Multi-state amine sensing by electron transfers in a BODIPY probe

Article abstract of DOI:10.1039/c9ob02466b

Amines are ubiquitous in the chemical industry and are present in a wide range of biological processes, motivating the development of amine-sensitive sensors. There are many turn-on amine sensors, however there are no examples of turn-on sensors that utilize the amine's ability to react by single electron transfer (SET). We investigated a new turn-on amine probe with a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore. BODIPY fluorescence is first preprogrammed into an off state by internal photoinduced electron transfer (PET) to an electron-deficient quinolinium ring, resulting in fluorescence quenching. At low concentrations of aliphatic amine (0 to 10 mM), this PET pathway is shut down by external SET from the amine to the photoexcited charge-transfer state of the probe and the fluorescence is turned on. At high concentrations of amine (50 mM to 1 M), we observed collisional quenching of the BODIPY fluorescence. The probe is selective for aliphatic amines over aromatic amines, and aliphatic thiols or alcohols. The three molecular processes modulate the BODIPY fluorescence in a multi-mechanistic way with two of them producing a direct response to amine concentrations. The totality of the three molecular processes produced the first example of a multi-state and dose-responsive amine sensor.

Full text of DOI:10.1039/c9ob02466b

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Multi-state amine sensing by electron transfers in
a BODIPY probe†
Cite this: DOI: 10.1039/c9ob02466b
Katherine L. VanDenburgh,
Christopher R. Benson, Natalie M. Cox, Sundus Erbas-Cakmak, § Bo Qiao,
Xinfeng Gao, Maren Pink, Krishnan Raghavachari and Amar H. Flood
Yun Liu, ‡ Tumpa Sadhukhan,
¶
*
Amines are ubiquitous in the chemical industry and are present in a wide range of biological pro-
cesses, motivating the development of amine-sensitive sensors. There are many turn-on amine
sensors, however there are no examples of turn-on sensors that utilize the amine’s ability to react by
single electron transfer (SET). We investigated a new turn-on amine probe with a 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene (BODIPY) fluorophore. BODIPY fluorescence is first preprogrammed
into an off state by internal photoinduced electron transfer (PET) to an electron-deficient quinolinium
ring, resulting in fluorescence quenching. At low concentrations of aliphatic amine (0 to 10 mM), this PET
pathway is shut down by external SET from the amine to the photoexcited charge-transfer state of the
probe and the fluorescence is turned on. At high concentrations of amine (50 mM to 1 M), we observed col-
lisional quenching of the BODIPY fluorescence. The probe is selective for aliphatic amines over aromatic
amines, and aliphatic thiols or alcohols. The three molecular processes modulate the BODIPY fluorescence
in a multi-mechanistic way with two of them producing a direct response to amine concentrations. The
totality of the three molecular processes produced the first example of a multi-state and dose-responsive
amine sensor.
Received 14th November 2019,
Accepted 12th December 2019
DOI: 10.1039/c9ob02466b
rsc.li/obc
nism utilizes the reactivity of amines as sacrificial electron
Introduction
donors, resulting in turn-off fluorescent sensors.17–27 For
Amines are critically important across numerous chemical and example, collisional quenching of perylene diimide fluo-
biological processes1,2 and are potentially toxic pollutants.3–6 rescence has been observed with a range of organic amines
For these reasons, the need to monitor amines motivates the like aniline and triethylamine across 0.1 to 1 M concentrations
development of complementary molecular sensors.7,8 There in tetrahydrofuran.22 An unexplored area in amine sensing is
are two common mechanisms used for amine sensing. The the use of the electron transfer to generate an irreversible turn-
first relies on the reactivity of the nitrogen as a nucleophile, on response with a probe.
