previously studied Arrhenius photobases, NO₂-Acr-OH undergoes ESHID reactions in protic solvents as well as in polar aprotic solvents. Solvent-dependent photo-induced reactions of NO₂-Acr-OH are comprehensively studied by time-resolved fluorescence spectroscopy. Molecular designs for visible light triggered acridinol-based Arrhenius photobases with a large ΔpK_b value are proposed.

Full text of DOI:10.1039/c9nj05404a

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A new visible light triggered Arrhenius photobase
and its photo-induced reactions†
Cite this:DOI: 10.1039/c9nj05404a
Joonyoung F. Joung,‡ Jeeun Lee,‡ Joungin Hwang, Kihang Choi* and
Sungnam Park
*
Arrhenius photobases are of potential use for excited state hydroxide ion dissociation (ESHID), photo-
induced pOH jump experiments, and base-catalyzed reactions. However, previously studied Arrhenius
photobases have to be excited by UV light and undergo ESHID reactions only in protic solvents. These
characteristics have become a disadvantage to their application in many fields of research. In this work,
we have designed and synthesized a new Arrhenius photobase (NO
which is readily triggered by visible light excitation. In contrast to previously studied Arrhenius
photobases, NO -Acr-OH undergoes ESHID reactions in protic solvents as well as in polar aprotic
solvents. Solvent-dependent photo-induced reactions of NO -Acr-OH are comprehensively studied by
time-resolved fluorescence spectroscopy. Molecular designs for visible light triggered acridinol-based
Arrhenius photobases with a large DpK value are proposed.
2
-Acr-OH), the ESHID reaction of
Received 29th October 2019,
Accepted 18th December 2019
2
DOI: 10.1039/c9nj05404a
2
rsc.li/njc
b
fluorometric pH indicators in aqueous and alcohol solutions
because their optical properties are sensitive to pH. Furthermore,
I. Introduction
29
A sudden change in the pH of a solution can be made optically light-driven pOH jump experiments have been carried out by using
25
by using various photoacids which release protons upon electronic Arrhenius photobases. As such, Arrhenius photobases are of
excitation because their pK value is significantly decreased in potential use for various fundamental studies and applications.
their electronic excited state.
a
1
–16
As a counterpart of photo- However, for previously reported Arrhenius photobases, their
17,18
19
20
acids, quinolines,
aminoanthraquinone, 3-styrylpyridine,
ESHID reactions have been shown to be triggered by UV light
2
1
22
23,24
xanthone, curcumin, and Schiff bases
have been reported excitation and are found to occur only in protic solvents. UV light
as Brønsted-Lowry photobases which accept protons upon excitation may hinder the potential use of currently available
max
electronic excitation because their pKa value is increased in Arrhenius photobases (lAbs o B310 nm) for applications in
their electronic excited state. Based on their interesting photo- materials sciences because highly energetic UV photons can cause
chemical properties, the photoacids and Brønsted-Lowry photo- degradation of materials under study. Furthermore, biological
bases have been extensively used for a model system to investigate applications of existing Arrhenius photobases may be limited
the excited-state proton transfer reaction, proton solvation, due to their spectral interferences with amino acids such as
ionic effect on the proton transfer reaction, and pH jump tryptophan, tyrosine, and phenylalanine which have strong
1
–14,17–24
induced protein folding.
absorption at 260–310 nm.
As opposed to the Brønsted-Lowry photobases, Arrhenius
In this work, we have designed and synthesized a new
À
photobases are capable of releasing hydroxide ions (OH ) directly Arrhenius photobase, the photoreactions of which can be readily
2
5–28
upon electronic excitation. Triphenylmethanol derivatives,
triggered by visible light excitation. Photo-induced reactions of
9-phenyl-acridin-9-ol the newly designed Arrhenius photobase NO -Acr-OH shown in
have been studied as Fig. 1 are studied in various solvents in great detail.
