Conformational switching: Via an intramolecular H-bond modulates the fluorescence lifetime in a novel coumarin-imidazole conjugate

Article abstract of DOI:10.1039/c7cp07274k

Achieving synthetic control over light-driven molecular dynamics is essential for designing complex molecule-based devices. Here we design a novel coumarin-imidazole conjugate (1) whose excited state structural dynamics are primarily controlled by a distant intramolecular H-bonding interaction within the backbone. The coumarin conjugate is based on a 1,2,4,5-aryl substituted imidazole framework (aryl = -Ph and -PhOH) covalently connected to the coumarin moiety via a C-N bond. A carefully positioned OH group in the aryl part of the imidazole fragment resulted in achieving two dissimilar O-H?N and O-H?O distal intramolecular hydrogen bonding interactions. NMR studies in conjunction with density functional theory (DFT) at the B3LYP/6-311G(d,p) level of theory show the existence of two ground state conformers with a rotational barrier of 6.12 kcal mol^-1. Due to the presence of conformational isomers of 1, the local excited state dynamics of the parent coumarin get biased towards a long-lived fluorescence state with diminished non-radiative decay channels. Time-resolved emission studies show an ～4-5 times increase in the excited state lifetime in 1 when compared to coumarin-imidazole conjugates, 2 and 3, without the OH group. Solvent dependent studies show that solvent polarity, the H-bond donating ability and viscosity dictate the conformational distribution in the ground state and the dynamical evolution to the final emissive state. Our studies highlight the importance of rotamerism around the C1-C4 single bond, which leads to rigidification along the coumarin-imidazole backbone through a combination of distal H-bonding and solvent interactions. The concept of new emission signaling pathways caused by conformational switching between two states offers a new paradigm to introduce functional allostery in macromolecular backbones.

Full text of DOI:10.1039/c7cp07274k

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Conformational switching via intramolecular H-bond modulates
fluorescence lifetime in a novel coumarin-imidazole conjugate
Indranil Bhattacharjeea, Nita Ghoshb, Abhinav Raina,a Jyotishman Dasguptab and Debdas Ray*a
Received 00th January 20xx,
Accepted 00th January 20xx
Achieving synthetic control on light-driven molecular dynamics is essential for designing complex molecule-based devices.
Here we design a novel coumarin-imidazole conjugate 1 whose excited state structural dynamics is primarily controlled by
a distant intramolecular H-bonding interaction within the backbone. The coumarin conjugate is based on 1,2,4,5-aryl
substituted imidazole framework (aryl = −Ph and −PhOH) covalently connected to the coumarin moiety via a C−N bond. A
carefully positioned OH group in the aryl part of the imidazole fragment resulted in achieving two dissimilar O−H•••N and
O−H•••O distal intramolecular hydrogen bonding interacꢀons. NMR studies in conjuncꢀon with density functional theory
(DFT) at the B3LYP/6-311G(d,p) level of theory shows the existence of two ground state conformers with rotational barrier
of 6.12 kcal.mol-1. Due to the presence of conformational isomers of 1, the local excited state dynamics of the parent
coumarin gets biased towards a long-lived fluorescent state with diminished non-radiative decay channels. Time-resolved
emission studies show a ca. 4-5 times increase in the excited state lifetime in 1 when compared to coumarin-imidazole
conjugate, 2 and 3, without OH group. Solvent dependent studies show that, effect of solvent polarity, H-bonding donating
ability and viscosity dictate the conformational distribution in the ground state as well as the dynamical evolution to the
final emissive state. Our studies highlight the importance of rotamerism around C1−C4 single bond which leads to
rigidification along courmarin-imidazole backbone through a combination of distal H-bonding and solvent interactions. The
concept of new emission signaling pathways caused by conformational switching between two states offers a new
DOI: 10.1039/x0xx00000x
www.rsc.org/
paradigm
to
introduce
functional
allostery
in
macromolecular
backbones.
the presence of these dissimilar H-bonds
1
may lead to
Introduction
conformational isomers (syn/anti)36-44 caused by rotamerism
around the C1−C4 single bond rotaꢀon. NMR studies in
conjunction with density functional theory (DFT) at the
Radiative pathways in photoactive chromophores can be
tuned by introducing rigidity of the molecular backbone
through either covalent or non-covalent interactions.1-5 Such
tailoring of chemical interactions provides opportunities to
optimize the optoelectronic properties inherent to the
electronic structure of a fluorophore.6-21 An increasing number
of sensory systems have now been built on this principle, in
which conformational change along the molecular backbone is
tightly coupled to changes in the optical or electrochemical
signal output.22-35 Continuing challenges in such efforts lie in
the invention of novel molecular systems in which
fluorescence signalling pathway is controlled via
conformational switching.
