Photophysical and theoretical studies of diphenylacetylene derivatives with restricted rotation

Article abstract of DOI:10.1016/j.molstruc.2017.02.096

Diphenylacetylene derivatives containing electron-donor (amino) and electron-acceptor (ester) groups in 2,2′ or 3,2′ positions in phenyl rings were synthesized to study the effects of intramolecular charge transfer and stiffening of the conformation by in

Full text of DOI:10.1016/j.molstruc.2017.02.096

Accepted Manuscript
Photophysical and theoretical studies of diphenylacetylene derivatives with restricted
rotation
Małgorzata Szyszkowska, Cezary Czaplewski, Wiesław Wiczk
PII:
S0022-2860(17)30255-7
10.1016/j.molstruc.2017.02.096
MOLSTR 23490
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 23 November 2016
Revised Date: 27 February 2017
Accepted Date: 27 February 2017
Please cite this article as: M. Szyszkowska, C. Czaplewski, W. Wiczk, Photophysical and theoretical
studies of diphenylacetylene derivatives with restricted rotation, Journal of Molecular Structure (2017),
doi: 10.1016/j.molstruc.2017.02.096.
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H2N
H3CO
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30
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dihedral angle [degrees]

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Photophysical and theoretical studies of diphenylacetylene derivatives
with restricted rotation
Małgorzata Szyszkowska, Cezary Czaplewski, Wiesław Wiczk*
University of Gdańsk, Faculty of Chemistry
Wita Stwosza 63, 80-308 Gdańsk
* corresponding author: e-mail wieslaw.wiczk@ug.edu.pl

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Abstract
Diphenylacetylene derivatives containing electron-donor (amino) and electron-
acceptor (ester) groups in 2,2` or 3,2` positions in phenyl rings were synthesized to
study the effects of intramolecular charge transfer and stiffening of the conformation
by intramolecular hydrogen bond on the photophysical properties. Additionally, the
derivatives with and without a steric hindrance were studied to determine the effect of
conformational freedom on photophysical properties. The persistence of the
intramolecular hydrogen bond and the corresponding twisted conformations as well
as the conformational flexibility were also established by DFT calculations.

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Introduction
Conjugated organic polymers and their widespread application have been
recently subject of the intense research [1,2]. The rather rigid conjugated
poly(phenyleneethynylene)s have been found to be substitutes for conventional
materials in electronic or photonic devices [3], molecular wires [4], light emitting
diodes [5-7], solar cells [8-11]. Diphenylacetylene as a building block for a larger
structures, oligo- and poly(phenyleneethynylene)s is frequently used. They are rigid,
and their only degree of freedom is the rotation of the arene unit around the central
alkyne. This rotational freedom has an impact on the optical and photophysical
properties
of
diphenylacetylene
and
its
derivatives
as
well
as
poly(phenyleneethynylene)s [12-18]. The rather small ground-state torsional barriers
of diphenylacetylene, about 2.5 kJmol-1, obtained from the experiments [19,20] give
rise to the multitude of accessible conformations. However, in the first excited state
the torsional barrier increases by about factor of ten because of an increase of the
molecule cumulenic character [15]. The direct substitution at sp-carbons by an
electron-donating on one side and an electron-withdrawing group on the other side
results in large increase of the torsional barrier. For aminoaldehyde derivative of
acetylene the torsional barrier is equal to 24.4 kJmol-1 [20,21] whereas for
dimethylacetylene only about 0.070 kJmol-1 [21,22]. Moreover, the insertion of an
extra acetylene unit decreases the barriers by about 40-70% [21,22]. The
enhancement of rotational barrier was predicted by introducing bulky substituent in
ortho-position in the phenyl ring of diphenylacetylene due to destabilization of the
transition state of the rotation process [16,17,21,23-25]. However, the enhancement
is more difficult for acyclic alkynes than for cyclic ones attributed to the flexibility in
the former [16,23-25]. The presence of substituents which may form a hydrogen bond

