Effect of conformational flexibility on photophysics of bis-coumarins

Article abstract of DOI:10.1039/c8cp01084f

The fluorescence intensity of bis-coumarins linked via CONH and COO functionalities are shown to exhibit a strong dependance on solvent polarity. The presence of the intramolecular hydrogen bond between the CO oxygen atom of coumarin and amide-NH moieties reduces the number of thermodynamically populated conformations in both ground and electronically excited states and an anti-arrangement of coumarin units is favored. Additionally, the rigidity of the linker raises the barrier to the conical intersection with the ground state, and in non-polar solvents strong fluorescence is observed. Although changing the CONH linking position from 3-7′ to 3-6′, does not remarkably affect the photophysics, replacement with a flexible ester linker allows the molecule a greater degree of conformational freedom due to the absence of the intramolecular hydrogen bonding interaction. The photophysical effect of this is the appearance of two fluorescence bands, the relative intensity and spectral positions of which are sensitive to the environment. Theoretical explorations of the excited-state potential energy surfaces performed with the aid of the ADC(2) ab initio electronic structure theory method revealed an exceptional wealth of concomitant photophysical processes. In particular, two channels for radiationless deactivation of the excited state were found; the first is related to the inter-ring twist of the coumarin units, and the second is associated with the excited-state intramolecular proton-transfer (ESIPT) from the CONH linker to the coumarin core.

Full text of DOI:10.1039/c8cp01084f

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Effect of conformational flexibility on photophysics of bis-
coumarins
Łukasz Kielesiński,a,b Daniel T. Gryko,*,a Andrzej L. Sobolewski*,b and Olaf W. Morawski*,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The fluorescence intensity of bis-coumarins linked via CONH and COO functionalities are shown to exhibit a strong
dependance on solvent polarity. The presence of the intramolecular hydrogen bond between the C=O oxygen atom of
coumarin and amide-NH moieties reduces the number of thermodynamically populated conformations in both ground and
electronically excited states and an Anti-arrangement of coumarin units is favored. Additionally, the rigidity of the linker
raises the barrier to the conical intersection with the ground state, and in non-polar solvents strong fluorescence is
observed. Although changing the CONH linking position from 3-7' to 3-6', does not remarkably affect the photophysics,
replacement with a flexible ester linker allows the molecule a greater degree of conformational freedom due to the
absence of the intramolecular hydrogen bonding interaction. The photophysical effect of this is the appearance of two
fluorescence bands, the relative intensity and spectral positions of which are sensitive to the environment. Theoretical
explorations of the excited-state potential energy surfaces performed with the aid of the ADC(2) ab initio electronic
structure theory method revealed an exceptional wealth of concomitant photophysical processes. In particular, two
channels for radiationless deactivation of the excited state were found; the first is related to the inter-ring twist of the
coumarin units, and the second is associated with the excited-state intramolecular proton-transfer (ESIPT) from the CONH
www.rsc.org/
linker
to
the
coumarin
core.
possibilities, the most intense research has focused on 2-
amino-6-acylnaphthalenes,6 4-amino-1,8-naphthaleneimides,7
stilbenes,8 coumarins9 and others.10 The latter, especially
those possessing a electron-donating group at position 7 and
electron-withdrawing at position 3 have been investigated
photophysically and practically utilized at an unprecedented
frequency.11 The fluorescence of coumarins substituted with
an amino group in the 7-position is one of the most intensively
studied and most puzzling topics in photophysics of organic
chromophores. Jones discovered that the emission
characteristics are strongly influenced by substituents on and
in the proximity of the amino-nitrogen atom.12 The in-depth
analysis of this topic was further developed by numerous
authors13 with critical contributions coming from Rettig14 and
Cole.15 Very recently Lavis16 and Ahn17 independently
developed two structural motifs which retain reasonably high
fluorescence quantum yields of 7-aminocoumarins in very
polar solvents.
1. Introduction
The properties of covalently linked multichromophoric
assemblies depend not only on the discrete structures of the
comprising units but also on their relative spatial
arrangement.1
Electronic
communication
between
chromophoric units in such assemblies is dependent on the
chemical properties of the linker such as its length, rigidity and
hydrogen bonding propensity.2 Over the last three decades
these features have been extensively explored in in
multichromophoric assemblies in regard to electron transfer
and charge recombination.3 The linking of disparate
chromophores has been explored in the context of the design
of functional dyes with particular interest being paid to the
linkage of highly polarized molecules.4 Their ground state
dipole moment, its change in the excited state and π-π
stacking interplay often leads to unforeseeable and
serendipitous final properties.5 Among a countless number of
The use of amide functionality as a linking group holds a
prominent position in the chemistry of self-assembled systems
including foldamers,18 electrets,19 tapes20 and helixes21 etc.
The very special case of such interactions, has been recently
encountered by us.22 The CoumDimer comprised of two
coumarin units linked by a tertiary amide bridge was the first
example in which two polarized coumarin units were linked
together by an amide bridge in a donor-acceptor-donor-
acceptor (D-A-D-A) orientation. Such a highly polarized system
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is the source of the strong internal electric field that shall, in may provide additional degrees of freedom for subunits
principle, allow splitting of an exciton and separate charges. mutual movements. Finally we designed Amide-6 (Scheme 3)
Using single, sterically hindered coumarins as a building block, differing from Amide-7 in the location of CONH group at the
we wanted to obtain a structure with linearly arranged second coumarin unit. It is well known that moving amino
molecular units. Theoretical calculations and X-ray diffraction group on the coumarin scaffold from position 7 to position 6
analysis showed that one of the privileged conformations of results in tremendous changes in optical properties.23
such structure is one in which the coumarin units form a
wedge. We called this conformation a Clin.22 The perturbation
of electronic states due to this arrangement prompted us to
perform in-depth studies of the photophysical properties of
this family of structures comprised of two coumarin moieties
linked in a D-A-D-A orientation. The key focus was the
modification of the spatial arrangement of coumarin units via
introduction of a hydrogen bond. Herein present the results of
this modification. Optical spectroscopic investigations are
accompanied by theoretical calculations to gain a deeper
insight into photophysical processes occurring in the studied
molecules. The aim of our investigation is to elucidate how the
rigidity of the linker influences the geometry and photophysics
of a molecular system comprised of strongly polarized units
connected in D-A-D-A orientation.
2. Methods
The molecules were synthesised with the strategy explained
below and methods described in the Electronic Supplementary
Information (ESI). All experimental details of reported optical
spectra and fluorescence decay acquisition are provided in ESI
along with a description of theoretical calculation methods.
3. Results and discussion
3.1 Design and Synthesis
We have chosen coumarins possessing an electron-donating
group at position 7 as a key building block, as they represent
classical push-pull chromophores at the same time being
photophysically challenging in spite of numerous studies.10
Me
N
CO2Et
O
O
O
Et
Et2N
O
O
CoumDimer
Fig. 1 Structure of CoumDimer.22
On the basis of the photophysical properties of CoumDimer
(Fig. 1), we decided to introduce slight modifications to its
structure. We assumed, that the presence of a intramolecular
hydrogen bond between the amido-NH in one unit and
carbonyl group in the other, will energetically favor the anti
conformation of this molecule. Replacement of the tertiary
amide with a secondary amide leads to Amide-7 (Scheme 1).
On the other hand we envisioned that replacement of the
amide bond with an ester functionality (Ester-7) (Scheme 2)
Scheme 1 Synthesis of Amide-7.
The retrosynthetic analysis revealed that the key building block
in the synthesis of Amide-7 was coumarin
primary amino group at position 7. Thus, starting from 2-
hydroxy-4-nitrobenzaldehyde we performed the
Knoevenagel condensation with diethyl malonate furnishing
3 possessing
(1)
nitrocoumarin
2
(Scheme 1).
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CO2H
O
diethyl malonate,
pyrrolidine, EtOH
CO2Et
O2N
CHO
OH
O2N
CO2Et
O
+
reflux, 3 h, 75%
Et2N
O
HO
O
O
O
4
6
10
9
EDC, DMAP, DCE
rt, 24 h, 25%
.
SnCl2 2H2O, EtOH
reflux, 1.5 h, 98%
CO2Et
O
CO2H
O
H2N
CO2Et
O
O
O
O
+
Et2N
O
O
Et2N
O
O
Ester-7
11
4
Scheme 2 Synthesis of Ester-7.
EDC, DMAP, DCE
rt, 24 h, 69%
The subsequent reduction of nitro group turned out to be
cumbersome. Initially we used formic acid in the presence of
palladium on carbon,24 but we did not observe any conversion
CO2Et
O
of the substrate
2 into product 3. Ganem’s procedure with
sodium borohydride and tetrahydrate of nickel(II) acetate
failed too.25 The methodology26 employing Co2(CO)8 allowed to
obtain the desired product but the yield was rather moderate.
O
O
O
N
H
Finally, we obtained aminocoumarin 3 in almost quantitative
yield using tin(II) chloride dihydrate in ethanol.27 In the final
Et2N
O
step we sought to synthesize Amide-7 via formation of amide
Amide-6
bond between aminocoumarins
3 and 4. Initially we used
Scheme 3 Synthesis of Amide-6.
