Solvatochromic characteristics of dansyl molecular probes bearing alkyl diamine chains

Article abstract of DOI:10.1016/j.saa.2020.118413

A series of dansyl-based fluorescent probes bearing linear alkyl-1,n-diamine chains of different length (DA_1.n, n = 2–8, 10, 12) was characterized in terms of the absorptive and emissive features in solvents of different polarity and hydrogen bond donor/hydrogen bond acceptor character. The probes show solvent-dependent absorption, a feature that is uncommon among dansyl derivatives. The dual emission of DA_1.n probes is strong in non-aqueous solvents and is influenced by the chain length and interactions with the solvent. Solvent effects on the spectral parameters were rationalized on the basis of the Kamlet-Taft and Catalán solvatochromic models, in order to quantify the degree of polarity-driven and hydrogen bonding interactions. A comparative discussion of the results predicted by the two models was made. In ground state, the DA_1.n probes act as hydrogen bond acceptors. In excited state, hydrogen bonding is less favoured, the solute-solvent interactions being governed by the increasing polarity of the solvent that results in a large bathochromic shift of the emission. A comparison was made with the spectral features previously reported for the corresponding series of bis-dansyl fluorescent probes (2DA_1.n).

Full text of DOI:10.1016/j.saa.2020.118413

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118413
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Solvatochromic characteristics of dansyl molecular probes bearing alkyl
diamine chains
⁎
Sorin Mocanu, Gabriela Ionita, Iulia Matei
“Ilie Murgulescu” Institute of Physical Chemistry of the Romanian Academy, 202 Splaiul Independentei, Bucharest 060021, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of dansyl-based fluorescent probes bearing linear alkyl-1,n-diamine chains of different length (DA1.n
n = 2–8, 10, 12) was characterized in terms of the absorptive and emissive features in solvents of different po-
larity and hydrogen bond donor/hydrogen bond acceptor character. The probes show solvent-dependent absorp-
tion, a feature that is uncommon among dansyl derivatives. The dual emission of DA1.n probes is strong in non-
aqueous solvents and is influenced by the chain length and interactions with the solvent. Solvent effects on the
spectral parameters were rationalized on the basis of the Kamlet-Taft and Catalán solvatochromic models, in
order to quantify the degree of polarity-driven and hydrogen bonding interactions. A comparative discussion
of the results predicted by the two models was made. In ground state, the DA1.n probes act as hydrogen bond ac-
ceptors. In excited state, hydrogen bonding is less favoured, the solute-solvent interactions being governed by the
increasing polarity of the solvent that results in a large bathochromic shift of the emission. A comparison was
made with the spectral features previously reported for the corresponding series of bis-dansyl fluorescent probes
(2DA1.n).
,
Received 25 March 2020
Received in revised form 22 April 2020
Accepted 24 April 2020
Available online 27 April 2020
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
solvatochromic models can be found in the paper published by
Reichardt more than two decades ago [2] and in a more recent book
Solute-solvent interactions are at the core of solution chemistry, af-
fecting reaction rates, positions of chemical equilibria and product
yields [1], as well as altering molecular geometries, electronic distribu-
tions and, consequently, spectroscopic data [2]. Interactions with the
microenvironment are reflected in changes in the position, shape and
intensity of the absorption and emission bands, which are generally
termed as solvatochromic effects. The mechanism responsible for
these effects is the differential solvation of the ground and excited states
of solute molecules. The extent of solvation depends on the intermolec-
ular forces between solute and neighbouring solvent molecules, which
can be classified into non-specific (global) interactions, determined by
polarity and polarizability effects, and specific (local) interactions such
as hydrogen bonding involving hydrogen bond donor (HBD)/hydrogen
bond acceptor (HBA) and electron-pair donor (EPD)/electron-pair ac-
ceptor (EPA) molecules.
chapter [6]. The reliability of the Kamlet-Taft and Catalán
multiparametric models over other solvatochromic models reported
in the literature is that they employ solvent scales obtained by averag-
ing solvent effects over a variety of standard molecular probes, specific
to each model.