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in ortho position of diphenylacetylene further stabilizes a rigid conformation by
increasing the height of the energy barrier associated with rotation [26-30]. For
methyl 2-((2-acetamidophenyl)ethynyl)benzoate the calculated rotation energy barrier
is about 30 kJmol-1 [27] and increased further with chain at both substituents [29].
While the spectral and photophysical properties of symmetrical derivatives of
acetylene and 1,3-diyne have been extensively studied [18,31-40], the spectral and
photophysical properties of diphenylacetylene derivatives having substituents in the
ortho and/or meta positions have not been investigated. Therefore, the
corresponding derivatives shown in Fig. 1 were synthesized and their spectral and
photophysical properties are presented.
Fig. 1
Material and methods
Synthesis
Compounds 1c-4c and 5d were prepared by Sonogashira coupling reaction
[41-42] of appropriate acetylene derivatives with the respective halogenoarenes
according to the following Scheme 1.
Scheme 1.
Methyl 2-iodobenzoate (Sigma-Aldrich), 2-iodoaniline (Alfa Aesar), 3-
aminophenylacetylene
(Alfa
Aesar),
2-methyl-3-butyn-2-ol
(Fluka),
dichlorobis(triphenylphosphine)-palladium (II) (Sigma-Aldrich), iodophenol (Sigma
Aldrich) and heptanoic acid (P.P.H) were commercially available.
The progress of the reaction was checked by means of TLC (Merck plates,
Kieselgel 60 F254). The products were isolated by means of column chromatography

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(Merck, Kieselgel 60 (0,040-0,063 mm)) and semi-preparative RP-HPLC (Kromasil
column, C-8, 5ꢀm, 250 mm long, i.d. 20 mm). The product identification was based
1
on: H and 13C NMR spectra recorded on Bruker AVANCE III (500 MHz)
spectrometer in CDCl3, mass spectra recorded on Bruker Daltonics (HCTultra)
instrument and Raman spectra recorded on Raman FRA-106 instrument coupled to
IFS 66 (Bruker). Excitation source was Nd:YAG laser (1064 nm) and Ge detector
cooled to liquid nitrogen was used. In the spectral range from 3500 cm-1 to 50 cm-1,
512 scans with a resolution of 1 cm-1 were collected.
Procedure A
Synthesis of methyl 2-(3-hydroxy-3-methylbut-1-yn-1-yl)benzoate (1a), 4-(2-
aminophenyl)-2-methylbut-3-yn-2-ol (2a) and heptyl 4-(3-hydroxy-3-methylbut-1-yn-1-
yl)benzoate (5b)
Appropriate halogenoarene (1 eq) was dissolved in triethylamine (TEA) and
the solution was cooled to 0oC. Then a trace amount of copper iodide and
bis(triphenylphosphine)palladium(II) dichloride, Pd[(C6H5)3P]2Cl2 as catalysts were
added. After a few minutes acetylene derivative (2 eq) was added. The reaction
mixture was stirred for about 9 h and then was filtered, extracted with ethyl acetate
and washed with water. Combined organic layers was dried over anhydrous
magnesium sulfate, filtered and the solvent was evaporated. The pure products 1a-
2a were isolated by: crystallization from a mixture of ethyl acetate and petroleum
ether (1a) and ethyl acetate (2a) or by column chromatography on silica gel
(petroleum ether/ethyl acetate 20:1 (v/v) (5b)).

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Synthesis of dimethyl 2,2'-(ethyne-1,2-diyl)dibenzoate (1c) and methyl 2-((2-
aminophenyl)ethynyl)benzoate (2c), methyl 2-((3-aminophenyl)ethynyl)benzoate (3c)
Compounds 1c-3c were prepared by Sonogashira coupling reaction [41-42] of
appropriate acetylene derivative with the respective iodoarene according to
procedure A. The pure products 1c-3c were isolated by column chromatography on
silica gel (petroleum ether/ethyl acetate 7:1 (1c) or 5:1 (v/v) (2c-3c)).
Procedure B (Removing the protective group) [43-44]
A mixture of 1a or 2a or 5b (1 eq) and potassium hydroxide (1 eq) in toluene
was heated under reflux for 2 h in argon atmosphere. The reaction mixture was
cooled, filtered, extracted with ethyl acetate and washed with 0.1 M KHSO4 and
water. Organic layer was dried over anhydrous magnesium sulfate, filtered and the
solvent was evaporated The pure products were isolated by multiple crystallization
from a mixture of petroleum ether and ethyl acetate (1b and 2b) or by column
chromatography on silica gel (petroleum ether/ethyl acetate 10:1 (v/v) (5c)).
Procedure C
Synthesis of 2,2'-(ethyne-1,2-diyl)dibenzoic acid (4c)
Compound 4c was prepared according to procedure described in literature
[45].
To a solution of 1c (1 eq) in CH2Cl2/MeOH (9/1 (v/v)) mixture, a methanolic solution
of 2 N NaOH (3 eq) was added (the final concentration of the alkali was about 0.1-
0.2 N). The reaction mixture was stirred for 9 h and then the solvents were removed
under vacuum. The residue was diluted with water and the aqueous solution was
extracted with diethyl ether. The aqueous phase was acidified to pH 2-3 with 1 N HCl