B3LYP/6-311G(d,p) level show
a distinct conformational
distribution in the ground state. Fluorescence upconversion
along with time resolved emission studies ensure that the
excited state dynamics of the parent coumarin gets biased
Here we describe the synthesis of a new series of
coumarin-imidazole conjugates having tunable emission
lifetimes dictated by a carefully positioned –OH group in the
phenyl attached to C1 atom of the imidazolyl ring. Our design
feature is highlighted in molecule
1 (Scheme 1) which is
constructed by conjugating 1,2,4,5-aryl substituted imidazole
(aryl = –Ph and –PhOH) with 7-diethylaminocoumarin moiety
Scheme 1 Conformational changes of 1 through distal H-bond.
via a C−N bond. The exciꢀng possibility of stabilizing two towards a long-lived fluorescent state with diminished non-
dissimilar O−H•••N and O−H•••O distal intramolecular radiative decay channels in , when compared to coumarin-
hydrogen bonding (H-B) interactions in the same framework is conjugate and (Scheme 2), without OH group. The distal
tested by synthesizing the corresponding functional group intramolecular H-bond leads to rigidification along coumarin-
substituents (–OCH3, and –H, ) at the –OH position. Due to imidazole backbone.
1
2
3
2
3
Synthesis and Characterization
The synthesis of
1 was achieved in three steps (Scheme 2).
First, 4-diethylaminosalicyaldehyde is treated with ethyl
nitroacetate to obtain 1a 45
.
Reduction (SnCl2) of 1a in water
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followed by multicomponent condensation reaction46 with attached to C2), 6.6(5)o (phenyl ring attached to C3) and -
salicyalaldehyde, benzil and ammonium acetate, purification 92.1(4)o (coumarin ring attached to N2) when viewed along
by column chromatography (SiO2, hexanes/toluene, 7:3, the N(2)–C(2)–C(19)–C(20), N(1)–C(3)–C(30)–C(29) and the
respectively) gives
Similarly, two controlled compounds,
following the above protocol to yield
yellow powder. All compounds were characterized by 1H and is almost ca. 25 times bigger compared to those of similar
13C NMR spectroscopies, high-resolution mass spectrometry, intramolecularly hydrogen-bonded imidazole derivatives48-50
1
in 55% isolated yield as yellow powder. C(2)–N(2)–C(10)–C(12) atoms, respectively. More importantly,
and were synthesized the analysis shows that the measured torsion angle in when
(62%) and
(70%) as viewed along the N(1)–C(1)–C(4)–C(5) atoms is 24.5(4)o which
2
3
1
2
3
,
MALDI-TOF and X-ray crystallography (in the ESI†).
indicating that the hydroxyphenyl group in 1 significantly
deviates from planarity resulting in a weaker H-bond. For
comparison, the corresponding torsion angles of reference
X-ray crystal structures
compound
3
(Fig. 1b, and Fig. S20†) are 76.1(3)o, 21.9(3)o, -
83.3(3)o, and -27.0(3)o respectively. It is clear that orientation
of the phenyl and coumarin moieties attached to C1, C2, C3
and N2 atoms of imidazolyl ring are significantly affected due
to absence of the intramolecular hydrogen bonding
Crystals of
slow evaporation of toluene solution resulted in the
rectangular shaped yellow crystals. similar procedure
resulted in the cubic shaped yellow crystals of second
reference compound, 3 47
The crystal structure determination
1 suitable for X-ray analysis were obtained from a
A
interaction. Further, the crystal structure of
1 indicates that
.
the O1 atom from coumarin group is hydrogen bonded with
atom H12 of adjacent coumarin ring (C12−H•••O1, 2.381 Å,
153.63ᵒ). Moreover, an intermolecular C−H•••C interaction
between atoms C5 and H23 (C23−H23•••C5, 2.9 Å, 126.71ᵒ),
respectively is also present in the crystal lattice. In addition to
data are tabulated in Table S1†, and selected bond lengths and
bond angles are summarized in Table S2.† Single
crystallographic analysis (Fig. 1a) of
1 reveals that there is an
intramolecular H-bond (O–H•••N, 1.895 Å, 147.3o) between
the imidazolyl nitrogen (N1) and the OH proton, which
confirms syn configuration in the solid state. The measured
dihedral angles indicate that the phenyl, coumarin and
imidazolyl rings deviate from planarity 67.2(4)o (phenyl ring
this, a head to head
π-π stacking (3.748 Å) interaction between
–PhOH groups of neighbouring molecules is also seen. Thus,
1
Scheme 2 Synthetic scheme and chemical structure of 1, 2, and 3.