Ulven procedure in which the coupling reagent was
chlorodipyrrolidinocarbenium hexafluorophosphate (PyClU),28
but it turned out to be ineffective. Next attempts with classical
peptide coupling reagents such as N-(3-diethylaminopropyl)-N-
ethylacrbodiimide hydrochloride (EDC) or N,N,N',N'-
tetramethyl-O-(1H-benzotriazol-1-yl)uranium
The synthesis of Amide-6 possessing amide bridge localized
between the position 3 of one coumarin unit and position 6 of
second coumarin unit was undertaken following an analogous
procedure to that developed for Amide-7. The Knoevenagel
hexafluorophosphate (HBTU) did not give desired product.29,30
Finally we adopted the method of Ringling and co-workers.31
Employment of a propylphosphonic anhydride (T3P) solution
condensation of 2-hydroxy-5-nitrobenzaldehyde (
9) led to
nitrocoumarin 10 30,42
.
In next step the nitro functional group
was reduced in the same manner as described previously27
obtaining product 11 with excellent yield (Scheme 3).
Interestingly in the synthesis of Amide-6 we obtained the best
result when we used EDC as a coupling reagent while in case of
Amide-7 this reagent did not work. The plausible rationale
behind the three-fold higher yield of the final coupling in the
case of Amide-6 is the decreased nucleophilicity of NH2 group
in the molecule of Amide-7 due to the strongly electron-
withdrawing effect of carboxylic group.
in DCM as a coupling reagent allowed us to isolate Amide-7
.
Because the yield was low (21%), we investigated another
synthetic pathway (Scheme 1). For this purpose, we obtained
amidocoumarin
832
from
2-hydroxy-4-
diethylaminobenzaldehyde and cyanoacetamide, as well as
coumarin 733-35 with triflic group. In the key step we tested N-
arylation of the CONH2 group using various catalysts e.g.
palladium(II) acetate, tris(dibenzylidenoacetone)dipalladium(0)
or copper(I) iodide, to no avail.36-39
The second targeted dye differed from Amide-7 in chemical
nature of the linker i.e. amide bond has been replaced with
ester functionality. We obtained the Ester-7 in a three-step
synthesis (Schemes 1-2). As a coupling agent for the reaction
of coumarin 6 with acid 4 N,N’-dicyclohexylcarbodiimide
(DCC)40 was trialled but it turned out that the corresponding
urea is difficult to separate from the desired product and we
observed trace amounts in the NMR spectrum of Ester-7
Gratifyingly the use of EDC41 allowed us to isolate pure Ester-7
.
.
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3.2 Theoretical results
3.2.1 Amide-7
Table 1b Relevant molecular orbitals involved in the lowest electronic excitations
of the Amide-7 (Anti).
HOMO
Three stable conformers of Amide-7 (Fig. 2) were localized on
the ground-state potential-energy surface. This number of
possible structures has to be multiplied by relative orientations
of the coumarin and amide scaffolds, and by different
rotations of the methoxy and methyl groups.
105a
112a
110a
113a
Anti (0.0/16.7)
LUMO
114a
115a
Syn (0.20/5.2)
Clin (0.39/4.2)
Inspection of results presented in the Tables shows that the
absorption spectrum of the system is dominated by the fully
allowed (f=1.146) transitions to the S1 state of the ππ*
electronic nature. The transition involves a substantial charge-
transfer (CT) character as it is indicated by a remarkable rise of
the dipole moment from 16.65 Debye in the S0 state to 25.07
Debye in the lowest excited singlet state (S1). An increase of
the stacking interaction between the two coumarin moieties in
the Clin and Syn conformers is manifested by distribution of
the intensity of absorption transitions to higher singlet states
(Table S1 of the ESI), and in the latter conformer the S0-S1
transition is of pure CT in nature.
Fig. 2 Stable conformers of Amide-7 determined at the MP2/cc-pVDZ level. Numbers in
parentheses denote their relative energy in eV and dipole moment in Debye,
respectively.
The conformer Anti represents the global minimum of the
potential-energy (PE) landscape of the ground state in the
isolated phase. This is also the most polarized conformation,
so it is expected to be additionally stabilized by interactions
with a polar environment. Its absorption spectrum and the
relevant molecular orbitals are presented in Tables 1a and 1b,
respectively. Similar data computed for other conformers are
presented in Table ST1 of the ESI.
The ADC(2) geometry optimization of the first excited singlet
state of the Amide-7 conformers has clearly revealed that the
Clin conformer is unstable in the S1 state and spontaneously
relaxes to the Syn conformation. Moreover, two additional
isomers of the molecule were revealed to have local minima
on Potential Energy Surface (PES) of the S1 state. The first
conformation (abbreviated as Clinx) is not expected to be of a
Table 1a. Transition energy (ΔE), oscillator strength (f), dipole moment (μ), and leading
electronic configurations, of Amide-7 (Anti) computed with ADC(2)/cc-pVDZ method at spectroscopic relevance, but to provide a source of an efficient
the ground-state MP2/cc-pVDZ equilibrium geometry.
radiationless deactivation of electronic excitation due to
negligibly small energy gap to the ground state. The second
Electronic
isomer (AntiT) represents a conformer formed due to the
State
ΔE\eV
f
μ/Debye
configurations
transfer of the proton from the NH moiety of the amide linker
to the nearby coumarin unit along the hydrogen bond. At this
point it is worth emphasizing that excited-state intramolecular
proton transfer (ESIPT) with amide-NH is a very rare case and
to the best of our knowledge it is up to date restricted to 1-
acylaminoantraquinone derivatives.43 Since the ‘vertical’
energy gap to the S0 state is only 0.6eV, this conformation is
also not expected to be of spectroscopic relevance, but
provides an additional source of radiationless decay. The
overall photophysical diagram of Amide-7 is presented in Fig. 3
and the relevant details are presented in Table ST2 of the ESI.
S0
-
-
16.65
25.07
(113a)2
0.67(113a-114a)-
0.63(112a-114a)
0.58(113a-115a)
+0.55(112a-114a)
1ππ*
3.32
1.146
1ππ*
3.74
0.251
16.16
1nπ*
1ππ*
4.06
4.17
0.2
7.91
0.85(105a-114a)
0.65(110a-114a)
0.008
16.86
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presume that emission from this form is hardly observed due
to the CT nature and small energy gap.
In order to estimate barriers that protect the emitting
conformer Anti on the S1 PES against the radiationless decay
channels (AntiT and Clinx conformations) a minimum-energy
reaction paths leading to these conformations were
determined at the ADC(2)/def-SV(P) level of theory. Obtained
potential-energy (PE) profiles are presented in Figures SF1 and
SF2 of the ESI. The adiabatic barrier estimated for the ESIPT
reaction is about 0.3 eV (Figure SF1). The zero-point
vibrational-energy correction may significantly reduce this
value due to high-energy quanta of the NH stretching
vibration. PE profiles determined along the CCNC dihedral
angle (Figure SF2) are much more complex. One can notice
upon inspection of the figure that there is an interplay
between the optically populated S1(LE) state and the
“reactive” S2(CT) state along the reaction path, and only the
latter closes the gap with the ground state. The intersections
between the LE and CT states visible in Figure SF2 are only
apparent since these reaction pathways differ in many other
intramolecular coordinates and this is reflected by large
difference between the “vertical” PE profiles of the S0 state
computed along these paths. This is indicated by a strong
curvature of the multidimensional minimum-energy reaction
Fig. 3 Photophysical scheme of Amide-7 computed at the MP2/ADC(2) level of theory.
Solid levels denote adiabatic (optimized) energy of a given state while dashed levels
denote ‘vertical’ energy of the absorbing/fluorescing state computed at the optimized
geometry of the respective doorway state. Numbers denote energy(eV)/dipole
moment (Debye) of a given state. Dotted lines denote a diabatic correlation between
states of the closed/open-shell electronic nature and solid/dashed arrows denote
radiative/radiationless decay pathways, respectively.
Inspection of the computational results presented in Figure 3
leads to three main conclusions: (i) Optical excitation of the
most stable conformer (Anti) may result in strong fluorescence
from the S1 state. The S1-S0 fluorescence is fully allowed
(f=0.989) and possesses Stokes shift of about 0.48eV. These
values are typical for a locally-excited (LE) electronic transition
and this is supported by analysis of results presented in Table
ST2a of the ESI. (ii) The fluorescing state of the Anti conformer
represents a local minimum on the potential-energy surface
(PES) of the S1 state but the energy gap to the ground state
may significantly be reduced by distortion of molecular
geometry along particular intramolecular coordinates. First of
all, torsion around the CCNC dihedral angle of the linking
amide group results in deletion of the S1-S0 energy gap around
the perpendicular mutual orientation of the coumarin rings
path that connects the reactant (Anti) and the product (Syn
)
conformers and may result in a much higher barrier than
estimated on the base of one-dimensional PE profiles shown in
Figure SF2.