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and extracted with CH2Cl2. Combined organic layers were dried over anhydrous
sodium sulfate, filtered and the solvent was removed. The pure product 4c was
obtained by crystallization from ethanol.
Procedure D
Synthesis of 4-iodophenyl heptanoate (5a)
To a mixture of heptanoic acid (1 eq) and N,N`-diisopropylcarbodiimide
(DIPCI) (2 eq) in dioxane, iodophenol (0.5 eq) was added. The reaction mixture was
stirred for a couple of days at room temperature. After the reaction was completed
the crude product was filtered, extracted with ethyl acetate and washed with
saturated Na2CO3 and water. Organic layer was dried over anhydrous magnesium
sulfate, filtered and the solvent was evaporated. The pure product was isolated by
column chromatography on silica gel (petroleum ether/ethyl acetate 20:1 (v/v) (5a)).
Identification data for all studied compounds
1
Raman and H NMR spectra of studied compounds are placed in Supporting
Information. In the manuscript the chemical shifts values and Raman frequency of
C≡C bond are given.
Dimethyl 2,2'-(ethyne-1,2-diyl)dibenzoate (1c) (yield 52 %)
1H NMR (400 MHz, CDCl3):
δH (ppm): 3.95 (s, 6H, (CH3)2); 7.38 (t, 2H, C5H, C5’H, J=4 Hz, J=8 Hz); 7.47 (t, 2H,
C6H, C6’H, J=8 Hz, J=4 Hz); 7.53 (d, 2H, C7H, C7’H, J=4 Hz); 7.94 (d, 2H, C4H, C4’H,
J=8 Hz).
13C NMR (100 MHz, CDCl3)

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δC (ppm): 52.12 C9, C9’; 80.92 C1, C1’; 123.27 C4, C4’,127.90 C2, C2’, 130.36 C5, C5’;
131.65 C7, C7’; 132.01 C6, C6’; 133.90 C3, C3’; 166.70 C8, C8’.
MS (m/z): 294 [M+]
Raman νmax (cm-1): 2232 C≡C
Methyl 2-((2-aminophenyl)ethynyl)benzoate (2c) (yield 63 %)
1H NMR (400 MHz, CDCl3):
δH (ppm): 3.94 (s, 3H, CH3); 4.75 (singlet, 2H, NH2); 6.70 (t, 1H, C5’H, J=2 Hz); 6.77
(d, 1H, C4’H, J=8 Hz); 7.18 (t, 1H, C6’H, J=8 Hz); 7.39 (d, 1H, C7’H, J=8 Hz); 7.41 (t,
1H, C5H, J=4 Hz); 7.55 (t, 1H, C6H, J=2 Hz); 7.69 (d, 1H, C7H, J=8 Hz); 8.05 (d, 1H,
C4H, J=8 Hz).
13C NMR (100 MHz, CDCl3)
δC (ppm): 52.32 CH3; 92.50 C1’; 93.68 C1; 107.42 C2’; 114.28 C4’; 117.29 C6’; 124.66
C4; 127.20 C2; 130.23 C5’; 130.56 C5; 131.95 C7’; 132.06 C7; 133.71 C6; 133.89 C3;
149.42 C3’; 166.30 C8.
MS (m/z): 252 (M+H)+
Raman νmax (cm-1): 2211 C≡C
Methyl 2-((3-aminophenyl)ethynyl)benzoate (3c) (yield 56.5 %)
1H NMR (400 MHz, CDCl3):
δH (ppm): 3.88 (s, 3H, CH3); 5.72 (singlet, 2H, NH2); 6.90 (d, 1H, C5’H, J=4 Hz); 7.14
(d, 1H, C7’H, J=16 Hz); 7.16 (s, 1H, C3’H); 7.18 (t, 1H, C6’H, J=4 Hz, J=5 Hz); 7.30 (d,
1H, C6H, J=4 Hz); 7.41 (t, 1H, C5H, J=8 Hz, J=4 Hz); 7.54 (d, 1H, C7H, J=8 Hz); 7.89
(d, 1H, C4H, J=8 Hz).
13C NMR (100 MHz, CDCl3)

