Planarized and Twisted Intramolecular Charge Transfer: A Concept for Fluorophores Showing Two Independent Rotations in Excited State

Article abstract of DOI:10.1002/chem.201700566

TICT (twisted intramolecular charge transfer) compounds are characterized by showing a rotation around a single bond in the excited state: starting from an almost planar geometry in the ground state, a twisted system is formed in the electronic excited state. The previously reported PLICT (planarized intramolecular charge transfer) compounds show inverse behavior: starting from a twisted geometry in the electronic ground state, a planarized system is formed in the excited state by rotation around a single bond. Here, a concept for planarized and twisted intramolecular charge transfer (PLATICT) states is presented which amalgamates both (TICT and PLICT) effects. Due to an intramolecular charge transfer, both a twisting around one single bond and a planarization around another one occurs. In sum, the PLATICT system shows two independent rotations around different axes in the excited state. By means of quantum chemical calculations (TD-cam-B3LYP and CC2) and experimental studies, it is demonstrated that N-aryl-substituted 1-aminoindoles are able to form photoinduced PLATICT states. In the fluorescence spectra of N-aryl-substituted 1-aminoindoles with a methoxycarbonyl or a cyano group as substituent in the aryl ring, very large Stokes shifts (ca. 18 000 cm^?1; >250 nm) are observed. The two independent rotations in the excited state, the very large Stokes shifts and their easy availability starting from indoline, make them very attractive for use as optical switches and motors in various fields of chemistry.

Full text of DOI:10.1002/chem.201700566

A Journal of
Accepted Article
Title: Planarized and Twisted Intramolecular Charge Transfer - A
Concept for Fluorophores Showing Two Independent Rotations in
Excited State
Authors: Gebhard Haberhauer
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10.1002/chem.201700566
Chemistry - A European Journal
Planarized and Twisted Intramolecular Charge Transfer – A Concept for
Fluorophores Showing Two Independent Rotations in Excited State
Gebhard Haberhauer†
† Institut für Organische Chemie, Universität Duisburg-Essen, Universitätsstr. 7, D-45117
Essen, Germany
Email address: gebhard.haberhauer@uni-due.de
ABSTRACT: TICT (twisted intramolecular charge transfer) compounds are characterized by
showing a rotation around a single bond in excited state: starting from an almost planar
geometry in ground state, a twisted system is formed in the electronically excited state. The
previous reported PLICT (planarized intramolecular charge transfer) compounds show
inverse behavior: starting from a twisted geometry in the electronically ground state, a
planarized system is formed in the excited state by rotation around a single bond. Here, we
present a concept of the planarized and twisted intramolecular charge transfer (PLATICT)
states which amalgamates both (TICT and PLICT) effects. Due to an intramolecular charge
transfer, both a twisting around one single bond and a planarization around another one
occurs. In sum, the PLATICT system shows two independent rotations around different axes
in the excited state. By means of quantum chemical calculations (TD-cam-B3LYP and CC2)
and experimental studies, we could demonstrate that N-aryl-substituted 1-aminoindoles are
able to form photoinduced PLATICT states. In the fluorescence spectra of N-aryl-substituted
1-aminoindoles with a methoxycarbonyl or a cyano group as substituent in the aryl ring, very
large Stokes shifts (ca. 18000 cm-1; >250 nm) are observed. The two independent rotations
in excited state, the very large Stokes shifts and their easy availability starting from indoline,
make them very attractive for the use as optical switches and motors in various fields of
chemistry.
Table of Contents graphic:
1
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Introduction
Fluorophores are used to detect, identify and characterize structures and functions of
inorganic and organic molecules as well as biological systems.[1],[2] In general they show a
small Stokes shift, which is defined as the gap between the maximum of the first absorption
band and the maximum of fluorescence.[1] Larges Stokes shifts are observed when an
intramolecular relaxation process in an electronic excited state leads to an energy minimum,
which is far away from the initial geometric parameters. Such a relaxation could be a rotation
around a single bond in the excited state. A very prominent example for an organic molecule
showing this type of rotation is 4-(dimethylamino)benzonitrile (DMABN, 1).[3],[4],[5] In non-polar
solvents, the fluorescence spectrum of DMABN exhibits only a single band from locally
excited (LE) state. The electronic excited molecule in the LE state shows differences only in
bond length and bond angles, but not in the relative orientation of parts of the molecule.