Further explorations of photophysically relevant PES of Amide-
7
have revealed that the conformer obtained by rotation of
the amide group with respect to the Me2N-coumarin unit
AntiB) possesses a PE diagram (Figure SF3) qualitatively
(
similar to the parent Anti conformer (Figure 3) with the only
absence of the PT reaction path (lack of the hydrogen bond),
and generally all conformers have higher energy than their
counterparts in Figure 3. Another branch of the photophysical
diagram develops if the rotation of the amide group occurs
after the PT (Figure SF4). The net effect of this reaction is
transfer of the HB-forming hydrogen atom between two
heteroatoms (N and O) of the amide group. The photophysical
scheme of this reaction is similar to the working principle of
the molecular switch driven by excited-state hydrogen
transfer.44
(Clinx). Secondly, proton transfer from the amide nitrogen to
the coumarin C=O group (AntiT) stabilizes the S1 state
remarkably and reduces energy gap to only 0.63eV. Such small
values of energy gap provide a source of efficient radiationless
decay of electronic excitation (internal conversion) to the
ground state that excludes observation of any fluorescence
from these conformations. (iii) The vibrationally ‘hot’ S0 state
populated at these conformations can further evolve on the
PES of the ground state. Although, the ground-state
optimization of the AntiT conformer results in restoring the
Anti form, the Clinx conformer relaxes to the stable Clin
conformation. This conformation absorbs at a wavelength
similar to the Anti conformer (Table ST1a), but it is unstable on
the PES of the S1 state and spontaneously relaxes to the Syn
conformation with a significantly reduced gap to the S0 state
(∆E=1.53 eV). Analysis of the electronic nature of the S1-S0
radiative transition at this configuration (Table S2c) reveals its
charge-transfer (CT) character that is also manifested by very
small oscillator strength (f=0.005). One can reasonably
All these theoretical considerations lead to the conclusion that
only the Anti conformer of the Amide-7 is expected to be
fluorescent from its S1(LE) state.
3.2.2 Amide-6
The first branch of the photophysical diagrams of Amide-6
determined at the MP2/ADC(2)/cc-pVDZ level are presented in
Figures SF5 and SF6, and in Tables ST5 and ST6 of the SI. The
diagrams are qualitatively similar to the relevant schemes
obtained for Amide-7 (Figure 3, SF3, and SF4). There are only
marginal differences in energetic location of electronic states
and their dipole moments. Thus this chemical modification of
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the molecular system is not expected to lead to any major the conformers. Now the Syn conformer represents a global
change in the photophysical behavior.
minimum of the system on the ground-state PES, and the Anti
conformer is higher in energy by 0.16 eV. However, interaction
with a polar environment may additionally stabilize the latter
conformer to become the global minimum of the system due
to its higher dipole moment. Calculated absorption spectra of
the conformers and the relevant molecular orbitals are
presented in Table ST5 of the SI and are qualitatively similar to
the Amide-7 case (Table ST1). The overall photophysical
diagram of the Ester-7 is presented in Figure 5 and the
relevant details are presented in Table ST6 of the ESI.
3.2.3 Ester-7
Three stable conformers of Ester-7 localized on the ground-
state potential-energy surface are shown in Figure 4. Once
again this number of possible structures has to be multiplied
by relative orientations of the coumarin and amide scaffolds,
and by different rotations of the methoxy and methyl
groups.
Inspection of the diagram reveals two important differences
with respect to Amide-7 (Figure 3). First of all, there is no
ESIPT reaction channel due to the lack of the proton-donating
NH group, and secondly, the channel for radiationless decay
has a different conformation which is denoted as Antix in
Figure 5 (instead of Clinx in Figure 3). This is related to a
difference in the electronic nature of the S1 state in this
conformation. Namely, the S1-S0 transition is dominated by
the nπ* electronic configuration (Table ST6c). The adiabatic
energy of this state is almost degenerate with the adiabatic
energy of the S1 state of the Anti conformer, but the vertical
energy of the ground state computed at these geometries are
very different. The increase of the ground-state energy at the
equilibrium geometry of the 1nπ* state is mainly related to
elongation of the C=O bond length of the Me2N-coumarin from
1.22Å in the former state to 1.38Å in the latter. The orbital
nature of the state and small energy gap (0.72 eV) precludes
observation of any fluorescence from this state. Thus
population of this state opens the channel for radiationless
decay of electronic excitation.
Anti (0.16/10.7)
Fig.
4
Stable
confor
mers
of
Ester-
7
Syn (0.22/7.2)
Clin (0.0/7.4)
deter
mined
at the
MP2/s
s-
pVDZ
level.
Numb
ers in
paren
theses denote their relative energy in eV and dipole moment in Debye.
Photophysically relevant PE diagram of the Ester-7 conformer
obtained by rotation of the amide group with respect to the
Me2N-coumarin unit (AntiB) is shown in Figure SF7 of the SI
and is qualitatively similar to the parent Anti conformer (Figure
5) with all conformers having higher energy than their
counterparts in Figure 5.
Summarizing the theoretical explorations one may conclude
that the three molecules studied have a fluorescent S1(LE)
state. On the other hand, the high degree of conformational
freedom allows the formation of numerous conformers and
opens paths for efficient radiationless deactivation of
electronic excitation. In Amide-7 and Amide-6 the
intramolecular hydrogen bond between proton of amide NH
moiety and oxygen of coumarin rigidifies the linker but
introduces the ESIPT process which leads to
a conical
intersection with the S0 state which opens an additional non-
radiative channel of deexcitation.
Fig. 5 Photophysical scheme of Ester-7 computed at the MP2/ADC(2) level of theory.
Solid levels denote adiabatic (optimized) energy of a given state while dashed levels
denote ‘vertical’ energy of the absorbing/fluorescing state computed at the optimized
geometry of the respective doorway state. Numbers denote energy(eV)/dipole
moment(Debye) of a given state. Dotted lines denote a diabatic correlation between
states of the closed/open-shell electronic nature and solid/dashed arrows denote
radiative/radiationless decay pathways, respectively.
3.3 Experimental results
3.3.1 Amide-7
Amide-7 forms yellow crystals that are soluble in polar or
polarizable solvents forming yellowish solutions. The
absorption spectrum of Amide-7 (Figure 6a) is dominated by
An important difference with respect to the Amide-7 system
discussed above is related to different energetic ordering of
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the low energy band of a strong S0 → S1 transition positioned
in toluene at 22800 cm-1, other much weaker bands are
located at 26880, 28410, 32000, 33700 and 42500 cm-1. These
display a good match with the theoretical results presented in
Table 1a– the transition to S1 is much stronger than transitions
to S2 and S3 (5.4 and 6.3 times respectively) and excitation
energies observed in absorption (3.33, 3.52 and 3.97 eV) are
lower than those calculated for an isolated molecule in
vacuum by 0.5 eV, a typical difference between ADC(2) results
computed for vacuum and experiments in solvents. The first
excited singlet state of Amide-7 has an energy 1650 cm-1 lower
than the S1 state of CoumDimer reported previously,22
indicating that linking through a secondary amide bridge
results in stronger coupling between polarized coumarin units
than through linkage with a sterically hindered tertiary amide.
This may confirm the expected planar conformation of the
molecule and its expected D-A-D-A alignment of both units.
The small Stokes shift points to rather negligible excited state
relaxation, which was predicted in the theoretical calculation
for the Anti form. The calculation also predicts the Anti form to
have the lowest lying absorption band among stable
conformers of Amide-7 (Figure 3, Table ST2). Moreover,
according to theoretical results, the lowest excited singlet
states of the Syn form have negligible transition moments, and
this form would show very large Stokes shift. Additional
arguments for the Anti form come from analysis of the IR
absorption and Raman spectra of Amide-7 in the solid state
(Figures SF21 and SF23), as the spectra calculated for the Anti
form are a better match to the observed ones.
From the solvatochromic shift one may evaluate the change
ꢀꢁ between ground state ꢁg and excited state dipole moment
Fig. 6 Normalized absorption (a) and fluorescence (b) spectra of Amide-7 in selected
solvents (toluene, chloroform, 1,2-DCB, acetonitrile, methanol). The legend specifies
colors of the spectra. Insert: Lippert-Mataga plot of Stokes shift of Amide-7 spectra
versus solvent polarity parameter f(ε,n) = (ε-1)/(2ε+1) – (n2-1)/(2n2+1). Line represents
ꢁ
ꢀν
e. The change ꢀꢁ = ꢁe – ꢁg can be related to the Stokes shift
=
νabs
–
ν
flu by a Lippert-Mataga equation45-47
ꢁꢂ ꢃꢅꢅꢅꢅꢅꢆꢇ ꢅꢃꢅꢅꢅꢈꢅꢆꢉꢊ
ꢊ
ꢑ ꢒꢇꢓ ꢖ ꢁꢗꢗꢊꢇꢕꢓꢓꢘ
(1)
ꢄ
linear fit to experimental data. Slope of the line is 2698±990 cm-1
.
