Photophysical properties of symmetrically substituted diarylacetylenes and diarylbuta-1,3-diynes

Article abstract of DOI:10.1039/c5pp00197h

A series of symmetrically substituted diarylacetylenes and diaryl-1,3-butadiynes were prepared and studied with an emphasis on their spectral and photophysical properties. The photophysical characteristics of these compounds were studied in relation to their structures and the influence of solvent or temperature. The observed spectral and photophysical properties are explained using potential energy maps of the ground and excited states obtained from density functional theory calculations. The structure-property relationships of all of the compounds are discussed and compared with the appropriate diphenylacetylene and diphenyl-1,3-butadiyne derivatives.

Full text of DOI:10.1039/c5pp00197h

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Photophysical properties of the
symmetrically substituted
diarylacetylenes and diarylbuta-1,3-
diynes
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Irena Bylińska, Małgorzata Wierzbicka, Cezary Czaplewski,
www.rsc.org/
Wiesław Wiczk*
The first two authors contributed equally to the work
A series of symmetrically substituted diarylacetylenes and diaryl-1,3-
butadiynes were prepared and studied with an emphasis on their spectral
and photophysical properties. The photophysical characteristics of these
compounds were studied in relation to their structures and influence of
solvents or temperature. The observed spectral and photophysical
properties are explained using potential energy maps in the ground and
excited states obtained from density functional theory calculations. The
structure-property relationships of all of the compounds are discussed and
compared with appropriate diphenylacetylene and diphenyl-1,3-butadiyne
derivatives.
Introduction
Asymmetrically substituted acetylene derivatives containing an
aromatic or heteroaromatic substituents are fluorescent
compounds which have been applied to a variety of purposes.
They can be used as polarity-sensitive probes1, for labelling
biological molecule2 and intracellular imaging3, as
oligonucleotide analogues4, metal or pH sensors5-8
fluorescence electrochemiluminescence
swiches9,10
materials11-13 or to synthesize of novel fluorescent probes14-17
,
,
.
Moreover, so-called pull–push chromophores containing
electron donors and acceptors connected by a π-electron
bridge (D–π–A) show a large nonlinear optical response (NLO)
which allows their use in second-order NLO applications18-20
.
Poly(aryleneethylene)s find wide application as components of
light-emitting diodes, solar cells21, as well as for the
construction of nanoarchitectures22 and molecular machines23-
26
.
Buta-1,3-diyne derivatives are less applicable than
acetylene derivatives, however, substituted buta-1,3-diynes
undergo 1,4-addition reaction in the solid state to give
polydiacetylene27 which can be used as a label-free fluorogenic
chemosensors28. While asymmetrically substituted derivatives
of acetylene and buta-1,3-diyne are fluorescent, their
symmetrically substituted phenyl counterparts show very
weak or practically no measurable fluorescence at room
temperature and their fluorescence quantum yields show
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1c
strong dependence on temperature. This applies in particular
to diphenylacetylene (DPA) and diphenyl-1,3-butadiyne and
their derivatives containing an electron-donating substituents
in position 4, 4` in the phenyl ring29-37. These phenomena have
been explained by Zgierski and Lim38,39 based on theoretical
calculations. The authors founded that the excited singlet
states of DPA demonstrate the existence of a low energy
crossing between the initially excited ππ* (B1u) state in the
linear D2h symmetry and the “non-fluorescent, dark πσ* state”
(11Au) in the bent C2h symmetry, which, as demonstrated
Saltiel and Kumar40 is weakly fluorescent state. The state
switch from the linear ππ* state to the bent πσ* state
accounts for all the unusual photophysical and spectroscopic
properties of DPA and its derivatives. The experimental
confirmation of the presence of πσ* state was a strong
πσ*←πσ* absorpꢀon at about 700 nm in picosecond transient
absorption for different DPA derivatives containing electron-
donating substituents and lack of such absorption for electron-
withdrawing substituents38,39 as it was predicted by theoretical
calculations. Moreover, for 1,4-diphenylbuta-1,3-diyne,
similarly to DPA, dramatic reduction of the fluorescence
quantum yield when the photoexcitation energy becomes
higher than about 2000 cm-1 above the 11B1u state was
observed and it was explained assuming the presence of close
location of the 11A1u state above 11B1u state35.
2c
3c
4c
A big differences between photophysical properties of DPA
and diphenylbuta-1,3-diyne on one side and its non-
symmetrical substituted derivatives containing an polycyclic
aromatic/heteroaromatic groups raises the question about the
reason for such differences. To find the answer to this
question, symmetrical derivatives of acetylene and buta1,3-
diyne containing polycyclic aromatic substituents were
synthetized and its spectral and photophysical properties were
examined (Fig. 1). Moreover, there is a lack of the systematic
studies of the spectral and photophysical properties of
symmetrically substituted diarylacetylene, as well as
diarylbuta-1,3-diyne derivatives, and at the same time
5c
6c
7c
considerable interest of systems containing diyne motif41,42
.