Therefore, the LE emission band exhibits only a small Stokes shift. In more polar solvents,
beside the "normal" band, an additional red-shifted band is observed. This "anomalous"
emission band can be interpreted by following model: The excitation by irradiation leads to
an intramolecular charge transfer from the donor group D to the acceptor moiety A (see
Figure 1a).[4],[5] The resulting diradical I* stabilizes itself by rotation of ca. 90° around the
single bond resulting in the twisted intramolecular charge transfer (TICT) state II*. The
driving force for this rotation around the single bond is the minimization of the Coulomb
interaction between the two unpaired electrons.[4] The geometric relaxation results in large
Stokes shifts of the TICT fluorescence.[6],[7],[8],[9] As the rotation leads to orbitally decoupled π
systems of the donor and acceptor units, the emission from TICT state to ground state is
overlap forbidden.[4],[5] A small TICT fluorescence quantum yield is therefore expected. The
observed TICT fluorescence occurs mainly from molecules in excited vibrational states.[4],[5]
a)
twisted ICT
(TICT)
b)
planarized ICT
(PLICT)
rotation
rotation
A
A
D
D
A
A
A
A
A
D'
D
D'
D'
D
D
I*
I
II*
II
I*
II*
II
h
'
h
'
h
h
rotation
rotation
A
D'
D
D
D
I
4
CH3
1
CH3
CH3
H
H
H
1
4
3
N
2
N
N
NC
NC
N
N
N
2
3
H
CH3
I
II
I
II
(C1-N2-N3-H4) -150°
~
~
~
(C1-N2-N3-H4) -60°
~
~
~
(C1-C2-N3-C4) 0°
(C1-C2-N3-C4) 90°
~
~
Figure 1. Schematic representation of the change in the geometrical arrangement of the
donor groups D and D’ as well as the acceptor group A upon going from the ground state to
a TICT (a) and PLICT state (b). As examples 4-(dimethylamino)benzonitrile (DMABN, 1) and
1-aminoindole (2) are shown.
2
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Recently, we introduced a second concept for a rotation of ca. 90° around a single bond in
an electronic excited state. As model compounds 1-aminoindole (2) and its derivatives (see
Figure 1b) were used.[10] Here, two donor groups (D and D') are connected by a single bond.
In the electronic ground state (I), the repulsive interaction between the electron pairs of the
donor groups D' and D results in a twisting of these groups. The electronic excitation leads to
the intramolecular charge transfer from the donor group D' to the acceptor moiety A. The
thus formed state I* does not represent a minimum on the potential energy surface and
immediately passes into the planarized intramolecular charge transfer (PLICT) state (II*) via
rotation around the N-N single bond.[10] This adiabatic relaxation process leads to a strong
Stokes shift which can also be recorded in non-polar solvents. Furthermore, in contrast to the
TICT compounds, the emission from the PLICT state to the ground state is allowed and
hence the quantum yields for the observed fluorescence are high.[10]
Both the TICT and the PLICT systems can be considered as ultrafast fluorescence
rotors.[11],[12],[13] However, as they exhibit only a 90° forward and back rotation, they are not
suitable for the design of an ultrafast molecular motor. Here, either a unidirectional 360°
rotation process around one axis or a combination of two independent rotations around
different axes is required.[14] The first case describes a molecular rotation motor,[15] the latter
case a molecular pushing motor.[16] In search of suitable rotations for the design of fast
molecular pushing motors[17] we got interested in systems showing two independent rotations
around different axes in the excited state. Here we describe a concept in which we merge the
principle of the TICT states and the PLICT states. The resulting system undergoes in the
excited state both a twisting around one single bond and a planarization around another
single bond. These two rotation processes should also result in very large Stokes shifts of
the fluorophores. We assumed that systems meeting the above combination (TICT & PLICT)
are substituted N-aryl-1H-indol-1-amines. In the present work the fluorescence of some N-
aryl-substituted 1-aminoindoles were investigated by means of quantum chemical
calculations (TD-B3LYP, TD-cam-B3LYP and CC2). Furthermore we synthesized some N-
aryl-substituted 1-aminoindoles, recorded their fluorescence spectra in different solvents and
compared the experimentally data with the calculated ones.
Results and Discussion
a) Concept
Our idea for the combination of both states (TICT and PLICT) is depicted in Figure 2a. From
the chemical point of view, such a system should have one donor group D coupled to the
acceptor group A and to the donor group D'. The latter is fused to the acceptor unit A'. In the
ground state (I), the donor group D and the acceptor group A are in the same plane (θ2(N1-
3
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N8-C9-C10) = 0°), whereas the donor group D' is perpendicular to D (θ1(C2-N1-N8-C9) =
90°). The concurrent presence of both structural elements (D-A for TICT state and D-D'-A' for
PLICT state) should lead to an amalgamation of the TICT and the PLICT process in the
electronic excited state. Let's assume that the excitation of one electron from donor D to the
acceptor A leads to the state I* in which the donor D exhibits a positive charge. The
stabilization of this state can occur in different ways. In the first case (path a), the TICT state
II* is formed by rotation around the single bond between the donor D+ and the acceptor A-
which corresponds to a twisting of the D+-A- subunit (θ2(N1-N8-C9-C10) = 90°).