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δC (ppm): 52.26 CH3; 88.53 C1’; 93.82 C1; 118.26 C2’; 120.76 C5’; 123.55 C6’; 124.52
C4; 124.88 C2; 125.74 C6’; 127.99 C5; 129.12 C7’; 129.56 C7; 130.48 C6; 131.71 C3;
131.90 C3’; 134.06 C4’; 166.70 C8.
MS (m/z): 252 (M+H)+
Raman νmax (cm-1): 2221 C≡C
2,2'-(Ethyne-1,2-diyl)dibenzoic acid (4c) (yield 60 %)
1H NMR (400 MHz, CDCl3):
δH (ppm): 7.36 (t, 2H, C5H, C5’H, J=8 Hz); 7.58 (t, 2H, C6H, C6’H, J=8 Hz); 7.65 (d,
2H, C7H, C7’H, J=8 Hz), 8.11 (d, 2H, C4H, C4’H, J=8 Hz); 10.49 (s, 2(OH).
13C NMR (100 MHz, CDCl3):
δC (ppm): 70.67 C1, C1’; 129.07 C3, C3’,129.29 C5, C5’, 129.67 C7, C7’; 130.82 C4, C4’;
134.31 C6, C6’; 137.92 C2, C2’; 168.63 C8, C8’.
MS (m/z): 266 [M+]
Raman vmax (cm-1): 2261 C≡C
Diheptyl 4,4'-(ethyne-1,2-diyl)dibenzoate (5d) (yield 38%)
1H NMR (400 MHz, CDCl3):
δH (ppm): 1.10 (t, 6H, C15H, C15’H, J=4 Hz); 1.33 (m, 8H, 2C13H, 2C13’H, 2C14H,
2C14’H); 1.64 (m, 8H, 2C12H, 2C12’H, 2C11H, 2C11’H); 1.68 (m, 4H, 2C10H, 2C10’H);
4.21 (t, 4H, C9H, C9’H), J=4 Hz, J=8 Hz); 7.45 (d, 4H, C3H, C3’H, C6H, C6’H, J=8 Hz),
7.92 (d, 4H, C4H, C4’H, C7H, C7’H, J=8 Hz).
13C NMR (100 MHz, CDCl3):
δC (ppm): 19.12 C15, C15’; 22.57 C14, C14’; 26.05 C11, C11’; 28.68 C12, C12’; 30.28 C10,
C10’; 31.70 C13, C13’; 65.53 C9, C9’; 90.15 C1, C1’; 119.70 C4, C4’, C7, C7’; 127.67 C2,
C2’;128.41 C5, C5’; 130.69 C3, C3’; C6, C6’; 166.61 C8, C8’.
MS (m/z): 463 [M+H]+

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Raman νmax (cm-1): 2231 C≡C
Spectroscopic measurements
Fluorescence quantum yields were measured using: 2-aminopyridine in 0.05
M H2SO4 (QY=0.60) (for 1c and 2c) or quinine sulphate in 0.5 M H2SO4 (QY=0.546)
(for 3c, 4c and 5d) as a reference and were corrected for different refractive indices
of solvents.
The fluorescence lifetimes were measured with a time-correlated single-
photon counting FT300 PicoQuant fluorescence lifetime spectrometer using
subnanosecond pulsed diodes: PLS-290 (for 1c), PLS-320 (for 3c, 5d) and LDH 375
(for 2c).
Theoretical calculations
The ground state conformational search was done with RDkit software [46].
For each studied compound 1000 of conformers were generated and the maximum
number of attempts to try embedding were also 1000. Embedding algorithm enforced
the correct chirality if chiral centers were present and imposed experimental torsion
angle preference and basic knowledge such as flat rings. Only the conformations that
are at least 0.1 RMSD (computed on the heavy atoms) far apart from each other,
were retained. All generated conformations were minimized using MMFF94 force
field [47] (up to 10000 iterations), and then dived into families using Butina clustering
algorithm and Torsion Fingerprints [48] with 0.01 threshold. The structure
optimization of the lowest energy conformer as well as the rotation energy barriers in
the ground and the excited states were performed using density functional theory
(DFT) within the Turbomole v. 6.4 suite programs on a PC cluster. Energy-minimized

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structures for the ground and the lowest excited states were calculated using a B3-
LYP hybrid functional with def2-TZVP basis set of triple-ζ quality. The optimization of
the excited state was performed using a grid parameter equal to 4. The local minima
confirmed through vibrational frequency calculations. No imaginary frequencies were
observed. The rotation energy barriers in the ground and the excited states were
calculated by imposing constraints on the dihedral angle formed by the phenyl rings
and optimizing the energy of the remaining part of the molecule.
Results and discussion
Theoretical calculations
To understand the influence of substituents in the phenyl rings on torsional
motions, we performed conformational analysis of studied compounds. The torsional
potential energy of phenyl ring, particular substituents (ester, amino or carboxyl
group), as well as substituent component: methoxy group of ester or hydroxyl group
of carboxyl in the ground state were calculated and analyzed. Conformation analysis
using RDkit software with MMFF94 force field indicated that for 2c there are four
conformational families. Moreover, the lowest energy conformation is the one with
strong hydrogen bond between ester and amino substituents, whereas the
conformation in which the substituents in the phenyl rings are in “trans” position is the
one with the highest energy. The energy difference between these two conformations
is about 12.5 kJmol-1. Similarly, for 3c also four conformational family can be
distinguish, however, the energy difference between conformations falls within the
range of 1 kJmol-1. Far more conformations can be distinguish for 1c. The energy
difference between them does not exceed the value of 0.9 kJmol-1. The largest