The prerequisite for a molecular probe is its sensitivity to the proper-
ties of the microenvironment, leading to characteristic solvatochromic
behaviour. Continuous development of new probes has allowed re-
searchers to focus their attention on systems that have been less studied
in the past, including solvent mixtures [7], micellar solutions [8], bio-
polymers [9] and solid surfaces [10]. Other applications of
solvatochromic probes include identifying the solvent composition at
which the phase transition occurs in polymers [11] and gels [12], and
the investigation of micropolarity and local mobility of amino acid
chains [13].
The overall solvent effect recorded experimentally can be unraveled
into its various contributions by the use of solvatochromic models such
as those developed by Kamlet et al. [3] and, more recently, Catalán [4,5].
These models are based on linear solvation energy relationships be-
tween a measured spectroscopic parameter and several solvent scales
of polarity, HBD and HBA character. Comprehensive reviews of
In our previous studies, we reported the synthesis and photophysical
parameters of several series of molecular probes: i) dual probes bearing
a pyrene fluorescent moiety and a TEMPO paramagnetic moiety liked by
oligoethylene glycol chains [14,15], ii) dual probes bearing a dansyl
fluorescent moiety and a TEMPO paramagnetic moiety liked by alkyl
chains [16], and iii) bis-dansyl fluorescent probes bearing alkyl-
diamine linkers of increasing length (2DA1.n) [17]. The spectral features
of these probes were analysed in water solutions of cyclodextrins
[14,16,17], pluronics [15] and polymeric gels [14,16], proving their
⁎
Corresponding author.
E-mail address: iuliamatei@icf.ro (I. Matei).
https://doi.org/10.1016/j.saa.2020.118413
1386-1425/© 2020 Elsevier B.V. All rights reserved.

2
S. Mocanu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118413
with respect to a standard quinine sulfate solution in 1 N sulfuric acid
(Φstd. = 0.55 at 25 °C) [19], according to Eq. (1) [20]:
H
N
O
NH2
(CH2)n
S
!
ꢀ
ꢁꢀ
ꢁ
nX2
n2std:
N
SX
AX
Astd:
Sstd:
ΦX ¼ Φstd:
ð1Þ
O
In Eq. (1), subscripts X and std. refer to the probe and standard, re-
spectively, S represents the integrated fluorescence intensity, A is the
absorbance at the excitation wavelength and n is the refractive index
of the solvent (sulfuric acid 1 N: 1.3403; water: 1.3330; ethanol:
1.3617; DCM: 1.4250; DMSO: 1.4775; chloroform: 1.4476; isopropanol:
1.3750).
Scheme 1. Chemical structure of the dansyl-based fluorescent probes DA1.n (n = 2–8, 10,
12) analysed in this study.
potential for offering information on a multitude of non-covalent inter-
actions occurring in simple to complex systems.
In this paper, we perform a detailed analysis of the solvatochromic
properties of a series of mono-dansyl probes bearing linear alkyl-1,n-di-
amine linkers of different lengths (probes DA1.n in Scheme 1). We aim to
understand how the microenvironment influences the ground and ex-
cited state properties of these probes. The dansyl fluorophore (5-
dimethylamino-1-naphthalene sulfonate) exhibits structural and spec-
tral properties such as intense absorption in the near UV spectral region,
strong solvent-dependent fluorescence in the visible domain and the
presence of HBD/HBA and EPD/EPA molecular moieties. Linking this
fluorophore to an aliphatic chain provides the probes with increasing
conformational mobility that can influence their adaptability to differ-
ent microenvironments. The relevance of this study resides in the
acute need of effective solvatochromic reporters for complex molecular
systems in which non-covalent and conformational interactions play
major roles.
3. Results and discussions
3.1. Characteristics of the electronic absorption of DA1.n dansyl probes
The DA1.n molecular probes bearing one dansyl fluorophore linked
to an alkyl-1,n-diamine chain of variable length (Scheme 1) are charac-
terized in the electronic absorption spectrum by the presence of two
bands located at ~250 nm (characteristic to π-π* transitions of the naph-
thalene moiety) and ~330 nm (having intramolecular charge transfer
character, typical of aminonaphthalene-sulfonates) (Fig. 1A) [21–23].