Figure 2. Principle of the planarized and twisted intramolecular charge transfer (PLATICT)
states. a) Schematic representation of the change in the geometrical arrangement of the
donor groups D and D' as well as the acceptor group A upon going from the ground state (I)
to planarized and twisted intramolecular charge transfer (PLATICT) states (IV*). The direct
path b from I* to IV* corresponds to a concerted pathway. b) Substituted N-aryl-1H-indol-1-
amines are possible examples for PLATICT compounds.
In the second case (path c), the PLICT state III* is formed due to interaction between the
positively charged D+ and the donor group D'. The rotation around the D+-D subunit results in
4
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a planarization of this unit (θ1(C2-N1-N8-C9) = 0°). State II* as well as state III* can be further
stabilized: the TICT state (II*) by planarization of the subunit D+-D and the PLICT state (III*)
by twisting of the subunit D+-A-. In both cases the result is the formation of the planarized
and twisted intramolecular charge transfer (PLATICT) state IV* (θ1(C2-N1-N8-C9) = 0° and
θ2(N1-N8-C9-C10) = 90°). Furthermore, a direct path (b) from I* to IV* corresponding to a
concerted pathway is also possible (Figure 2).
The deexcitation of the PLATICT state (IV*) can occur either by radiative or non-radiative
transition. Due to the twofold rotations in the excited state, the resulting fluorescence should
feature a very large Stokes shift. The then formed state IV is labile and converts through two
independent rotations (via path d, e or f in Figure 2) into the starting state I.
b) Theoretical Investigation of the PLATICT States
We assumed that N-aryl-substituted 1-aminoindoles represent systems, which should be
able to form a planarized and twisted intramolecular charge transfer (PLATICT) state (Figure
2b). Here, two planar systems are linked via their donor units. In the ground state, the indole
unit (donor group D' and acceptor group A') should be perpendicular to the aniline system
(donor group D and acceptor group A). The whole system possesses both a TICT unit, which
is represented by the aniline, and a PLICT system, which is represented by the 1-aminoindol.
The nitrogen atom N8 belongs to both systems (TICT and PLICT) and is therefore the key
element for the whole PLATICT system. In order to verify the concept, the N-aryl-substituted
1-aminoindoles 3-6 (Figure 3) unsubstituted or having strongly electron withdrawing groups
in the para position to the nitrogen atom N8 were investigated by means of quantum
chemical methods. The geometry optimization of the electronic ground states S0 of 3-6 were
performed using Becke's three-parameter hybrid functional (B3LYP[18]). The geometrical
parameters of the excited states S1 were calculated by means of the time-dependent density
functional theory (TD-DFT[19]). It is known that TD-B3LYP tends to underestimate charge-
transfer excitation energies[20] due to spurious self-interaction.[21] However, the use of TD-
B3LYP yields good agreement for the vertical excitation energies of the first excited singlet
states of TICT and PLICT compounds such as 4-(dimethylamino)benzonitrile[22],[23]
,
dialkylaminopyridines[24] and 1-aminoindoles[10]. Furthermore, the electronic ground states S0
and the excited states S1 were geometrically optimized by means of cam-B3LYP[25]. This
hybrid exchange-correlation functional using the Coulomb-attenuating method performs well
for charge transfer excitations in systems, which B3LYP underestimates enormously.[25] As
basis set, the def2-TZVP[26] was used for both functionals. Subsequent frequency
calculations were carried out in the ground states and the electronic excited states to verify
the nature of the stationary points. All optimized states have no imaginary frequencies.
Furthermore, the electronic excitation energies of the B3LYP- and cam-B3LYP-optimized
5
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structures were computed at the approximate second-order coupled cluster (CC2[27],[28]
levels. Here again the def2-TZVP basis set was used.
)
A comparison of the optimized ground state structures reveals that the structural parameters
of all 1-aminoindoles 3-6 are very similar independent on the used functional (B3LYP or cam-
B3LYP). Selected structural parameters and the molecular structures of the ground state of 3
and 6 obtained by using cam-B3LYP/def2-TZVP are depicted in Figures 3 and 4. In both
molecules the most important requirement for the ground state of compounds, which are able
to be transformed into PLATICT states, is fulfilled: The nitrogen atom N8 shows sp2
hybridisation, its lone pair interacts with the phenyl ring and it is perpendicular orientated to
the π system of the indole unit. Accordingly, the dihedral angle θ2(N1-N8-C9-C10) amounts to
values close to 0° (-20° for 3 and -16° for 6), whereas the dihedral angle θ1(C2-N1-N8-C9) is
close to 90° (109° for 3 and 106° for 6).
Figure 3. a) Structures of the 1-aminoindoles 3-6. b) Structural parameters of the lowest
singlet electronic states (ground, PLICT and PLATICT states) of the 1-aminoindoles 3 and 6
calculated using (TD)-cam-B3LYP.
6
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Using the MO approximation, the transition from the ground state of 3 to the first exited single
state (vertical excitation) is dominated by the (HOMO - 1) → LUMO excitation according to
CC2 results. Both orbitals are π and π* orbitals and located at the indole system. In the case
of the 1-aminoindole 6 the first exited single state (vertical excitation) corresponds to an
excitation from (HOMO - 2) → (LUMO + 1), whereby both orbitals are localized at the
aminobenzonitrile unit.