2 | J. Name., 2012, 00, 1-3
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Fig. 1 Oak Ridge thermal ellipsoid plots (50% probability ellipsoids) of the X-ray structures of (a) 1 and (b) 3. The hydrogens except OH are omitted
for clarity.
crystallizes into cylindrical structure through head to head
stacking (3.753 Å), as seen along the b-axis (Fig. S19†). The that both
crystals of shows two intermolecular hydrogen bond region compared to S1 minimum from which the emission
between atoms of N1 and H20 from two neighbouring occurs.57 The excitation spectrum of
recorded in cyclohexane
molecules (C20−H20•••N1, 2.656 Å, 145.9o) and atoms of O1 is independent of
ex and overlaps exactly with (Fig. S21a†) the
π
-π
shifts of ca. 31-80 nm. This Stoke’s shift leads us to conclude
1
and are less polar in the Franck-Condon (FC)
2
3
1
λ
and H31 (C31−H31B•••O1, 2.347 Å, 143.1o). Moreover, two absorption, which indicates the presence of a single species in
C−H•••C interactions between atoms of C26 and H9 ground state in cyclohexane solution. However, excitation
(C9−H9•••C26, 2.891 Å, 132.6o) and C14 and H7 of two spectra of
1
in polar solvents become dependent on λem and
adjacent molecules (C7−H7•••C14, 2.835 Å, 160.26o), does not exactly overlap with absorpꢀon spectra (Fig. S21b†),
respectively are also present in the crystal lattice. Thus, indicating its origin from the presence of either an impurity or
prefers to pack in head to tail fashion, as viewed along a-axis more than one coexisting ground-state conformers. Since,
3
(Fig. S20†).
purity of thee samples was confirmed by NMR and
crystallization, we eliminate the presence of any impurity,
thereby assigning the observed features due to possibility of
co-existing ground state conformers.
Steady State Absorption and Emission
All the synthesized compounds
characterized by steady-state absorption spectroscopy (Fig. 2).
For comparison, steady state absorption spectra of and are
1, 2 and 3 were completely
Time correlated single photon counting (TCSPC)
1
2
measured in solvents of different polarity (Table 1). The Time-correlated single-photon counting (TCSPC) (see
absorption feature peaked at ca. 400 nm is assigned to the experimental section for details) was used to monitor the
coumarin absorption51 and it arises from the charge transfer excited state lifetimes of
1, 2, and 3 in solvents of varying
(CT) transition within the coumarin fragment.51 For
1, λmax of polarity. Solvent dependent behaviour of coumarin dyes is well
the absorption bands located at ca. 373, 391, 395, and 403 nm, studied in the literature.20,21,57,58 However, lifetime of
respectively, ongoing from nonpolar to polar solvents coumarin emissive state can be modulated by other dominant
(cyclohexane (CHx), ACN, EtOH, ethyleneglycol (EtGly)), are non-radiative pathways. From our measurements, we find out
shifted bathochromically. Likewise, for compound 2, similar that compounds 2 and 3 have radiative lifetimes ranging from
trend was noticed. The intramolecular charge transfer ca. 400 ps to 1.29 ns, which is distinctively shorter than the
character of the absorption bands52-56 was further supported parent coumarin chromophore having a fluorescence lifetime
by the observation of blue shift in non-polar solvent of ca. 3-5 ns (Table 1). These values indicate that non-radiative
cyclohexane (CHx). Following the same line of thought, steady decay channels dominate the excited state lifetime of
state emission measurements were also done in solvents imidazole-coumarin conjugates
having disparate polarity. Emission spectra of both and λex compound 1, which has a carefully designed –OH substitution,
400 nm) show broadband features with positive the lifetime in all the solvents is 4 to 5 times longer than its –
solvatochromism effects (Fig. 2), leading to moderate Stoke’s OCH3 and –H counterparts. Additionally steady state emission
2 and 3. On the contrary, for
1
2 (
=
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Fig. 2 UV/vis and steady state emission spectra (a, b) of 1, and (c, d) of 2. All spectra were recorded in various solvents at 298 K. Solvents are highlighted in color: CHx, blue; ACN,
Dark Cyan; EtOH, Black; CHCl3, Red; EtGly, Magenta.
spectra of
1 is highly broader than that of the other two the emission observed in all the three compounds. Fig. 3 and
substituents. To confirm the origin of the broadened red-edge, Fig. S25-S26† show the emission dynamics of all three
we collected time-resolved emission data at 538 nm along compounds at 475 nm. The long-lived emission from an
with data recorded at 475 nm, close to the peak maximum of intramolecular charge-transfer coumarin excited state at 475
Fig.3 TCSPC decay profile of 1 and 2 in a) EtOH (λem = 475 nm), b) EtGly (λem = 475 nm).
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Table 1 Steady state photophysical data of 1 and 2.