As can be seen from the data in Table 1, the longest wavelength (λa) ab-
sorption band does not vary significantly in position with respect to the
chain length. This trend is common to all tested solvents. Interestingly,
the behaviour of DA1.n probes is different from that of bis-dansyl 2DA1.
n probes (Scheme S1) in water. In the case of 2DA1.n probes, a 10 nm
bathochromic shift of λa occurs for 2DA1.12 as compared to 2DA1.2 [17].
In the case of DA1.n probes, the position of the longest wavelength
absorption band λa varies within a 15 nm range as a function of solvent
(Fig. 1B and Table 1). This solvatochromic behaviour contrasts with that
reported for other mono-dansyl derivatives obtained by the reaction of
dansyl chloride with 4-vinylbenzyl-amine [24] or with ethylene di-
amine and acryloyl chloride [12], which show unchanged absorption
maxima, at 360 nm and 335 nm, respectively, in a wide range of sol-
vents. A similar behaviour was reported for some hyperbranched poly-
mers labelled with multiple (6 to 12) dansyl groups [23]. These
derivatives show absorption maxima at ~340 nm irrespective of solvent,
which indicates little solvent effect on the ground state of the dansyl
chromophore. By difference, the solvent-dependent absorption of DA1.
n probes can provide additional information on the microenvironment
of the dansyl moiety, as discussed in the following.
2. Material and methods
The mono-dansyl molecular probes DA1.n, n = 2–8, 10, 12, were
synthesised by reacting dansyl chloride with one amino group of
alkyl-1,n-diamines, in the presence of 1,4-diazabicyclo[2.2.2]octane,
as described in ref. [16]. Chloroform, dichloromethane (DCM),
isopropanol, ethanol and dimethylsulfoxide (DMSO) were pur-
chased from Sigma Aldrich.
Electronic absorption spectra and steady-state fluorescence spectra
were recorded on a V-560 Jasco UV–Vis spectrophotometer and a
JASCO FP-6500 spectrofluorimeter, respectively, at 25 °C. Fluorescence
quantum yields (Φ) were determined for solutions having absorbances
lower than 0.1 in order to avoid inner-filter effects at wavelengths larger
than the excitation wavelength [18]. Quantum yields were computed
Fig. 1. (A) Absorption spectra of DA1.n dansyl probes in water (solution absorbance in the range 0.05–0.10). Spectra normalized at the longest wavelength absorption band are given as
inset. (B) Absorption spectra of DA1.2 in solvents of different polarity.

S. Mocanu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118413
3
Table 1
analysis is reflected in a good correlation of the theoretical ν values, pre-
dicted from the computed p, a and b coefficients, with the experimental
ν values [2].
Wavelengths of the longest wavelength absorption maximum (λa, in nm) of DA1.n dansyl
probes in different solvents.
Probe
Water
Ethanol
Isopropanol
DMSO
DCM
Chloroform
The DA1.n probes are susceptible to form hydrogen bonds with the
solvent with the participation of the dimethylamino and sulfonamido
groups of the dansyl moiety [26,27], and/or the terminal amino group.
While the latter can interact with either HBA solvents (via the hydrogen
atoms) or HBD solvents (via the lone pair electrons on nitrogen), the
dimethylamino group (electron-pair donor) linked to a naphthalene
moiety might interact with HBD solvents [28], and the sulfonamido
group (electron-pair acceptor) may interact with HBA solvents [29].
We performed comparative Kamlet-Taft and Catalán analyses on the
νa values of DA1.n probes, and the results are listed in Table 2. Plots of
theoretical vs. experimental νa values (Fig. 2) reveal very good correla-
tions for all DA1.n probes (R N 0.99). From the data in Table 2, one ob-
serves that both solvatochromic models predict that the HBA ability of
DA1.n probes is responsible to the greatest extent for the experimental
shift of νa, as reflected by the large contribution of the a coefficient
(60–80% as predicted by Catalán). This indeed explains the trends ob-
served experimentally. As mentioned above, water and DMSO have
similar, high polarity (SPP = 0.681 for water, 0.830 for DMSO). They
have, however, drastically different HBD ability (SA = 1.062 for water,
0.072 for DMSO), which appears to be decisive in determining the posi-
tion of the absorption band of DA1.n probes. In what concerns the lesser
influence of polarity-driven interactions on the spectral shifts, the
Kamlet-Taft model predicts a slightly higher contribution (up to 44%)
as compared to the Catalán model (up to 31%). Fig. 3A shows graphical
representations that allow a straightforward comparison between the
ground state solute-solvent interactions predicted by the two
solvatochromic models.