On the basis of obtained results the similarities and differences
between diarylacetylene and diarylbuta-1,3-diyne derivatives
are discussed. Moreover, properties of diarylacetylene and
diaryl-1,3-butadiyne are compared to published before,
properties of DPA and diphenylbuta-1,3-diyne.
8c
Experimental
Materials and methods
All halogenoarenes: 1-iodonaphtalene (Sigma-Aldrich), 2-
bromonaphtalene (Fluka AG), 9-bromoanthracene (Fluka), 1-
bromopyrene (Sigma-Aldrich) and acetylene derivatives: 2-methyl-
3-butyn-2-ol (Fluka), trimethylsilylacetylene (Sigma-Aldrich), 1-
Fig. 1 Structure and atom numbering of studied compounds
.
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ethynylpyrene (Alfa Aesar) were commercially available and used as states, a gradient of <1*10−3 au was achieved. The low-lying excited
received. Symmetrical monoacetylene derivatives (1c-4c) were states were treated within the adiabatic approximation of time-
prepared based on the Sonogashira-Hagihara method43,44 using a dependent density functional theory (DFT-RPA) using a B3-LYP
respective halogenoarenes (procedure
A
and A’), while functional. The geometry optimization of the excited state was
symmetrically substituted 1,3-butadiyne derivatives (5c-8c) were performed using grid parameter equal to 4. The convergence of all
made based on the Glaser method45 (procedure C) (see ESI). The studied systems was checked by harmonic vibrational analysis. No
progress of reaction was monitored by means of TLC (Merck plates, imaginary frequencies were observed. The rotation energy barriers
Kieselgel 60 F254). The products were isolated by means of column in the ground and excited states were performed by imposing
chromatography (Merck, Kieselgel 60 (0.040-0.063 mm) and semi- constraints on the dihedral angle optimizing the energy of
preparative RP-HPLC (Kromasil column, C-8, 5μm, 250 mm long, i.d. molecule. It does not imposed additional constraints such as the
20 mm). The identification of product was based on: 1H and 13C symmetry of the molecule. In a similar way the energy of the bent
NMR spectra recorded on
a Brucker AVANCE III (500 MHz) conformation of molecule was calculated imposing the bent angle
spectrometer in CDCl3, mass spectra recorded on a Bruker Daltonics value and optimizing the remaining part of the molecule; The same
(HCTultra) instrument and Raman spectra recorded on a Bruker functional and bases set were used as in the case of energy
FRA-106 instrument (see details in ESI).
optimization.
Spectroscopy and fluorescence measurements
Results and discussion
Absorption spectra were measured using a Perkin-Elmer Lambda-
40P spectrophotometer, whereas emission spectra were measured
using a FluoroMax-4 (Horriba-Jobin-Yvon) spectrofluorimeter. Low
temperature luminescence spectra were obtained using an FL-1013
liquid nitrogen dewar assembly.
Absorption spectra
Fluorescence quantum yields were calculated using quinine
sulphate in 0.5 M H2SO4 (QY=0.52±0.02)(for 1,2-di(naphtalen-1-
yl)ethyne (1Nac), 1,2-di(naphtalen-2-yl)ethyne (2Nac), 1,4-
di(naphtalen-1-yl)buta-1,3-diyne
(1Nac2),
1,4-di(naphtalen-1-
yl)buta-1,3-diyne (2Nac2)), fluorescein in 0.1 M NaOH (QY= 0.87)
(for 1,2-di(anthracen-9-yl)ethyne (Aac), 1,2-di(pyren-1-yl)ethyne
(Pac), 1,4-di(anthracen-9-yl)buta-1,3-diyne (Aac2)) and coumarin
480 in acetonitrile (QY= 0.91) (for 1,4-di(pyren-1-yl)buta-1,3-diyne
(Pac2)) as references and were corrected for different refractive
indices of solvents. The fluorescence lifetimes and TRES spectra
were measured with a time-correlated single-photon counting
fluorescence lifetimes spectrometer FT300 (PicoQuant) using
subnanosecond pulsed diodes: PLS-340 (for 1Nac, 2Nac, 1Nac2,
2Nac2) or diode laser LHD 420 (for Aac, Pac, Aac2, Pac2).
Theoretical calculations
All calculations were performed using density functional theory
(DFT) within the Turbomole v. 6.4 suite programs on a PC cluster,
similarly to published before36,37. The structures of all compounds
were prepared and initially optimized using the TmoleX program.
Energy-minimized structures were searched for the ground and the
lowest excited states using a B3-LYP hybrid functional with def2-
TZVP basis set of triple-ζ quality. This procedure was considered
satisfactory if the energy difference between optimized cycles was
<1*10−6Hartree for ground state structure optimization and
<1*10−7Hartree for excited state structure optimization. In both
Fig. 2 Normalized absorption spectra of acetylene and buta-
1,3-diyne derivatives in methylcyclohexane (MCH).