Solvent (ET(30))
λabs
(nm)
ε
λem
(nm)
ꢀν
(nm)
φf (%)
τ (ns)
(L mol-1 cm-1)
λ475 (nm)
λ
538 (nm)
1
CH (31.2)
MeCN (46.0)
EtOH (51.9)
EtGly (53.8)
373, 393
391
3.0 × 104
4.6 × 104
1.0 × 105
3.0 × 104
404, 420b
464a
31
73
77
82
78.8
82.5
88.1
91.5
-
-
3.19
4.0
3.56
4.08
4.23
395
472a
403
485a
4.11
2
CH (31.2)
MeCN (46.0)
EtOH (51.9)
372, 387
400
1.1 × 104
3.5 × 104
3.6 × 104
1.1 × 104
425b
473a
465a
486a
nm,
58
74
63
18.1
20.6
23.9
26.4
-
-
0.73
0.37
0.76
0.42
402
EtGly (53.8)
aexcitation
411
wavelength
75
0.99
(
1.19
375
(
λex
)
400
bexcitation
wavelength
λex
)
nm
nm in
channels have been slowed down. One possibility could be an ultrafast timescale for
that the distal intramolecular H-bonding in
leads to measurements61 (see experimental section for details) were
1
implies a mechanism by which non-radiative decay To gain a detailed understanding of the emission dynamics on
1,
fluorescence upconversion
1
rigidification of the coumarin unit thereby diminishing non- performed. To correlate with steady state emission spectra,
radiative decay channels. Since we detected a red-shifted upconversion decays were collected at two wavelengths, 475
emission at 538 nm with similar lifetimes, we suggest that a nm and 538 nm upon selective coumarin excitation at 400 nm.
new radiative decay path which is a subtle function of Transients were fit with four exponential decay function
molecular structure and viscosity, dictates the photophysics.
convoluted with an instrument response function of 250 fs.
Relevant fitting parameters are summarized in Table 2. Fig. 4
and S23† shows fluorescence upconversion transients of
1 and
Quantum yield
2, respectively in solvents of different polarity. Furthermore,
the decay kinetics of this state measured in polar aprotic
solvents indicate a clear polarity dependence of lifetime.
However, in polar protic solvents, we see a rise time. The rise
time further increases in ethylene glycol which indicates
viscosity dependence to the evolution to this state. It is clear
from the time-resolved emission data that viscosity and proton
accepting ability of the solvent lead to two opposite effects,
thereby playing critical role in determining the excited state
evolution pathways for 1. The dependence of excited state
dynamics on protic solvent can be explained by the possibility
Fluorescence quantum yields (
determined by comparing the spectrum with that of standard
Fluorescin59-60
φf = 0.85 in 0.1(N) NaOH) taking total area
under the emission spectra. It is noteworthy that, a significant
variation in the quantum yield of and (Table 1), were
observed. Compound shows lower f, while shows
φf) in different solvents were
(
1
2
2
φ
1
remarkably high quantum yield in all solvents, which indicates
that the intramolecular H-bond (O−H•••N) between OH
proton and nitrogen atom of imidazolyl ring plays a crucial role
to maintain structural rigidity of 1. Interestingly, measured φf
value of further increases on going from non-polar to polar
of distal intramolecular H-bonding in 1. In this framework, the
1
upconversion transients suggest that decay at 475 nm channel
is associated with extension of conjugation along coumarin
backbone which is directed by an intramolecular H-bond
between OH proton of hydroxyphenyl group and N1 atom of
imidazolyl ring. Our conjecture is that this process is preceded
by a rotation around C1−C4 single bond on the conjugate. The
timescale for this geometrical re-adjustment suggests emission
from locally excited state. Previous line of arguments for
emission channel at 475 nm also holds true for emission
collected at 538 nm. Moreover, we observed rise time of ca.
26 ps measured in ethylene glycol (Table 2) that is significantly
higher when compared with ethanol. This observation clearly
suggests that emission in this channel is originating from a
and viscous solvents (cyclohexane, CH3CN, EtOH,
ethyleneglycol (EtGly). These results suggest that a severe
structural re-adjustment of 1 might be established by distal
intramolecular H-bond (O−H•••O) between OH proton and
keto carbonyl group of coumarin segment in the excited state.
In contrast to 1, the φf of 2 are also increased with solvent
polarity, which ensures pronounced intramolecular charge
transfer (ICT)58 and reduced rotational motion associated with
the backbone. Thus, increased φf of
1 in solvents of different
polarity results from the combination of structural rigidity
caused by intramolecular distal H-bond and ICT. These results
are also consistent with the TCSPC measurements which show
a ca. 4-5 times enhancement in the lifetime of the excited
state.
state of completely new characteristics that follows
a
rotational reaction coordinate. This geometric re-adjustment
might be assisted with torsion along with atoms of
N1−C1−C4−C5 introducing increased planarity along the
Upconversion measurement
backbone. On the other hand, 2 shows, almost similar
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Table 2 Fluorescence upconversion data of 1 and 2.