DA1.2
DA1.3
DA1.4
DA1.5
DA1.6
DA1.7
DA1.8
DA1.10
DA1.12
328
326
325
326
326
325
324
324
326
336
336
335
336
335
336
335
335
335
336
335
335
335
335
334
334
335
336
341
340
340
339
340
340
338
339
340
342
339
342
342
343
343
342
343
340
342
340
341
341
341
339
341
341
341
Surprisingly, no straightforward correlation with a solvent parame-
ter, in particular the solvent polarity, can be made in order to account
for the variation of λa in DA1.n probes. The longest wavelength absorp-
tion band is located around 325 nm in water, 335 nm in ethanol and
isopropanol, and 340 nm in DMSO, DCM and chloroform. Thus, water
and DMSO, two highly polar solvents, determine opposite shifts in λa.
Another approach is therefore needed in order to rationalize the spec-
tral features.
The use of solvatochromic models such as those developed by
Kamlet et al. [3] (Eq. (2)) and Catalán [4] (Eq. (3)) allows one to distin-
guish between and quantify the effects of polarity and of hydrogen
bonding to the solvent on the absorptive and emissive features of mo-
lecular probes.
ν ¼ ν0 þ pðπ ꢀ þdδÞ þ aα þ bβ
ν ¼ ν0 þ pSPP þ aSA þ bSB
ð2Þ
ð3Þ
3.2. Emissive characteristics of DA1.n dansyl probes
In Eqs. (2) and (3), ν denotes the absorption (νa) or fluorescence (νf)
wavenumber, or the Stokes shift (ΔνStokes = νa − νf) (in cm−1). The sol-
vent parameters π* (or SPP), α (or SA) and β (or SB) are independent,
yet complementary, and describe the polarity/polarizability of the sol-
vent (its ability to stabilize a neighbouring charge/dipole through non-
specific dielectric interactions), as well as its ability to donate a proton
in a solvent-to-solute hydrogen bond (hydrogen bond donor acidity,
HBD) and to accept a proton in a solute-to-solvent hydrogen bond (hy-
drogen bond acceptor basicity, HBA), respectively. The term δ = 0.5 is a
polarizability correction introduced for aliphatic chlorinated solvents
[25]. ν0 is the wavenumber independent of solvent effects (in gas
phase or in the reference solvent cyclohexane, for which all solvent pa-
rameters are zero).
The fluorescence emission of DA1.n dansyl probes is characterized by
a dual feature with two bands located at ~485 nm and ~520 nm (Fig. 4
and Table 3). Dual fluorescence of dansyl derivatives has been previ-
ously ascribed to emissions from the locally excited state (LE,
~485 nm) and from the charge transfer excited state (CT, ~520 nm,
solvent-dependent) [17]. In dansyl derivatives, charge transfer in the
first excited singlet state occurs from the N,N′-dimethylaminonaphtyl
moiety to the substituted sulfonamide [30]. Interestingly, in water,
dual emission is only observed for the probes bearing long alkyl chains,
DA1.10 and DA1.12. This suggests that the LE emission is favoured in these
probes, most likely due to the coiled conformation adopted by the
chains (vide infra). The band at ~485 nm appears as a shoulder in
water, ethanol, isopropanol and DMSO (F485/F520 = 0.5–0.9, Table 4),
is equal in contribution to the band at ~520 nm in DCM (F485/F520 ~ 1),
and dominates the emission in chloroform (F485/F520 ~ 1.2). These differ-
ences in the F485/F520 value can be explained as follows. It is known that
polar solvents stabilize the dipolar CT states, while non-polar or weakly
polar solvents stabilize LE states that lack charge separation [19]. Thus, a
The values of the p, a and b coefficients (in cm−1) are obtained by
multiple regression analysis for a series of solvents. The solvents we se-
lected for this purpose vary in polarity and HBD/HBA character. The p, a
and b values are characteristic to the photophysical process under study
and reflect the relative sensitivity of the DA1.n spectral parameter ν to
the solvent polarity, HBD and HBA character, respectively. An accurate
Table 2
Predicted contributions (% in brackets) of solvent polarity (p), HBD acidity (a) and HBA basicity (b) to the electronic absorption (νa) of DA1.n probes; R2 is the coefficient of multiple
determination.