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number of conformational families was obtained for 4c (about 58). Moreover, they are
characterized by the most diverse energy. The energy difference between the lowest
and the highest energy conformations is about 83 kJmol-1. Similarly to 2c, the lowest
energy conformation of 4c is this with strong hydrogen bond between substituents
whereas the conformation in which the substituents in the phenyl rings are in “trans”
position is the one with the highest energy. The lowest energy conformations
obtained using molecular mechanics were then used as the starting conformations in
the studies of structure and the torsional potential energy in both the ground and
excited states within the framework of the local mode approximation using a DFT
method. Appropriate lowest energy structures of studied compounds in the ground
and excited states, except 5d which is similar to 4,4`-substituted diphenylacetylene
derivatives [18], are presented in Figs 12-19 ESI. Figs 20-23 ESI present DFT-based
potentials energy for rotation of ester, amino or carboxyl group in the ground and
excited states. Rotation of ester group around the single C-C bond connected to the
phenyl ring required a dozen kJmol-1 to overcome the energy barrier. Moreover, the
height of this barrier depends on whether this group forms a hydrogen bond. Thus,
e.g. for 2c (Fig. 20 ESI) it is about 20 kJmol-1, whereas for 3c (Fig. 21 ESI) 12 kJmol-
1. Also rotation barrier of the carboxyl group in 4c (Fig. 22 ESI) is larger than that of
ester group in 1c (Fig. 23 ESI). The energy of rotation of methoxy and hydroxyl
groups were also calculated. The results are presented in Fig. 24 ESI. Rotation of
methoxy group around O-C single bond in aromatic ester, regardless of whether or
not it forms a hydrogen bond, requires a lot of energy (about 50 kJmol-1). The curves
showing the dependence of the potential energy of rotation are presented in Fig. 24
ESI. They have a poorly developed maximum which in practice means a well-defined
position of the methoxy group against a carbonyl group (see Figs 12-14 ESI).

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However, well defined maximum on the potential energy of rotation of hydroxyl group
(about 25 kJmol-1) is observed. It suggests that hydroxyl group in 4c has higher
conformational freedom then methoxy group in remaining studied compounds. The
energy barrier of amino group rotation in the ground state is higher than that of ester
group. Thus, e.g. for 2c (Fig. 20 ESI) it is about 35 kJmol-1, whereas for 3c (Fig. 21
ESI) 25 kJmol-1. Moreover, the maximum of barrier accrues not to 90 degrees but is
displaced due to the pyramidal shape of an amino group in the ground state. The
energy barriers of rotation of functional groups (ester and amino) were also
calculated in the excited state (Figs 20-23 ESI). Like in the ground state, ester group
is coplanar with phenyl ring and a maximum of the energy barrier is for perpendicular
orientation of the ester group relative to the phenyl ring. Similarly is for the amino
group. This implies that in the excited state it becomes flat, losing a pyramidal shape.
The height of the energy barrier of rotation of ester and amino group in the excited
state is much higher than in the ground state. A charge transfer in the excited state
(see e.g. Figs 25 ESI, 27 ESI and 29 ESI) seems to be responsible for stronger
coupling of these groups with the phenyl ring.
Not only the functional groups or fragments thereof may rotate but the entire
phenyl ring together with substituents. Mutual position of the phenyl rings of
diphenylacetylene derivative has a big impact on their mutual coupling and thereby
spectral and photophysical properties. Therefore, energy of rotation of the phenyl
rings in the ground and excited states were calculated. Fig. 2 presents DFT-based
torsional potentials for 2c in the ground and excited states. The torsional potential
energy in the ground and excited states is not approximated by a simple periodic
function like for 4,4`-substituted diphenylacetylene derivatives [18].
Fig. 2