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Absorption spectra of the studied compounds measured in disorder and quadratic coupling between the ground and first
methylcyclohexane (MCH) are presented in Fig. 2, whereas excited states52or the exciton model developed by Liu et
measured in 2-methyltetrahydrofurane (2-MeTHF) and al.53,54. The red shift of the fluorescence maximum depends on
acetonitrile (MeCN) in Figs 1-8, ESI. All studied compounds, the size of substituent for both acetylene and buta-1,3-diyne
except for 2Nac and 2Nac2, show complicated vibrational derivatives in the following order: 2-naphthyl, 1-naphthtyl,
structure. Among some of them it can be recognized as pyrenyl and anthryl (Table 1 ESI).
vibrations associated with the vibration of the triple bond
(about 2000 cm-1) and the ring breathing mode of an aromatic
ring (at about 1400 cm-1). However, for acetylene and buta-
1,3-diyne derivatives containing 2-naphthyl substituents (2Nac
and 2Nac2 and to a lesser extent in the case of 1Nac2) a clear
long progression of C≡C stretching mode (ca. 2150 cm-1), which
is an indication of the large displacement of the C≡C bond
upon the electronic excitation, is seen. It is similar to the
absorption spectra of simple phenyl derivatives of acetylene
and buta-1,3-diyne30,34-37,46. The positions of long-wavelength
absorption bands measured in MCH are presented in Table 1
ESI. The position of the absorption maximum depends on the
size of aromatic substituent. The biggest it is, the more
bathochromially
shifted
spectrum
is
observed
(2Nac<1Nac<Pac<Aac). The introduction of the second
acetylene unit does not change the position order in the
absorption spectra of studied compounds, albeit it shifts the
spectra to the longer wavelengths for about 10 nm (15 nm for
1Nac). However, for 1Nac2 a substantial changes in the shape
of absorption spectrum, comparing to that of 1Nac are
observed, which pointing to a larger participation of the triple
bonds in conjugation between naphthalene substituents. More
polar solvent induce small changes in the position of the
absorption bands, however, in the case of anthryl derivatives
band shape changes (Figs 1-8, ESI). In 2-MeTHF a small
bathochromic shift of the absorption spectrum is observed,
whereas in MeCN a blue shift for a few nanometers occurs.
Simultaneously, with the increase of solvents polarity,
vibrating structure is blurred and the ratio of the vibronic band
changes. This is particularly evident for anthryl and to a lesser
degree for pyrenyl, as well as for di(naphtalen-1-yl)
derivatives.
Fig. 3 Normalized fluorescence spectra of acetylene and buta-
1,3-diyne derivatives in methylcyclohexane (MCH).
Room temperature fluorescence
Similarly as for absorption, second acetylenic unit shifts the
emission spectrum to the red for about a dozen nanometers
for naphthyl and for several nm for anthryl and pyrenyl
derivatives (Table 1 ESI). The emission spectra in 2-MeTHF are
similar to that in MCH, however, they are shifted by several
nm to the red. A greater impact on the shape and position of
the emission band has MeCN. The vibrational structure is
blurred and for anthryl derivatives it completely disappears.
Moreover, emission bands are broader and the ratio of the
intensity of the vibrational peaks changes, in comparison to
that in 2-MeTHF. Such influence of a polarity of solvent
indicates on a fairly large contribution of the emission from
the charge transfer excited state. This statement is supported
The fluorescence spectra of acetylene and buta-1,3-diyne in
MCH are presented in Fig. 3, whereas in 2-MeTHF and MeCN in
Figs 9-16, ESI. These spectra contain all the characteristic
features of the emission spectra of symmetrical derivatives of
acetylene and buta-1,3-diyne, as well as phenyleneethylene
oligomers30,34-37,47-51. Contrary to the DPA for which two
emission band form 11Bu and 11Au were observed40, for studied
compound only one emission band was recorded. The
fluorescence spectra are narrow and exhibit clear vibronic
bands. Moreover, the optical spectra are characterized by a
substantial asymmetry between absorption and fluorescence.
These effects have been explained in terms of torsional
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by femtosecond time-resolved studies of fluorescence spectra acetylene have high fluorescence quantum yield, for pyrenyl
of 1,2-bis(9-anthryl)-acetylene in solvents of different derivative the value is closed to 1. However, 1,3-diyne
polarity51. Fluorescence quantum yield of all acetylene and derivatives, except for pyrenyl one, have much lower
buta-1,3-diyne derivatives in MCH, 2-MeTHF and MeCN are fluorescence quantum yield than acetylene derivatives (Table
gathered in Table 1. Moreover, in Table 2 ESI fluorescence 1 and Table 2 ESI). This indicates on the participation of an
quantum yields and fluorescence lifetimes measured in five additional deactivation process of the excited state. The
additional non-polar solvents differing in viscosity are polarity of solvent has minor influence on the fluorescence
presented.