i.e., chemidosimeter sensing.9–16 For example, the reaction of
When designing molecular sensors, turn-on fluorescent
electron-rich amines, e.g., ethylamine, with 1,3-dimethyl- responses (Fig. 1a) are preferred. They are less prone to false
barbiturate displaces a covalently-linked fluorophore to turn positives and are easier to detect with the naked eye.28
on its fluorescence.14 The second common sensing mecha- However, they are also much rarer. To date, there are no
examples of amine sensors with a turn-on response that utilize
electron transfer from the nitrogen’s lone pair. To the best of
our knowledge, the closest examples29–33 utilize metallo-
Department of Chemistry, Indiana University, 800 E. Kirkwood Avenue, Bloomington,
macromolecules, such as polymers29–32 and metal–organic
IN 47405, USA. E-mail: aflood@indiana.edu
†Electronic supplementary information (ESI) available: full experimental details,
frameworks,33 where the turn-on response results from metal
Fig. S1–S34, and Tables S1–S4 (PDF), pdb files for computed structures, and the
coordination from the amine’s lone pair. For example, a poly
crystal structure of BQuMe·PF6 (CIF). CCDC 1965321. For ESI and crystallo-
(9,9-dioctylfluorene) polymer was quenched upon incorpor-
ation of a palladium porphyrin; the fluorene polymer’s fluo-
rescence was restored when amines coordinate to the palla-
dium to prevent the quenching.29,31 Fundamentally, these
systems did not use electron transfer to generate the turn-on
response.
graphic data in CIF or other electronic format see DOI: 10.1039/c9ob02466b
‡Present address: Beckman Institute for Advanced Science and Technology,
University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
§Present address: Department of Molecular Biology and Genetics, Konya Food
and Agriculture University, Konya, Turkey
¶Present address: Research Laboratory of Electronics, Massachusetts Institute of
Technology, Cambridge, MA 02139, USA
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Fig. 1 (a) Changes in fluorescence intensity for various sensing mechanisms and the multi-state mechanism described in this work. (b) Black-light
photograph of the multi-state sensing mechanism with BQu (10 μM), BQuMe+ (10 μM), BQuMe+ (10 μM) with 5 mM of triethylamine, and BQuMe+
(10 μM) with 1 M of triethylamine (MeCN). (c) General cartoon and the structural representation of the probe, BQuMe+, undergoing fluorophore
quenching by PET. (d) Representation of changes in multi-state fluorescence: intramolecular PET quenching, fluorescence turn-on by single elec-
tron transfer (SET) with an amine, and collisional quenching with excess amine.
Sensing systems that display turn-off responses (Fig. 1a) are trolled way. Fortunately, A. P. de Silva developed a general
common. They are easier to identify by simply screening a fluo- approach (Fig. 1c) to create turn-on fluorescent molecular
rescent compound against various stimuli for quenching. sensors,8 which he originally pioneered with cations.34
Sometimes, they may even self-identify by serendipity after Typically, a fluorophore is designed to be initially quenched by
exposure to a chemical agent. By contrast, the identification of
a preprogrammed photoinduced electron transfer (PET).
a turn-on sensor is less straightforward. It could be pursued by Therefore, light absorption excites the fluorophore followed by
screening every potential but non-fluorescent compound PET to a covalently-linked acceptor and subsequent back elec-
against various analytes of interest to find examples that turn tron transfer to reset the sensor. These electron transfer steps
on the fluorescence; not only must the potential sensor be sen- open up a non-radiative decay pathway to quench the fluo-
sitive to the analyte, but the product of the reaction also needs rescence. Subsequent analyte binding or interaction with the
to be highly emissive. The ideal case, by contrast, is to electron acceptor inhibits the PET pathway to produce an
program a mechanism into the sensor that will enable the analyte-dependent shut down of the non-radiative decay to
fluorescence to be turned off, and then on again, in a con- turn on fluorescence. The essence of this approach is the com-
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bination of tunable electron-transfer processes with analyte
interactions (Fig. 1).
