Influence of an electron-acceptor substituent type on the photophysical properties of unsymmetrically substituted diphenylacetylene

Article abstract of DOI:10.1016/j.jphotochem.2016.03.023

Unsymmetrically substituted diphenylacetylene derivatives containing a different type of an electron-acceptor substituent were synthetized and studied using spectroscopic methods in order to establish the influence of electron affinity of substituent on t

Full text of DOI:10.1016/j.jphotochem.2016.03.023

Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Influence of an electron-acceptor substituent type on the
photophysical properties of unsymmetrically substituted
diphenylacetylene
ꢀ
*
Małgorzata Szyszkowska, Irena Bylinska, Wiesław Wiczk
ꢀ
ꢀ
Faculty of Chemistry, University of Gdansk, Wita Stwosza 63, 80-308 Gdansk, Poland
A R T I C L E I N F O
A B S T R A C T
Article history:
Unsymmetrically substituted diphenylacetylene derivatives containing a different type of an electron-
acceptor substituent were synthetized and studied using spectroscopic methods in order to establish the
influence of electron affinity of substituent on the photo physics of donor-acceptor pairs. Spectral and
photophysical properties were studied in 14 solvents (non-polar, polar aprotic and protic). Influence of
solvent properties on spectral and photophysical properties of studied compounds was analyzed using
Catalan multi-parameter solvent scale. The change of the excited state dipole moment was estimated
based on Bilot-Kawski theory.
Received 25 November 2015
Received in revised form 14 March 2016
Accepted 25 March 2016
Available online 30 April 2016
Keywords:
Diphenylacetylene derivatives
Fluorescence
Internal charge transfer
Solvatochromism
ã 2016 Elsevier B.V. All rights reserved.
Excited state dipole moment
1. Introduction
solution. The calculations made by Zgierski and Lim [24,25] also
predicted that the attachment of an electron-withdrawing groups
Highly conjugated poly(p-phenylenethynylene)s are strongly
fluorescing [1–11]. Likewise, asymmetric acetylene derivatives
containing a polycyclic aromatic or hetero-aromatic hydrocarbon
substituent on one side and phenyl or its derivative on the second
side show intense fluorescence [12–20]. However, 1,2-diphenyla-
cetylene (tolane, DPA) behave quite differently [21–28]. One of the
most interesting photophysical properties of DPA is the loss of
fluorescence that occurs following excitation of higher vibronic
to DPA would increase the energy of the ps* state and the
pp*/ps* state switch would not be expected to occur. Moreover,
the attachment of an electron-donating groups to DPA enhanced
the state switch from the initially excited pp* to the ps* state.
These theoretically predictions have been confirmed experimen-
tally [24,29,30]. Diphenylacetylene and its symmetrically substi-
tuted derivatives were mostly studied with an emphasis on
theoretical calculations and picosecond absorption spectroscopy,
while unsymmetrically substituted diphenylacetylene derivatives
in phenyl rings were mostly studied by picosecond transient
absorption spectroscopy [24,31–33]. Moreover, only for p-cyano-
p0-methylthiodiphenylacetylene [34] and p-cyano-p0-N,N-dime-
thylaminodiphenylacetylene [35] the fluorescence in various
solvents was reported.
1
levels of the B1u state under the collision-free conditions of a
supersonic free jet [21]. It also exhibits a strong temperature
dependence of fluorescence quantum yield in solution [22,23]. The
results of theoretical calculations made by Zgierski and Lim [24–
26] indicate that while the lowest-energy excited singlet state is
the B1u (pp*) state in the linear D2h symmetry, the weakly
fluorescent ps* state of Au symmetry is the lowest in energy in the
bent C2h symmetry. This leads to the crossing of the fluorescent
pp* and weakly fluorescent ps* state potential energy curves
[27,28]. The transition from an initially excited pp* state to the
ps* state requires crossing a small energy barrier, which explains
the loss of fluorescence in the gas phase at higher excitation
energies and the thermally activated quenching of fluorescence in
1.1. Catalan solvent polarity scale
A general model which describes the properties of solvent and
solvation does not exists. As a result, several empirical solvent
polarity scales were proposed to characterize and rank empirically
the polarity of the solvent. To study solute-solvent interactions
multiparameter solvent polarity scales are widely used. The
multiple linear regression approach based on a three-parameter
scale of polarity of the solvent was introduced by Kamlet et al. [36].
* Corresponding author.
E-mail address: wieslaw.wiczk@ug.edu.pl (W. Wiczk).
http://dx.doi.org/10.1016/j.jphotochem.2016.03.023
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
77
Another, extended to a four-parameter, solvent polarity scale was
ꢂ
ꢃ
introduced by Catalan [37] according to the equation:
1
memg
m1
m2
cos
C
¼
ð
mg2
þ
m2e Þ ꢀ
ð
mg2
ꢀ
me2
Þ
ð9Þ
y ¼ y0 þ aSPSP þ bSdPSdP þ cSASA þ dSBSB
ð1Þ
2
where y denotes a solvent-dependent physicochemical property in
a given solvent and y0 the statistical quantity corresponding to the
value of property in the gas phase; SP,SdP, SA, and SB represent
independent solvent parameters accounting for various types of
solute-solvent interactions (SP denotes solvent polarizability, SdP—
solvent dipolarity, SA—solvent’s hydrogen bond donor strength and
SB—hydrogen-bond acceptor strength); aSP, bSdP, cSA, and dSB are
adjustable coefficients that reflect the sensitivity of the physical
quantity y in a given solvent to the various solvent parameters.
One of the most popular solvent polarity scale is the one
parameter solvent polarity scale ET(30) or the normalized ET
N
parameter introduced by Reichardt et al. [41,42]. For non-hydrogen
N
bond donor solvents, solvent polarity parameter ET(30) or ET
describes almost exclusively the electrostatic forces between
solute and solvent [43]. Apart from the dependence of different
spectral and photophysical properties, the correlation of the Stokes
shift with this parameter can be applied to calculate the dipole
moment change between the excited and ground state based on
the equation proposed by Ravi et al. [44,45]:
1.2. Determination of excited state dipole moment of molecule by
solvatochromic method
"
#
ꢀ
ꢁ
2
ꢄ
ꢅ
3
Dm
DmB
aB
a
Dn
¼
ꢀ
nem ¼ 11307:6
ENT þ const
ð10Þ
~
nabs
~
In the case of different electron densities in the electronic
ground and excited state of a light-absorbing molecule, its dipole
moment varies in these two states. Thus, a change of the solvent
affects the ground and excited state differently. For non-polariz-
able solute, a linear dependence of absorption and emission
frequency on the solvent polarity is predicted. Taking into
consideration the linear and quadratic Stark's effect for spherical
molecules with a radius a, and with approximation that the
where mB and aB are the dipole moment change and Onsager’s
radius, respectively, for a pyridinium N-phenolatebetaine dye used
to determine the ET values (mB = 9 D and aB = 6.2 Å),whereas Dm
N
and a are the corresponding quantities for the molecule under
study. The advantage of this equation is to minimize the problem
associated with the Onsager’s radius estimation since a ratio of two
Onsager’s radii is involved in the equation. Thus, based only on the
Stokes’ shift the change of the dipole moment in excited state can
be calculated.