In order to optimize the geometric parameters of the excited state S1 by means of cam-
B3LYP/def2-TZVP, the five energetically lowest excited singlet states were solved in each
step and the lowest of them were used for optimization. As starting geometries structures
showing different dihedral angles θ1 and θ2 were used. In the case of the aminoindoles 3 and
4 two [III* (PLICT) and IV* (PLATICT)] of the possible four (I*-IV*) states were identified as
being minima on the energetic potential surface. For the aminoindoles 5 and 6 only the
PLATICT state (IV*) shows an energetic minimum. Please note, that there are in each case
two diastereomers of the PLICT and PLATICT states. Here, only the anti isomers in which
the acceptor group A is orientated away from the phenyl ring of the indole unit are
considered. The syn isomers show almost the same transition energies, Stokes shifts and
oscillator strengths and are slightly energetically disfavored (Figure S1 and Table S1). In all
cases, the TICT states of the aminoindoles 3-6 were no minima on the energetic potential
surface.
a)
b)
Figure 4. a) Molecular structure of the ground (left), PLICT (middle) and PLATICT (right)
states of the 1-aminoindole 3 calculated using (TD)-cam-B3LYP. b) Molecular structure of
the ground (left) and PLATICT (right) states of the 1-aminoindole 6 calculated using (TD)-
cam-B3LYP.
7
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The PLICT state of 3 and 4 can be considered as an intramolecular charge transfer from the
pyrrole ring to the benzene ring of the indole unit[29], whereby the thus generated positive
charge in the pyrrole ring is stabilized by the lone pair of the nitrogen atom N8, which is part
of the aniline system (see Figures 3 and 4). The display of the electron density difference
between excited state and ground state at the PLICT geometry corroborated this assumption
(Figure S3). This π → π* transition results in an increase of the bond lengths C4-C5, C6-C7,
N1-C7a and N1-C2 as well as a decrease of the bond lengths C5-C6 and C3-C3a in the
indole ring. The bond lengths in the phenyl ring do not change significantly (Figure 3). The
stabilization of the positive charge in the pyrrole ring by the lone pair of the nitrogen atom N8
causes a reduction of the N1-N8 distance. In the ground state the N1-N8 bond length
amounts to 1.374 Å, whereas in the PLICT state the N-N bond has a value of only 1.325 Å.
The PLICT state of 3 exhibits a dihedral angle θ1 of 42°, which means that starting from the
ground state a rotation of 67° has occurred (Figure 4). This intramolecular charge transfer
can also be confirmed by the increase of the dipole moment. There is a significant change
between the permanent dipole moments of the ground state of 3 (1.32 D) and the optimized
PLICT state of 3 (6.71 D) (Table 1). The fact, that no PLICT states of 5 and 6 were found,
can be easily explained as following: In the PLICT states the lone pair of the nitrogen atom
N8 acts as electron donating unit. However, the methoxycarbonyl and cyano groups of 5 and
6 in para position also withdraw electrons from the nitrogen atom N8 causing a drastic
reduction of its electron donating character.
The PLATICT states of the 1-aminoindoles can be considered as electron transfer from the
lone pair of the nitrogen atom N8 to the phenyl group. The resulted singlet diradical is
stabilized by two processes. The first one is a rotation around the dihedral angle θ2(N1-N8-
C9-C10) in order to minimize the Coulomb interaction between the two unpaired electrons.
The second process is a rotation around the dihedral angle θ1(C2-N1-N8-C9) to stabilize the
positive charge at the nitrogen atom N8 by conjugation with the electron-rich π system of the
indole unit. The dihedral angles θ1 and θ2 of the PLATICT compounds amounts to 0° and
100°, respectively, (see Figures 3 and 4). Starting from the ground states So, this
corresponds to two independent rotations by more than 100° in each case around two
different axes.
The intramolecular charge transfer process is manifested by changes in the bond length. The
occupation of a π* orbital of the phenyl ring causes an increase in the distances C9-C10 and
C11-C12 as well as a decrease in the distance C10-C11. With the exception of N1-C7a, the
changes in the bond lengths of the indole ring are small. The stabilization of the positive
charge of the nitrogen atom N8 by the electron-rich π system of the indole unit leads to an
reduction of the N1-N8 bond distance by ca. 0.05 Å.
8
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Table 1. Comparison between CC2 calculated and experimentally determined spectral properties of the aminoindoles 3-6. The vertical singlet
excitation (S0 → S1) and emission (S0 ← S1) energies ∆E [cm-1], Stokes shifts [cm-1], oscillator strengths (f) and permanent dipole moments
[D] of
µ
each state were computed by means of CC2/def2-TZVP//(TD)-cam-B3LYP/def2-TZVP. The absorption and fluorescence maxima as well as the
quantum yields (Φ) were determined in different solvents.