Probe
Kamlet-Taft model
Catalán model
p
a
b
R2
p
a
b
R2
DA1.2
DA1.3
DA1.4
DA1.5
DA1.6
DA1.7
DA1.8
DA1.10
DA1.12
815 (39)
1489 (41)
602 (33)
710 (41)
174 (12)
−30 (2)
788 (42)
265 (15)
1034 (44)
1039 (50)
1344 (37)
985 (54)
883 (52)
768 (50)
720 (45)
942 (50)
748 (41)
1108 (47)
226 (11)
776 (22)
0.996
0.989
0.990
0.975
0.986
0.934
0.978
0.983
0.999
−464 (25)
−454 (28)
−81 (5)
593 (25)
−79 (4)
−113 (7)
947 (31)
740 (27)
−250 (16)
1199 (65)
1143 (69)
1468 (84)
1441 (62)
1426 (80)
1407 (84)
1669 (55)
1690 (61)
1267 (80)
177 (10)
−46 (3)
203 (11)
304 (13)
279 (16)
144 (9)
419 (14)
343 (12)
62 (4)
0.994
0.971
0.991
0.980
0.983
0.929
0.983
0.986
0.996
−246 (13)
−116 (7)
−586 (38)
−857 (53)
−159 (8)
−785 (44)
219 (9)

4
S. Mocanu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118413
Fig. 2. Plots of the absorption maximum wavenumber, νa (A) and fluorescence maximum wavenumber, νf (B) predicted using the Kamlet-Taft and Catalán models vs. experimental values.
polar solvent such as ethanol has a stabilizing effect on the CT state of
DA1.n probes (Fig. 4B), while chloroform, a solvent of lower polarity, sta-
a shoulder at 485 nm in the case of the DA1.10 and DA1.12 probes. The po-
sition of the band at ~485 nm is independent on the solvent (except for a
5 nm hypsochromic shift in water) and on the chain length.
bilizes the LE state of the probes (Fig. 4C). We can thus employ the F485
/
F520 ratio to estimate the extent of hydrophobicity in the system. In the
case of ethanol, the contribution of the band at 485 nm increases as the
chain length increases (Table 4). This can be explained by the increased
hydrophobicity sensed by the dansyl moiety (vide infra), which leads to
a progressive stabilization of the LE excited state, and thus to an increase
in the F485/F520 value. This effect can only manifest in polar, HBD sol-
vents like ethanol and water, in the latter case with the observation of
The longest wavelength (λf) fluorescence band of DA1.n probes dis-
plays a much more interesting solvatochromism. This band is located
around 515 nm in chloroform, DCM and isopropanol, 520 nm in ethanol,
525 nm in DMSO and 545 nm in water (Table 3). In contrast to the band
at ~485 nm, the increase of the chain length determines a systematic
hypsochromic shift in the λf value. This could be explained by the
alkyl chains creating an increasingly hydrophobic microenvironment
(A)
(B)
100
Kamlet-Taft
100
Catalán
p
p
a
a
b
b
80
Catalán
80
60
40
20
0
Kamlet-Taft
60
40
20
0
2
3
4
5
6
8
12
10
10 12
8
7
2
3
5
6
5
6
4
4
7
6
7
2
3
7
8
2
3
4
5
10
12
12
10
8
Fig. 3. Percentage contributions of solvent polarity (p), HBD acidity (a) and HBA basicity (b) to the (A) electronic absorption (νa) and (B) fluorescence maximum wavenumber (νf) of DA1.n
probes, estimated by two solvatochromic models.