Because there is not much information about fluorescence
spectroscopy of unsymmetrically substituted DPA derivatives, here
we present results of the photophysical studies of substituted DPA
derivatives containing an electron-accepting substituent (nitrile,
ester or aldehyde group) or an electron-donor group (N,N-dimethyl
isotropic polarizability
a of the solute of a molecule is
2
a
=4pe0a3 ¼ 1, where a is Onsager’s radius and e0 vacuum
permittivity, following equations, obtained by Bilot–Kawski
[38–40], allow to compute dipole moment in the excited state:
~
nabs
~
ꢀ
nem ¼ m1fBKð
er; nÞ þ const
ð2Þ
amino) as well as an electron-donor acceptor pairs (D-p-A) in
~
nabs
~
þ
nem ¼ ꢀm2½fBKð
e
r; nÞ þ 2gðnÞꢁ þ const
ð3Þ
which as the electron-donor N,N-dimethyl amino group and the
electron-acceptor nitrile or ester or aldehyde groups are present.
The studies include measurements of the absorption and emission
spectra as well as time-resolved fluorescence in different solvents
to investigate the effects of type of substituent(s) on the
photophysical properties of compound studied. Moreover, the
the solvent polarizability functions fBK
(3) are given by Eqs. (4) and (5):
(
er,n) and g(n) in Eqs. (2) and
ꢀ
ꢁ
2n2 þ 1 er ꢀ 1 n2 ꢀ 1
fBK
ð
e
r; nÞ ¼
ꢀ
ð4Þ
n2 þ 2 er þ 2 2n2 þ 1
influence of solvents properties on the photophysics on D-p-A
compounds using the Catalan solvent polarity scale are analyzed.
Based on the solvatochromic method the excited state dipole
moment are calculated.
The structure, atom numbering and abbreviation of studied
compounds are presented in Fig. 1.
3ðn4 ꢀ 1Þ
gðnÞ ¼
ð5Þ
2
2ðn2 þ 2Þ
Using a slope obtained from Eqs. (2) and (3) and knowing the
ground-state dipole moment mg, the excited-state dipole moment
me, can be calculated using the following equations:
2. Materials and methods
2
ð
ꢀ
Þ
ð
me2
þ
m2g ꢀ 2memgcos
pe0hca3
CÞ
~
me
~
mg
2.1. Synthesis
m1
¼
¼
ð6Þ
2
pe0hca3
2
Studied compounds were synthetized according to Scheme 1.
Iodobenzene, 4-iodobenzonitrile, methyl-4-iodobenzoate,
phenylacetylene, 4-ethynyl-N,N-dimethylaniline and bis(triphe-
nylphosphine)palladium(II) dichloride (Sigma–Aldrich) and
4-bromobenzaldehyde (Lancaster) were commercially available
and used without further purification.
The progress of all reactions was monitored by TLC using Merck
plates, Kieselgel 60 F254. The products were isolated by means of
column chromatography (Merck Kieselgel 60 (0.04–0.063 mm))
ð
me2
ꢀ
mg2
Þ
m2
¼
ð7Þ
2
pe0hca3
In these equations me and mg are the dipole moments in the excited
and ground state, respectively, a is Onsager’s interaction radius of
the solute, h is Planck’s constant, c is the velocity of light in vacuum,
n—refractive index, er—relative dielectric constant and e0 is the
permittivity of vacuum, thus 2pe0hc = 1.105110440 ꢂ10ꢀ35 C2.
Generally, the dipole moments me and mg are not parallel to each
and/or semi-preparative RP-HPLC (Kromasil column, C-8, 5
250 mm long, i.d. 20 mm).
mm,
other but make an angle
c
. The use of Eqs. (6) and (7) leads to:
The identification of the product was based on: 1H and 13C NMR
spectra (Bruker AVANCE III 500 MHz spectrometer) in CDCl3, mass
spectra (Bruker Daltonics HCTultra instrument) and Raman spectra
(FRA-106 instrument).
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
me
¼
ð
mg2
þ
m2hca3
ð8Þ
2

78
M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
Fig. 1. Structure, atoms numbering and abbreviation of the studied compounds.
Scheme 1. Synthesis of unsymmetrical diphenylacetylene derivatives.

M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
79
1,0
0,9
1,0
0,8
0,6
0,4
0,2
0,0
COOMe
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
MeCN
CH
MeOH
iPrOH
MeCN
CH
Me-THF
DMF
DMSO
iPrOH
260
280
300
320
340
360
380
240
260
280
300
320
340
Wavelength [nm]
Wavelength [nm]
Fig. 3. Normalized absorption spectra of DMAPhacPh in selected solvents.
Fig. 2. Normalized absorption spectra of PhacPhCOOMe in selected solvents.
Details of synthesis and identification of studied compounds
are presented in ESI.
exponents according to Eq. (11):
X
IðtÞ ¼
aiexpðꢀt=ti
Þ
ð11Þ
i
2.2. Spectroscopic measurements
where ai is the pre-exponential factor obtained from the
fluorescence intensity decay analysis and ti the decay time of
the i-th component, using a software supported by the manufac-
turer. The adequacy of the exponential decay fitting was judged by
visual inspection of the plots of weighted residuals as well as by the
statistical parameter x2R and shape of the autocorrelation function
of the weighted residuals and serial variance ratio (SVR).
Linear and multi-parametric correlations were performed using
Origin v. 9.0 software.
The UV–vis absorption spectra of studied compounds in various
solvents were measured using
a PerkinElmer Lambda 40P
spectrophotometer, whereas emission spectra were measured
using a FluoroMax-4Horriba-Yobin spectrofluorimeter. Solvents of
the highest available quality (spectroscopic or HPLC grade) were
applied.
Fluorescence quantum yields were calculated using: 2-amino-
piridine in 0.1 M H2SO4 (QY = 0.60) (for DMAPhacPh, PhacPhCHO
and PhacPhCN, PhacPhCOOMe) and quinine sulphate in 0.5 M
H2SO4 (QY = 0.546) (for DMAPhacPhCHO, DMAPhacPhCOOMe and
DMAPhacPhCN) as reference and were corrected for different
refractive indices of solvents. In all fluorimetric measurements, the
optical density of the solution does not exceed 0.1. The fluorescence
lifetimes were measured using a time-correlated single-photon
counting apparatus (FT300 PicoQuant fluorescence lifetimes
spectrometer) using subnanosecond pulsed diodes as the excita-
tion source: PLS-290 (for PhacPhCHO, PhacPhCOOMe and
PhacPhCN), PLS-320 for DMAPhacPh, PLS-340 (for DMAPhAcPh-
COOMe) and LDH-375 (for DMAPhacPhCHO and DMAPhacPhCN).
The half-width of the response function of the apparatus,
measured using a Ludox solution as a scatter, was about 1.0 ns
for pulse diode and about 150 ps for a diode laser. The emission
wavelengths were isolated using a monochromator. Fluorescence
decay data were fitted by the iterative convolution to the sum of
3. Results and discussion
3.1. Absorption spectroscopy
Absorption spectra of PhacPhCOOMe in selected solvents is
presented in Fig. 2 while for PhacPhCN and PhacPhCHO in
Figs. 1 and 2 ESI.