CC2 calculation
State
Experimental
Solvent
Transition
energy [cm-1]
Stokes shift
[cm-1]
f
Absorption Emission Stokes shift
µ
[D]
Φ
[cm-1]
[cm-1]
[cm-1]
3
4
S0
PLICT
PLATICT
1.32
6.71
8.48
39100
28000
21400
0.045
0.107
0.0000003
n-hexane
1-chlorobutane
dioxane
37200
36900
36800
36000
27200
26200
25800
24900
10000
10700
10900
11100
0.13
0.20
0.30
0.27
11100
17700
acetonitrile
S0
1.78
5.03
9.96
37600
28100
21800
0.042
0.095
0.000023
n-hexane
1-chlorobutane
dioxane
37500
37000
36800
35600
27400
26200
25700
24800
10100
10800
11100
10800
0.10
0.13
0.15
0.11
PLICT
PLATICT
9500
15800
acetonitrile
5
6
S0
1.25
14.07
38800
20100
0.015
0.000012
n-hexane
1-chlorobutane
dioxane
37300
36900
36500
22500
19200
18600
14800
17700
17900
0.02
0.0003
0.0001
PLATICT
18700
19000
S0
5.26
16.63
38300
19300
0.022
0.000009
n-hexane
1-chlorobutane
dioxane
38000
37600
37300
22300
19400
19200
15700
18200
18100
0.02
0.0006
0.0004
PLATICT
9
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For the PLATICT states, the change in the permanent dipole moments is even more
pronounced than for the PLICT states: The permanent dipole moment of the PLICT state of 3
is 6.7 D; for the PLATICT state a value of 8.5 D is calculated. In the case of the PLATICT
state of 6, the permanent dipole moment has a value of even 16.6 D (Table 1). It is also
interesting to compare the relative energies of the PLICT and PLATICT states of the 1-
aminoindoles 3 and 4 (Table S2). If the B3LYP-optimized structures are used for the CC2
calculations, the PLATICT states are energetically preferred by ca. 6 kcal/mol. However, if
the cam-B3LYP-optimized structures are taken, the PLATICT states of 3 and 4 are slightly
energetically disfavored by 1-2 kcal/mol (Table S2). Hence, according to the CC2
calculations, both states can be formed for the aminoindoles 3 and 4.
In Figure 5, the dominant natural transition orbital (NTO) pairs[30] of the optimized PLICT
state of 3 and of the PLATICT states of 3 and 6 calculated by means of cam-B3LYP are
shown. The depicted orbitals can be considered as those orbitals which are occupied by the
two unpaired electrons of the singlet diradical. The transition from S1 to S0 corresponds to a
transition of an electron from the virtual to the occupied natural transition orbital. As the
emissions are dominated by LUMO → HOMO transitions, the dominant natural transition
orbital pairs resemble the corresponding HOMOs and LUMOs (Figure S2).
In Table 1 the vertical emission (S0 ← S1) energies, the Stokes shifts and the oscillator
strengths of each state calculated by means of CC2/def2-TZVP//TD-cam-B3LYP/def2-TZVP
are listed. Furthermore, in Table S2 the Stokes shifts computed using different
approximations are given. A comparison shows that the TD-B3LYP values of 3 and 4
strongly differ from the values obtained by cam-TD-B3LYP and CC2. This is properly due to
the fact that TD-B3LYP overestimate the stabilization of charge transfer excitation states.[25]
The data from the TD-cam-B3LYP calculations agree well with those from the CC2//TD-cam-
B3LYP approximation. The deviation of the CC2 values calculated on different geometries
(TD-B3LYP and TD-cam-B3LYP) is small; this means that also the optimization using TD-
B3LYP with subsequent CC2 single point calculation leads to reliable results for the Stokes
shifts (Table S2).
The calculated (TD-cam-B3LYP and CC2) Stokes shifts of the PLICT states range between
9000 and 11000 cm-1. Having oscillator strengths of about 0.1, the PLICT states can be
regarded as electronically allowed transitions (Table 1). This is also evident, when the
corresponding natural transition orbitals of 3 are considered (Figure 5a). As both orbitals are
located in the same plane, the π systems are coupled and the resulting fluorescence is
electronically allowed.