2
9,30
9
-phenyl-xanthen-9-ol derivatives,
2
3
1,32
33,34
derivatives,
and fluoren-9-ol
Arrhenius photobases. The excited state hydroxide ion dissocia-
tion (ESHID) reactions of several Arrhenius photobases have been
30
studied in water and acetonitrile (ACN) mixtures. In addition,
Arrhenius photobases have been studied for colorimetric and
II. Results and discussion
II-A. Designing an Arrhenius photobase with visible light
absorption
Department of Chemistry and Research Institute for Natural Science, Korea
University, Seoul, 02841, Korea. E-mail: kchoi@korea.ac.kr, spark8@korea.ac.kr
Acridinol-based photobases (X-Ph-Acr-OH, X = CN, CF
OCH , N(CH and Y = H) shown in Fig. 1 have been studied
as Arrhenius photobases, the ESHID reactions of which can
3
, H,
3
3 2
)
†
‡
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj05404a
These authors contributed equally to this work.
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II-B. Photophysical properties of NO
2
-Acr-OH in water
-Acr-OH
2
-Acr in water, which are significantly red-shifted
Fig. 2(a) displays the UV-visible absorption spectra of NO
2
+
and NO
due to the nitro group which increases the conjugation length
of the acridine backbone. In addition, the nitro group is found
+
to destabilize NO
2
-Acr because of its electron withdrawing
ability which leads to a significant increase in the pKb value
of NO -Acr-OH. The pKb value of NO -Acr-OH in water was
2
2
theoretically estimated to be 7.69 by using eqn (S3) (ESI†) and
DFT calculations, and was experimentally determined to be
35–41
7.63 at 25 1C as described in the ESI† (Fig. S1–S5).
Obviously,
the pK value of NO -Acr-OH is larger than that of X-Ph-Acr-OH
b
2
shown in Fig. 1(a) which is in the range of 2.3 to 3.3.
Fig. 2(b) shows the photoluminescence (PL) spectra of
+
NO -Acr-OH and NO -Acr in water. The PL quantum yields of
2
2
+
NO -Acr-OH and NO -Acr in water are determined to be
2
2
4
2
FPLQY = 0.003 and 0.010, respectively (Fig. S6, ESI†). The PL
of NO -Acr-OH in water is negligibly weak. Two peaks are
observed in the PL spectrum of NO -Acr-OH in water (see the
inset of Fig. 2(b)). The PL peaks at B450 nm and B550 nm are
2
2
+
2 2
found to result from photo-generated NO -Acr and NO -Acr-OH,
respectively, as will be discussed in the next section. Because of
Fig. 1 (a) Molecular structures of acridinol-based photobases and their
ESHID reactions. (b) Frontier orbitals of Ph-Acr-OH and NO
2
-Acr-OH.
NO -Acr-OH exhibits more charge transfer (CT) character than Ph-Acr-OH.
2
3
2
occur by absorption of UV light at B310 nm. The functional
groups (X = CN, CF , OCH , and N(CH ) ) are introduced at the
3
3
3 2
para position (X) of the 9-phenyl group in X-Ph-Acr-OH. DFT
calculations indicate that the 9-phenyl group in X-Ph-Acr-OH is
perpendicular to the acridine backbone as shown in Fig. 1(b)
and it does not influence the conjugation of the acridine
backbone significantly. Accordingly, the absorption spectra of
acridinol derivatives (X-Ph-Acr-OH) are found to be not much
different. However, the absorption and emission spectra of
+
X-Ph-Acr are significantly influenced by the functional groups
at the X position because the 9-phenyl group is placed in the
+
same plane with the acridine backbone in X-Ph-Acr forming
an extended conjugated structure. Based on the relationship
between the optical properties and the molecular structures of
acridinol-based photobases, we designed a new visible light
triggered photobase by introducing a nitro group at the Y
position of the acridine backbone shown in Fig. 1 and extending
the conjugation of the acridine backbone. This newly designed
2
Arrhenius photobase (NO -Acr-OH) in various solvents is found
+
Fig. 2 (a) UV-visible absorption spectra of NO
water (38 mM). (b) PL spectra of NO -Acr-OH (black) and NO
water (lex = 375 nm) and their relative intensities are compared. The inset
shows the emission spectrum of NO -Acr-OH on a different intensity scale.
2
-Acr-OH and NO
2
+
-Acr in
to be readily excited by B400 nm light and undergoes ESHID
reaction and intersystem crossing, which will be discussed
below in great detail.
2
2
-Acr (red) in
2
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such negligible fluorescence intensity, the photo-induced reaction
2
of NO -Acr-OH in water is unable to be studied by time-resolved
fluorescence experiments.