We describe the multi-state dose-response of a BODIPY-
based probe. Multi-state sensing in this context is of a single
We saw an opportunity to implement these design BODIPY probe such that the fluorescence is modulated by
principles with 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene three different processes. The sensor is dose-responsive as a
a
(BODIPY) fluorophore. These dyes have garnered interest as result of switching between the two processes over different
one of the most versatile fluorescent sensors with high amine concentration ranges. Prior examples of dose-responsive
quantum yields and photostability.35 These dyes are widely sensors use two recognition sites to bind the analyte; for
used as labeling agents,36–39 laser dyes,40,41 and sensors.42–44 example using heteroditopic ligands for zinc sensing.57–59 The
PET-quenched BODIPY dyes45 have been used as turn-on fluorescence of the described BODIPY probe is modulated by
sensors for a range of analytes, including anions (e.g., hypo- amine concentrations, but not by a discrete recognition site.
chlorite),46 cations (e.g., zinc, magnesium, and H+),47–49 and First, the BODIPY fluorophore was preprogrammed to be
neutral compounds (e.g., cysteine, thiols, and phosgene).50–52 quenched (turned off) by intramolecular PET to the methyl
We were drawn to previous work that showed the BODIPY’s quinolinium acceptor (Fig. 2a). We then discovered two con-
fluorescence could be quenched by PET using a covalently- centration-dependent mechanisms by which the BODIPY fluo-
linked methyl-pyridinium as electron acceptor.53,54 We orig- rescence is modulated by amines (Fig. 1d). The initial turn-on
inally set out to use this platform to create a turn-on of fluorescence at lower amine concentration requires light
fluorescent sensor for chloride. For this reason, we modified irradiation and involves modulation of the PET from the
the original dye structure by replacing the methyl-pyridinium BODIPY to the quinolinium. Specifically, single electron trans-
with methyl-quinolinium to utilize the latter’s dose- fer (SET) from the amine to the BODIPY core and putative
dependent response to halides by collisional quenching.55 decomposition of the quinolinium radical shuts down the
Serendipitously, we identified that it was the amine impurities PET-based non-radiative decay to turn on the fluorescence
in our tetraethyl ammonium chloride salt56 that produced the (Fig. 2b). At higher concentrations of amine (50 mM–1 M), we
turn-on response we initially observed. Thus, we report here observed collisional quenching of the fluorescence. The
the design, discovery, properties, and the mechanism of behav- balance of the preprogrammed quenching mechanism and the
ior displayed by this new class of amine-sensitive quinolinium- two amine sensing mechanisms results in a multi-state fluo-
BODIPY probe (Fig. 1).
rescence response.
Fig. 2 Hypothesized mechanisms of the first two modes of BODIPY fluorescence modulation. (a) Fluorescence quenching by internal PET from the
photoexcited BODIPY to the methyl-quinolinium and non-radiative back electron transfer (BET). (b) Fluorescence turn-on by single electron transfer
(SET) from the amine to the charge-transfer excited state and decomposition of the methyl-quinolinium radical.
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transfer state by internal PET (Fig. 2b), the amine undergoes a
SET into the BODIPY core. The putative methyl-quinolinium
Results and discussion
The synthesis of the probe (BQuMe+, Fig. 3a) involved the tra- radical that is produced appears to decompose. The net result
ditional one-pot preparation60 of the BODIPY core by reaction of this process is removal of the PET quenching that reinstates
of 3-ethyl-2,4-dimethylpyrrole with 4-quinolinecarboxaldehyde. BODIPY fluorescence (Fig. 2b).
As expected, the resulting intermediate, BQu, was highly emis-
Starting from the initially dark form of the probe, BQuMe+,
sive. This species served as an essential control compound for at a low concentration of 1 μM (acetonitrile, MeCN), we
this study. Following methylation, the hexafluorophosphate observed a turn-on response to 5 mM of triethylamine (TEA,
(PF6−) salt was prepared by metathesis, as verified by 19F NMR Fig. 4). We observed the turn-on response grow during con-
(Fig. S29†). A crystal structure of the probe, BQuMe·PF6, stant light irradiation over the course of five hours (Fig. 4a and
(Fig. 3b) shows the BODIPY and quinolinium moieties are Fig. S6†). Without the light irradiation, we did not observe any
orthogonal. Thus, the electron donor (BODIPY) and acceptor fluorescence turn-on with exposure to the amines in the dark
(methyl-quinolinium) moieties are likely to be poorly coupled, over a two hour period (Fig. S6†). Consistent with our expec-
which is a typical requirement of PET. Here, the coupling is tation (Fig. 2b), the light dependence of the turn-on response
broken geometrically rather than by using a non-conjugated indicated that the reaction between the amine and the probe
linker.