Due to the independent rotations around two different axes, the calculated (CC2) Stokes
shifts of the PLATICT states are very large (14000 to 19000 cm-1; see Tables 1 and S2). As
the rotation around the dihedral angle θ2(N1-N8-C9-C10) leads to orbitally decoupled π
10
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systems, whereby one electron is located in the 1-aminoindole system (occupied NTO) and
the other one in the phenyl ring (virtual NTO) being perpendicular to the 1-aminoindole
system (Figures 5 and S3). The orbitally decoupled π systems lead to the fact that the
emission to the ground state is electric dipole forbidden resulting in very low fluorescence
quantum yields.
a)
b)
c)
Figure 5. Occupied (lower panel) and virtual (upper panel) natural transition orbitals of the
optimized PLICT state of 3 (a) and of the PLATICT states of 3 (b) and 6 (c) calculated by
means of cam-B3LYP.
c) Experimental Investigation of the PLATICT States
To examine the above concept and the corresponding quantum chemical calculations, we
synthesized the 1-aminoindoles 3-6 and investigated their spectroscopic properties. The
synthesis of 3-6 were conducted by the treatment of indoline (7) with the corresponding
nitroso compound 8-11 in the presence of benzoic acid in toluene as solvent (Scheme 1).
The 1-aminoindoles 3-6 could be isolated in yields of 46-70%. Prior to the fluorescence
measurements, the 1-aminoindoles 3-6 were crystallized twice from a mixture of n-hexane
and tetrahydrofuran. The fluorescence spectra were recorded in the solvents n-hexane, 1-
chlorobutane, dioxane and acetonitrile (Figures 6 and S5-S7). The experimentally
determined spectral properties of the aminoindoles 3-6 are also listed in Table 1.
11
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Scheme 1. Synthesis of the aminoindoles 3-6 (46-70% yield) starting from indoline (7) and
the nitroso compounds 8-11.
a)
b)
Figure 6. a) Absorption and fluorescence spectra of 1-aminoindoles 3 (red) and 4 (blue) in n-
hexane. b) Absorption and fluorescence spectra of 1-aminoindoles 5 (blue) and 6 (red) in n-
hexane. The excitation wavelength is 280 nm.
If the experimentally determined Stokes shifts and the fluorescence quantum yields are
considered, it can be noticed two different types (Table 1, Figures 6 and S5-S7). The 1-
aminoindoles 3 and 4 belongs to the first type showing Stokes shifts in the range from 10000
to 11000 cm-1 (100-120 nm) and fluorescence quantum yields of about 0.1 to 0.3. Let's
consider as comparison the fluorophores indole and 1-aminoindole. In the case of indole, no
rotation relaxation can occur in the excited state and accordingly the Stokes shift has only a
value of about 5000 cm-1 in cyclohexane.[31] 1-Aminoindole (2) is a pure PLICT compound
and exhibits a Stokes shift of 9300 cm-1 in cyclohexane.[10] Hence, the recorded Stokes shifts
of 3 and 4 in the range from 10000 to 11000 cm-1 corresponds to the transition from the
PLICT state to the ground state. A comparison of the calculated (CC2//TD-cam-B3LYP) and
12
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10.1002/chem.201700566
Chemistry - A European Journal
experimentally determined Stokes Shift shows a good match between calculation and
experiment (Tables 1 and S2). As predicted by the CC2//TD-cam-B3LYP computations, only
the 1-aminoindoles 3 and 4 have PLICT states as minima on the energy surface and these
PLICT states are energetically more stable than the PLATICT states. The emission from the
PLICT states is allowed and the bands are red-shifted with increasing polarity of the solvents
(Table 1). This underpins the higher dipole moment of the excited state. The fluorescence
quantum yields for the PLICT states are only marginally influenced by the polarity of the used
solvents.
The second type of emission is found for the 1-aminoindoles 5 and 6. The bands show very
large Stokes shifts ranging from 15000 to 18000 cm-1 (180-260 nm) as well as fluorescence
quantum yields being lower than 0.02. The quantum yields strongly depend on the polarity of
the used solvents. The higher the polarity of the solvent is, the lower are the quantum yields.
In acetonitrile as solvent, fluorescence cannot be observed neither for 5 nor for 6. The low
fluorescence quantum yields and their dependence on the solvent indicate that the
corresponding transitions are forbidden ones.[23] As comparison let us consider the
fluorescence spectra of the TICT compound DMABN (1). In n-hexane as solvent, only the
"normal" band showing a Stokes shift of 6200 cm-1 can be recorded (Figure S3). In the
medium polar, aprotic solvent 1-chlorobutane an emission band from the TICT state can be
observed as a shoulder of the "normal" band (Figure S3).[5,4] The Stokes shift of this TICT
band amounts to about 9800 cm-1. Opposite to this, the emission bands of 5 and 6 show
even in n-hexane very large Stokes shifts (ca. 15000 cm-1) and in 1-chlorobutane their values
are higher by ca. 8000 cm-1 than that measured for DMABN. This allows the conclusion that
the recorded emission bands of 5 and 6 stem from PLATICT states, and not from PLICT or
TICT states. As expected for the highly polar PLATICT states, the magnitude of the Stokes
shifts and fluorescence quantum yields strongly depends on the polarity of the solvent.
A comparison of the calculated (CC2//TD-cam-B3LYP) und experimentally recorded Stokes
Shift of the PLATICT states shows a fairly good match between calculation and experiment
(Table 1), whereby the calculated values are slightly higher than the experimental observed
ones.