S. Mocanu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118413
5
Fig. 4. Fluorescence spectra of DA1.n dansyl probes evidencing the effect of the chain length on the emissive features in different solvents: (A) water, (B) ethanol and (C) chloroform. Each
spectrum has been normalized at the emission maximum.
that is sensed by the dansyl moiety. This behaviour could allow us to
gather additional information related to the local mobility and confor-
mation adopted by the alkyl chains (quasi-linear vs. coiled). A similar
behaviour was reported for some dansylated copolymers of maleic
acid and alkyl vinyl ethers [28,31]. In water, the copolymer bearing a
butyl chain is hypsochromically shifted by 60 nm with respect to the co-
polymer bearing a methyl group. This shift is accompanied by a strong
increase of the fluorescence emission. In the case of the DA1.n probes,
the maximum hypsochromic shift is 2 nm in DMSO and chloroform,
4 nm in ethanol and isopropanol, and 8 nm in water. The sole exception
is DCM, where no change in band position is observed. Differently from
the absorption case, where no apparent connection to solvent polarity
could be established, in the case of λf a large bathochromic shift of
30 nm occurs with increasing solvent polarity, indicating positive
solvatochromism (Fig. 5). Such solvatochromic behaviour is associated
with the stabilization of the excited state in polar solvents [2].
When comparing the emissive features of DA1.n probes to those re-
ported for the corresponding series of bis-dansyl 2DA1.n probes [17],
several observations can be made. The dual emission feature stands
for both series of probes, however the LE state is more strongly favoured
in long-chain probes in the case of the 2DA1.n probes; for DA1.n probes,
the F485/F520 ratio is independent on chain length, with the exception
of ethanol. Interestingly, in the case of 2DA1.n probes, increasing chain
lengths determine hypsochromic shifts of both LE (8 nm) and CT
Table 4
Table 3
Ratio of the fluorescence intensities of the two emission bands of DA1.n dansyl probes
(F485/F520) in different solvents.
Wavelengths of the emission maxima of DA1.n dansyl probes in different solvents.
Probe Water
Ethanol
Isopropanol DMSO
DCM
Chloroform
Probe
Water
Ethanol
Isopropanol
DMSO
DCM
Chloroform
DA1.2 545
DA1.3 544
DA1.4 544
DA1.5 543
DA1.6 544
DA1.7 543
DA1.8 541
523; 485sh 517; 485sh
522; 485sh 517; 484sh
521; 485sh 517; 484sh
520; 485sh 516; 485sh
520; 485sh 515; 485sh
520; 485sh 515; 485sh
519; 485sh 515; 484sh
524; 485sh 513; 485 514; 483
524; 485sh 513; 484 514; 482
522; 485sh 513; 485 512; 482
523; 485sh 513; 485 512; 481
523; 485sh 513; 485 511; 481
522; 485sh 514; 485 512; 482
522; 485sh 512; 485 512; 482
522; 485sh 514; 486 511; 481
522; 485sh 513; 485 512; 481
DA1.2
DA1.3
DA1.4
DA1.5
DA1.6
DA1.7
DA1.8
DA1.10
DA1.12
–
–
–
–
–
–
–
0.46
0.54
0.67
0.70
0.74
0.75
0.76
0.77
0.78
0.79
0.80
0.82
0.81
0.85
0.85
0.86
0.85
0.86
0.88
0.84
0.61
0.62
0.64
0.64
0.64
0.64
0.67
0.66
0.66
1.01
1.07
1.04
1.05
1.05
1.04
1.04
1.06
1.06
1.13
1.18
1.14
1.17
1.17
1.15
1.15
1.18
1.19
DA1.10 536; 480sh 519; 485sh 515; 484sh
DA1.12 537; 480sh 519; 485sh 513; 482sh
sh = shoulder.

6
S. Mocanu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118413
Fig. 5. Fluorescence spectra of DA1.n dansyl probes evidencing the solvent effect on the emissive features. Each spectrum has been normalized at the emission maximum of the most intense
band (wavelengths of these maxima are also shown).