The control compound, BQu, was found to be highly fluo-
is mediated by the probe’s excited state.
In order to characterize the fully turned-on state of the
rescent (ϕ = 0.91 0.02, ε530 = 60 000 M−1 cm−1, τ = 7.12 0.03 probe, we added a large excess (5 mM) of TEA in solution
ns, Fig. S2 and S4†). Upon methylation to the N-methyl quino- (MeCN) to produce the on state of the probe. The result was a
linium, BQuMe+, fluorescence was quenched (ϕ = 0.051
190-fold increase in fluorescence intensity (Fig. 4a) after five
0.004, ε530 = 14 000 M−1 cm−1, τ = 6.40 0.01 ns, Fig. S3 and hours of light irradiation. The quantum yield of the probe
S4†). These values are consistent with the changes seen pre- grew to 0.89 0.10, which is well within error of the quantum
viously as the pyridine analogue is methylated to the pyridi- yield measured from the fluorescent control compound, BQu.
nium, e.g., ϕ = 0.5 drops to 0.03 and ε = 59 000 to 26 000 This similarity implied that the PET quenching mechanism
M−1 cm−1, with quantum yields reduced by a factor of 17 and is fully shut down, completely restoring the BODIPY fluo-
the molar absorptivity by two.53
Assuming the quenching arises from a PET mechanism conversion.
rescence to the entire sample in solution; quantitative
(Fig. 2a), the probe is set up for a de Silva style mechanism of
We hypothesized that the amine reacted after formation of
turn-on sensitivity with a complementary analyte disrupting the charge-transfer state of the probe (Fig. 2b) and examined
the preprogramed off state. After formation of the charge- the photophysical changes over the course of the reaction in
Fig. 3 (a) Synthesis of the fluorescent control compound, BQu, and the probe, BQuMe·PF6. (b) Crystal structure of BQuMe·PF6.
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quinolinium ring system (Fig. 2b). To examine the pertur-
bation of the quinolinium, we monitored the reaction by
absorption spectroscopy. Over the course of the turn-on
period, we saw the absorption spectra of the probe, BQuMe+,
shift over time to more closely resemble the spectral line
shape observed for the fluorescent control compound, BQu
(Fig. 4c and Fig. S7†). We observed shifts in the characteristic
BODIPY π–π* transition at ∼530 nm toward the peak
maximum of the fluorescent control compound, BQu (Fig. 4c).
We also observed shifts away from the absorption peak
assigned to the methyl-quinolinium at 317 nm (Fig. 4c and
Fig. S7†). Overall, the combination of the spectral shifts and
quantum yield measurements provide evidence that, over the
course of the reaction, the BODIPY core is no longer participat-
ing in PET to the quinolinium ring.
We hypothesized that the photodriven reaction with amine
chemically altered the quinolinium moiety to shut down the
PET process of the parent BQuMe+ thereby restoring the fluo-
rescence of the BODIPY core. If the amine is undergoing SET
to the probe (Fig. 2b), we hypothesized that we could model
the product of the SET with the amine by using electro-
chemistry. That is, the photoproduct and electrochemically
reduced states are both expected to be the same.
We compared the redox properties of the probe, BQuMe+,
to that of methyl-quinolinium, QuMe+ (Fig. 5). An irreversible
one-electron reduction of methyl-quinolinium, QuMe+,
occurred at −0.63 V (Fig. 5c). Prior work shows the reduced
methyl-quinolinium forms a radical and that the radical rever-
sibly dimerizes at the carbon para to the ring’s nitrogen.61 The
authors proposed that it goes via a radical-mediated mecha-
nism. Substitution at the carbon para to the nitrogen prevents
dimerization.61 In our case, this para site is substituted by
BODIPY and is expected to inhibit the reaction of the
reduction product. Consistently, the redox properties of
BQuMe+ shows the appearance of an electrochemically revers-
ible one-electron reduction at −0.59 V to form a putative
radical with its corresponding reoxidation at −0.50 V (Fig. 5a).
Only when we hold the potential at −1.0 V for 60 seconds do
we provide more time for the reduced quinolinium to react as
seen in the loss of the reoxidation peak (Fig. 5b). The irrevers-
ible reduction of the quinolinium on the probe, BQuMe+, in
the electrochemistry indicates that the reduction of the quino-
linium upon photoreaction with the amine would also be
irreversible.