For diphenylacetylene derivatives containing an electron-
acceptor substituent the long-wave absorption band is shifted to
the longer wavelength in comparison with parent molecule, while
at shorter wavelengths than for symmetrically di-substituted DPA
derivatives [29]. The polarity of solvent has a minor influence on
the position of the absorption spectrum. For PhacPhCOOMe the
maximum of absorption is located at 294 nm in polar acetonitrile,
while at 297 nm for non-polar cyclohexane (Table 1). Changing a
Table 1
Spectral (nabs and
(
nfluo) and photophysical parameters (fluorescence quantum yield (w), fluorescence lifetime (t), Stokes shift (Dn)), pre-exponential factor (a), quality of fit
x2R) and radiative (kf) and non-radiative (knr) rate constants of PhacPhCOOMe in different solvents.
solvent
nabs (cmꢀ1
)
nfluo (cmꢀ1
)
Dn (cmꢀ1
)
w
t
(ns)
a
x2
kf ꢂ 10ꢀ8 (sꢀ1
)
knr ꢂ 10ꢀ8 (sꢀ1
)
R
MeOH
i-PrOH
hexane
CH
hexadecane
1-Cl-hexadecane
MeCN
32072
31867
31766
31586
31516
31526
32092
27503
28316
31348
31133
31027
30581
28265
4569
3551
418
453
489
0.305
0.398
0.420
0.421
0.429
0.441
0.277
0.62
0.64
0.55
0.59
0.55
0.59
0.52
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.14
1.05
0.98
1.19
1.03
1.10
1.19
4.9
6.1
7.6
7.1
7.6
7.5
5.3
11.2
10.0
10.5
9.8
10.4
9.5
945
3827
14.9

80
M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
Table 2
Spectral (nabs and nfluo) and photophysical parameters (fluorescence quantum yield (w) fluorescence lifetime (t), Stokes shift (Dn)), pre-exponential factor (a), quality of fit
(
x2R) and radiative (kf) and non-radiative (knr) rate constants of DMAPhacPh in different solvents.
solvent
nabs (cmꢀ1
)
nfluo (cmꢀ1
)
Dn (cmꢀ1
)
w
t
(ns)
a
x2
kf ꢂ 10ꢀ8 (sꢀ1
)
knr ꢂ 10ꢀ8 (sꢀ1
)
R
MeOH
i-PrOH
Iso-octane
hexadecane
1-chlorohexadecane
CH
toluene
acetone
1,4-dioxane
THF
30864
31114
31746
31556
30303
31556
30647
30358
30618
30414
26882
27397
29155
28820
27740
28986
27322
24361
26846
25907
3982
3717
2592
2735
2563
2570
3324
5998
3773
4508
0.010
0.011
0.280
0.372
0.409
0.350
0.390
0.103
0.320
0.244
>0.15*
>0.17*
0.35
0.43
0.66
0.38
0.58
1.83
0.71
0.34
1.52
0.37
1.26
0.35
1.21
1.16
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
0.789
0.211
0.951
0.049
0.929
0.071
1.000
0.906
0.094
0.988
0.012
0.525
0.475
1.26
1.16
1.17
0.93
0.98
0.99
0.94
1.13
1.12
1.28
–
–
–
–
8.0
8.7
6.2
9.2
6.7
0.6
4.5
–
20.6
14.6
9.0
17.1
24.1
4.9
9.6
–
MeTHF
Et2O
30572
30845
26490
27137
4082
3708
0.314
0.110
1.16
–
–
–
–
0.97
AcOEt
DMF
30534
30003
26144
23895
4391
6108
0.250
0.300
0.95
1.10
–
–
–
–
1.06
3.02
0.71
4.65
0.77
1.78
DMSO
MeCN
29735
30312
23613
24067
6123
6245
0.290
0.102
0.98
1.10
–
–
–
–
solvent from non-polar CH to polar MeCN did not cause a
significant changes in the shape of the long-wavelength absorption
band however, in polar acetonitrile and protic solvents a small
blue-shift and less distinct vibronic structure is observed. Similarly
absorption spectra were recorded for nitrile and aldehyde
derivatives with the difference that for aldehyde derivative the
maximum of absorption is additionally red-shifted for about 10 nm
compared to that of nitrile and ester (Tables 1 and 2 ESI). Moreover,
absorption spectra of PhacPhCHO possess a long-wavelength tail
(Fig. 2 ESI).
maximum of absorption band to the longer wavelength, while in
protic one a small blue-shift is observed (Fig. 3).
A similar influence of solvents on position of absorption band
was observed by Hirata et al. for aminophenyl(phenyl)acetylene
[33].
Normalized absorption spectra of D-p-A pairs in selected
solvents are presented in Figs. 4–6.
For compounds containing an electron-donor and an electron-
acceptor substituents (D- -A) absorption spectra are red-shifted
p
for about 40 nm (for DMAPhacPhCN and DMAPhacPhCOOMe)
(Figs. 4 and 5), while for aldehyde derivative for about 60 nm
(Fig. 6) compared to that of DMAPhacPh (Fig. 3). In non-polar
solvents the vibronic structure is preserved although the shape of
the spectrum is changed depending on the substituent. In the
remaining solvents studied absorption bands are unstructured and
red-shifted with increasing solvent polarity. The maxima of
absorption spectra in all solvents studied are gathered in
Tables 3–5 for DMAPhacPhCN, DMAPhacPhCOOMe and DMA-
PhacPhCHO, respectively.
A greater impact on the position of the absorption band exerts
an electron-donor substituent (Fig. 3).
The position of maximum of absorption is red-shifted compared
to the derivatives containing an electron-acceptor substituent
(Table 2). Moreover, in non-polar solvents absorption spectrum
has a less clearly marked vibronic structure than for an electron-
acceptor containing derivatives, which disappears in more polar
non-protic and protic solvents. Additionally, polar solvents shift the
MeCN
CH
1,0
1,0
0,9
MeTHF
DMF
DMSO
iPrOH
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
0,8
MeCN
CH
0,7
MeTHF
0,6
0,5
0,4
0,3
0,2
0,1
0,0
DMF
DMSO
iPrOH
280
300
320
340
360
380
400
420
440
280
300
320
340
360
380
400
420
440
Wavelength [nm]
Wavelength [nm]
Fig. 4. Normalized absorption spectra of DMAPhacPhCN in selected solvents.
Fig. 5. Normalized absorption spectra of DMAPhacPhCOOMe in selected solvents.

M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
81
3.2. Fluorescence spectroscopy
MeCN
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
CH
Fluorescence spectra for PhacPhCOOMe are presented in Fig. 7,
while for PhacPhCN in Fig. 3 ESI. Because of PhacPhCHO is very
weakly fluorescent its spectra are not presented.
MeTHF
DMF
DMSO
iPrOH
For diphenylacetylene derivatives containing an electron-
acceptor group their fluorescence spectra in the same solvent
are similar to each other. In non-polar solvents emission spectra
shown the same features as fluorescence spectra of symmetrically
substituted diphenylacetylene [29] with well-formed vibronic
structure. Moreover, in non-polar solvents substantial asymmetry
between absorption and emission spectra is observed. Such
phenomenon have been observed also for arylethynyl derivatives
of N,N-dimethylaniline [19,20] as well as for phenylene ethynylene
oligomers [29,46–49] and have been explained in terms of
torsional disorder and quadratic coupling between the ground
and the first excited state [50] or the exciton model developed by
Liu et al. [51]. In polar protic and aprotic solvents the emission
spectrum shift bathochromically and lose their vibration structure.
There is a certain difference in the position of emission band in
protic solvents for ester and nitrile derivatives. For PhacPhCN the
position of maximum of fluorescence spectrum in acetonitrile
coincidence with methanol, whereas for PhacPhCOOMe with that
280 300 320 340 360 380 400 420 440 460 480
Wavelength [nm]
Fig. 6. Normalized absorption spectra of DMAPhacPhCHO in selected solvents.