Conclusion
In sum, we could show by means of quantum chemical calculations (TD-cam-B3LYP and
CC2) that N-aryl-substituted 1-aminoindoles can form photoinduced planarized and twisted
intramolecular charge transfer (PLATICT) states. These states are an amalgamation of the
well-known TICT and the previous reported PLICT states. The investigated N-aryl-substituted
1-aminoindoles show both a TICT and a PLICT unit, whereby one nitrogen atom belongs to
both systems. The intramolecular charge transfer causes both a twisting around one single
13
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10.1002/chem.201700566
Chemistry - A European Journal
bond and a planarization around another one. In sum, the PLATICT system shows two
independent rotations around different axes in the excited state. The experimentally
determined absorption and fluorescence spectra of the N-aryl-substituted 1-aminoindoles
confirm the quantum chemical predictions: N-aryl-substituted 1-aminoindoles with a
methoxycarbonyl or a cyano group as substituent in the aryl ring show emission bands which
stem from the PLATICT states. Due to two rotations in the excited state, these bands have
very large Stokes shifts (>250 nm; >18000 cm-1). As N-aryl-substituted 1-aminoindoles can
be easily synthesized from indoline, they are attractive candidates both for optical switches
and for molecular machines.
Experimental Section
Computational Details
All calculations were performed by using the program packages TURBOMOLE[32] and
Gaussian 09[33]. The geometrical parameters of the electronically ground states were
optimized by means of B3LYP[18] and cam-B3LYP[25]. The optimized geometries of the
excited states were obtained on the basis of time-dependent density functional theory (TD-
DFT)[19] with the functionals B3LYP and cam-B3LYP. The def2-TZVP[26] basis set was
employed for both functionals. To optimize the geometric parameters of the excited states S1
by means of cam-B3LYP/def2-TZVP, the five energetically lowest excited singlet states were
solved in each step and the lowest of them were used for optimization. In order to locate the
minima of the excited states S1, starting geometries structures with different dihedral angles
θ1 and θ2 were used. For all stationary points no symmetry restriction was applied. Frequency
calculations were carried out at each of the structures to verify the nature of the stationary
point. All optimized states have none imaginary frequency. Furthermore, the energies, the
electronic excitation energies, the permanent dipole moments and the oscillator strengths for
the transitions were computed at the approximate second-order coupled cluster (CC2[27],[28]
)
levels on the B3LYP- and cam-B3LYP-optimized structures. Here again def2-TZVP[26] was
used.
General remarks: All chemicals were reagent grade and were used as purchased.
Reactions were monitored by TLC analysis with silica gel 60 F254 thin-layer plates. Flash
1
13
chromatography was carried out on silica gel 60 (230-400 mesh). H and C NMR spectra
were recorded on a 600 MHz spectrometer. All chemical shifts ( ) are given in ppm. The
δ
spectra were referenced to the peak for the protium impurity in the deuterated solvents
indicated in brackets in the analytical data. HR-MS spectra were recorded using a time of
flight (TOF) detector. The 1-aminoindoles were synthesized starting from indoline (7) and the
nitrosobenzene derivatives 8-11 in analogy to a known procedure.[34] The nitrosobenzene
derivatives 8-11 were either purchased or prepared from the corresponding anilines.[35] Prior
14
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10.1002/chem.201700566
Chemistry - A European Journal
to the fluorescence measurements, the 1-aminoindoles 3-6 were crystallized twice from a
mixture of n-hexane and tetrahydrofuran.
General procedure: To a mixture of indoline (298 mg, 2.5 mmol) and benzoic acid (92 mg,
0.75 mmol) in dry toluene (5 mL), the nitrosobenzene derivative (2.5 mmol) in 4 mL dry
toluene was added dropwise over 1 hour at 110 °C. After 1 h stirring at 110 °C, the reaction
mixture was cooled to room temperature. The product was isolated by column
chromatography over silica gel (n-hexane/ethyl acetate) to yield the 1-aminoindole as an off-
white solid. The 1-aminoindole was further purified by crystallization from n-hexane/
tetrahydrofuran to give a white solid.
1
3
Aminoindole 3. Yield: 70%; M.p. 91-92 °C; H NMR (600 MHz, CDCl3):
δ = 7.65 (d, JH,H =
7.6 Hz, 1 H, Har), 7.29 (d, 3JH,H = 8.2 Hz, 1 H, Har), 7.22-7.14 (m, 5 H, Har), 6.91 (t, 3JH,H = 7.4
Hz, 1 H, Har), 6.64 (s, 1 H, NH), 6.55 (d, 3JH,H = 3.3 Hz, 1 H, Har), 6.53 (d, 3JH,H = 7.7 Hz, 2 H,
13
Har) ppm; C NMR (151 MHz, CDCl3):
δ = 147.32, 135.82, 129.38, 128.63, 126.60, 122.43,
121.20, 121.08, 120.35, 112.69, 109.40, 100.85 ppm; IR (ATR): 3311, 3129, 3100, 3049,
3030, 1600, 1510, 1494, 1473, 1457, 1386, 1365, 1317, 1291, 1238, 1221, 1179, 1158,
1128, 1089, 1045, 1026, 1007, 929, 885, 870, 847, 801, 763, 743, 735, 688 cm-1; HRMS
(ESI+): m/z calcd for C14H12N2+H+: 209.1079; found: 209.1081.