quantum yield. In acetonitrile it is a bit lower than that in 2-
MeTHF or MCH (Table 1). Also, the solvent viscosity does not
significantly affect the fluorescence quantum yield (Table 2
ESI). The fluorescence lifetimes of studied compounds are also
differentiated as fluorescence quantum yields (Table 1 and
Table 1 ESI). The fluorescence intensity decay for 1Nac, 1Nac2,
Pac and Pac2 is well described by mono-exponential function,
while for 2Nac and 2Nac2 and Aac in viscous solvents by bi-
exponential function. For 1Nac, fluorescence lifetimes are
included in the range from 1.82 to 1.46 ns whereas for 1Nac2
are much shorter (about 0.3 ns). 2-Naphthyl derivatives of
Table 1 Fluorescence quantum yield (ϕ), fluorescence lifetime
(τ), pre-exponential factor (α) and quality of fit χ2R of studied
compounds in MCH, 2-MeTHF and MeCN.
Compound
1Nac
Solvent
MeCN
MeTHF
MCH
ϕ
τ/ns
1.82
1.56
1.69
0.24
0.27
0.29
3.23
0.45
4.02
0.42
4.70
0.48
3.02
0.22
0.23
0.23
2.63
1.75
1.73
5.05
1.83
4.45
1.44
3.72
0.91
1.93
1.54
1.37
1.60
1.30
1.14
α
Χ2
R
0.745
0.841
0.836
0.083
0.165
0.160
0.109
1
1
1
0.98
0.99
0.95
0.89
0.96
0.97
0.95
MeCN
MeTHF
MCH
1
1
1
acetylene
and
buta-1,3-diyne
have
bi-exponential
1Nac2
2Nac
fluorescence intensity decay, however, 2Nac2 has a shorter
decay time. For 2Nac longer component is about 5 ns while
shorter one about 0.5 ns. It is worth to emphasize that
contribution of longer fluorescence lifetime seems to depend
on solvent viscosity. For solvents with a low viscosity, its
contribution to the fluorescence intensity decay is about 4-6%
while for viscous solvent increase to several percent (Table 1
ESI). The major component of the fluorescence lifetimes of
2Nac2 (contribution about 94%) is in the range from 0.2 to 0.3
ns, while for the minor component about 3 ns. Due to the very
low fluorescence quantum yield of this compound, measured
fluorescence lifetimes may be wholly flawed due to Raman
scattering and solvent impurities. The fluorescence lifetime of
Aac in low viscous solvents is about 2 ns while in viscous
solvents the second component (about 0.6 ns) appear with
several percent contribution, whereas Aac2 has two
fluorescence lifetimes in all studied solvents. The major
component (about 90%) is in the range from 0.9 to 1.4 ns
while the minor one in the range from 5 to 3.6 ns. For pyrene
derivatives of acetylene and buta-1,3-diyne mono-exponential
fluorescence intensity decay is recorded. For Pac fluorescence
lifetime is about 1.4 ns in non-polar solvents and 1.93 ns in
polar acetonitrile, while for Pac2 fluorescence lifetimes are
shorter, about 1.2 ns in non-polar solvents and 1.6 ns in
acetonitrile. The photophysical and theoretical studies suggest
that phenyleneethynylenes exist in planar and several twisted
conformations in their ground state due to the nearly free
rotation of the arene rings along the molecular axis. On the
other hand, their emission spectra are highly structured as a
consequence of the planarization of the molecule in the
excited state47-49,55,56. The emission properties (fluorescence
spectra, fluorescence excitation spectra and fluorescence
quantum yield) of 2Nac, 2Nac2 and Aac2 were found to be
independent of excitation/observation wavelengths (Figs 4-7
and Figs 17-24 ESI, Table 3, ESI). The bi-exponential
MeCN
0.063
0.937
0.067
0.933
0.060
0.940
0.014
0.986
1
MeTHF
MCH
0.119
0.110
0.004
0.92
1.12
1.14
MeCN
2Nac2
Aac
MeTHF
MCH
0.006
0.010
0.683
0.660
0.634
0.199
1.15
1.13
1.14
0.97
1.07
0.89
1
MeCN
MeTHF
MCH
1
1
1
MeCN
0.100
0.900
0.118
0.882
0.153
0.847
1
MeTHF
MCH
0.200
0.147
0.98
0.93
Aac2
MeCN
MeTHF
MCH
0.920
0.986
0.975
0.723
0.758
0.779
0.94
0.99
1.06
1.00
0.98
0.97
Pac
1
1
MeCN
MeTHF
MCH
1
1
1
Pac2
Studied compounds have various values of fluorescence
quantum yield which depend primarily on the structure of the
compound, the size of the substituent and substitution sites,
as well as the amount of triple bonds (Table 1). Thus, 1-
naphthyl derivative of acetylene has much higher fluorescence
quantum yield than 2-naphthyl derivative, (about one order of
magnitude). As can be seen from the absorption spectrum, this
seems to be due to the greater involvement of the triple bond
in conjugation with the aromatic system for 2-naphthyl than
for 1-naphthyl derivative. Anthryl and pyrenyl derivatives of
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fluorescence decays can be reasonably explained by the
delayed fluorescence caused by thermal repopulation of the
second excited state, which was reported for
diphenylacethylene derivatives57-59. However, for 2Nac2 and
Aac2, contrary to 2Nac, the energy barrier of rotation in the
excited state is relatively low (about 16 kJ/mol), Table 5 ESI
(for details see Theoretical calculation section,) causing the
crossing through the energy barrier several times during the
lifetime of the excited state. Thus, the recorded fluorescence
excitation/emission spectrum is averaged over emission of
different rotamers and fluorescence intensity decay
heterogeneous.