Fig. 4 (a) The change in fluorescence over time of BQuMe+ (1 μM,
degassed with argon, MeCN, λexc = 510 nm) upon addition of 5 mM of
triethylamine and UV irritation. The quantum yield and lifetime are high-
lighted for the probe alone (blue) and after being turned on (red). (b)
Representative fluorescence emission spectra of BQuMe+ (1 μM,
degassed with argon, MeCN, λexc = 510 nm) with triethylamine (5 mM)
and UV irradiation (λ = 365 nm) increasing over five hours. (c) Absorption
spectral changes of BQuMe+ (10 μM, MeCN) upon addition of TEA
(5 mM) and UV light irradiation (λ = 365 nm) over the course of two
hours.
Based on the electrochemistry, we aimed to characterize the
1
possible photoproduct with H NMR spectroscopy. To observe
the photoproduct, we repeated the reaction using a signifi-
cantly higher concentration of probe (5 mM) with excess
amine (100 mM). The reaction was monitored over ninety
minutes, during which time the sample was irradiated with
1
light. The H NMR spectrum revealed new peaks growing in at
order to further understand the reaction. We did not observe lower chemical shifts (∼7.5 ppm) than the aromatic peaks of
any significant changes in the spectral line shape of the emis- the parent probe, consistent with a loss of aromaticity on the
sion spectrum during the turn on (Fig. 4b). This finding quinolinium rings (Fig. 5d). The peaks corresponding to the
suggested that the BODIPY core is not chemically modified by BODIPY core were undisturbed by the reaction (Fig. S8†). The
1
the amine but rather the reaction is ultimately perturbing the new H NMR peaks did not align with the aromatic peaks of
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the fluorescent control compound, BQu, indicating the reac-
tion was not a simple loss of the methyl group from the
methyl-quinolinium ring (Fig. 5d). In an effort to increase the
product conversion, we increased the concentration of amine
to 1 M relative to the 5 mM of probe in solution. However, the
larger quantity of amine resulted in a third set of peaks
appearing, indicating multiple decomposition products
(Fig. S10†). The results of the NMR analysis are consistent with
a photodriven reaction chemically modifying the methyl-qui-
nolinium, likely a loss of aromaticity. The modified methyl-
quinolinium is thus not able to participate in the PET quench-
ing of the BODIPY.
We studied the concentration dependence of the turn-on
response, expecting that the probe could be used as a tra-
ditional sensor. However, we saw a new process that also
modulated the BODIPY fluorescence (Fig. 6a). At low concen-
trations (e.g., 1 mM), we observed the expected linear turn-on
response with light exposure for two hours. At high concen-
trations (>10 mM), the linear portion of the turn-on response
showed a saturation with just 15 minutes of light irradiation.
Surprisingly, the saturation level decreased as we raised the
concentration of amine. The decrease in fluorescence with
higher amine concentrations implied that a quenching mecha-
nism was activated.
Based on the structural similarities between the probe,
BQuMe+, and the fluorescent control compound, BQu, we
hypothesized that the quenching mechanisms of the two com-
pounds would be the same at high amine concentrations. To
test this idea, we conducted a Stern–Volmer analysis by titrat-
ing amine into a solution of the fluorescent control com-
pound, BQu. Consistent with previous literature on other dye
classes, e.g., coumarin and styryl pyridine,17,21 we observed
quenching of the fluorescent control compound, BQu, with
increasing concentration of amine in solution (Fig. 6b and
Fig. S17†). Our analysis with BQu provided a Stern–Volmer
Fig. 5 (a) Electrochemical analysis of BQuMe+ (1 mM, MeCN, no amine
present) by cyclic voltammetry and (b) with a 60 seconds hold at −1 V. (c)
Cyclic voltammetry of methyl-quinolinium ring (QuMe+, 1 mM, MeCN). (d)
Aromatic region of the 1H NMR spectra of BQuMe+ (5 mM, CD3CN,
degassed with argon) before and after adding triethylamine (100 mM) and
UV irradiation (λ = 365 nm) for 90 minutes in comparison to BQu (5 mM).