Table 3
Spectral (nabs and
and radiative (kf) and non-radiative (knr) rate constants of DMAPhacPhCN in different solvents.
nfluo) and photophysical parameters (fluorescence quantum yield w) fluorescence lifetime (t) Stokes shift (Dn)), pre-exponential factor (a )
), quality of fit (x2
R
solvent
nabs (cmꢀ1
)
nfluo (cmꢀ1
)
Dn (cmꢀ1
)
w
t
(ns)
–*
0.31
a
x2
kf ꢂ 10ꢀ8 (sꢀ1
–
–
)
knr ꢂ 10ꢀ8 (sꢀ1
–
–
)
R
MeOH
i-PrOH
27330
27200
18281
19608
9049
7592
0.006
0.032
–
–
0.250
0.750
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.16
0.53
0.59
0.62
1.54
0.67
1.52
1.26
2.27
1.26
2.55
2.05
2.25
0.77
0.60
0.64
iso-octane
hexadecane
1-chlorohexadecane
CH
toluene
acetone
1,4-dioxane
THF
MeTHF
Et2O
AcOEt
DMF
26900
26596
26738
26640
26820
27200
27320
26960
27160
27420
27260
26820
26550
27310
26316
25974
23607
26110
23419
18416
22421
19802
21367
22075
20747
17668
17452
17513
584
622
3131
530
0.300
0.33
1.07
0.93
0.98
1.11
1.07
1.10
1.17
1.12
1.06
1.17
1.10
1.00
1.12
1.07
5.1
5.3
3.1
5.7
3.2
0.5
1.8
0.8
1.1
1.6
0.8
0.4
0.4
0.4
11.9
10.8
3.5
9.3
3.4
7.4
2.6
7.2
2.8
3.3
3.7
12.6
16.3
15.3
0.455
0.380
0.490
0.066
0.400
0.099
0.290
0.330
0.170
0.029
0.024
0.024
3401
8804
4809
7158
5793
5345
6513
9152
9098
9797
DMSO
MeCN
Table 4
Spectral (nabs and
(
nfluo) and photophysical parameters (fluorescence quantum yield (w) fluorescence lifetime (t), Stokes shift (Dn)), pre-exponential factor (a), quality of fit
x2R) and radiative (kf) and non-radiative (knr) rate constants of DMAPhacPhCOOMe in different solvents.
solvent
nabs (cmꢀ1
)
nfluo (cmꢀ1
)
Dn (cmꢀ1
)
w
t
(ns)
–*
1.32
a
x2
kf ꢂ 10ꢀ8 (sꢀ1
–
)
knr ꢂ 10ꢀ8 (sꢀ1
–
)
R
MeOH
i-PrOH
hexane
iso-octane
hexadecane
1-Cl-hexadecane
CH
toluene
1,4-dioxane
THF
227855
27739
27435
27360
26991
27816
27064
27624
27739
27633
28209
28209
21505
22650
26490
27027
26144
23474
26247
23420
22222
20747
21528
21810
6350
5089
945
333
847
0.001
0.013
0.310
0.314
0.339
0.351
0.324
0.428
0.303
0.150
0.248
0.216
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
0.669
0.331
1.000
1.000
1.000
1.000
0.93
0.96
1.00
0.88
0.89
1.09
0.98
1.09
1.01
1.03
0.93
0.87
0.1
4.2
4.2
3.7
2.6
3.6
2.8
1.3
0.6
0.9
–
7.5
9.5
9.2
7.2
4.8
7.5
3.8
2.9
3.5
2.8
–
0.73
0.75
0.92
1.35
0.90
1.51
2.38
2.43
2.68
0.40
2.09
2.47
1.70
1.92
1.48
4342
817
4204
5517
6886
6681
6399
MeTHF
Et2O
AcOEt
DMF
DMSO
MeCN
28011
27211
27027
27739
21254
19084
18232
18382
6757
8127
8795
9357
0.140
0.065
0.053
0.134
0.84
0.99
1.03
1.00
0.6
0.4
0.3
0.9
3.5
5.5
4.9
5.8

82
M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
Table 5
Spectral (nabs and nfluo) and photophysical parameters (fluorescence quantum yield (w) fluorescence lifetime (t) Stokes shift (Dn)), pre-exponential factor (a), quality of fit
(
x2R) and radiative (kf) and non-radiative (knr) rate constants of DMAPhacPhCHO in different solvents.
solvent
nabs (cmꢀ1
)
nfluo (cmꢀ1
)
Dn (cmꢀ1
)
w
t
(ns)
a
x2
kf ꢂ 10ꢀ8 (sꢀ1
)
knr ꢂ 10ꢀ8 (sꢀ1
)
R
MeOH
i-PrOH
26281
26281
20277
20243
6004
6038
0.006
0.017
*
–
–
–
–
–
–
0.23
1.39
0.74
0.71
0.92
0.98
2.15
0.95
2.08
0.98
0.54
1.83
0.55
2.09
1.02
1.85
0.35
1.47
0.96
1.75
0.19
1.26
2.81
0.49
2.62
0.62
3.90
0.919
0.081
1.000
1.000
1.000
0.358
0.642
0.355
0.645
1.000
0.229
0.771
0.887
0.113
0.596
0.404
0.901
0.099
0.704
0.296
0.360
0.520
0.120
0.657
0.343
0.955
0.045
1.13
hexane
Iso-octane
hexadecane
1-Cl-hexadecane
26076
26110
25641
26212
25220
25126
24845
21622
856
984
796
0.044
0.056
0.093
0.363
1.18
1.00
1.09
1.13
0.6
0.8
1.0
–
12.9
13.3
9.8
–
4590
toluene
26212
21575
4637
0.337
0.92
–
–
CH
1,4-dioxane
25773
26666
24907
19881
866
6785
0.047
0.254
1.12
1.09
–
–
–
–
THF
26350
26178
26846
26666
26350
18726
19324
19880
19881
17513
7624
6854
6966
6785
8837
0.131
0.223
0.201
0.174
0.046
0.89
1.11
–
–
–
–
–
–
–
–
–
–
MeTHF
Et2O
0.97
0.94
0.94
AcOEt
DMF
DMSO
MeCN
25840
26666
17286
17360
8554
9306
0.037
0.012
1.04
1.11
–
–
–
–
measured in 2-propanol, while in methanol emission spectrum of
PhacPhCOOMe is most shifted to the red. This can be explained by
the hydrogen bond network formation with protic solvent
molecules. Both, the shape of emission band and the Stokes shift
(Table 1 and Table 1 ESI) indicates that in non-polar solvents the
emission of PhacPhCOOMe, PhacPhCN and DMAPhacPhCHO take
place from local excited state, whereas in polar solvents from
internal charge transfer state.
Fluorescence quantum yields of PhacPhCOOMe and PhacPhCN
are quite large in the range of 0.3–0.5 (Table 1 and Table 1 ESI).
Moreover, they are a little lower in polar and protic solvents
compared to non-polar ones. However, for PhacPhCHO fluores-
cence quantum yields are very low, lower than 0.001 (Table 2 ESI)
indicating that the emission occurs from the n
confirmed by absorption spectra in which the n
p
* with state. This is
* transition
manifests itself as the long-wavelength tail as a result of the
overlap of the intensive pp* with the n * transition. Similarly to
p
p
the fluorescence quantum yields also fluorescence lifetimes of
PhacPhCOOMe and PhacPhCN are similar (Table 1 and Table 1 ESI),
however for PhacPhCOOMe they are a little longer, especially in
polar and protic solvents. Fluorescence intensity decays for both
PhacPhCOOMe and PhacPhCN are mono-exponentials in all
studied solvents. For ester derivative are in the range of 0.5 ns
to 0.6 ns and are independent on solvent polarity, whereas for
PhacPhCN are about 0.5 ns in non-polar solvent and shorter (0.3 ns)
in polar and protic solvent.