Solvent
λ
475 (nm)
λ
538 (nm)
τ2(ps)
τ1(ps)
τ2(ps)
τ3(ps)
τ1(ps)
τ3(ps)
1
MeCN
EtOH
-
22.4 ± 0.8
1669.4 ± 120
0.4 ± 0.1 (rise)
20.82 ± 1.16 (0.54)
5000 ± 0 (0.46)
4584.1 ± 0
1.2 ± 0.5 (rise)
129.77 ± 31.6 (0.26)
1837.9 ± 0 (0.74)
2173.9 ± 0 (0.75)
0.7 ± 0.1 (rise) (0.54)
147.8 ± 1.8 (rise)
(0.46)
EtGly
0.8 ± 0.2 (rise)
48.9 ± 2.8 (0.25)
2.7 ± 0.7 (rise) (0.32)
26.2 ± 3.6 (rise) (0.68)
4187.4 ± 0
2
MeCN
EtOH
EtGly
2.01± 0.1
5.4 ± 0.1
5.1 ± 0.5
14.7 ± 0.4
105.2 ± 2
66.3 ± 3.5
275.5 ± 13.4
4.9 ± 0.2
5.4 ± 0.6
-
75.36 ± 2.95
-
1806.2 ± 320
1025.4 ± 98.4
202.7 ± 0
-
-
482.1 ± 0
aexcitation at 400 nm, and emission at 475 nm and 538 nm, respectively.
Fig. 4 Upconversion decay profile of 1 and 2 in a) EtOH (λem = 475 nm), b) EtOH (λem = 538 nm), c) EtGly (λem = 475 nm), and d) (λem = 538 nm). λex = 400
nm.
decay kinetics, which indicates that emission at 475 and 538 the effect of viscosity and protic dependence are much
nm, respectively have same origin. The decay kinetics were pronounced compared to emission at 475 nm channel,
also comparable to the decay transients at 475 nm (Table 2 indicating geometrical re-adjustment along the line of proton
and Fig. 4) which further ensure that emission at these transfer reaction co-ordinate which might be the limiting
channels are coming from a single emissive state of
Evaluation to this state involves rotational motion along the be noted that no difference in the excited state dynamics of
backbone, indicating more sterically hindered motion. (Fig. S24†) is observed compared to indicating OH
However, for , other than the decay kinetics being different, chromophore of the phenyl group attached to C1 atom of
2.
factor towards evolution to this new emissive state. It should
3
2,
1
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change associated with this process (syn/anti) was calculated62
to be
G# = 1.6 kcal.mol-1 at RT (Fig. S18†), which closely
matches with computational results (
G# = 0.69 kcal.mol-1).
ꢀ
imidazolyl ring plays a dominant role for such intriguing
geometrical re-adjustment. This further substantiate our
observation in the TCSPC measurements.
ꢀ
Hence, we conclude that presence of a dynamic equilibrium
between two conformers gives rise to two different kind of
hydroxyl groups (due to dissimilar hydrogen bonding) that
have disparate chemical shift values. Interestingly, when the
NMR Analysis
1H NMR spectrum of compound
2 in CD3CN (Fig. 5e and Fig.
The ground state conformational heterogeneity of molecule
1
S10†) was compared with that in CDCl3 (Fig. S8†), we observed
evidence of only a single species. This result is in stark contrast
was proved using a combination of both 1D and 2D-NMR
spectroscopy along with temperature dependent spectral
1
changes. The H NMR spectrum of
with the behaviour of compound
1 in similar environment,
1 in CDCl3 shows a broad
confirming the presence of conformational heterogeneity that
originates due to the –OH appendage on molecular backbone.
Finally, from our detailed NMR study we conclude that a
carefully positioned –OH group in the phenyl attached to C1
atom of the imidazolyl ring is responsible for such intriguing
conformational distribution in the ground state of compound
signal at 13.26 ppm (Fig. 5a), indicating presence of an
exchangeable proton. In CD3CN, however, we observe two
distinct peaks at 13.08 and 8.12 ppm assigned to –OH protons
(Fig. 5b). Since –OH protons are ionizable, the sample in CD3CN
was mixed with small amount of D2O (30:1 v/v) to confirm
exchange of the ionizable protons with deuterons. As shown in
Fig. 5c, peaks at 8.12 and 13.08 ppm in CD3CN as well as 13.26
ppm in CDCl3 disappeared confirming our assignment of –OH
group in the conjugate. The observation of an additional OH
proton peak at 8.12 in CD3CN indicates the presence of
another conformer in solution. The aromatic peaks that appear
at 6.55, 6.68, 7.02, 7.81, and 8.49 ppm in the 1H NMR
spectrum recorded in CD3CN at RT (Fig. 5b) were difficult to
assign due to dynamic equilibrium between the conformers.