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As shown in the Figure, the most stable (for about 17 kJmol-1 in comparison to the
“trans” rotamer) conformation of 2c in the ground state is the planar “cis” rotamer. It
can be a result of the formation of intramolecular hydrogen bond. This value is similar
to the energy of rotation in the ground state published by Yang et al. [27] for 2-((2-N-
acetamidophenyl)ethynyl)benzoate. Moreover, the transition from “trans” to “cis”
rotamer requires additional activation energy of about 5 kJmol-1. In the excited state
the most stable rotamer has twisted, nonplanar “cis” conformation with dihedral angle
formed by phenyl rings in the range from 35 to 60 degrees. It is worth noting that the
“trans”–“cis” transition occurs with barrier less than 0.5 kJmol-1 and hydrogen bond is
stronger (about 53 kJmol-1) than in the ground state. The energy minimum is wide
which means that in the solution different individuals with a wide range of dihedral
angles are present. This should results in a lack of vibronic structure of the
absorption and emission spectra and heterogeneous of fluorescence intensity decay.
Similarly looks the potential energy of rotation for 4c presented in Fig. 3, with
the exception that in the ground state the most stable rotamer is twisted, nonplanar
“cis” rotamer with the minimum for about only 2 kJmol-1 at dihedral angle formed by
two fenyl rings equal to 45 degrees.
Fig. 3
However, in the excited state the dihedral angle increases to about 65 degrees and
energy decreases to about 24 kJmol-1, comparing to that of “trans” rotamer.
In Fig. 4 the torsional potential energy for 3c in both the ground and excited states is
presented.
Fig. 4.

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The curve shape of potential energy of rotation around Ph-C≡C single bond for 3c in
the ground and excited states is similar to the shape of the torsional potential energy
calculated for 4,4`-substituted diphenylacetylene derivatives [18]. The energy barrier
of rotation is low in both states. Moreover, the “cis” rotamer is a little more stable than
“trans” one, indicating on a weak ICT interaction between amino and ester groups
predominated by the through-bond mechanism [49] and O..H-C hydrogen bond (see
Fig. 27 ESI). For 5d potential energy of rotation around Ph-C≡C bonds can be
described by periodical function like for 4,4`-substituted diphenylacetylene derivatives
[18] and the energy barrier of rotation in the ground and excited states is equal about
4 kJmol-1 and 30 kJmol-1, respectively.
The potential energy of rotation around a single Ph-C≡C bond for 1c (Fig. 32
ESI) in the ground state shows that more stable conformation is “trans” rotamer. On
the potential energy curve, there is a second minimum at dihedral angle about 35
degrees (∆E ~5.5 kJmol-1). Thus, in the ground state at room temperature different
rotamers can exist in the solution. Unfortunately, the calculations of the potential
energy curve associated with rotation in the excited state were unsuccessful due to
the change of compound structure as a result of an intramolecular cyclization
reaction. Therefore, the conformational freedom is unknown in the excited state.
Differences in the stability and structure (conformation) of the studied compounds in
the ground state and excited states are a consequence of different charge
distribution in both states. It influence on differentiation of bond lengths in these two
states what is particularly evident for acetylene units (Figs 26 ESI, 28 ES, 30 ESI, 31
ESI). This leads to an increase of cumulenic character of a molecule and results in
the increase of torsional barrier in the excited state [15].

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Absorption spectroscopy
Absorption spectra of diphenylacetylene containing methylcarboxylate groups
in ortho position whereas an amino group in ortho (2c) or meta (3c) position in the
phenyl ring in selected solvents are presented in Fig. 5 and Fig. 6, respectively.
Fig. 5
Fig. 6.
Compound 2c possesses a long-wavelength absorption band in the range of
320-420 nm, similarly to para substituted derivatives containing an electron-donor
and electron-acceptor groups [32,39,49-52], and the second absorption band below
300 nm. Moreover, between these bands a weak structured absorption band can be
seen like in the para-substituted diphenylacetylenes [18] (Fig. 5). The presence of the
long-wavelength bell-shaped absorption band, disappearing in a methanol solution
containing a few drops of trifluoroacetic acid (Fig. 33 ESI), indicates on the efficient
internal charge-transfer (ICT) process. Also, the calculated, using DTF method,
change of the electron density between the ground and excited states presented in
Figs 25 ESI, 27 ESI and 29 ESI confirms that. Moreover, for 2c, similarly to methyl 2-
((2-acetamidophenyl)ethynyl)benzoate, intramolecular hydrogen bond exist [26-27].
This is indicated by blue-shift of both absorption bands in protic solvents in
comparison to non-polar ones, as a result of disruption of intramolecular hydrogen
bond in favor of the intermolecular one. Moreover, the short-wavelength absorption
band shift depends on alcohol polarity and it is larger in more polar solvent. The
possibility of intramolecular hydrogen bond formation, causing the stiffness and
flattening the structure and enhancing the ICT process in di-meta derivative, has also
relevant influence on the absorption spectrum of this derivative. Ma et al. [49] found