∆
υ
ꢀ
ꢋ
πεꢌꢍꢎꢏꢐ ꢁꢔꢕꢓ
in which ε0 is the permittivity of free space, h is Planck’s
constant, c is the velocity of light, a represents the radius of
The fluorescence excitation spectra are similar to the
absorption spectra (Figure SF10). Fluorescence decay curves
are single exponential and depend on the solvent (Figure
SF11). Fluorescence quantum yields of Amide-7 in polar
solvents are much lower than in non-polar ones (Table 2). Such
dramatic changes originate from a large increase of the non-
radiative rate constant knr observed in polar solvents (Table 2).
This underlines that the solvent’s dielectric constant governs
the opening of the radiationless decay channel. This
phenomenon can be related to the mutual interaction of two
polarized units of Amide-7 and the energetics of excited states
in the preferred geometry of the structure. Although the
influence of viscosity on the lifetime τF and fluorescence
quantum yield ΦF is much smaller than the influence of
dielectric constant (Figures SF12), it reveals that the non-
radiative channel is related to motions of coumarin units.
Theoretical calculations predict that in the S1 state the PE
profile for the N-H bond stretching is shallow and behind the
small barrier is diffusive (Figure SF1). This makes the amide
bridge less rigid in the excited state. Moreover, in the Anti
form, the twist around the CCNC dihedral angle decreases the
the cavity in which the solute resides and
ε and n are the
solvent’s dielectric constant and refractive index respectively.
The solvatochromism of the absorption and fluorescence
spectra is small, from the slope of the line fitted to
experimental points (inset to Figure 6a), equation (1) and from
the Onsager cavity radius a (a = 7.75 Å for Anti and a = 5.5 Å
for Syn) one may evaluate the increase in dipole moment
occurring upon electronic excitation; ꢀꢁ = 11.2 D for the Anti
and ꢀꢁ = 6.7 D for the Syn forms. This is not far from the
theoretical values (8.4 D and 5.0 D respectively), however the
assumption of spherical shape of the cavity is problematic,
especially for the elongated Anti form, so the values obtained
from experimental data are roughly approximate.
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energy of the S2(CT) state lying only slightly above the S1(LE) spherical cavity for the solute leads to the overestimation of
and leads to intersection with the S0 state (Figure SF2), which ꢀꢁ
opens an efficient internal conversion channel to the ground
state. In such a model, the knr rate is sensitive to solvent’s
dielectric constant which influences the energy of states, and
to the solvent’s viscosity which affects a large amplitude
movement of both units. This is indeed observed – in
acetonitrile, which has smallest viscosity and the highest
dielectric constant, τF and ΦF are the lowest, and they grow
continuously in the methanol to butanol series with decreasing
.
ε
and increasing viscosity of solvent (Table 2, Figure SF12).
Both changes (growth of viscosity and a decrease of ε) slow
down the radiationless process, so exact quantitative
evaluation of their impact on knr is not feasible.
Table 2 Fluorescence quantum yield (
rate constant (kr)* and non-radiative rate constant (knr
with different viscosity and dielectric constant
Φ
F), fluorescence decay times (
τ
F), radiative
of Amide-7 in solvents
*
)
η
ε.
Solvent
1,2-DCB
CHCl3
η
/cP
ε
ΦF
τ
F/ns
k
r/ns-1
k
nr/ns-1
1.324
0.563
9.93
5.62
0.85
0.99
0.84
1.57
1.64
1.5
0.539
0.608
0.561
0.098
0.0015
0.105
62.4
Toluene
ACN
0.586 2.379
0.343
0.59
38.8
32.66 0.006 0.031 0.195
24.3 0.032 0.074 0.437
0.002 0.016 0.120
Methanol
Ethanol
Propanol
Butanol
32.1
1.074
13.1
1.959 20.33 0.083 0.152 0.545
2.573 17.51 0.142 0.247 0.574
6.03
3.47
* kr = ΦF/τF and knr = 1/τF - kr
3.3.2 Amide-6
Fig. 7 Normalized absorption (solid lines) and fluorescence (dashed lines) spectra of
Amide-6 in selected solvents (toluene, chloroform, 1,2-DCB, acetonitrile, methanol).
The legend specifies colors of the spectra. Insert: Lippert-Mataga plot for Amide-6
spectra. The straight line represents linear fit to experimental data. Slope of the line is
Amide-6 is highly soluble in polar or polarizable solvents. Its
absorption and fluorescence spectra are shown in Figure 7.
The absorption band in toluene is located at ca. 23300 cm-1
and shows little (300 cm-1) solvatochromism in solvents used.
The absorption spectrum of Amide-6 is blue-shifted by 500 cm-
4055±636 cm-1
.
1
to the spectrum of Amide-7 (Figure 6), which reveals that
In all solvents the fluorescence excitation spectra of Amide-6
follow the absorption ones and both reveal second absorbing
singlet state S2 at 34000 cm-1, significantly above the first
singlet state S1 (Figure SF13 of the ESI). This agrees with the
theoretical results obtained for the Anti form of Amide-6, in
which only three absorbing states were predicted for energies
up to 4.0 eV: fully allowed S1 at 3.37 eV, weakly absorbing S2 at
3.54eV, and S3 lying much higher at 4.0 eV and absorbing
better than S2 (Table ST3). Calculations for other conformers
predict excited states at other energies, so the Anti form is a
better fit to the experimental results. A similar conclusion can
be drawn from IR and Raman spectra (Figures SF24 and SF 26).
Fluorescence decay profiles are single exponential (Figure
SF14). In non-polar solvents the decay time is about 2 ns
whereas in alcohols or ACN it is several times shorter (Table 3).
Fluorescence quantum yields are very high in non-polar
solvents but drop down to just few percent in polar ones
(Figure SF15). These results resemble those for Amide-7, with
substituting the second coumarin unit at position 6 results in
weaker coupling between the polarized units than substitution
at position 7. Its fluorescence spectrum consists of a single
band that exhibits moderate (1200 cm-1) solvatochromic shift
pointing to an increase of dipole moment in excited state of
Amide-6
The insert in Figure 7a presents plot of Stokes shift versus
solvents polarity parameter f( ,n). The slope of the line fitted
.
ε
to experimental points is 4055 cm-1. Taking this value and
cavity radii from the theoretically calculated geometry of the
Anti form, a = 10 Å, from equation (1) one obtains ꢀꢁ = 22.8 D,
a value distinctly larger than the theoretical 7.6 D. For the Syn
structure the radii is a= 5.5 Å and equation (1) leads to ꢀꢁ
=
9.3 D, a value also larger than the theoretical 5.6 D. Similar
discrepancies are obtained for other structures shown in
Figures SF5 to SF7. This reveals that for complex structure like
Amide-6 equation (1) cannot be used, as the assumption of a
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the difference that in polar solvents ΦF and τF of Amide-7 are
lower and shorter than that of Amide-6. This is because in
polar solvents the non-radiative rate constant knr is about two
times higher for Amide-7 than for Amide-6 (Tables 2 and 3).
Such difference can be related to substitution of the second
coumarin at position 6 instead of position 7 what affects the
mutual interaction between two units of Amide-6, may change
the rate of the twist around the CCNC dihedral angle that leads
to the decrease of S2(CT) energy and opens a radiationless
relaxation channel. As for Amide-7, the influence of viscosity
on lifetime τF and fluorescence quantum yield ΦF in polar
solvents (Figure SF15) confirms the role of each molecule
units’ motion in the non-radiative process of deexcitation.
Solvent
η
/cP
ε
ΦF
τ
F/ns
k
r/ns-1
k
nr/ns-1
1,2-DCB
CHCl3
1.324
0.563
0.586
0.343
0.59
9.93
0.92
2.24
2.48
0.41
0.38
0.48
0.55
0.46
0.38
0.41
0.42
0.034
0.021
0.29
35.2
23.9
10.1
5.83
3.71
5.62
0.94
Toluene
ACN
2.379
38.8
0.62
1.29
0.015
0.019
0.036
0.066
0.102
0.028
0.041
0.095
0.160
0.242
Methanol
Ethanol
Propanol
Butanol
32.66
24.3
1.074
1.959
2.573
20.33
17.51
Table 3 Fluorescence quantum yield (
rate constant (kr)* and non-radiative rate constant (knr
with different viscosity and dielectric constant
Φ
F), fluorescence decay times (
τ
F), radiative
of Amide-6 in solvents
*
)
η
ε.
* kr = ΦF/τF and knr = 1/τF - kr
3.3.3 Ester-7
The absorption spectrum of Ester-7 has a strong band at 23200
cm-1 and three weak bands at 29500, 34000 and 38000 cm-1,
all showing a small degree of solvatochromism (Figure 8a).