Therefore, 1H NMR along with NOESY measurements were
carried out at 273 K. We find NOESY correlation in the 1D-
NOESY spectrum between proton H29 from the phenyl
attached to C3 atom of imidazolyl ring and -OH proton at 13.08
ppm thereby confirming that 1syn conformation is one of the
molecular geometries in soluꢀon at 273 K (Fig. S14a†). We
confirm that the peak appeared at 7.81 corresponds to H12
proton of coumarin unit, as this signal shows a typical NOESY
correlaꢀon (Fig. S14b†) with the H14 proton signal at 6.55
ppm. We come to the same conclusion when we compare a
correlation spectroscopy (COSY) interaction between proton at
6.68 (H15) and 6.55 (H14) ppm (Fig. S13†). Similarly, the peak
appeared at 7.02 ppm corresponds to H17 proton of coumarin
unit, as this signal also shows typical NOESY correlation with
the H33 proton signal at 3.41 ppm (Fig. S14c†). The assignment
of the peak at 8.49 ppm was unsuccessful at this stage as this
signal in CD3CN showed remarkable reduction in intensity at
273 K (Fig. 5d and Fig. S17e†). We believe that the signal at
8.49 ppm originates from H12 proton of the other conformer,
which is in dynamic equilibrium at RT.
1.
Computational Calculation
The initial ground state (S0) and first singlet excited state (S1)
geometry of 1, 2 and 3 were optimized using density functional
theory (DFT for S0) and time-dependent density functional
theory (TDDFT for S1).63 Vertical excitation energies at ground
state geometries were calculated using TDDFT with different
functional (B3LYP, CAM-B3LYP, M06-2X).64-69 All calculations
have been enumerated using 6-311G(d,p) basis set. DFT and
TDDFT calculations were carried out using GAUSSIAN09,
Revision D.01 package.70 First, using self-consistent reaction
field method (SCRF)71,72 model, we have been able to optimize
ground state (S0) geometry of
1 in ethanol (Fig. 6). Optimized
geometry of S0 state shows that the phenyl and coumarin
groups attached to C3 and N2 atoms of imidazolyl ring further
deviates from crystal geometry (Fig. 6). However, calculated
torsional angle (0.13o) viewed along N1−C1−C4−C5 atoms
indicate that hydoxyphenyl group attached to C1 atom of
imidazolyl ring is almost planar. Calculated intramolecular H-
bond (O3−H•••N1, 1.65 Å, 148.62o) (Table S3†) and presence
of a pseudo-six membered aromatic ring contributes to the
planarity of hydroxyphenyl group with imidazolyl ring. Rotation
around the C1−C4 single bond is very promising as per as
rotational isomerization is concerned. For that, a potential
energy surface (PES) scan analysis has been carried out, which
is used for relaxed optimizations in the ground electronic state
(S0) along with fixing the N1−C1−C4−C5 torsion in a series of
values (0 to 360o), which yielded to two different rotational
conformers with 6.12 kcal.mol-1 potential energy barrier (Fig.
7). Moreover, calculated torsions (Table S3†) suggest that a
structural rearrangement occurs upon rotation of C1−C4 single
bond. Interestingly, a new putative intramolecular hydrogen
bond (O3
In order to gain further insight into the dynamic equilibrium
between two proposed conformers of
1, temperature
dependent 1H NMR spectroscopy in CD3CN was carried out.
The observed changes in intensity of the H12 signal at 7.81 and
8.49 ppm as a function of temperature (Fig. S17†) was used to
−H•••O1, 1.84 Å, 153.4o) between O1 atom of keto
track the thermal equilibrium of 1syn
→ 1anti conversion. The
equilibrium ratio calculated from peak integration of each of
the conformers was 90:10 (syn/anti) at 273 K, which decreases
gradually with increase of temperature. At 283 K, this ratio
becomes 56:44 (syn/anti). No significant change in equilibrium
ratio was observed after 283 K and bellow 273 K. Free energy
carbonyl and OH proton is perceived in 1anti isomer, which is
shorter in distance compared to the H-bond (O3−H•••N1,
1.890(1) Å, 148.06(0)ᵒ) obtained in single crystal (Fig. 1, and
Table S3†). Our calculation also predicted that 1syn has
significantly lower rotational barrier73 (6.12 kcal.mol-1) along
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Fig. 5 1HNMR spectra (400 MHz) of 1 (a) in CDCl3; (b) partial peak assignments in CD3CN; (c) CD3CN/D2O (30:1), recorded after measuring in CD3CN; (d) CD3CN at 273 K; and (e)
1HNMR spectra (400 MHz) of 2 in CD3CN.
with low Gibbs free energy change (ꢀ
G# = 0.69 kcal.mol-1) due mechanism around the C1−C4 single bond was based on
to formation of a new stabilizing intramolecular O−H•••O formation of a new stabilizing O−H•••O hydrogen bonds in
hydrogen bond upon rotation around the C1−C4 single bond. 1anti isomer, which lowers their rotational barriers. In table S4,
This strong intramolecular H-bond manifests itself by the we have shown NBO74 (B3LYP/6-311G(d,p)) interactions, which
1
formation of two conformers which were detected in H NMR may lead to activate the rotation process around the C1−C4
of
1 in CD3CN. This result is consistent with NMR analysis. The bond.
final observation in support of the proposed rotation
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Fig. 6 Optimized geometry of 1syn and 1anti. All H atoms except OH are removed for
clarity.