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that the magnitude of the internal charge transfer coupling is sensitive to substitution
position in terminal donor/acceptor-substituted diphenylacetylene. Meta substituted
compounds show relatively higher ICT coupling value. It can be attributed to the more
intense cumulenic character of conjugated bridge in the relaxed excited state of
meta-isomers, thus results in stronger charge localization (the meta-effect) [53].
For ortho and meta substituted diphenylacetylenes with an amino and methyl
carboxylate groups (3c) the long-wavelength absorption band is substantially blue-
shifted in comparison to that of di-ortho derivative, despite the same substituents
(Fig. 6). However, a large overlap of two absorption bands causes that the accurate
determination of maximum for long-wavelength absorption band is difficult. Similarly
to 2c, the long-wavelength absorption band seems to be a result of ICT transition
(Fig. 27 ESI). However, the position of substituents results in lower participation of
CT transition. This is evidenced by the relatively small band-shift after protonation of
the amino group in the comparison to 2c (Fig. 3 ESI). In non-polar solvents, such as
cyclohexane and 1-chlorohexadecane, a poorly marked vibronic structure was
observed (Fig. 6). However, blue-shift of absorption band with simultaneous
formation of long-wavelength tail in polar protic and aprotic solvents indicates on the
formation of intermolecular hydrogen bonds by amino [51] and/or methylcarboxylate
group.
The absorption spectra of 4c in selected solvents are presented in Fig. 34 ESI.
This symmetrically substituted diphenylacetylene derivative forms a strong
intramolecular hydrogen bond, so that even a polar protic solvent like methanol
causes only a small blue-shift of the maximum of the absorption band. This hydrogen
bond stiffens the structure of di-carboxyl derivative which is reflected in more
bathochromic shifts of the absorption band in comparison to the ester derivative (Fig.

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34 ESI) which has a similar steric hindrance but without the ability to form
intramolecular hydrogen bonds. As can be seen in Fig. 35 ESI, the vibronic structure
of the long-absorption band is blurred in aprotic solvent and even more in protic
ones. The maximum of absorption band is blue-shifted in comparison to 1,2-bis(2-
methoxyphenyl)ethyne [16] because of higher steric hindrance. Absorption spectra of
5d, presented in Fig. 36 ESI, are shifted to longer wavelength in comparison to di-
meta ester derivative. Moreover, the long-wavelength absorption band has all
features typical for symmetrically substituted diphenylacetylene derivatives, with a
very well-developed vibronic structure [18,39]. A single progression of the most
prominent vibronic structure of about 2000 cm-1 has been assigned to the C≡C
stretching mode. Furthermore, other vibronic bands (about 1000 cm-1) are visible in
the absorption spectrum, and may be attributed to the total symmetric vibration of the
phenyl ring as for methylcarboxylate described in [18]. Similarly, as for the above-
described 4c (Fig. 34 ESI), a small influence of protic solvents on the position of the
maximum of absorption band is observed, indicating on the formation of a weak
hydrogen bond between ester group and solvent(s) molecule.
Emission spectroscopy
Fluorescence spectra of 2c in selected solvents are presented in Fig. 7.
Fig. 7
The presented above emission spectra were recorded at excitation
wavelength equal to 370 nm. In all studied solvents including non-polar saturated
hydrocarbons (cyclohexane, hexane, methylcyclohexane and hexadecane, 1-
chlorohexadecane) they are structureless. Moreover, increasing solvent polarity
causes a substantial red-shift of the spectrum. The maximum of fluorescence in

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cyclohexane is located at about 416 nm, while in polar acetonitrile at 514 nm.
Observations outlined above suggests emission from ITC state (Fig. 25 ESI) which
seems to have a greater dipole moment than the ground state. Since a limited
number of solvents was used and their properties does not differ much (two groups of
solvents) as well as specific interactions between solvent and solute take place, the
influence of the solvent properties on the spectral and photophysical properties is not
treated in a more quantitative manner. Despite the inhibition of the internal rotation of
phenyl rings by internal hydrogen bond, the fluorescence quantum yields of 2c are
low (Table 1). The largest values are recorded for aprotic solvents, while the lowest
ones in alcohols. It seems that in alcohols, beside intramolecular hydrogen bond,
intermolecular hydrogen bonds are partially severed. Thus, formation of hydrogen
bond network results in reduced fluorescence quantum yield. Furthermore, a small
impact of the solvent viscosity on the fluorescence quantum yield can be seen. In
more viscous solvents like hexadecane, 1-chlorohexadecane or viscous protic
solvents like t-BuOH or glycols the fluorescence quantum yield increases.
Table 1
A similar dependence of the fluorescence quantum yield on the solvent properties is
observed for 3c, except that the values of fluorescence quantum yield are slightly
higher in nonpolar solvents, and lower in protic ones. Its fluorescence spectra in
selected solvents are presented in Fig. 37 ESI. Similarly to the previous described
isomer, 3c shows a large red-shift with solvent polarity increase. The emission
maximum in cyclohexane is located at about 371 nm, while in polar acetonitrile at 539
nm, indicating on the intramolecular charge transfer in the excited state. However,
comparing the red-shift between the emission spectra measured in non-polar
cyclohexane and weakly polar 1-chlorohexadecane, it is much bigger for 2c (about