Fig.
6
The normalized emission spectra of Aac2 in
methylcyclohexane recorded for different excitation
wavelengths. The shoulder in the spectrum recorded at
λex=465 nm is caused by scattered light.
Fig.
4 The normalized emission spectra of 2Nac in
methylcyclohexane recorded for different excitation
wavelengths. The shoulder in the spectrum recorded at
λex=330 nm is caused by scattered light.
Fig. 7 The normalized fluorescence excitation spectra of 2Nac
in methylcyclohexane recorded for different observation
wavelengths. The shoulder in the spectrum recorded at
λobs=475 nm is caused by scattered light
For compounds having
a mono-exponential fluorescence
intensity decay, it is possible to calculate the radiative (kr) and
non-radiative (knr) rate constants, according to equations:
,
where φ is the fluorescence
kr = φ /τ and knr = (1−φ) /τ
quantum yield and τ is fluorescence lifetime. For 1Nac, 2Nac2,
Aac, Pac and Pac2 the corresponding rate constants for the
three solvents used (MeCN, 2-MeTHF and MCH) are
summarized in Table 4 ESI. The fluorescence rate constant is
very similar for all above-mentioned derivatives in the non-
polar 2-MeTHF and MCH while a little lower in MeCN. In
contrast, there is a big difference in the knr rate constant
between acetylene and buta-1,3-diyne derivatives. The non-
Fig. 5 The normalized fluorescence excitation spectra of 2Nac
in methylcyclohexane recorded for different observation
wavelengths.
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radiative rate constant for buta-1,3-diyne is one order higher This may indicate on the transformation to orthogonal TICT
than that for appropriate acetylene derivative. This indicates state, all the more, that the energy barrier of rotation
on the additional deactivation channel of the excited state, decreases with increasing the solvent polarity50. A such
probably related with the greater conformational freedom. dependence is not determined for Aac (Table 2). However,
Also, values of the fluorescence rate constant in acetonitrile, according to Gutierez-Arzuluz et al.51 the relative orientation
smaller than in nonpolar solvents, seem to confirm that the of the two anthracenic groups of the Aac molecule occurs
emission of anthryl and pyrenyl derivatives of acetylene and rapidly over time of about 40 ps, shorter than the resolving
1,3-diyne in this solvent comes from the charge-transfer (CT) power of our instrument. Based on measurements of the
state. Presented in Figs 25-28, ESI time-resolved emission fluorescence quantum yield, it is difficult to conclude about
spectra (TRES) for anthryl and pyrenyl derivatives show the the influence of the solvent viscosity on the photophysical
bathochromic evolution of the fluorescence in a short time properties, however, the measurements of the fluorescence
after excitation. Such a process has been observed for 1,2- intensity decay indicate that the 2-naphthyl and 9-anthryl
bis(9-anthryl)acetylene by Gutierez-Arzuluz et al.51. These derivatives of acetylene can be used as molecular rotor to
authors demonstrated on the basis of femtosecond study the solvent viscosity (particularly for viscous solvents at
fluorescence up-conversion experiments that in acetonitrile lower temperature54).
spectral shift is biphasic, with first sub-picosecond decay time
dictated by solvent response and a much slower one, about 37
Low temperature luminescence
ps, connected with the relaxation of the excited state structure
leading to a change in the relative orientation of the two
anthracenic end group which allows for further stabilization of
the CT state. Thus, before the system is able to relax to the full
CT state, an initial equilibrium between the local excited state
and partial charge-transfer excited state takes place. It seems
that the process of solvent-mediated symmetry reduction in
the excited state, also takes place for anthryl derivative of
buta-1,3-diyne as well as pyrenyl derivative of acetylene and
buta-1,3-diyne and to some extent for the naphthyl derivatives
of acetylene. The broad structureless fluorescence spectra of
anthryl derivatives in MeCN suggests the possibility of the
formation of orthogonal twisted-internal charge transfer (TICT)
state51. The dependence of fluorescence decay times on the
observation wavelengths found for Aac2 (Table 2) suggests the
relatively slow conformation changes.
In order to check the influence of the environment stiffness on
the fluorescence fluorescence intensity, the luminescence
spectra of the studied compounds were measured in glass of
MCH and 2-MeTHF. As the measurements at liquid nitrogen
temperature in 2-MeTHF gave the same results as in MCH,
they are not presented here. Luminescence, fluorescence
excitation and room temperature fluorescence spectra of 1-
naphthyl and 2-naphthyl acetylene and buta-1,3-diyne
derivatives in MCH are presented in Figs 8-11, while for anthryl
and pyrenyl derivatives in Figs 29-32, ESI.