Above presented results indicate that diphenylacetylene
derivatives containing one an electron-acceptor group behave
like symmetrically substituted derivatives in which the lowest
excited state is the pp* of a linear structure which does not
intersect with the sp* for the bent structure [24–31]. It seems that
the linear structure in the excited state is stabilized by the internal
charge transfer from the phenyl group to the substituted phenyl
group. However, a small solvatochromic effect indicated that
excited state dipole moment is not much bigger than in the ground
state.
1,0
0,8
0,6
0,4
0,2
0,0
COOMe
MeCN
CH
Hexane
Hexadecane
1-Cl-hexadecane
MeOH
Fluorescence spectra of diphenylacetylene derivative contain-
ing an electron-donor substituent (DMAPhacPh) in selected
solvents are presented in Fig. 8
iPrOH
N,N-Dimethylamino as an electron-donor substituent influ-
enced on the emission spectra of diphenylacetylene quite
differently than an electron-acceptor substituent does. The
emission spectra are much more bathochromically shifted and
their shape is strongly modified by the solvent polarity. In the case
of DMAPhacPh the maxima of emission bands are at 346 nm in CH,
360 nm in 1-Cl-hexadecane, 419 nm in MeCN, 427 nm in DMSO and
420 nm in MeOH (Fig. 8). A characteristic features include the dual-
band emission in polar solvents and long-wavelength tail in
medium polarity solvents like: MeTHF, 1,4-dioxane or ethyl
325
350
375
400
425
450
475
Wavelength [nm]
Fig. 7. Normalized fluorescence spectra of PhacPhCOOMe in selected solvents,
lexc = 295 nm.

M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
83
MeOH
MeCN
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
CH
1,0
MeOH
MeCN
CX
Dioxane
Me-THF
0,9
AcOEt
DMF
Dioxane
MeTHF
AcOEt
DMF
0,8
DMSO
iPrOH
0,7
1-Cl-hexadecane
hexadecane
DMSO
iPrOH
0,6
1-Cl-hexadecane
hexadecane
0,5
0,4
0,3
0,2
0,1
0,0
400
450
500
550
600
650
700
350
400
450
500
550
600
650
Wavelength [nm]
Wavelenght [nm]
Fig. 10. Normalized fluorescence spectra of DMAPhacPhCOOMe in selected
solvents, lexc = 350 nm.
Fig. 8. Normalized fluorescence spectra of DMAPhacPh in selected solvents,
lexc = 320 nm.
acetate. Such a phenomenon was observed by Hirata et al. [33] for
aminophenyl(phenyl)acetylene and interpreted as an emission
from both the locally excited and charge-transfer state. It is worth
noting that the phenomenon of dual emission does not occur for
acetylene derivatives containing N,N-dimethylaminophenyl and
polycyclic aromatic hydrocarbon as substituents [20]. Fluores-
cence quantum yields as well as fluorescence lifetime are diverse
and depend on the type of solvent (Table 2). In protic solvents both
fluorescence quantum yield and fluorescence lifetime are small
compared to the other solvents studied probably due to the
formation of hydrogen bond with solvent as in the case of
derivatives containing polycyclic aromatic hydrocarbon as substit-
uent [20].
For the donor model compound DMAPhacPh fluorescence
lifetimes are more diversified and depend on the solvent (Table 2).
In protic solvent they are too short to be measured using our
equipment, and according to Hirata et al. [31,33] are shorter than
60 ps. For the saturated hydrocarbon solvents a single exponential
fluorescence intensity decay is observed with the lifetime about
0.5 ns. In medium polar and polar solvent bi-exponential function
is needed to correctly fitted to the fluorescence intensity decay
(Table 2). However, is a certain inconsistency between our results
and those published by Hirata et al. [31,33]. Contrary to us, Hirata
et al. using a streak-camera technique found that in DMF, THF,
diethyl ether DMAPhacPh has a single fluorescence lifetime.
Fluorescence spectra of D-p-A pairs are presented in Figs. 9–11.
In these cases emission spectra have the form of a single band.
Fluorescence spectra in saturated hydrocarbons possesses vibronic
structure, while a structureless broad fluorescence with a quite
large solvatochromic shift in polar solvents is observed (Tables
3–5). For DMAPhacPhCN they are similar to previously published
by Hirata [31] and the maxima of emission bands are at 383 nm in
CH, 425 nm in 1-Cl-hexadecane, 474 nm in MeTHF, 548 nm in
MeCN, 570 nm in DMSO and 545 nm in MeOH (Fig. 9 and Table 3).
In the similar range are located the emission spectra of
DMAPhacPhCOOMe, except that they are shifted slightly toward
the short-wavelength (the maxima of emission bands are at
381 nm in CH, 427 nm in 1-Cl-hexadecane, 465 nm in MeTHF,
540 nm in MeCN, 547 nm in DMSO and 465 nm in MeOH) (Fig. 10
and Table 4). The most bathochromic shift of emission spectra are
recorded for DMAPacPhCHO (at 401 nm in CH, 462 nm in 1-Cl-
hexadecane, 517 nm in MeTHF, 576 nm in MeCN, 580 nm in DMSO
and 477 nm in MeOH) (Fig. 11 and Table 5). The absorption
DMAAcPhCHO
MeOH
MeCN
1,0
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
CH
MeOH
1,0
Dioxane
MeTHF
EtOAc
MeCN
CH
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
dioxane
MeTHF
AcOEt
DMF
DMSO
iPrOH
1-Cl-hexadecane
hexadecane
DMF
DMSO
iPrOH
1-Cl-hexadecane
hexadecane
400
450
500
550
600
650
700
400
450
500
550
600
650
700
Wavelength [nm]
Wavelength [nm]
Fig. 9. Normalized fluorescence spectra of DMAPhacPhCN in selected solventsm,
lexc = 355 nm.
Fig. 11. Normalized fluorescence spectra of DMAPhacPhCHO in selected solvents,
lexc = 350 nm.

84
M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
spectrum shows only a little red shift and a slight broadening with
increasing solvent polarity, while the emission spectra show a
large bathochromic shift. Also, Stokes shift which is large in polar
solvent and small in non-polar ones. These results indicate that in
non-polar solvents the emission take place from the local excited
state whereas in polar solvent from the internal charge transfer
state [31]. A trough-bond charge-transfer mechanism in the
excited state for DMAPhacPhCN was also postulated by Ma et al.
[52] based on the theoretical calculations.
Table 1 ESI for PhacPhCOOMe and PhacPhCN, respectively, may be
noted that the electron-affinity of electron-acceptor substituent
modifies photophysical properties of compounds studied. Lower
fluorescence quantum yield of PhacPhCN containing a stronger
electro-acceptor substituent than that of PhacPhCOOMe are
caused be higher values of non-radiative rate constant at
comparable values of radiative rate constant. Due to the fact that
for DMAPhacPh, containing electron-donor substituent, mono-
exponential decays of the fluorescence intensity was recorded in
other solvents than for derivatives containing an electron-acceptor
substituent the influence of type of the substituent on the
photophysical properties cannot be evaluated. Based on
Tables 3 and 4 is difficult to unambiguously evaluate the effect
of electron affinity of the substituent on photophysical properties
of D-A pairs. However, in polar solvents for both compounds the
radiative rate constant is lower than in non-polar solvents while
non-radiative rate constant is higher in nonpolar solvents with the
exception for strongly polar DMF, DMSO and MeCN for DMA-
PhacPhCN (Table 3).