2
II-C. Photo-induced reaction of NO -Acr-OH in organic solvents
The UV-visible absorption and emission spectra of NO
2
-Acr-OH
are measured in various solvents (dimethyl sulfoxide (DMSO),
acetonitrile (ACN), methanol, ethanol, tetrahydrofuran (THF),
and toluene) as shown in Fig. 3. The absorption peaks of
2
NO -Acr-OH are near 400 nm and are gradually red-shifted as
the solvent polarity is increased (solvatochromic shift). The
+
absorption spectra of NO -Acr in organic solvents, which is
2
prepared by the reaction of NO -Acr-OH with sulfuric acid
2
+
+
(
NO
Fig. 3(b). The DpK
estimated to be 2.9, 4.5, 4.4, and 4.7 in DMSO, ACN, methanol,
2
-Acr-OH + H - NO
2
-Acr + H
2
O), are displayed in
b
(=pK
b
À pK
b
*) values of NO
2
-Acr-OH are
1
4
and ethanol, respectively, by using the F o¨ rster cycle method.
The emission spectra of NO -Acr-OH (Fig. 3(c)) are solvent-
2
dependent, which results from different photo-induced processes
occurring in various solvents as will be presented in the next section.
To study the photo-induced reaction of NO -Acr-OH in
2
different solvents, we carried out time-resolved fluorescence
(
TRF) experiments by using the time-correlated single photon
counting (TCSPC) method. Fig. 4 shows the TRF signals measured
with NO -Acr-OH in different solvents following excitation by
75 nm pulses. Note that the TRF signals measured at B400 nm
2
3
are found to be readily quenched by molecular oxygen (O ) as
2
shown in Fig. S7 (ESI†) and are attributed to the phosphorescence
of NO -Acr-OH. This feature has been commonly observed with
2
3
0–34
previously studied Arrhenius photobases.
TRF signals measured at B600 nm, which are not quenched
by O as shown in Fig. S8 (ESI†), result from the fluorescence
of NO -Acr-OH. Accordingly, to analyze the TRF signals of
NO -Acr-OH, the ESHID reaction and intersystem crossing (ISC)
process are included in the kinetic model presented in Fig. 4(a).
In the kinetic model shown in Fig. 4(a), kROH, kR , and ktriplet
are the rate constants for the radiative decay of NO -Acr-OH*,
However, the
2
2
2
+
2
+
3
NO
for ESHID and reverse ESHID reactions are kOH and krev
respectively, and kISC is the ISC rate constant. By using the
2 2
-Acr *, and [NO -Acr-OH*], respectively. The rate constants
,
9
,43–45
global fitting analysis based on the kinetic model,
signals of NO -Acr-OH were fully analyzed and all the rate con-
stants were extracted as summarized in Table 1. The individual
the TRF
2
+
3
contributions of NO -Acr-OH*, NO -Acr *, and [NO -Acr-OH*] to
2
2
2
each TRF signal exhibit distinct spectral and temporal features as
shown in Fig. 4.
The global fitting analysis of TRF signals reveals a few
interesting and important features. First, the ESHID reaction
of NO
and toluene). In this case, the produced NO
stabilized in THF (e = 7.52) and toluene (e = 2.38) with small
dielectric constants (e). Second, the ESHID reaction and the
ISC process occur competitively in polar solvents but the ESHID
reaction is relatively faster than the ISC process (kOH 4 kISC).
2
-Acr-OH does not occur in weakly polar solvents (THF
+
2
-Acr may not be
Fig. 3 (a) UV-visible absorption spectra of NO
2
-Acr-OH in various
+
solvents. (b) UV-visible absorption spectra of NO
c) PL spectra of NO
2
-Acr in various solvents.
3
7
(
2
-Acr-OH in various solvents (lex = 375 nm).
2
Third, the ESHID reaction of NO -Acr-OH is much faster in aprotic solvents (ACN and DMSO). This implies that the hydro-
polar protic solvents (methanol and ethanol) than in polar xide ion dissociation reaction of NO
2
-Acr-OH should occur via
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Fig. 4 (a) Kinetic model for analyzing the TRF signals of NO
2
-Acr-OH in various solvents. TRF signals measured with NO
2
-Acr-OH in (b) DMSO, (c) ACN,
(
(
2
d) methanol, (e) ethanol, (f) THF, and (g) toluene (lex = 375 nm). The spectral components and time-dependent population decay of NO -Acr-OH*
+
3
blue), NO
2
-Acr * (red), and [NO
2
-Acr-OH*] (green), which are extracted from the global fitting analysis, are presented in the right and bottom panels,
respectively.