Fluorescence spectra depend on the wavelength of excitation
and are comprised of one or two bands. Fluorescence
excitation spectra depend strongly on the wavelength of
observation: spectra observed at low energy fluorescence
band are similar to the absorption spectrum, spectra observed
at 400 nm (violet fluorescence band) reveal an absorbing state
around 29000 cm-1 (Figure SF16). This indicates the existence
of two conformers of Ester-7 in solution and confirms the
expectation that the ester linker between chromophores
provides a greater freedom of mutual orientation. The violet
band at 400 nm is stronger in polar solvents whereas the blue
band at 450-470 nm dominates in toluene and chloroform
(Figure 8b and Figure SF17). Solvatochromic shift of the low-
energy fluorescence band is substantially larger than the shift
of the high-energy band (Figure S19) showing that the change
of dipole moment in conformers is different.
Fig.
8
Normalized spectra of Ester-7 in selected solvents: (a) absorption, (b)
fluorescence recorded with excitation at 340 nm (29411 cm-1). The legend specifies
colours of the spectra.
A larger conformational freedom makes the photophysics of
Ester-7 more complex than that of Amide-6 or Amide-7
.
Nevertheless, the general trends of fluorescence quantum
yield and lifetimes are similar to other compounds studied. ΦF
is high in non-polar solvents and drops down significantly in
polar ones (Table 4). Moreover absorption and emission
maxima of Ester-7 are shifted bathochromically ca 25 nm
versus that of ethyl 7-aminocoumarin-3-carboxylates. Also the
fluorescence decay time is much shorter in polar than in non-
polar solvents, and both quantities ΦF and τF change to some
extent. The additional complication with Ester-7 is that the
fluorescence quantum yield obtained for excitation at 394 nm
(
Φ394F), within the first absorption band, is higher than Φ340
F
obtained for excitation at 340 nm, where both conformers
absorb (Table 4). This means that the radiationless processes
rate constant increases at high excitation energy. Another
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complexity in the photophysics of Ester-7 is that the decay bit less red-shifted with respect to the Anti form (0.84eV) and
times do not behave similarly to the quantum yields as in other is slightly less forbidden (f=0.029). In both conformers the ratio
molecules (Figure SF20). This reveals the variety of relaxation of the dipole moments of the fluorescing state and the ground
paths that compete in population and/or depopulation of state of the Syn conformer (µ1/µ0) is about 1.6-1.7, and is
conformers. Indeed there are four different forms of Ester-7
-
higher than the respective ratio for fluorescence of the Anti
Anti, AntiB, Clin and Syn and several pathways of mutual conformer (1.4-1.5). This may be responsible for the larger
conversions between them (Figures 5 and SF8). This is the solvatochromic shift observed for the blue-band fluorescence
expected consequence of bridging coumarins in Ester-7 by a as compared to the violet-band.
more flexible ester bond that provides a greater degree of
freedom for mutual motion of both subunits, allowing the There exists, however, another possible explanation for the
formation of different structures. However, even in this appearance of the blue-fluorescence of Ester-7
.
The
complex case, the solvent-driven photophysics observed for photophysical scheme of this molecule is qualitatively similar
other molecules is observed here too – the nonradiative rate to the CoumDimer system investigated in our previous
knr increases significantly in changing from non-polar to polar paper.22 Both molecular systems, due to a lack of hydrogen
solvents (Table 4). The phenomenon can be connected with bonding between the molecular linker and the coumarin units,
the change of excited state geometry and proves the strong favour the Syn conformation in the gas phase. It was shown
interaction of polarized subunits with themselves and solvent.
that in such case an explicit inclusion of a single solvent
molecule stabilizes the Clin conformation which may exist as a
As in the Amide-7 case, the Clin form of the Ester-7 system has local (emitting) minimum at the PES of the S1 state. A similar
no stable minimum in the S1 state and relaxes to the Syn situation was also observed for Ester-7 where inclusion of a
conformer in the course of geometry optimization. The vertical single solvent molecule (benzene) can block the Clin form in
fluorescence of this conformer, if any, is predicted to be red- the S1 state and make this form emissive. Moreover, the Clin
shifted by about 1eV with respect to fluorescence of the Anti conformation of the system represents a global minimum on
form (Figure 5) and very weak due to the forbidden nature of the ground-state PES of the system (Figure SF8 of the SI). For
this transition (f=0.003). A similar scheme is also predicted for technical reasons to make the ADC(2) computations tractable
the other photophysical branch related to the rotation of the it was necessary to truncate methyl groups present in Ester-7
ester group with respect to the DMA-coumarin unit (Figure SF7 and use a more compact basis set def-SV(P).
of the SI). In this case fluorescence of the Syn conformer is a
Table 4 Fluorescence quantum yield (ΦF), decay times (τF), radiative rate constant (kr) and non-radiative rate constant of Ester-7 in several solvents.*
a
b
Solvent
Φ394
0.87
Φ340
0.63
λ
/nmc
520
380
480
380
480
400
520
400
520
400
520
400
520
τ1
/
ns
τ2
/ ns
F
A1/A2
kr / ns-1
0.28
knr / ns-1
0.17
F
F
F
CHCl3
2.23
0.496
2.00
0.93
0.020
1.91
2.87
1.98
2.66
2.00
2.85
2.05
2.17
0.79
1.2219
0.30
Toluene
ACN
-
0.39
0.20
0.008
0.37
1.06
-
0.007
0.23
48.6
0.12
0.40
Methanol
Ethanol
Propanol
Butanol
0.27
0.12
0.10
0.16
0.054
0.057
0.082
0.127
1.0
0.37
0.075
0.10
5.09
13.4
0.018
2.08
0.48
0.036
0.055
0.064
15.46
0.47
0.028
1.27
10.92
0.46
0.031
1.25
6.62
*radiative rate constant (kr = ΦF/τF), non-radiative rate constant (knr = 1/τF - kr), fluorescence decay times τ1F , τ2F and amplitude ratio A1/A2 obtained from decay profiles
with deconvolution, a Φ394F - fluorescence quantum yield for excitation at 394 nm, b Φ340F - fluorescence quantum yield for excitation at 340 nm, c λ - the wavelength at
with fluorescence decay was recorded, decay profiles were obtained with excitation at 334 nm.
One can notice upon inspection of Figure SF8 that the spectral luminescence bands assigned to both conformers (Figure 8b).
shift of fluorescence bands between Anti and Clin conformers The overall absorption/fluorescence pattern of the truncated
of the truncated Ester-7 is only 0.33eV. A similar value, 0.34 eV Ester-7 system as a benzene complex is shown in Figure SF8 of
(2730 cm-1), is obtained from fluorescence spectra when the the SI. In general, three fluorescence bands are theoretically
shift is defined as the difference between maxima of predicted, however the fluorescence of the Syn conformer is
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essentially forbidden and may occur at the spectral region viscosity in the non-radiative process which we relate to the
beyond the detection. With the exception of the third twist around the CCNC dihedral angle of the amide bridge.
conformer and the third near IR fluorescence band, the However, the slopes of lines fitted to experimental points of
theoretically estimated photophysical behavior of Ester-7 is in Anti forms (1.06, 1.34 and 0.75) are higher than the theoretical
accord with the experimental observation shown in Figures 8, 2/3 of the viscosity-controlled process. This means than
SF16, and SF17. The calculated change of dipole moment for another mechanism also influences the radiationless process.
fluorescence is larger for the Clin than for the Anti structure Indeed, in the series of solvents used in Figure 9 (acetonitrile,
(Figure SF8). The solvatochromic shift of the blue fluorescence methanol, ethanol, propanol, butanol) two parameters
band assigned to the Clin structure is indeed larger than that of change: viscosity increases but dielectric constant decreases. A
the violet fluorescence band assigned to Anti conformer small ε decreases stabilization and therefore population of the
(Figures 8b and SF18). The comparison of IR absorption and S2(CT) state which leads to the conical intersection with the
Raman spectra in powdered samples with the spectra ground state. This effect contributes to further slowdown of
calculated for Anti, Clin and Syn forms (Figures SF27 and SF the non-radiative rate and manifests itself as an increase of the
29), excludes the Syn conformer and confirms the presence of exponent x.
Anti and Clin conformers. Thus we assign the fluorescence
bands to Anti and Clin structures.
Fig. 10 Normalized fluorescence (dashed lines) and fluorescence excitation (solid lines)
spectra of Ester-7 in powder. The legend specifies colours and wavelengths of
excitation of fluorescence spectra or wavelengths of observation of fluorescence
excitation spectra.
Fig.
9 Decimal logarithm of nonradiative rate constant Log(knr) versus decimal
logarithm of solvent’s viscosity for molecules studied: Amide-6 (NH-6), Amide-7 (NH-7)
and Ester-7 conformers (O-7 400) and (O-7 480). Results for polar solvents only are
includes (left to right: acetonitrile and methanol to butanol alcohols). Legend specifies
colors and symbols. The slopes of the lines are -1.06±0.07, -1.34±0.05, -0.75±0.22 and -
0.27±0.20 for Amide-6, Amide-7, Ester-7 (conformer 480 nm) and Ester-7 (conformer
400 nm) respectively.