As shown by Krystkowiak et al. [53] 1-chloro-n-alkanes are the
best solvents that interact only non-specifically with many
fluorescence probes [20,54–56] because they do not have
p
-electrons, do not form hydrogen bond and do not have
charge-transfer character. However, comparing the emission
spectra of DMAPhacPh and D- -A pairs recorded in hexadecane
p
and 1-chloro-hexadecane (Figs. 9–11) one can see a big difference
in the shape and position of maximum of emission spectra in these
two solvents. In hexadecane, like in other saturated hydrocarbon
solvents, emission spectra possess a clear vibronic structure, while
in 1-chloro-hexadecane are structureless, and substantial Stokes
shift is recorded (Tables 3–5), similarly to the CT emission in polar
solvents. However, a such differences are not seen in emission
spectra of diphenylacetylene derivatives studied containing an
electron-acceptor substituent (Fig. 7 and Figs. 3 and 4 ESI). This can
be explained by interactions between the permanent solvent
dipole and the permanent and induced dipoles of studied
compounds [57], which results in emission from the charge-
transfer. If a such weak interactions of bond dipole of solvent cause
the states inversion this means that the difference of energy
between these states is small.
3.3. Multiple-linear correlation
In order examine in more detail the solute-solvent interactions
and know which solvent properties have the greatest impact on the
spectral and photophysical parameters studied compounds
multiple-linear correlation according to Catalan equation
(Eq. (1)) for DMAPhacPh and D-p-A pairs was carried out and
obtained results are gathered in Table 6.
For DMAPhacPh a good correlation of the absorption band
maximum was obtained (R2adj = 0.9341) with the largest impact of
solvent polarizability and dipolarity.
Also noteworthy is a significant blue-shift of emission spectra in
protic solvent observed for DMAPhacPhCOOMe and DMAPhacPh-
CHO in comparison to that of DMAPhacPhCN. It seems that the
hydrogen bonding in protic solvents may be responsible for this. As
However, for D-p-A pairs the correlation is much worse
(R2adj = 0.5794, R2adj = 0.3232, R2adj = 0.5133 for DMAPhacPhCN,
DMAPhacCOOMe and DMAPhacCHO, respectively). The largest
impact has the solvent polarizability, causing the bathochromic
shift. Moreover, for DMAPhacPhCHO absorption some impact has
also solvent dipolarity, whereas for DMAPhacPhCN and DMA-
PhacPhCOOMe solvent basicity. Emission band maximum and
Stokes shift correlate well with Catalan solvent polarity scale for all
studied compounds. For DMAPhacPh and DMAPhacPhCN exclu-
sively solvent dipolarity causes the bathochromic shift of fluores-
cence spectra and increase Stokes shift. For DMAPhacPhCHO
beside solvent dipolarity also solvent basicity play substantial role,
whereas for DMAPhacPhCOOMe solvent acidity with opposite
effect to solvent dipolarity.
ꢀ
shown by Bylinska at al. [20] and Hirata et al. [35] molecules
containing N,N-dimethylaniline and aromatic hydrocarbon linked
by the acetylene unit form in excited state a hydrogen bond
between amino group and protic solvent causing blue-shift and
broadening of emission band. In the case of the studied
compounds, lower electron affinity of ester and aldehyde group
than that of nitrile increase the electron density on nitrogen atom
of amino group increasing the strength of hydrogen bond and
thereby reducing the charge separation. Additionally, there is the
possibility of hydrogen bond formation by the ester or aldehyde
group with protic solvent.
The fluorescence intensity decay of DMAPhacPhCN and
DMAPhacPhCOOMe in all studied solvents (except in alcohols
for DMAPhacPhCN and in ethyl ether for DMAPhacPhCOOMe) are
For two D-A pairs for which received a sufficient number of
points correlations of radiative (kr) and non-radiative rate (knr
)
constant with Catalan solvent polarity scale was carried out. The
radiative rate constant in both cases correlates well R2adj ꢃ 0.9 and
solvent dipolarity and solvent basicity play substantial role
lowering its value. In contrast, non-radiative rate constant
indicates a weak correlation 0.5 ꢄ R2adj ꢃ 0.7, and the value of this
constant depends only on solvent acidity and basicity (with the
opposite signs).
Due to various standard deviations of the measured values and
the independent solvent parameters coefficients in Catalan
equation, more quantitative measure of the impact of the
individual properties of the solvent are standardized coefficients.
These factors allow to determine the percentage share of the
solvent polarity parameters on the measured solvent-dependent
physicochemical property. The relevant data are presented in
Table 7.
described by
a single-exponential function. In the case of
DMAPhacPhCN (Table 3) the fluorescence lifetimes are about
0.6 ns in polar solvents and saturated hydrocarbons and in the
range from 1.3 ns to 2.5 ns in the remaining studied solvents. In the
case of DMAPhacPhCOOMe (Table 4) shorted the fluorescence
lifetimes are in saturated hydrocarbons solvents (0.7–0.9 ns) and in
polar solvents they are in the range from 1,3 to 1,5 ns.
DMAPhacPhCHO is characterized by complex photophysics
(Table 5). Only in saturated hydrocarbon solvents fluorescence
intensity decay is mono-exponential (fluorescence lifetime are in
the range of 0.7–1 ns) while for the remaining studied solvent two-
or three exponentials function is needed to correctly describe the
decay. This seems to be connected with the small difference of
energy of close lying np* and pp* states.
When fluorescence quantum yield is known single exponential
fluorescence intensity decay allows to determine the radiative and
As shown in the above Table, the position of absorption band
maximum for DMAPhacPh is determined in 55% by solvent
dipolarity and in nearly equal amount by solvent polarizability and
basicity, whereas for DMAPhacPhCN and DMAPhacPhCOOMe
non-radiative rate constants according to the equations; kr =
w/t
and knr = (1 ꢀ )/ . Analyzing the data contained in Table 1 and
w
t

M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
85
Table 6
Estimated from Eq. (1) adjusted coefficients (nx)0, aSP, bSdP, cSB, dSA their standard error and adjustable correlation coefficient (rAdj2) of the multiple linear correlation analysis
of absorption (nabs), fluorescence ( fluo) wavenumber, Stokes shift (DnSS of studied compounds as a function of Catalan-four parameter solvent polarity scale. Additionally, the
radiative rate constant (kr) and non-radiative rate constant (knr) for DMAPhacPhCN and DMAPhacPhCOOMe are presented.