Table 2 Fluorescence lifetime (τ), pre-exponential factor (α)
and quality of fit (χ2 ) of Aac and Aac2 in acetonitrile
determined for different observation wavelengths.
R
Observation
wavelength
/nm
λexc= 420 /nm
α
τ /ns
Χ2
R
5.05
1.83
5.40
1.88
6.06
1.87
1.87
1.87
0.10
0.90
0.04
0.94
0.007
0.993
1
495
515
530
1.10
0.97
0.99
1.09
1.03
560
575
Fig. 8 The normalized emission (solid red line), and excitation
(solid green line) spectrum measured at 77 K and fluorescence
spectrum measured at room temperature (black solid line) of
1Nac in methylcyclohexane.
1
Aac
1
1
1
1
460
475
500
530
570
2.65
2.62
2.65
2.64
2.63
1.11
1.12
1.12
1.03
1.08
1
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Fig. 9 The normalized emission (solid red line), and excitation
(solid green line) spectrum measured at 77 K and fluorescence
spectrum measured at room temperature (black solid line) of
2Nac in methylcyclohexane.
Fig. 11 The normalized emission (solid red line), and excitation
(solid green line) spectrum measured at 77 K and fluorescence
spectrum measured at room temperature (black solid line) of
2Nac2 in methylcyclohexane.
Fluorescence spectra measured at 77
K have a richer
fluorescence excitation spectra for pyrenyl derivative indicates
that long absorption band is a superposition of at least two
absorption transitions. The mirror symmetry indicates on a
hindered ring torsion in the MCH organic glass. It is worth
mentioning that for 2-naphtyl derivatives only a very weak
phosphorescence is detected indicating a low intersystem
crossing rate constant in both acetylene and buta-1,3-diyne
derivatives. For all the studied compounds, the room
temperature fluorescence intensity is less intense than that
recorded at 77 K. The exception are pyrene derivatives (Pac
and Pac2, Figs 31 and 32, ESI) for which room temperature
fluorescence is much more intense, probably because of
solubility at low temperature. It should be noted that for
compounds with high or moderate value of the fluorescence
quantum yield (acetylene derivatives), the increase of
fluorescence intensity with decreasing temperature is
relatively small, contrary to buta-1,3-diyne derivatives (Figs 8-
11). Thus, the measurements in stiff organic glasses confirm
the greater conformational freedom of buta-1,3-diyne
derivatives in comparison to acetylene derivatives56.
vibrational structure than those recorded at room
temperature and are red-shifted for a few nanometers.
Furthermore, they exhibit very small Stokes` shift. In contrast
to the room temperature results, for all studied compounds,
except pyrenyl derivatives, almost mirror symmetric images
between the fluorescence and fluorescence excitation spectra
are recognizable at 77 K. The lack of mirror symmetry between
fluorescence and
Theoretical calculations
To understand the influence of the substituents dimension on
torsional motions of acetylene and buta-1,3-diyne derivatives,
we studied the torsional potential in both the ground and
excited states within the framework of the local mode
approximation using a DFT method. Figs 33-38, ESI show DFT-
based torsional potentials for 1Nac, 2Nac, Aac, 1Nac2, 2Nac2
and Aac2 in the ground and excited states. The torsional
potential in the ground state is well approximated by a simple
periodic potential of the form:
Fig. 10 The normalized emission (solid red line), and excitation
(solid green line) spectrum measured at 77 K and fluorescence
spectrum measured at room temperature (black solid line) of
1Nac2 in methylcyclohexane.
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are also lower than that for diphenylacetylene. The DFT
method gives overestimated values of the energy barrier of
rotation56. However, the experimentally determined
magnitude of the rotational barrier is found to be the 40%
larger than that in a gas phase29, thus they may be close to the
barrier of rotation in low viscous solvent, especially for
polyaromatic hydrocarbon substituents. The rate constant of
the conversion through the energy barrier during the rotation
(kc→t) can be calculated on the basis of transition-state theory
for unimolecular reaction using the equation:
Vmax
(1)
(1− cos2Θ)
V (Θ) =
2
where: Vmax is the barrier height of the torsional potential and
θ is the ring-to-ring torsional angle. The above equation well
describes the torsional potential in the ground state, while
poorly in the excited state, in which the approximation by
Gauss or Lorentz function is much better (Fig 38 ESI). The
energy barrier of rotation in the excited state is much higher
than in the ground state indicating a hampered rotation of
polyaromatic hydrocarbon units in the excited state, which is
attributed to the breaking of the cylindrical structure of the
electron density along the axis of symmetry61 and the adoption
of the partial cummulene-like structure55. For this reason, the
rotation energy barrier for 1- and 2-naphthalene derivatives in
the excited state is high (close to 40 kJ / mol) contrary to
diphenylbutadiyne for which the rotation energy barrier is low
κ
kT
h
(2)
∗e ∗exp(ꢀS / R)∗exp(−ꢀEa / RT)
kc→t
=
where: k is the Boltzmann constant, T is the absolute
temperature, h is Planck`s constant, R is the gas constant, ΔS is
the entropy change, and ΔEa is the energy of activation. It is
difficult to assign a value to the transmission coefficient, κ,
which incorporates all correction factors and uncertainties. We
chose κ=0.4 as for ring inversion66 and ΔS=0 and for these
values the rate constant and rotamer lifetime were calculated.