Fig. 6 (a) Fluorescence response of BQuMe+ (1 μM, MeCN, λexc = 510 nm) with addition of various concentrations of triethylamine (UV-irradiation
with λ = 365 nm between time points, under argon). (b) Stern–Volmer plot showing the fluorescence quenching of the control compound, BQu
(1 μM, MeCN), in the presence of triethylamine.
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of amine (Fig. S17†) were consistent with collisional quench-
ing.28 As expected, the quenching of BQu was observed over the
same concentration range as the plateau seen with BQuMe+.
In order to analyze the plateau response of BQuMe+, we
plotted the concentration (0.05, 0.1, and 1 M) versus the
average intensity loss (I0/I) after the intensity had stabilized
(>45 minutes). A Stern–Volmer analysis of the data generated
an apparent diffusion-limited rate constant (1.7 0.1 × 109
M−1 s−1, Fig. S18†). The similarity in the quenching rate con-
stants between the two compounds over the same concen-
tration regime implied the same collisional quenching mecha-
nism was operating.
Overall, the BQuMe+ probe has two amine sensing mecha-
nisms. The linear turn-on response with time occurs at the low
amine concentrations while a plateau response is present at
high amine concentrations. The fluorescence is turned on at
low concentrations of amine (0.1–10 mM) by SET from the
amine to the BODIPY core of the charge-transfer state (Fig. 2b)
generating an emissive photoproduct. The initial photoproduct
is believed to be the reduced state of the methyl-quinolinium,
which decomposes into a non-aromatic heterocycle. At higher
amine concentrations (≥50 mM), we also observed bimolecular
collisional quenching to become an active process. The colli-
Fig. 7 Stern–Volmer calibration curve for amine sensing with the
probe. Fluorescence emission turn-on response of BQuMe+ (1 μM,
MeCN, degassed with argon) after addition of triethylamine at two time
points of UV irradiation (λ = 365 nm, black = 15 minutes, red =
45 minutes).
quenching constant of 20.6
bimolecular rate constant of 2.9 0.1 × 109 M−1
0.2 M−1 and a corresponding sional quenching modulates the fluorescence response of
−1 (eqn (S2)†). BQuMe+ across the same concentration range as the amine
s
The changes in the fluorescence lifetime of BQu upon addition quenches the fluorescence of BQu. At the higher concen-
Fig. 8 (a) Turn-on response of BQuMe+ (10 μM, MeCN) in the presence of various aliphatic amines (5 mM) after two hours of UV irradiation (λ =
365 nm). (b) Selectivity screen of BQuMe+ (10 μM, MeCN) against triethylamine, aniline, p-(dimethylamino)benzaldehyde, pyridine, 1-butanethiol,
and ethanol (5 mM) after two hours of UV irradiation (λ = 365 nm). (c) Solvent dependent turn-on response of BQuMe+ (10 μM) with trimethylamine
(5 mM) after two hours of UV irradiation (λ = 365 nm).
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trations, the rate of SET and the subsequent irreversible
decomposition to the on-state of the probe are hypothesized to
occur to a substantial extent. It is then the collisional quench-
ing of this on state that generates the initially high plateau at
the mid-range (50 mM) concentrations, which decreases in
intensity as amine concentrations are raised.
The multi-state sensitivity to amine concentration has the
potential to be used for sensing. The sensor can be used
directly to detect amines up to 50 mM. If the dynamic range
needs to be extended further, however, a different approach is
needed. Comparing the fluorescence intensity change at two
time points of UV-irradiation (Fig. 7) allows for discrimination
of high amine concentrations. For example, a 15-minute
measurement (black trace) of the intensity turn-on (I/I0) of 15
could correspond to either
5 mM or 1 M of amine.
Differentiation between the two concentrations could be made
after further light exposure. The 5 mM amine sample would
grow in fluorescence intensity while the latter would remain
constant.
We tested the generality of the turn-on response to amines
by reacting the probe, BQuMe+, with a range of amines in
different solvents (Fig. 8). All of the aliphatic amines tested
resulted in a turn-on response. As expected, quaternary
ammonium groups had no impact on the fluorescence, con-
sistent with the involvement of the amine’s lone-pair in the
observed reaction-based sensing mechanism (Fig. 8a).