In the solid state, rotation of Ester-7 units is inhibited,
fluorescence is strong and the fluorescence decay curves are
dozens of nanoseconds long. Decay profiles depend on the
wavelength of observation and are not exponential. Profiles
can be fitted to the three exponential decay curves with the
dominant 40-50 ns component (Table 5). These results prove
that the freezing of molecular rotations hinders internal
conversion. Fluorescence and fluorescence excitation spectra
of Ester-7 in powdered samples are significantly red-shifted
(Figure 10) which indicates strong intermolecular interactions
between packed molecules. Excitation spectra reveal the
existence of two sites or conformers. This is further confirmed
by multiexponential fluorescence decays. Let us note that
reabsorption may also influence the decay profiles making
them non-exponential and longer in the red part of the
fluorescence spectrum.
Figure 9 shows the non-radiative rate constant knr values
determined in polar solvents (acetonitrile and alcohols) for the
four conformers: Anti is observed in all molecules studied and
Clin assigned to the blue band of Ester-7 fluorescence. Log-Log
plots are used for molecular rotors, in which the rate of
radiationless deexcitation is viscosity controlled, and the plot
allows the determination of exponent x relating to the
fluorescence quantum yield ΦF with viscosity
η in the power
law formula ΦF = C×ηx, with C being a constant, and x ≈ 0.66
for viscosity-controlled process.48 The knr of the Ester-7 Clin
conformer is 10 times lower than for its Anti structure and
does not change much with solvent. This is distinctly different
from the behaviour in the Anti conformer and supports the
above discussed theoretical results for the Clin conformer with
a moderate dipole moment and structure stabilized by a
solvent molecule wedged within it. For Anti structures the knr
decreases with solvent viscosity what points to the role of
Table 5 Fluorescence decay times and amplitudes of Ester-7 in powder.*
λ
obs /nm
τ1F / ns
τ2F / ns
τ2F / ns
A1
A2
A3
460
0.619
7.8
39.2
23.2
36.2
40.6
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500
600
0.845
1.39
5.89
6.51
39.5
48.7
8.7
3.7
28.2
63.1
76.1
direct
comparison
of
Ester-7
with
ethyl
7-
20.10
diethylaminocoumarin-3-carboxylate or the corresponding
acid, shows that the presence of the second coumarin moiety
significantly modulates the excited states and as a result
emission is less dependent on solvent polarity and viscosity.12-
15,49
* decay times (τF) and amplitudes (A) obtained from fitting 3-exponential decay to
fluorescence decay profiles observed at wavelength λobs. Excitation at 394 nm.
3.5 Discussion
Conclusions
We relate the very complex photophysics of these three
molecules to interactions between the subunits that lead to
several conformers in the ground as well as electronically
excited state. In Amide-6 and Amide-7, where subunits are
linked with an amide bond, the mutual interactions of both
subunits are stronger than in the previously studied
CoumDimer derivative in which units were linked by a tertiary
amide bridge with steric hindrance.22 The difference between
Amide-6 and Amide-7 proves that the position of the bridge
matters and the position 6 of the “second” coumarin results in
a weaker interaction between subunits than position 7.
Amide-7 is more linear and enables better conditions for the
coumarin cores interaction in the planar Anti form. Both forms
have the S2(CT) state in close proximity to the S1(LE) state, thus
in polar solvents the state of the CT character is lowered and
can be populated. In that state the twist around the amide
bond is allowed and leads to conical intersection with the
ground state, enhancing internal conversion in polar solvents.
In Ester-7 the flexible ester bridge allows formation of two
conformers which is clearly seen in fluorescence and
fluorescence excitation spectra. Interestingly, the flexibility of
the ester linker does not increase the rate constant of internal
conversion in this molecule in comparison to Amide-6 and
Amide-7 as seen in Figure 9. This can be related to the CT-LE
energy gap which in Ester-7 (Table ST5) is larger than in
Amide-6 and Amide-7 (Tables ST3 and ST1) and only an
intramolecular geometry change (without solvent-induced
state interaction) increases the internal conversion. This
supposition is supported by smaller change of knr in solvents
(Table 4 and Figure 9).
Three dyes, comprising of two coumarin units linked in a D-A-
D-A fashion, have been synthesized and thoroughly studied.
The compounds have been studied in both solution and the
solid state to determine how the geometry and stiffness of the
linker influences the relevant photophysical processes
occurring upon photoexcitation. The key conclusion is that the
photophysical properties of these molecules strongly depend
on chemical nature of the linker and not only on the coumarin-
based building blocks. Mere replacement of a tertiary amide
linker with secondary amide has the power to alter geometries
(and hence energies) of various states, switching from weakly
fluorescent compound to strongly emissive one. The degree of
sensitivity of fluorescence vs. solvent polarity and viscosity can
be fine-tuned by altering the linker. The experimental and
theoretical results point to the conclusion that intramolecular
interactions tend to bend the polarized structure and they
significantly influence the geometry of the system. This
process is particularly effective in polar solvents where the
low-lying S(CT) state is lowered to, or below, the first S1(LE)
singlet state. In the CT state a spontaneous (barrierless)
rotation around the CCNC bond of amino bridge leads directly
to the conical intersection with the ground state and opens an
efficient channel of radiationless deactivation. This process is
hindered in the solid state, while in solution it can be slowed
down by an increase of solvent viscosity. In the molecule with
a flexible ester bridge the intramolecular interaction of both
subunits of the polar structure are effective enough to form
the more compact conformers of Ester-7 already in the ground
state. This indicates that in order to build extended oligomers
of aligned polarized molecules one should rigidify the bridges
between units of polar chains. We want to stress that linking
polarized π-units in D-A-D-A orientation leads to an
exceptional wealth of photophysical processes.
Although the fluorescence properties of 7-aminocoumarins
and related coumarins possessing other electron-donating
groups at the 7-position were investigated numerous times,
these studies were aimed at elucidating photophysics of dyes
lacking an additional aromatic unit. Our bis-coumarins Amide-
7, Amide-6 and Ester-7 differ significantly in that they enable
Conflicts of interest
the study of the influence of both the intramolecular hydrogen
bonding and electrostatic interactions on the preferred
conformation and hence, optical properties. In general the
There are no conflicts to declare.
fluorescence intensity of Amide-7, Amide-6 and Ester-7 is very
Acknowledgements
sensitive to the solvents’ polarity, which stands in analogy with
ethyl 7-diethylaminocoumarin-3-carboxylate and analogous
compounds.12-15 The solvatofluorochromism of Amide-7 and
Amide-6 is even stronger, however, in contrast to this
coumarin derivative, the fluorescence quantum yields are
below 0.02 in acetonitrile. Similarly to analogous coumarin
derivatives49 the increase in solvents viscosity improves the
fluorescence quantum yields of Amide-7 and Amide-6. The
This work was supported by the National Centre for Research
and Development (PBS3/A1/17/2015) and the National
Science
Centre
(Grants
and
2012/04/A/ST2/00100,
PRELUDIUM UMO-
2012/06/A/ST5/00216
2017/25/N/ST5/00227). We thank Global Research Laboratory
Program (2014K1A1A2064569) through the National Research
Foundation (NRF) funded by Ministry of Science, ICT & Future
12 | J. Name., 2012, 00, 1-3
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7
(a) S. Saha and A. Samanta, J. Phys. Chem. A, 2002, 106
4763-4771; (b) P. Nandhikonda, S. Paudel and M. D.
Heagy, Tetrahedron, 2009, 65 2173-2177; (c) S.
,
Planning (Korea). We also thank Dr. David Young for amending
the manuscript.
,
Banthia and A. Samanta, Chem. Lett., 2005, 34, 722-
723.
Notes and references
8
9
H. Meier, Angew. Chem. Int. Ed., 2005, 44, 2482-2506.
(a) X. Liu, J. M. Cole, P. G. Waddell, T. C. Lin, J. Radia
and A. Zeidler, J. Phys. Chem. A, 2012, 116, 727-737; (b)
M. S. Schiedel, C. A. Briehn and P. Bäuerle, Angew.
Chem. Int. Ed., 2001, 40, 4677-4680; (c) D. J. Yee, V.
1
(a) I. Huc, Eur. J. Org. Chem., 2004, 17-29; (b) D. W.
Zhang, X. Zhao, J. L. Hou and Z. T. Li, Chem. Rev., 2012,
112, 5271-5316; (c) P. N. Wyrembak and A. D.
Balsanek and D. Sames, J. Am. Chem. Soc., 2004, 126
,
Hamilton, J. Am. Chem. Soc., 2009, 131, 4566-4567; (d)
J. S. Nowick, D. M. Chung, K. Maitra, S. Maitra, K. D.
Stigers and Y. Sun, J. Am. Chem. Soc., 2000, 122, 7654-
7661; (e) E. A. Archer, A. E. Sochia and M. J. Krische,
2282-2283; (d) M. Tasior, I. Deperasińska, K.
Morawska, M. Banasiewicz, O. Vakuliuk, B.
Kozankiewicz and D. T. Gryko, Phys. Chem. Chem.
Phys., 2014, 16, 18268-18275; (e) G.-J. Zhao and K.-L.
Han, J. Phys. Chem. A, 2007, 111, 2469-2474; (f) A. E.
Lanterna, M. González-Béjar, M. Frenette and J. C.