Aminoindole 4. Yield: 62%; M.p. 115-116 °C; 1H NMR (600 MHz, CDCl3):
δ =
= 7.65 (d, 3JH,H
7.7 Hz, 1 H, Har), 7.26 (d, 3JH,H = 7.4 Hz, 1 H, Har), 7.21-7.18 (m, 2 H, Har), 7.15 (dt, 3JH,H = 7.5
Hz, 4JH,H = 1.1 Hz, 1 H, Har), 6.92-6.88 (m, 2 H, Har), 6.58 (s, 1 H, NH), 6.54 (d, 3JH,H = 3.3 Hz,
1 H, Har), 6.49-6.46 (m, 2 H, Har) ppm; 13C NMR (151 MHz, CDCl3):
δ
= 158.00 (1JC,F = 239.0
Hz), 143.60, 135.83, 128.63, 126.82, 122.66, 121.33, 120.60, 116.13 (2JC,F = 22.9 Hz),
114.11 (3JC,F = 7.8 Hz), 109.46, 101.12 ppm; IR (ATR): 3331, 3137, 3056, 1867, 1605, 1503,
1473, 1455, 1391, 1367, 1328, 1316, 1298, 1245, 1224, 1208, 1155, 1126, 1102, 1090,
1045, 1006, 927, 881, 866, 823, 764, 748, 729 cm-1; HRMS (ESI+): m/z calcd for
C14H11FN2+H+: 227.0985; found: 227.0975.
Aminoindole 5. Yield: 57%; M.p. 149-150 °C; 1H NMR (600 MHz, CDCl3):
δ = 7.90-7.88 (m,
2 H, Har), 7.66 (d, 3JH,H = 7.3 Hz, 1 H, Har), 7.24 (d, 3JH,H = 8.2 Hz, 1 H, Har), 7.21-7.15 (m, 3
H, Har), 6.90 (s, 1 H, NH), 6.58 (d, 3JH,H = 3.3 Hz, 1 H, Har), 6.52-6.50 (m, 2 H, Har), 3.86 (s, 3
H, OCH3) ppm; 13C NMR (151 MHz, CDCl3):
δ = 166.73, 151.09, 135.58, 131.53, 128.32,
126.65, 122.75, 121.26, 120.71, 111.67, 109.18, 101.49, 51.81 ppm; IR (ATR): 3315, 3101,
3058, 2950, 1684, 1602, 1515, 1475, 1460, 1432, 1389, 1307, 1280, 1261, 1216, 1194,
1173, 1114, 1090, 1049, 1007, 967, 937, 881, 846, 798, 764, 740, 728, 698, 662 cm-1;
HRMS (ESI+): m/z calcd for C16H14N2O2+H+: 267.1134; found: 267.1133.
Aminoindole 6. Yield: 46%; M.p. 171-172 °C; 1H NMR (600 MHz, CDCl3):
δ
= 7.66 (dt, 3JH,H
4
= 7.4 Hz, JH,H = 1.2 Hz, 1 H, Har), 7.48-7.46 (m, 2 H, Har), 7.22-7.16 (m, 3 H, Har), 7.15 (d,
3JH,H = 3.4 Hz, 1 H, Har), 6.98 (s, 1 H, NH), 6.59 (d, 3JH,H = 3.4 Hz, 1 H, Har), 6.53-6.50 (m, 2
15
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10.1002/chem.201700566
Chemistry - A European Journal
H, Har) ppm; 13C NMR (151 MHz, CDCl3):
δ
= 150.69, 135.36, 133.89, 128.02, 126.68,
122.96, 121.40, 120.94, 119.28, 112.35, 108.99, 103.68, 101.93 ppm; IR (ATR): 3323, 3132,
3105, 3057, 2215, 1604, 1518, 1510, 1472, 1456, 1417, 1394, 1370, 1324, 1304, 1261,
1222, 1173, 1130, 1090, 1051, 1007, 967, 957, 932, 882, 849, 825, 762, 747 717 cm-1;
HRMS (ESI+): m/z calcd for C15H11N3+H+: 234.1031; found: 234.1032.
Supporting Information
NMR spectra of 3-6, calculated properties, absorption and fluorescence spectra, cartesian
coordinates and absolute energies for all calculated compounds.
Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft (DFG; HA 2973/9-1).
The author would like to thank Helma Kallweit, Petra Schneider and Katrin Steiger for the
support they provided in the synthesis and measurements of the absorption and
fluorescence spectra.
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