Rate constant of barrier crossing for the energy barrier of
rotation in the ground state at room temperature (300 K),
gathered in Table 4 ESI, are high and rotamer lifetimes are as
short as tens of femtoseconds, which is tantamount with the
free rotation in the ground state. The energy barriers of
rotation in the excited state (Table 5, ESI) for acetylene
derivatives are in the range from 40 kJmol-1 to 30 kJmol-1,
therefore similar to these for symmetrically substituted
(∆ E~3 kJ/mol). Therefore replacement of the phenyl group
with naphthyl groups eliminates an energy minimum at the
orthogonal geometry in the excited state observed for
diphenylbuta-1,3-diyne62. For 1Nac in the ground and excited
states, the most stable is trans conformation, while cis
conformer is non-planar (the ring-to-ring torsional angle is
about 27 degrees) and less stable for about 0.5 kJmol-1 (Fig. 33
ESI). Due to the lack of steric hindrance for the acetylene and
diacetylene derivatives, the place of substitution in the
naphthalene does not differentiate the rotation energy barrier
as it does for ethylene derivatives63. Theoretical calculations at
the density functional level of theory indicate that in the
ground and excited state anthryl derivative of acetylene is not
fully planar. An angle between the two anthryl unit of about 38
degrees exists in the ground state, with the flat maximum at
90 degrees, in accordance with the data published by Gutierez-
Arzaluz et al.50. However, in the excited state this dihedral
angle decreases to a value of 20 degrees (Fig. 35 ESI). For all
studied buta-1,3-diyne derivatives in the both ground and
excited states the most stable are linear planar conformations
(Figs 36-38 ESI). The barrier heights of the torsional potential
calculated using the equation (1), for all the studied
compounds, are gathered in Table 5, ESI. For acetylene
derivatives the energy barriers of rotation in the ground state
of are slightly higher than these for buta-1,3-diyne derivatives
diphnenylacetylene
derivatives36
or
1,4-
bis(phenylethynyl)benzene55,56. Moreover, the energy barrier
of rotation in the excited state for studied buta-1,3-diyne
derivatives are about 10 kJmol-1 lower than that for acetylene
derivatives (Table 5 ESI) and much lower than that for
symmetrically substituted diphenyl derivatives of buta-1,3-
diyne37. The calculated rate constant of rotation for acetylene
derivatives are low and rotamer lifetimes are long, a hundred
of nanosecond for naphthyl derivatives and about 20 ns for
anthryl derivative (Table 5 ESI). Thus, during the fluorescence
lifetime there is no rotation of aryl substituents in the excited
state. However, lower energy barrier of rotation for buta-1,3-
diyne causing that rotamer lifetime for naphthyl derivatives
are about 100 ps, while for anthryl derivative about 800 ps,
which are comparable to the fluorescence lifetime.
Accordingly, rotation in the excited state may provide to
appear an additional radiationless channel for excited state
deactivation. However, as in the case of the ground state, the
energy barrier of rotation in the excited state seems to be
overestimated. However, taking into account the size of the
substituents and the viscosity of the solvent with a high degree
of probability it can be assumed that during the lifetime of the
excited state the acetylene derivatives do not change their
conformation contrary to the buta-1,3-diyne derivatives.
and
are
comparable
to
the
values
for
diphenylacethylene29,36,37,64 and its symmetrically substituted
derivatives36. Lower values of rotational energy barriers
obtained for 1,3-diyne are in the conformity with the
statement that the insertion of extra –C≡C- unit decreases the
barrier by 40-70%61. The rotational energy barriers calculated
for the polycyclic aromatic hydrocarbon derivatives of buta-
1,3-diyne are comparable with the data obtained for
diphenylbuta-1,3-diyne29,37,46 and its derivatives37. Moreover,
the value of the energy barrier of rotation determined
experimentally for covalently-linked multi-porphyrin arrays65
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A low value of rotation barriers in the ground state and at the
same time large its high value in the excited state should cause
that in the ground state the two conformers of 1Nac and 2Nac
should be almost equally populated and should be distinct
isomers in the excited state. Because two conformers belong
to different symmetry groups and should have different
electronic transition and consequently the different absorption
and emission spectra as in the case of trans-1,2-di(2-
naphthyl)ethane67,68. However, theoretically calculated, using
TD DFT method, the vertical transitions in absorption and
emission for fully optimized structure for both conformers (cis
and trans) of 1Nac and 2Nac reveal a great coincidence of the
vertical transition in absorption and emission of the rotamers
(Table 3). Moreover, the value of the angle between long axis
of molecule and transition dipole moment for both rotamers
differs slightly (Figs 39-42 ESI). Thus, our earlier statement that
fluorescence and fluorescence excitation spectra are excitation
and monitoring wavelength independent is fully consistent
with the calculations.