Fig. 8 Calculated molecular orbital energy levels, transition energies and oscillator
strengths of the π-π* and CT transitions of the syn/anti conformers of 1.
major electronic transition from HOMO to LUMO (HOMO→
LUMO, f = 0.88) was observed for 1anti conformer. For 1anti, it is
important to note that corresponding HOMO and LUMO is
predominantly delocalized over coumarin. The calculated
energy of the LUMO in 1anti conformer is lower compared to
the 1syn, which would explain significant stabilization of 1anti
conformer in the excited state due to distal intramolecular H-
bond between OH proton and O1 atom of keto carbonyl from
coumarin segment. The corresponding contour surface (Fig.
Fig. 7 Optimized potential energy surface scan when viewed along the dihedral angle
[N−C1−C4−C5] of 1.
S27†) and excitaꢀon energy of
results (Table S7 and S8†).
2 and 3 shows almost similar
Computational results further suggests that a significant
orbital re-distribution is observed between two conformers,
indicating structural re-adjustment caused by distal
intramolecular H-bonds. Bassed on an extensive study of NMR,
X-ray data, ultrafast spectroscopy measurements, and
computations using density functional theory (DFT) at the
B3LYP/6-311G(d,p), an overall scenario for solvent induced
TD-DFT calculations were used to access frontier molecular
orbitals, excitation energy, and oscillator strength (f) for
significant vertical transitions of two conformers (syn/anti). All
calculated excitation energy and oscillator strength are
compared in Fig. 8, Fig. S27† and Tables S5-S8†. The excitation
energy of 1syn and 1anti were 3.9 eV (318 nm), 3.22 eV (385
nm), 3.01 eV (411 nm), and 3.1 eV (400 nm) respectively,
rotamerization of 1 is summerised in Fig. 9.
which closely match with the broad absorption spectra of
1 in
polar solvents (Fig. 2). These excitations should be
undoubtedly assigned to the ICT transitions as judged by
electron density perturbations between HOMOs and LUMOs.
Conclusions
Interestingly, this results indicate that three electronic In summary, we successfully demonstrate a novel design
transitions from highest occupied molecular orbitals (HOMO) principle for tuning lifetime of a coumarin fluorophore which is
delocalized over the imidazole core along with hydroxyphenyl conjugated to tri-phenyl-imidazole moiety, 1, by introducing
group attached to the C1 atom of imidazolyl ring and coumarin an –OH group on the imidazolyl fragment remote to the dye.
(HOMO-1) moieties to the lowest unoccupied molecular Although the –OH functionality renders two ground state
orbitals (LUMO and LUMO+1) delocalized over coumarin and conformers of
imidazole core along with phenyl groups attached to C1 and C3 fluorescence lifetime change to the establishment of
1
as confirmed by 1H-NMR, we attribute
a
atom of imidazolyl ring (HOMO → LUMO, f =0.12; HOMO-1 → putative O–H•••O interaction subsequent to local
LUMO, f = 0.76; and HOMO → LUMO+1, f = 0.70) make major photoexcitation at the coumarin. Spectroscopic measurements
contributions to the excited states (Fig. 7). However, only one tracking the excited state dynamics provide unequivocal
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Fig. 9 Model for the ground state and excited state behaviour of 1.
evidence for bond rotation, which was affected by solvent (HRMS) were carried out using an Agilent 6540 accurate–mass
viscosity. Although the barrier for ground state rotation is 6.12 Q-TOF LC/MS (Agilent Technologies, U.S.A.). MALDI-TOF mass
Kcal/mol in
rotation along the C1–C4 bond leading to the formation of Daltonics). Samples in buffer (0.5
long-lived anti state. Systematic measurements on two other solution of 3,5-Dimethoxy-4-hydroxycinnamic acid (1:1
coumarin-imidazole conjugates and , showed ca. 4-5 times MeCN:H2O with 0.1% trifluoroacetic acid, 0.5 L) and applied
1, photoexcitation of coumarin leads to facile spectra were obtained on a Bruker UltrafleXtreme(TM) (Bruker
ꢁ
L) were mixed with a
2
3
ꢁ
reduction in the emission lifetimes thereby supporting our on the MALDI plate. Samples were allowed to dry completely
design principle. The novelty of our work therefore lies in before analysis. Infrared spectroscopy had been implemented
highlighting the role of a distal functionality in tweaking the by using Thermo Fisher scientific FTIR. Steady state absorbance
fate of localized coumarin emission. In principle, our work were measured by Agilent Technologies Cary 8454 UV/vis
provides a new scheme to implement conformational control instrument. Steady-state emission was recorded in
on the photophysics of molecular fluorophores, and should fluorescence spectrometer (Model: FL3-2-IHR from Horiba)
inspire novel framework to control function through rational with standard FL3 PMT R928P detector were used for steady
design of chemical interactions.
state emission. Fluorescence quantum yields in each case were
determined by comparing the corrected spectrum with
standard Fluorescin22
under the total emission.