Scaiano, Photoch. Photobio. Sci., 2017, 16, 1284-1289;
(g) A. Mukhopadhyay, T. Hossen, I. Ghosh, A. L. Koner,
W. M. Nau, K. Sahu and J. M. Moorthy, Chem. Eur. J.,
2017, 23, 14797-14805; (h) A. Mukhopadhyay, V. K.
Maka and J. N. Moorthy, Phys. Chem. Chem. Phys.,
2017, 19, 4758-4767; (i) B. Ventura, Y. M. Poronik, I.
Chem. Eur. J., 2001, 7, 2059-2065; (f) B. Xia, D. Bao, S.
Upadhyayula, G. Jones II and V. I. Vullev, J. Org. Chem.,
2013, 78, 1994-2004; (g) L. Sebaoun, V. Maurizot, T.
Granier, B. Kauffmann and I. Huc, J. Am. Chem. Soc.,
2014, 136, 2168-2174.
2
(a) D. Holten, D. F. Bocian and J. S. Lindsey, Acc. Chem.
Res., 2002, 35, 57-69; (b) X. Li, N. Markandeya, G.
Jonusauskas, N. D. McClenaghan, V. Maurizot, S. A.
Denisov and I. Huc, J. Am. Chem. Soc., 2016, 138
,
Deperasińska and D. T. Gryko, Chem. Eur. J., 2016, 22
15380-15388; (j) I. Kim, D. Kim, S. Sambasivan and K. H.
Ahn, Asian J. Org. Chem., 2012, , 60-64; (k) D. Kim, Q.
P. Xuan, H. Moon, Y. W. Jun and K. H. Ahn, Asian J. Org.
Chem., 2014, , 1089-1096; (l) Y. W. Jun, H. R. Kim, Y. J.
Reo, M. Dai and K. H. Ahn, Chem. Sci., 2017, , 7696-
7704; (m) B. H. Jhun, K. Ohkubo, S. Fukuzumi and Y.
You, J. Mater. Chem. C, 2016, , 4556-4567; (n) Y. Sun,
,
13568-13578; (c) U. Lewandowska, W. Zajączkowski,
W. Pisula, Y. Ma, C. Li, K. Müllen and H. Wennemers,
Chem. Eur. J., 2016, 22, 3804-3809; (d) X. Huang, B.
Borhan, B. H. Rickman, K. Nakanishi and N. Berova,
1
3
Chem. Eur. J., 2000, 6, 216-224; (e) B. Saha, S. A. Ikbal,
8
A. G. Petrovic, N. Berova and S. P. Rath, Inorg. Chem.,
2017, 56, 3849-3860; (f) K. Ohkubo, H. Kotani, J. Shao,
Z. Ou, K. M. Kadish, G. Li, R. K. Pandey, M. Fujitsuka, O.
Ito, H. Imahori and S. Fukuzumi, Angew. Chem. Int. Ed.,
2004, 43, 853-856.
4
T. Wu, F. Zhang, R. Zhang, M. Wu, Y. Wu, X. Liang, K.
Guo and J. Li, Dyes Pigm., 2018, 149, 73-81; (o) T.
Sheshashena Reddy and M.-S. Choi, J. Photoch.
Photobio. A, 2018, 351, 108-114.
3
(a) M. R. Wasielewski, Chem. Rev., 1992, 92, 435-461;
(b) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem.
Res., 2001, 34, 40-48; (c) A. Harriman and J.-P. Sauvage,
Chem. Soc. Rev., 1996, 25, 41-48; (d) D. Gust, T. A.
Moore, A. L. Moore, A. N. Macpherson, A. Lopez, J. M.
De-Graziano, I. Gouni, E. Bittersmann, G. R. Seely, F.
Gao, R. A. Nieman, X. C. Ma, L. Demanche, D. K.
Luttrell, S.-J. Lee and P. K. Perrigan, J. Am. Chem. Soc.,
1993, 115, 11141-11152; (e) H. Imahori, D. M. Guldi, K.
Tamaki, Y. Yoshida, C. Luo, Y. Sakata and S. Fukuzumi, J.
Am. Chem. Soc., 2001, 123, 6617-6628; (f) S. Fukuzumi,
K. Ohkubo, E. Wenbo, Z. Ou, J. Shao, K. M. Kadish, J. A.
Hutchison, K. P. Ghiggino, P. J. Sintic and M. J. Crossley,
J. Am. Chem. Soc., 2003, 125, 14984-14985; (g) I. V.
Rubtsov, N. P. Redmore, R. M. Hochstrasser and M. J.
Therien, J. Am. Chem. Soc., 2004, 126, 2684-2685; (h) J.
Kroon, H. Oevering, J. W. Verhoeven, J. M. Warman, A.
M. Oliver and M. N. Paddon-Row, J. Phys. Chem., 1993,
97, 5065-5069; (i) K. Okamoto and S. Fukuzumi, J. Am.
Chem. Soc., 2004, 126, 13922-13923; (j) K. Okamoto
and S. Fukuzumi, J. Phys. Chem. B, 2005, 109, 7713–
7723.
10 (a) Y. Niko, S. Sasaki, S. Kawauchi, K. Tokumaru and G.
Konishi, Chem. Asian J., 2014, , 1797-1807; (b) L.
9
Lorbach, A. Ji, R. M. Edkins and T. B. Marder, J. Org.
Chem., 2015, 80, 5658-5665; (c) E. M. Kosower, H.
Dodiuk, K. Tanizawa, M. Ottolenghi and N. Orbach, J.
Am. Chem. Soc., 1975, 97, 2167-2178; (d) A. Neubauer,
J. Bendig and W. Rettig, Chem. Phys., 2009, 358, 235-
244; (e) I. López-Duarte, P. Chairatana, Y. Wu, J. Pérez-
Moreno, P. M. Bennett, J. E. Reeve, I. Boczarow, W.
Kałuża, N. A. Hosny, S. D. Stranks, R. J. Nicholas, K.
Clays, M. K. Kuimova and H. L. Anderson, Org. Biomol.
Chem., 2015, 13, 3792-3802.
11 (a) R. Nazir, P. Danilevicius, A. I. Ciuciu, M.
Chatzinikolaidou, D. Gray, L. Flamigni, M. Farsari and D.
T. Gryko, Chem. Mat., 2014, 26, 3175-3184; (b) K.
Sivakumar, F. Xie, B. M. Cash, S. Lung, H. N. Barnhill and
Q. Wang, Org. Lett., 2004, 6, 4603-4606; (c) M. Tasior,
Y. M. Poronik, O. Vakuliuk, B. Sadowski, M. Karczewski
and D. T. Gryko, J. Org. Chem., 2014, 79, 8723-8732; (d)
M. Tasior, D. Kim, S. Singha, M. Krzeszewski, K. H. Ahn
and D. T. Gryko, J. Mat. Chem. C, 2015, 3, 1421-1446;
4
(a) A. Lohr, M. Grüne and F. Würthner, Chem. Eur. J.,
2009, 15, 3691-3705; (b) A. Lohr, S. Uemura and F.
Würthner, Angew. Chem. Int. Ed., 2009, 48, 6165-6168;
(e) M. S. Schiedel, C. A. Briehn and P. Bäuerle, Angew.
Chem. Int. Ed., 2001, 40, 4677-4680; (f) G. J. Kim, K.
Lee, H. Kwon and H. J. Kim, Org. Lett., 2011, 13, 2799-
2801; (g) A. O. Gerasov, M. P. Shandura and Y. P.
Kovtun, Dyes Pigm., 2008, 77, 598-607; (h) M. El-
Kemary and W. Rettig, Phys. Chem. Chem. Phys., 2003,
(c) A. Lohr and F. Würthner, Chem. Commun., 2008, 19,
2227-2229; (d) A. Zitzler-Kunkel, E. Kirchner, D. Bialas,
C. Simon and F. Würthner, Chem. Eur. J., 2015, 21
,
14851-14861.
(a) F. Würthner, Acc. Chem. Res., 2016, 49, 868-876;
(a) H. M. Kim and B. R. Cho, Acc. Chem. Res., 2009, 42
5
, 5221-5228; (i) N. Narayanaswamy, M. Kumar, S. Das,
R. Sharma, P. K. Samanta, S. K. Pati, S. K. Dhar, T. K.
Kundu and T. Govindaraju, Sci. Rep., 2014, , 6476; (j)
5
6
,
4
863-872; (b) S. Singha, D. Kim, B. Roy, S. Sambasivan, H.
Moon, A. S. Rao, J. Y. Kim, T. Joo, J. W. Park, Y. M. Rhee,
T. Wang, K. H. Kim, Y. H. Shin, J. Jung and K. H. Ahn,
M. A. Kirpichenok, V. M. Baukulev, L. A. Karandashova
and I. I. Grandberg, Khim Geterotsikl., 1991, 11, 1480-
1487; (k) S. L. Gilat, A. Andronov and J. M. J. Frechet,
Chem. Sci., 2015, 6, 4335-4342.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 13
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Page 14 of 15
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DOI: 10.1039/C8CP01084F
ARTICLE
Journal Name
Angew. Chem. Int. Ed., 1999, 38, 1422-1427; (l) B.