n
quantity
(nx
)
0
aSP
bSdP
cSB
dSA
R2
adj
DMAPhacPh
nabs (cmꢀ1
)
33519 ꢅ 526
ꢀ(3003 ꢅ 792)
ꢀ(2739 ꢅ 706)
ꢀ(1091 ꢅ 199)
ꢀ(1036 ꢅ 179)
ꢀ(472 ꢅ 322)
ꢀ(586 ꢅ 284)
1423 ꢅ 1768
0.9341
0.9366
33371 ꢅ 4834
–
nfluo (cmꢀ1
)
30529 ꢅ 1688
28940 ꢅ 227
ꢀ(2350 ꢅ 2450)
ꢀ(4882 ꢅ 636)
ꢀ(4893 ꢅ 535)
256 ꢅ 1035
ꢀ(2691 ꢅ 5633)
0.9347
0.9409
–
–
–
DnSS (cmꢀ1
)
2990 ꢅ 1350
2499 ꢅ 190
ꢀ(653 ꢅ 20231)
3785 ꢅ 508
3457 ꢅ 295
ꢀ(728 ꢅ 828)
4159 ꢅ 4528
0.9181
0.9192
–
–
–
DMAPhacPhCN
nabs (cmꢀ1
)
28831 ꢅ 662
28848 ꢅ 639
29052 ꢅ 589
ꢀ(2892 ꢅ 996)
ꢀ(2932 ꢅ 961)
ꢀ(3288 ꢅ 858)
160 ꢅ 249
378 ꢅ 406
584 ꢅ 273
635 ꢅ 288
ꢀ(2301 ꢅ 2224)
ꢀ(1752 ꢅ 1984)
–
0.5398
0.5700
0.5794
–
–
nfluo (cmꢀ1
)
26270 ꢅ 2761
ꢀ(378 ꢅ 4153)
ꢀ(7705 ꢅ 1040)
1940 ꢅ 947
ꢀ(2872 ꢅ 9278)
0.9383
DnSS (cmꢀ1
)
225841 ꢅ 3448
2560 ꢅ 3022
1055 ꢅ 388
–
ꢀ(8277 ꢅ 534)
7864 ꢅ 1137
8467 ꢅ 601
–
–
0.9524
0.9300
0.9426
ꢀ(2513 ꢅ 4545)
1458 ꢅ 1852
507 ꢅ 10154
–
–
–
kf ꢂ 10ꢀ8 (sꢀ1
)
5.541 ꢅ 3.310
5.196 ꢅ 0.355
0.937 ꢅ 3.717
ꢀ(3.7714 ꢅ 0.9900)
ꢀ(3.1857 ꢅ 0.7174)
ꢀ(3.2898 ꢅ 1.3684)
ꢀ(3.6649 ꢅ 1.2199)
10.276 ꢅ 12.884
0.8982
0.9057
–
–
knr ꢂ 10ꢀ8 (sꢀ1
)
21.89 ꢅ 11.170
9.467 ꢅ 1.945
ꢀ(17.88 ꢅ 16.07)
0.1235 ꢅ 4.2859
ꢀ(9.153 ꢅ 5.936)
ꢀ(9.158 ꢅ 3.370)
212.72ꢅ 55.88
189.98 ꢅ 35.80
0.6692
0.6914
–
–
DMAPhacPhCOOMe
nabs (cmꢀ1
)
29850 ꢅ 1128
29324 ꢅ 933
ꢀ(3758 ꢅ 1659)
ꢀ(2984 ꢅ 1350)
141 ꢅ 430
753 ꢅ 596
733 ꢅ 341
ꢀ(705 ꢅ 709)
0.2600
0.3222
–
–
nfluo (cmꢀ1
)
28197 ꢅ 3244
26046 ꢅ 403
ꢀ(3226 ꢅ 4770)
ꢀ(7518 ꢅ 1237)
ꢀ(7738 ꢅ 700)
20 ꢅ 1714
4304 ꢅ 2037
4993 ꢅ 1651
0.8835
0.8984
–
–
DnSS (cmꢀ1
)
1652 ꢅ 3770
1399 ꢅ 461
ꢀ(527 ꢅ 5541)
7660 ꢅ 1437
8003 ꢅ 801
733 ꢅ 1991
ꢀ(5009 ꢅ 2367)
ꢀ(4853 ꢅ 1890)
0.8566
0.8787
–
–
kf ꢂ 10ꢀ8 (sꢀ1
)
4.064 ꢅ 1.527
4.031 ꢅ 0.209
3.962 ꢅ 0.206
ꢀ(0.050 ꢅ 2.291)
ꢀ(1.8338 ꢅ 0.5635)
ꢀ(1.8362 ꢅ 0.5207)
ꢀ(1.9005 ꢅ 0.5305)
ꢀ(3.665 ꢅ 0.922)
ꢀ(3.667 ꢅ 0.861)
ꢀ(3.200 ꢅ 0.79)
2.296 ꢅ 2.178
2.314 ꢅ 1.890
–
0.9274
0.9355
0.9322
–
–
knr ꢂ 10ꢀ8 (sꢀ1
)
13.746 ꢅ 5.696
7.974 ꢅ 0.780
ꢀ(8.630 ꢅ 8.435)
ꢀ(0.137 ꢅ 22.074)
ꢀ(6.857 ꢅ 3.392)
ꢀ(8.051 ꢅ 1.997)
19.450 ꢅ 8.018
22.878 ꢅ 7.006
0.5222
0.5641
–
–
DMAPhacPhCHO
nabs (cmꢀ1
)
27800 ꢅ 1161
28804 ꢅ 897
ꢀ(2703 ꢅ 1718)
ꢀ(4159 ꢅ 1341)
555 ꢅ 583
680 ꢅ 214
467 ꢅ 723
ꢀ(6270 ꢅ 6944)
0.2866
0.5133
–
–
nfluo (cmꢀ1
)
26655 ꢅ 3532
24779 ꢅ 452
ꢀ(2794 ꢅ 5224)
ꢀ(6181 ꢅ 1774)
ꢀ(5715 ꢅ 1081)
ꢀ(2976 ꢅ 2201)
ꢀ(3567 ꢅ 1748)
11096 ꢅ 21117
0.9024
0.9180
–
–
DnSS (cmꢀ1
)
1146 ꢅ 4569
1220 ꢅ 587
91 ꢅ 6758
6837 ꢅ 2295
5677 ꢅ 1401
3444 ꢅ 2848
4338 ꢅ 2269
ꢀ(117367 ꢅ 27317)
0.8574
0.8793
–
–
solvatochromism, while position of the fluorescence band maxi-
mum as well as the Stokes shift mostly depend on solvent
dipolarity, albeit solvent basicity and acidity have minor influence
too. A similar solvent influence on the spectral and photophysical
properties was observed for pull-push molecules containing N,N-
dimethylaniline and aromatic hydrocarbon linked by an acetylene
unit [20]. The kind of electron-acceptor substituent only slightly
modified the radiative rate constant. For DMAPhacPhCN the
solvent dipolarity in 60% influence kr while in 47%. for DMA-
PhacPhCOOMe. A more pronounced of solvent properties is
observed on the non-radiative rate constant. A solvent basicity
has 32% and 55% impact on it for DMAPhacPhCN and DMAPhacPh-
COOMe, respectively.
nearly in half by solvent polarizability and solvent basicity.
Moreover, for DMAPhacPhCHO in equal amount by solvent
polarizability and dipolarity. The position of the fluorescence
band maximum and Stokes shift for DMAPhacPh and DMA-
PhacPhCN in 100% depend on solvent dipolarity, while for
DMAPhacPhCOOMe in 80% on solvent dipolarity and 20% solvent
acidity, whereas for DMAPhacPhCHO in 70% on solvent dipolarity
and 30% on solvent basicity. Thus, not only electron affinity of
substituent but also the type of substituent expressed through the
proton-acceptor properties has a significant impact on the solute-
solvent interactions which modify their spectral and photophysical
properties. However, multiple-linear correlation analysis shows
that solvent polarizability has major influence on absorption

86
M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
Table 7
Estimated, based on data gathered in Table 6, standardized coefficients ASP, BSdP, CSB, DSA and their contribution (in%) to the dependence of absorption (nabs), fluorescence
fluo) wavenumber and Stokes shift (DnSS) of studied compounds on the Catalan-four parameter solvent polarity scale. Additionally, coefficients ASP, BSdP, CSB, DSA and their
contribution (in %) to the dependence of (kr) and (knr) for two D-A pairs are also presented.