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Table 1 Rate constants extracted from the global fitting analysis
À1
À1
À1
À1
À1
a
À1
b
kOH (ns
)
krev (ns
)
kISC (ns
)
kROH (ns
)
k
R
+
(ns
)
ktriplet (ns
)
F
OH
DMSO
ACN
Methanol
Ethanol
THF
5.03
4.50
10.63
8.19
—
0.18
0.27
0.05
0.05
—
0.93
3.71
3.40
4.53
1.00
3.20
0.099
0.103
0.247
0.167
0.078
0.091
0.651
0.428
0.542
0.534
—
0.015
0.013
0.012
0.016
0.009
0.012
0.83
0.54
0.74
0.64
—
Toluene
—
—
—
—
a
b
k
triplet is determined by TRF experiments with NO
2
-Acr-OH in degassed solvents.
FOH = kOH/(kOH + kISC + kROH).
Table 2 The pK
solvents
b
b
and pK * values of NO
2
-Acr-OH in various organic polar solvents. In addition, NO -Acr-OH was found to undergo ISC
2
process in various solvents but the ISC process of NO -Acr-OH was
2
not significantly solvent-dependent. Note that NO
2
-Acr-OH in
DMSO
ACN
Methanol
Ethanol
water exhibits weak fluorescence (FPLQY = 0.003) and undergoes
pK
pK
b
b
1.5
À1.4
3.3
À1.2
2.1
À2.3
2.5
À2.2
+
ESHID reactions producing NO
negligibly weak as well. Therefore, NO
non-fluorescent Arrhenius photobase in aqueous solutions
-Acr-OH and can be used in pOH jump experiments minimizing its
in various solvents were estimated by the rate constants, e.g., fluorescence interference with molecular systems under study.
pK
Visible light triggered photobases with large DpK values are
2
-Acr whose fluorescence is
*
2
-Acr-OH is nearly a
b 2
acid-catalytic processes. Lastly, the pK * values of NO
b
* = Àlog10(kOH/krev), by assuming that the ESHID equilibrium
b
was rapidly achieved in the electronic excited state. Subsequently, ideal for studying ESHID reactions in various molecular systems.
the pKb values were determined by using the F ¨o rster cycle NO -Acr-OH absorbs visible light but exhibits a relatively small
2
À
method and the pK * values (Table 2). The OH dissociation DpK value. In the case of Ph-Acr-OH, functionalizing the para
b
b
rate (kOH) (Table 1) is shown to become larger as the pK * value position of the 9-phenyl group is shown to increase the DpKb
b
gets smaller, which has been explained by the free-energy- value while functionalizing the acridine backbone induces a
3
0,32
reactivity relationship in the literature.
The quantum yield redshift in the absorption of photobases. Accordingly, we will
À
À
of OH production indicates that the OH dissociation in excited be able to develop various visible light triggered Arrhenius
state is more favored than the ISC process in polar solvents.
photobases with large DpKb values by introducing different
Before this section is closed, it should be noted that functional groups at the para position of the 9-phenyl group
acridinol-based photobases (X-Ph-Acr-OH, X = CN, CF , H, and in the acridine backbone.
3
CH O, and (CH ) N) in various solvents have been studied by
3
3 2
3
1,32
Glusac and co-workers.
in S
alcohol but not in polar aprotic solvents (e.g., ACN). In addition,
They have found that X-Ph-Acr-OH
1
state undergoes ESHID reaction and ISC process in Conflicts of interest
+
There are no conflicts of interest to declare.
they have claimed that the ground state X-Ph-Acr is directly
formed as a result of the ESHID reaction of X-Ph-Acr-OH. In
2
contrast, our new NO -Acr-OH is found to undergo ESHID
Acknowledgements
reaction in both polar aprotic and protic solvents producing
+
fluorescent NO -Acr . The nitro group introduced to the acridine
2
This study was supported by grants from the National Research
Foundation of Korea (NRF) funded by the Korean government
backbone makes significant changes in the chemical and photo-
physical properties of acridinol-based Arrhenius photobases
in terms of the absorption spectrum, pK value, and solvent-
b
(MSIP) (No. 2019R1H1A2079968 and 2019R1A6A1A11044070
for S. P. and No. 20100020209 for K. C.).
dependent ESHID reaction.
III. Concluding remarks
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