Aromatic amines, aliphatic thiols, and aliphatic alcohols did
not result in a turn-on response (Fig. 8b). We see that the for-
mation of the quenched charge-transfer state (Fig. 2b) was not
affected by solvent polarity on account of the fact that the
probe remained initially quenched in all the solvents we exam-
ined (Fig. 8c). We observe that the turn-on response to the
amine changes with a solvent and hypothesize that it reflects
the solvent’s hydrogen bond donating ability.62 Non-protic sol-
vents (toluene, dichloromethane, acetone) facilitated the turn-
on response, while polar protic solvents (water, methanol)
inhibited the change in fluorescence (Fig. 8c).
To better understand how methylation of the quinoline
moiety leads to the photophysical changes, we conducted
density functional theory (DFT) calculations of the frontier
molecular orbitals. In the fluorescent control compound, BQu,
both the HOMO and LUMO are localized on the BODIPY core
(Fig. 9a), which is consistent with the orthogonal arrangement
of the quinoline having a negligible effect on fluorescence.
The LUMO+1 is localized on the quinoline. Methylation to
BQuMe+ leads to dramatic stabilization of the quinolinium-
localized orbital by 1.5 eV leading to a re-ordering such that it
becomes the LUMO (Fig. 9b).
Fig. 9 Vertical transitions calculated using TDDFT for the emissions of
(a) BQu and (b) BQuMe+ including solvation effects (acetonitrile). All ver-
tical transitions were dominated by a single electronic transition (Tables
S2 and S3†). The orbitals shown are from natural transition orbital ana-
lysis. Oscillator strengths ( f ) for the electronic transitions are included
in parentheses; solid lines indicate higher oscillator strengths and
dashed lines indicate lower oscillator strengths.
chromophores undergo rapid de-excitation by intersystem
crossing to non-emissive triplet states. The low oscillator
strength of the charge transfer state is also consistent with the
suggested PET mechanism, while the absorption spectrum is
dominated by the BODIPY band for both the fluorescent
control compound, BQu, and the probe, BQuMe+.
The impact of the orbital re-ordering upon methylation was
examined using time-dependent DFT (TDDFT) calculations.
Upon methylation, we see a change in character of the lowest
energy S1 → S0 transition from highly allowed ( f = 0.78) ππ for
BQu to charge transfer for BQuMe+ with a lower oscillator
Conclusion
strength ( f = 0.03), supporting the experimentally-observed We discovered a multi-state and dose-dependent response of a
quenching. Consistently, other works54,63,64 have also BODIPY-based fluorophore towards amines. The multi-state
suggested that charge transfer states in BODIPY-containing character is based on three processes that modulate the
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BODIPY fluorescence. First, the fluorescence is prepro-
grammed to be quenched by internal PET to a quinolinium
ring system. Second, fluorescence is restored by SET from the
amine analyte to the photoexcited BODIPY core generating a
photoproduct such that the internal PET quenching mecha-
8 B. Daly, J. Ling and A. P. de Silva, Chem. Soc. Rev., 2015, 44,
4203–4211.
9 S. Udenfriend, S. Stein, P. Böhlen, W. Dairman,
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work was supported by the Chemical Sciences,
Geosciences, and Biosciences Division of the Basic Energy
Sciences Program of the US Department of Energy Office of
Science (DE-FG02-09ER16068). AHF and KLV also acknowledge
support from the Luther Dana Waterman Professorship. AHF
and SEC acknowledge support from the TUBITAK 2214-A fel-
lowship. AHF acknowledges early support by the Office of the
Vice Provost for Research, Indiana University with their Faculty
Research Support Program. A.H.F and K. L. V thank Dr Irina
Tsvetkova for assistance with the fluorescence experiments,
and Dr Jonathan A. Karty from the Indiana University Mass
Spectrometry Facility for support.
27 M. Venkatesan, H. Mandal, R. Bheerappagari and
P. R. Bangal, J. Photochem. Photobiol., A, 2019, 376, 212–
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