(
n
quantity
ASP
BSdP
CSB
DSA
%ASP
%BSdP
%CSB
%DSA
DMAPhacPh
nabs (cmꢀ1
)
)
ꢀ0.293
ꢀ0.671
ꢀ0.983
0.973
ꢀ0.252
–
–
–
24.3
0
0
55
100
100
20.7
0
0
0
0
0
nfluo (cmꢀ1
DnSS (cmꢀ1
–
–
–
–
)
DMAPhacPhCN
nabs (cmꢀ1
nfluo (cmꢀ1
DnSS (cmꢀ1
kr (sꢀ1
knr (sꢀ1
)
)
ꢀ0.796
–
ꢀ0.573
–
–
–
–
–
56.7
0
100
100
60.0
0
43.3
0
0
40.0
32
0
0
0
0
68
–
–
–
–
ꢀ0.989
0.984
ꢀ0.535
–
0
0
0
0
)
–
)
ꢀ0.362
ꢀ0.426
)
0.899
DMAPhacPhCHO
nabs (cmꢀ1
nfluo (cmꢀ1
DnSS (cmꢀ1
)
)
ꢀ0.751
0.733
ꢀ0.075
0.805
–
–
–
–
50.6
0
0
49.4
70.7
69.6
0
29.3
30.4
0
0
0
–
–
ꢀ0.031
0.352
)
DMAPhacPhCOOMe
nabs (cmꢀ1
nfluo (cmꢀ1
DnSS (cmꢀ1
kr (sꢀ1
knr (sꢀ1
)
)
ꢀ0.495
–
0.487
–
–
50.4
0
49.6
0
0
53.0
55.0
0
–
–
–
–
ꢀ1.075
1.063
ꢀ0.477
–
0.294
ꢀ0.267
–
0
0
0
0
78.5
79.9
47.0
0
21.5
20.1
0
)
–
)
ꢀ0.538
ꢀ0.945
)
0.765
45.0
3.4. Exited state dipole moment
state increase to 63ꢆ. Taking into account the calculated ground
state dipole moment (10 D) the excited state dipole moment is
equal mex = 25.4 D and is lower than that published by Hirata et al.
(36 D) [33], which was obtained using Lippert-Mataga equation,
however, greater than for p-cyano-p0methylthiodiphenylacetylene
The method depending on the internal electric field (solvato-
chromism) has been frequently used to determine the excited state
dipole moment. Herein, solvent dependence of absorption and
emission spectra is used to estimate the excited state dipole
moment based on the Bilot-Kawski theory. Table 8 presents the
values of slops (m1 and m2) obtained from a linear fit to Eq. (2)
(Stoke’s shift) and (3) (sum of position of absorption and emission
maxima) and quality of linear fit (r2). Moreover, dipole moment
change in excited state (Dmex) calculated from Eq. (8) and angle
between dipole moment in the ground and excited state calculated
from Eq. (9) as well as the ground state dipole moment calculated
using DFT method are also presented. Table 8 shows also the
Onsager radius (a) value used in the calculation.
In the calculations of excited state dipole moment polar protic
solvents were not included. Moreover, 1,4-dioxane was also
excluded due to the fact that it behaves as a pseudo-polar solvent
of variable polarizability function, which depends upon the
solute’s electric field, as result of conformation polarizability
[58]. The quality of fit to Eqs. (2) and (3) are good. The excited state
dipole moment changes show, that derivatives with the greater
electron affinity substituent possess a greater excited state dipole
moment. For DMAPhacPh dipole moment change is equal to
Dmex = 8.0 D and the angle between dipole moment in the ground
and excited state is about 23ꢆ. For DMAPhacPhCN Dmex = 15.4 D,
while angle between dipole moment in the ground and excited
(19 D) [58]. For two remaining studied D-p-A pairs, DMAPhacPh-
COOMe and DMAPhacPhCHO, dipole moment change in the
excited state is equal Dmex = 10.2 D and Dmex = 7.8 D, respectively,
with nearly the same angle between dipole moment in the ground
and excited state (about 71ꢆ). Such large change of the dipole
moment in the excited state and the high value of the angle
between the dipoles in the ground and excited state indicate a
substantial change in the charge distribution in the excited
molecule.
Excited state dipole moment change can be also calculated
using the relation between Stokes shift and ET solvent polarity
N
parameter according to Eq. (10). Thus, obtained values presented in
Table 9 are comparable (about 2D larger for PhacPhCOOMe,
PhacPhCHO and about 2D smaller for DMAPhacPhCN and
DMAPhacPh) to those obtained from solvatochromic shift.
4. Conclusion
Diphenylacetylene and its symmetrically substituted deriva-
tives containing an electron-donor groups are weakly fluorescing
compounds contrary to the derivatives containing an electron-
acceptor groups which show a relatively high fluorescence
Table 8
Values of slops (m1 and m2), quality of fit (r2), dipole moment change in the exited state (Dmex) and angle between dipole moments in the ground and excited state (
w)
obtained from solvatochromic studies for DMAPhacPh, DMAPhacPhCN, DMAPhacPhCOOMe and DMAPhacPhCHO. The theoretically calculated dipole moment for ground
state (mgcal) and Onsager radius value (a) are also presented.
compound
mgcal (D)
a (Å)
m1 (cmꢀ1
)
r2
m2 (cmꢀ1
)
r2
Dmex (D)
w
DMAacPhe
3.4
10.0
5.9
5.6
6.6
6.6
5.7
3659 ꢅ 285
8199 ꢅ 977
8229 ꢅ 1127
9001 ꢅ 1024
0.9428
0.8761
0.8001
0.8639
ꢀ6791 ꢅ 695
ꢀ8403 ꢅ 725
ꢀ8003 ꢅ 874
ꢀ8183 ꢅ 690
0.8968
0.9312
0.8643
0.9208
8.0
15.4
10.2
7.8
23ꢆ100
63ꢆ120
70ꢆ590
71ꢆ160
DMAacPheCN
DMAacPheCOOCH3
DMAacPheCHO
5.5

M. Szyszkowska et al. / Journal of Photochemistry and Photobiology A: Chemistry 326 (2016) 76–88
87
Table 9
Values of slops (b) obtained from a linear fits to Eq. (10), quality of fit (r2), dipole moment change in the exited state (Dmex) obtained from the dependence of Stokes shift on ET
N
for DMAPhacPh, DMAPhacPhCN, DMAPhacPhCOOMe and DMAPhacPhCHO.
compound
mgcal (D)
a (Å)
b
r2
Dmex (D)
DMAacPh
3.4
10.0
5.9
5.5
9.0
5.6
6.6
6.6
5.7
6.2
8582 ꢅ 930
0.8938
0.8439
0.8252
0.7875
–
6.7
13.0
12.6
10
DMAacPhCN
DMAacPhCOOCH3
DMAacPhCHO
betaine
19714 ꢅ 2656
18311 ꢅ 2412
18111 ꢅ 2802
–
quantum yield. Introducing only one substituent to the dipheny-
lacetylene skeleton changes its photophysical properties. Spectro-
scopic and photophysical studies of synthesized compounds show
that unsymmetrically substituted diphenylacetylene derivatives
are good fluorophores compounds. However, an electron-accept-
ing substituent has much lower influence on photophysical
properties of diphenylacetylene than an electron-donating one.
While, PhacPhCOOMe and PhacPhCN show only weak solvato-
chromic effect, DMAPhacPh shows a significant solvatochromic
effect caused by substantial charge redistribution in the excited
state the effect of which is a large change of the excited state dipole
moment. Moreover, dual emission band in moderate polar solvent
revealed that locally excited and charge-transfer states have
similar energy and simultaneous emissions from both states take
place. Introducing to the DMAPhacPh skeleton an electron-
acceptor substituent substantially increase excited state dipole
moment which value depends on the substituent electron affinity.
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Acknowledgement
This work was financially supported under grant UMO-2011/01/
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