Fig. 12 The potential-energy profiles of CPh-C
≡C bending angle
in the excited state for 1Nac (red dashed line) and 2Nac (black
solid line).
As shown in Fig. 12 for the naphthyl derivatives of acetylene
large excess of the excitation energy is needed to achieve the
σπ*/ππ* states intersection (about 36 kJmol-1 at bending angle
equal to 135 degrees and 42 kJmol-1 at bending angle equal to
145 degrees for 2Nac and 1Nac, respectively). Moreover, the
σπ* is less stable than linear ππ* for about 18 kJmol-1 for 2Nac
and 40 kJmol-1 for 1Nac. Other results were obtained for buta-
1,3-diyne derivatives. In this case, the presence of two
acetylene units causes that there are two possible bending of
the molecule. One comprising atoms CAr-C≡C, like in acetylene
derivative, and the second one containing C≡C-C atoms. The
results of calculations are presented in Fig. 13.
Table 3 Vertical transition in absorption and emission (in nm)
and oscillator strength (f) for the two rotamers of di-
naphthylacetylene isomers (1Nac and 2Nac) calculated using
TD DFT method (B3LYP, def2TZVP basis set).
1Nc cis
1Nac trans
2Nac cis
2Nac trans
abs
abs
377
328
310
299
299
f
abs
f
abs
350
329
317
303
284
f
f
0.625
378
326
309
300
299
0.655
0.92
351
329
318
304
285
1.01
0.3*10-4
0.5*10-3
0.2*10-5
0.9*10-4
0.2*10-5
0.510-6
0.3*10-4
0.6*10-6
0.1*10-4
0.2*10-3
0.11
0.6*10-6
0.2*10-2
1.0*10-3
0.60
0.56
emission
427
0.71
427
0.74
391
1.08
390
1.20
According to Zgierski et al.38,39 and Wierzbicka et al.36 low
fluorescence quantum yield of DPA and its derivatives and its
strong dependence on temperature is caused by the state
switch from initially excited linear fluorescing ππ* to the bent
σπ* state. The same phenomenon seems to occur for buta-1,3-
diyne and its symmetrically substituted derivatives containing
electron-donating substituents37. Similar calculations to those
described in the above cited works36-39 were carried out to
check the influence of the bending of CAr-C≡C angle on the
potential energy of the excited state, for naphthyl derivatives
of acetylene and buta-1,3-diyne. For other studied derivatives
(anthryl and pyrenyl substituent) such a calculations were not
performed due to the cost.
Fig. 13 The potential-energy profiles of CPh-C≡C (solid lines)
and C-C≡C (dashed line) bending angle in the excited state for
1Nac2 (black solid line and blue dashed line) and 2Nac2 (red
solid line and green dashed line).
Bending of the molecule at the site comprising atoms C≡C-C
atoms is less favourable energetically. The potential energy of
bending curve is increasing without a clear minimum for both
1Nac2 2Nac2. However, bending containing CAr-C≡C leads to
the σπ*/ππ* states intersection for a bending angle about 160
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degrees with the energy barrier about 11 kJmol-1 for 1Nac2
and 9 kJmol-1 for 2Nac2. Moreover, for 1Nac2 the shallow
minimum at bending angle about 135 degrees is observed and
σπ* state is less stable than linear ππ* state for about 7 kJmol-
1, while for 2Nac2 σπ* is a little more stable (1.7 kJmol-1). Thus,
theoretical calculations explain the differences in the
photophysics between acetylene and buta-1,3-diyne
derivatives of polycyclic aromatic hydrocarbon derivatives.
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Conclusions
The polycyclic aromatic hydrocarbon derivatives of acetylene
possess high fluorescence quantum yields and the weak
dependence of fluorescence intensity on temperature. This is
due to stiffening of the molecule in the excited state as a result
of a strong electron coupling between the polycyclic aromatic
substituents. As a consequence the bending of molecule
leading to a σπ* state is highly energetically unfavourable
which is confirmed by theoretical calculations. Thus, they do
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Su,
Synthesis,
electrochemiluminescence
electronic
of
properties
and
donor-substituted
behaviour. For big substituents like pyrenyl and to some extent
anhthryl, buta-1,3-diacetylene derivatives behave as
appropriate acetylene derivatives. However, bicyclic aromatic
substituents (naphthyl) exhibit all the characteristics of the
phenyl derivative of buta-1,3-diyne or acetylene. For 1- and 2-
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Acknowledgements
This work was financially supported under grant UMO-
2011/01/B/ST4/06094 from Polish National Science Centre
(NCN).
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