(φ = 0.85 in 0.1(N) NaOH taking area
Experimental Section
Nano-second lifetimes were measured using fluorescence
intensity decay of a TCSPC set up, coupled with a pico-second
laser. The excitation source was frequency doubled output of a
mode-locked picosecond Ti-sapphire laser (Tsunami, Spectra-
Physics, USA) (720-900 nm) pumped by a diode pumped CW
Nd-vanadate laser (532 nm) (Millennia X, Spectra-Physics,
USA). The pulse width of the exciting laser was typically 1-2 ps
with repetition rate of 80 MHz. Fluorescence decay curves
were obtained using a microchannel plate photomultiplier
(model R2809; Hamamatsu Corp.) coupled with a time-
correlated single photon-counting setup. The instrument
response functions (IRF) at 400 nm were obtained by collecting
scattering using a dilute colloidal suspension of dried non-dairy
coffee whitener. The width (fwhm) of the instrument response
Materials and Reagents
7-N,N-diethylaminosalicyaldehyde and ethyl nitroacetate were
purchased from Alfa aesar and rest all chemicals were
purchased from local source. All deuterated solvents were
purchased from Euroisotope. Thin-layer chromatography (TLC)
analyses were carried out on pre-coated silica gel plates
(Merck) and spots were visualized by UV irradiation. Column
chromatography was performed on glass columns loaded with
silica gel. All spectroscopic measurement had been
accomplished by using freshly distilled solvent.
Physical measurements and Instrumentation
Liquid state NMR were recorded in Bruker AVHDN 400 with
working frequencies of 400.245 MHz for 1H nuclei
and 100.6419 MHz for 13C nuclei, respectively, using CDCl3,
CD3CN and D2O as solvent. Chemical shifts were quoted in ppm
relative to tetramethylsilane, using residual solvent peak as a
reference standard. High Resolution Mass Spectroscopy
function (IRF) was ∼100 ps. The decay kinetics were fitted with
multi-exponential by iterative method using IGOR Pro 5.01
software.
Upconversion measurements23 for all samples were performed
in a rotating sample chamber of 1 mm path length, were
excited using the second harmonic (398 nm) of a mode-locked
10 | J. Name., 2012, 00, 1-3
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Ti-sapphire laser (Tsunami, Spectra Physics, average power 1W 3.43 (q, 4H), 1.22 (t, 6H) ppm. 13C NMR (100 MHz, CDCl3,
at 800 nm, pulse width 200 fs, repetition rate 80 MHz) pumped /ppm): 155.32, 154.21, 149.89, 146.31, 143.42, 139.53,
δ
δ
with a CW 5 W Millennia (Spectra Physics) DPSS laser. The 135.11, 130.66, 131.01, 130.79, 129.92, 128.95, 128.79,
fluorescence emitted from the samples was upconverted in a 128.52, 128.43, 128.02, 126.91, 126.82, 117.40, 109.38,
nonlinear crystal by mixing with a gate pulse, which consists of 107.01, 97.29, 45.03, 12.53 ppm. IR data (solid): 1715 (C=O)
a portion of the fundamental beam. The upconverted light was cm-1. HRMS(ESI): m/z calculated for C34H29N3O2: 511.2260,
+
dispersed in a monochromator and detected by photon found [M-H+]: C34H30N3O2 , 512.2328 (calcd 512.2333).
counting electronics. The femtosecond fluorescence kinetics
were fitted using a Gaussian function of 250 fs FWHM of the
Conflicts of interest
IRF for the excitation pulse using IGOR Pro 5.01 software.
There are no conflicts to declare.
Synthesis and Characterization
General procedures for compounds
compounds were synthesized using
1
,
2
and
3. These
Acknowledgements
a
modified multi
component cyclisation reaction.15 In a 25 ml round-bottomed
Financial support for this work was provided by the Science &
Engineering Research Board (SERB) (File No: SB/EMEQ-
004/2014 & YSS/2014/000923), DST, and Shiv Nadar University
(SNU), India. I. B. thanks SNU for fellowship. JD thanks TIFR and
the Department of Atomic Energy (DAE) for financial support.
We thank N. Sukumar for computational help.
flask, compound 1b (250 mg, 1.0 equiv.), substituted
benzaldehyde (1.0 equiv.) (salicylaldehyde for
1;
2-
methoxybenzaldehyde for 2; and benzaldehyde for
3), benzil
(1.0 equiv.) and ammonium acetate (6.0 equiv.) were taken in
10 mL acetic acid and then allowed to reflux for 18 h. After
completion, reaction mixture was poured into ice water,
precipitate was filtered out and dissolved in ethyl acetate. The
crude was purified by column chromatography using silica 100-
200 mesh and ethyl acetate/hexane mixture as eluent.
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Conformational switching via intramolecular H-bond modulates fluorescence lifetime in a novel coumarin-imidazole conjugate