(14 nm) emission bands on going from 2DA1.2 to 2DA1.12, in water. The
DA1.n probes display a different behaviour, with an 8 nm hypsochromic
shift observed for the CT band only. Regarding the solvent influence,
both series of probes display positive solvatochromism. In both cases,
the spectral effects are most poignant in water.
The two solvatochromic models, Kamlet-Taft and Catalán, have been
used to perform analyses on the νf and ΔνStokes values of DA1.n probes,
and the results are listed in Tables 5 and S1, respectively. Both models
lead to very good correlations (Fig. 2B and Fig. S1, R N 0.98), however
we observe striking differences between the models in what concerns
the predicted solvent influence on νf. The Catalán model predicts a
major contribution from polarity (p ~ 65%), while the Kamlet-Taft
model indicates the HBD ability of DA1.n probes as playing the dominant
role in the solute-solvent interaction (b = 50–90%). The negative values
of the coefficients predicted by the Catalán model indicate a decrease in
the transition energy due to the stabilization of the excited state by sol-
polarity-driven interactions with the solvent. Thus, in going from
ground to excited state, the solvent polarity imparts maximum stabili-
zation on the expense of the HBD ability of the solvent. The contribution
of b to νa and νf is small, indicating that hydrogen bonds formed by HBA
solvents with the amino hydrogens as HBD moieties or with the
sulfonamido group are not favoured in either ground or excited state.
In all cases, the coefficients do not vary with the chain length.
The group of Machado [34,35] also reported differences between the
solute-solvent interactions predicted by the Kamlet-Taft and Catalán
models. In the case of some nitro-substituted iminophenols [34] and
nitrostyryl phenols [35], both models led to good quality fits. However,
the Kamlet-Taft model predicted large b values that were not in agree-
ment with the molecular structure of the probes, which lacked HBD
acidic sites able to interact with HBA solvents. Differently, the Catalán
model correctly predicted large a and p values, thus describing correctly
the sensitivity of these probes to the acidity and polarity of the microen-
vironment. The two models also predicted opposite trends in the values
of a (dominant for Catalán) and b (dominant for Kamlet-Taft) in the
case of some derivatives bearing N,N-dimethylaminophenyl (electron
donor) and nitrophenyl (electron acceptor) groups [36].
In the case of DA1.n probes in ground state, the two models are in
overall agreement in what concerns the predicted solvatochromism.
Discrepancies between the models appear in the treatment of solute-
solvent interactions in excited state. In this respect, our results are in
line with the findings of Machado et al. and recommend the Catalán
model as most appropriate for describing the solvent effect on the emis-
sive features of DA1.n probes.
The fluorescence quantum yields (Φ) of the DA1.n dansyl probes are
listed in Table 6. In water, the Φ values are very low (around 0.02) and
increase linearly with the length of the alkyl chain (Fig. S3). Small Φ
values (below 0.1) have been reported for other mono-dansyl [37]
vation, and translate into
a
bathochromic shift (positive
solvatochromism) of the emission band with increasing solvent polar-
ity, in accordance to the experiment. In the case of ΔνStokes, the
Kamlet-Taft model predicts similar contributions from the three solvent
parameters, while the Catalán model indicates the solvent polarity and
HBD ability as having the largest contributions (Table S1 and Fig. S2).
Fig. 3B allows for a comparison of the solute-solvent interactions in
excited state, as predicted by the two models. The predominant interac-
tions of DA1.n probes as HBAs in the ground state (a = 60–80%, Fig. 3A)
are replaced in the excited state by polarity/polarizability effects
(p ~ 60%, Fig. 3B). Other authors have also reported different dominant
effects in ground and excited states [32,33]. In the case of DA1.n probes
in excited state, the Catalán model indicates a decreased probability of
hydrogen bond formation by the lone pairs of the nitrogen atoms
from the amino groups, and a stabilization of excited DA1.n probes via
Table 5
Predicted contributions (% in brackets) of solvent polarity (p), HBD acidity (a) and HBA basicity (b) to the fluorescence emission (νf) of DA1.n probes; R2 is the coefficient of multiple
determination.