Bangar Raju and T. S. Varadarajan, J. Phys. Chem.,
1994, 98, 8903-8905.
Gryko and V. I. Vullev, J. Am. Chem. Soc., 2016, 138,
12826-12832.
27 P. A. Panchenko, Y. V. Fedorov, O. A. Fedorova, V. P.
Perevalov and G. Jonusauskas, Russ. Chem. Bull. Int.
Ed., 2009, 58, 1233-1240.
28 M. E. Due-Hansen, S. K. Pandey, E. Christiansen, R.
Andersen, S. V. F. Hansen and T. Ulven, Org. Biomol.
Chem., 2016, 14, 430-433.
29 Y. Wang, A. D. Hollingsworth, S. K. Yang, S. Patel, D. J.
Pine and M. Weck, J. Am. Chem. Soc., 2013, 135,
14064-14067.
12 (a) G. Jones II, W. R. Jackson, C. Y. Choi and W. R.
Bergmark, J. Phys. Chem., 1985, 89, 294-300; (b) G.
Jones II, W. R. Jackson and A. M. Halpern, Chem. Phys.
Lett., 1980, 72, 391-395; (c) G. Jones II, W. R. Jackson
and S. Konaktanaporn, Opt. Commun., 1980, 33, 315-
320.
13 (a) J. A. Van Gompel and G. B. Schuster, J. Phys. Chem.,
1989, 93, 1292-1295; (b) B. Bangar Raju and S. M. B.
Costa, Phys. Chem. Chem. Phys., 1999,
(c) M. Sulpizi, P. Carloni, J. Hutter and U. Rothlisberger,
Phys. Chem. Chem. Phys., 2003, , 4798-4805; (d) K.
Azuma, S. Suzuki, S. Uchiyama, T. Kajiro, T. Santa and
K. Imai, Photochem. Photobiol. Sci., 2003, , 443-449.
14 M. El-Kemary and W. Rettig, Phys. Chem. Chem. Phys.,
2003, , 5221-5228.
15 X. Liu, Z. Xu and J. M. Cole, J. Phys. Chem. C, 2013, 117
1
, 3539-3547;
30 K. F. Wong, J. R. Deng, X. Q. Wei, S. P. Shao, D. P. Xiang
and M. K. Wong, Org. Biomol. Chem., 2015, 13, 7408-
7411.
31 J. R. Dunetz, Y. Xiang, A. Baldwin and J. Ringling, Org.
Lett., 2011, 13, 5048-5051.
32 X. He, Z. Yan, X. Hu, Y. Zuo, C. Jiang, L. Jin and Y. Shang,
Synth. Commun., 2014, 44, 1507-1514.
33 L. C. C. Vieira, M. Weber Paixao and A. G. Correa,
Tetrahedron Lett., 2012, 53, 2715-2718.
5
2
5
,
16584-16595.
16 J. B. Grimm, B. P. English, J. Chen, J. P. Slaughter, Z.
Zhang, A. Revyakin, R. Patel, J. J. Macklin, D.
Normanno, R. H. Singer, T. Lionnet and L. D. Lavis, Nat.
Methods, 2015, 12, 244-250.
17 S. Singha, D. Kim, B. Roy, S. Sambasivan, H. Moon, A. S.
Rao, J. Y. Kim, T. Joo, J. W. Park, Y. M. Rhee, T. Wang, K.
H. Kim, Y. H. Shin, J. Jung and K. H. Ahn, Chem. Sci.,
34 C. Szijjarto, E. Pershagen, N. O. Ilchenko and K. Eszter
Borbas, Chem. Eur. J., 2013, 19, 3099-3109.
35 A. L. S. Thompson, G. W. Kabalka, M. R. Akula and J. W.
Huffman, Synthesis, 2005, 4, 547-550.
36 T. Ikawa, T. E. Barder, M. R. Biscoe and S. L. Buchwald,
J. Am. Chem. Soc., 2007, 129, 13001-13007.
37 J. Yin and S. L. Buchwald, Org. Lett., 2000, 2, 1101-
2015,
6
, 4335-4342.
1104.
38 J. Yin and S. L. Buchwald, J. Am. Chem. Soc., 2002, 124,
6043-6048.
39 W. Deng, C. Zhang, M. Liu, Y. Zou, L. Liu and Q.-X. Gou,
Chin. J. Chem., 2005, 23, 1241-1246.
40 E. M. Kwon, C. G. Kim, A. R. Goh, J. Park and J.-G. Jun,
Bull. Korean. Chem. Soc., 2012, 33, 1939-1944.
41 Y. Wang, H. Liu, P. Lu, R. Mao, X. Xue, C. Fan and J. She,
Chem. Biol. Drug. Des., 2015, 86, 637-647.
42 K. Huang, M. Liu, Z. Liu, D. Cao, J. Hou and W. Zeng,
Dyes Pigm., 2015, 118, 88-94.
18 D. Zhao, L. Yang, Y. Yuan, H. Wang, H. Dong, and S. Li, J.
Phys. Chem. B, 2017 121, 10064-10072.
19 (a) D. Bao, S. Upadhyayula, J. M. Larsen, B. Xia, B.
Georgieva, V. Nuñez, E. M. Espinoza, J. D. Hartman, M.
Wurch, A. Chang, Ch.-K. Lin, J. Larkin, K. Vasquez, G. J.
O. Beran and V. I. Vullev, J. Am. Chem. Soc., 2014, 136
,
12966-12973; (b) J. M. Larsen, E. M. Espinoza, J. D.
Hartman, Ch.-K. Lin, M. Wurch, P. Maheshwari, R. K.
Kaushal, M. J. Marsella, G. J. O. Beran and V. I. Vullev,
Pure Appl. Chem., 2015, 87, 779-792.
20 (a) O. Khakshoor, B. Demeler and J. S. Nowick, J. Am.
Chem. Soc., 2007, 129, 5558-5569; (b) J. Zhu, J.-B. Lin,
Y.-X. Xu, X.-B. Shao, X.-K. Jiang and Z.-T. Li, J. Am. Chem.
Soc., 2006, 128, 12307-12313; (c) B. Gong, Acc. Chem.
Res., 2012, 45, 2077-2087; (d) E. A. Archer and M. J.
Krische, J. Am. Chem. Soc., 2002, 124, 5074-5083.
21 (a) A. Lamouroux, L. Sebaoun, B. Wicher, B. Kauffmann,
Y. Ferrand, V. Maurizot and I. Huc, J. Am. Chem. Soc.,
2017, 139, 14668-14675; (b) Y. Ferrand and I. Huc, Acc.
Chem. Res., 2018, DOI: 10.1021/acs.accounts.8b00075;
(c) E. Yashima, N. Ousaka, D. Taura, K. Shimomura, T.
Ikai and K. Maeda, Chem. Rev., 2016, 116, 13752-
13990.
43 Y. Ma, Y. Yang, R. Lan and Y. Li, J. Phys. Chem. C, 2017,
121, 14779-14786.
44 A. L. Sobolewski, Phys. Chem. Chem. Phys., 2008, 10
1243-1247.
,
45 E. Lippert, Z. Naturforsch, A: Phys. Sci., 1955, 10, 541-
545.
46 N. Mataga, Y. Kaifu and M. Koizumi, Bull. Chem. Soc.
Jpn., 1955, 28, 690-691.
47 N. Mataga, Y. Kaifu and M. Koizumi, Bull. Chem. Soc.
Jpn., 1956, 29, 465-470.
48 M. A. Haidekker and E. A. Theodorakis, J. Biol. Eng.,
2010, 4, 11.
49 A. Chatterjee and D. Seth, Photochem. Photobiol.,
2013, 89, 280-293.
22 Ł. Kielesiński, O. Morawski, Ł. Dobrzycki, A. L.
Sobolewski and D. T. Gryko, Chem. Eur. J., 2017, 23,
9174-9184.
23 (a) X. Liu, J. M. Cole and Z. Xu, J. Phys. Chem. C, 2017,
121, 13274-13279; (b) W. Rettig and A. Klock, Can. J.
Chem., 1985, 63, 1649-1653; (c) E. Krystkowiak, K.
Dobek, G. Burdziński and A. Maciejewski, Photochem.
Photobiol. Sci., 2012, 11, 1322-1330; (d) E. Krystkowiak,
R. A. Bachorz and J. Koput, Dyes Pigm., 2015, 112, 335-
340.
24 M. Li and M. E. Johnsos, Tetrahedron Lett., 1994, 35
,
6255-6258.
25 J. Osby and O. B. Ganem, Tetrahedron Lett., 1985, 26
,
6413-6416.
26 (a) H.-Y. Lee and M. An, Bull. Kor. Chem. Soc., 2004, 25
,
1717-1719; (b) A. Purc, E. M. Espinoza, R. Nazir, J. J.
Romero, K. Skonieczny, A. Jeżewski, J. M. Larsen, D. T.
14 | J. Name., 2012, 00, 1-3
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