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27 nm) than that for 3c (about 5 nm). This suggests the greater internal charge
transfer character of the excited state of 2c.
A third of the investigated derivatives, 4c, whose structure in the excited state
seems to be stabilized by internal hydrogen bond (Fig. 38 ESI), shows no
measurable fluorescence, contrary to ester derivative 1c, which fluorescence spectra
in selected solvents are presented in Fig. 38 ESI and fluorescence quantum yields in
Table 1. Ester derivative has two substituents of the same type. As a result solvent
polarity has a minor influence on the position of emission band maxima, thereby
confirming lack of the substantial internal charge transfer in the excited state. The
spectral shift of the fluorescence band between cyclohexane and acetonitrile is small
(about 10 nm only). Furthermore, a small red-shift of the fluorescence spectra in
protic solvents is observed, indicating on the weak intramolecular hydrogen bond
network formation. Due to the steric hindrance caused by the presence of two ester
groups in ortho position in the phenyl rings, this compound cannot take planar
structure and therefore, its fluorescence quantum yields are low, but their values
increase a little in protic solvents. These values are about one order of magnitude
lower than for 1,2-bis(2-methoxyphenyl)ethyne [16]. The steric hindrance preventing
the adoption of planar structure in the excited state seems to have a significant
impact on the photophysics of studied compounds. This is confirmed by high
fluorescence quantum yields of 5d (Table 1), which are comparable to that of di-
methyl 4,4'-(ethyne-1,2-diyl)dibenzoate [18]. Moreover, the fluorescence spectra
presented in Fig. 39 ESI are very similar to that of di-methyl 4,4'-(ethyne-1,2-
diyl)dibenzoate [18].
Time-resolved fluorescence spectroscopy

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For studied compounds for which the fluorescence quantum yields in a
particular solvent were higher than 0.01 the fluorescence intensity decays were
measured (Table 2). For 5d in all studied solvents and 1c in protic solvents, the time-
resolved fluorescence decays of the emission appear single exponential, whereas for
the remaining studied compounds bi-exponential function is needed to correctly
describe the fluorescence intensity decay. Fluorescence lifetimes of 5d are about 0.7
ns and do not depend on solvent properties, similarly to dimethyl 4,4'-(ethyne-1,2-
diyl)dibenzoate [18].
Table 2
For 1c, heterogeneous fluorescence intensity decays are observed in aprotic
solvents pointing to emission of different individuals, associated with conformational
freedom resulting from the rotation around a Ph-C≡ single bonds present in the
solution. Moreover, in protic solvents the hydrogen bond networks of bulky ester
groups with solvent molecules restrict the conformational freedom enough to record a
single decay time. In the case of both amino-ester derivatives of diphenylacetylene
(2c,3c) heterogeneous fluorescence intensity decays are probably a result of the
emission of many individuals forming hydrogen bonds of varying strengths present in
the solution.
Conclusion
Diphenylacetylene derivatives containing different substituents at ortho, and/or
meta and para positions were synthesized. It has been demonstrated that the ICT
state is formed in 2c and 3c causing red-shift of absorption and emission spectra in
comparison to 1c and 5d. Moreover, contrary to 3c, for 2c and 4c a strong
intramolecular hydrogen bond is formed. It stiffens otherwise flexible conformation of

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diphenylacetylene derivatives. The fluorescence quantum yields of diphenylacetylene
derivatives vary in dependence upon twisting angle of the two constituting phenyl
rings in the excited state (e.g. 1c and 5d). The intramolecular or intermolecular
hydrogen bond formation leads to a decrease of both fluorescence quantum yield
and fluorescence lifetime. Since the nitrogen atom in the amino group of the solute is
positively charged in the ICT state, an interaction of the hydrogen atom of the solvent
with nitrogen atom of the solute cannot be operative. The interaction between
positively charged nitrogen atom and the electron rich oxygen atom of the solvent
molecule should play important role leading to deformation of the amino group and
rapid charge recombination in protic solvents. However, the ester group can form a
classical intermolecular hydrogen bond with a proton of the hydroxyl group of the
solvent molecules modifying the photophysical properties of studied compounds.
Theoretical calculations fully confirmed the experimental results related to the
possibility of intramolecular hydrogen bond formation and conformational flexibility of
studied dipenylacetylene derivatives.
Acknowledgements
This work was financially supported under grant UMO-2011/01/B/ST4/06094 from
Polish National Science Centre (NCN).
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