Probe
Kamlet-Taft model
Catalán model
p
a
b
R2
p
a
b
R2
DA1.2
DA1.3
DA1.4
DA1.5
DA1.6
DA1.7
DA1.8
DA1.10
DA1.12
141 (8)
69 (4)
83 (5)
70 (5)
1455 (87)
1357 (91)
838 (58)
938 (67)
930 (62)
867 (59)
1096 (86)
502 (44)
1115 (83)
0.999
0.998
0.999
0.998
0.999
0.999
0.997
0.999
0.998
−3014 (63)
−2825 (62)
−2535 (60)
−2923 (63)
−3102 (63)
−2719 (63)
−2843 (64)
−2529 (64)
−2642 (66)
−1407 (30)
−1363 (30)
−1370 (32)
−1373 (30)
−1444 (30)
−1330 (31)
−1316 (29)
−1101 (28)
−1044 (26)
−328 (7)
−343 (8)
−318 (8)
−344 (7)
−353 (7)
−260 (6)
−319 (7)
−329 (8)
−304 (8)
0.999
0.996
0.997
0.996
0.990
0.989
0.998
0.977
0.971
−391 (27)
−346 (25)
−440 (29)
−437 (30)
−163 (13)
−510 (44)
49 (4)
−215 (15)
−108 (8)
−135 (9)
−170 (11)
17 (1)
−140 (12)
179 (13)

S. Mocanu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 237 (2020) 118413
7
Table 6
Fluorescence quantum yields of DA1.n dansyl probes.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Probe
Water
Ethanol
Isopropanol
DMSO
DCM
Chloroform
DA1.2
DA1.3
DA1.4
DA1.5
DA1.6
DA1.7
DA1.8
DA1.10
DA1.12
0.015
0.018
0.019
0.020
0.022
0.023
0.023
0.025
0.029
0.225
0.249
0.198
0.242
0.232
0.217
0.223
0.213
0.157
0.315
0.290
0.170
0.284
0.267
0.277
0.277
0.238
0.345
0.334
0.326
0.342
0.367
0.367
0.346
0.352
0.316
0.354
0.470
0.444
0.539
0.481
0.476
0.455
0.430
0.543
0.443
0.450
0.454
0.510
0.496
0.508
0.397
0.487
0.421
0.427
Acknowledgements
This work was supported by the Romanian Academy within the re-
search direction “Non-covalent interactions investigated by using dual
molecular probes” of the “Ilie Murgulescu” Institute of Physical
Chemistry.
Note: the errors in Φ have been estimated from duplicate measurements and are 10% of
the quoted values.
Appendix A. Supplementary data
and bis-dansyl derivatives [17,38] and explained on the basis of high po-
larity and specific hydrogen bonding interactions that favour the non-
radiative decay pathways, which corroborates with our findings. The
linear increase in Φ with the chain length is not observed in the non-
aqueous solvents tested, where the Φ values are much larger (around
0.2 in ethanol, 0.3 in isopropanol, 0.4 in DMSO and 0.5 in DCM and chlo-
roform) and independent on the chain length. Two observations can be
made. First, the behaviour of Φ in water correlates with the
hypsochromic shift of λf observed with increasing chain length. This
supports our previous assessment regarding the coiled conformation
adopted by the alkyl chains, creating a low polarity microenviroment
in the vicinity of the dansyl moiety. Secondly, the Φ values decrease
with increasing polarity of the solvent. For this reason, a solvatochromic
analysis was performed in the case of Φ as well, and the results are listed
in Table S2.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.saa.2020.118413.
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4. Conclusions
The spectral, photophysical and solvatochromic properties of DA1.n
probes were reported and analysed based on the models elaborated
by Kamlet-Taft and Catalán. Our results suggest that the Catalán
model is superior in describing the solute-solvent interactions of DA1.n
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CRediT authorship contribution statement
Sorin Mocanu: Investigation, Validation, Formal analysis, Writing -
original draft. Gabriela Ionita: Conceptualization, Methodology, Writ-
ing - review & editing. Iulia Matei: Conceptualization, Formal analysis,
Writing - review & editing.

8
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