Experimental and theoretical investigation of long-wavelength fluorescence emission in push-pull benzazoles: Intramolecular proton transfer or charge transfer in the excited state?

Article abstract of DOI:10.1039/c8cp05186k

This study presents the synthesis, characterisation and theoretical calculations of compounds that contain electron donor and withdrawing groups connected through a π-conjugated benzazolic structure. The compounds in solution show an absorption maximum in the UV-visible spectrum (380-390 nm) due to spin and symmetry allowed electronic ¹ππ? transitions with no clear evidence for charge transfer in either compound in the ground state. A fluorescence emission located in the violet-blue-green region, tailored by solvent polarity, with a large Stokes shift was observed. Taking the long-wavelength emission into account, the Lippert-Mataga plot indicates a positive solvatochromism in the solvent polarity function (Δf) range 0.02-0.20, related to the occurrence of an ICT mechanism in the excited state. At Δf greater than 0.20, the polarity of the medium seems no longer to increase the stabilization of the compounds, reaching a plateau. Time-dependent density functional theory (TD-DFT) and resolution-of-identity second-order approximate coupled-cluster (RI-CC2) calculations were also used to better understand the excited state of these compounds. The results indicated that ESIPT was disfavoured in the compounds, mainly in polar solvents, and the emission wavelengths were primarily associated with ICT. In summary, in these push-pull compounds, the electron donating and withdrawing groups do not favour the ESIPT process.

Full text of DOI:10.1039/c8cp05186k

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2019, DOI: 10.1039/C8CP05186K.
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
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DOI: 10.1039/C8CP05186K
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ARTICLE
Experimental and theoretical investigation of long-wavelength
fluorescence emission in push-pull benzazoles: intramolecular
proton transfer or charge transfer in the excited state?
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
a
Guilherme Wiethaus, Josene Maria Toldo, Fabiano da Silveira Santos, Rodrigo da Costa Duarte,
b
a
DOI: 10.1039/x0xx00000x
Paulo Fernando Bruno Gonçalves* and Fabiano Severo Rodembusch*
www.rsc.org/
This study presents the synthesis, characterisation and theoretical calculations of compounds that contain electron donor
and withdrawing groups connected through a -conjugated benzazolic structure. The compounds in solution show an
1
absorption maxima in the UV-visible spectrum (380-390 nm) due to spin and symmetry allowed electronic * with no clear
evidence for charge transfer in either compound in the ground state. A fluorescence emission located in the violet-blue-
green region, tailored by the solvent polarity, with a large Stokes shift was observed. Taking the long-wavelength emission
into account, the Lippert-Mataga plot indicates a positive solvatochromism in the solvent polarity function (f) range 0.02-
0
.20, related to the occurrence of an ICT mechanism in the excited state. At f greater than 0.20, the polarity of the medium
seems no longer to increase the stabilization of the compounds, reaching a plateau. Time-dependent density functional
formalism (TD-DFT) and resolution-identity second-order approximate coupled-cluster (RI-CC2) calculations were also used
for better understand the excited state of these compounds. The results indicated that ESIPT is disfavoured in the
compounds, mainly in polar solvents, and the emission wavelengths were primarily associated with ICT. In summary, in these
push-pull compounds, the electron donating and withdrawing groups do not favour the ESIPT process.
converted to the excited keto tautomer (K*) through an
intramolecular proton transfer. The K* tautomer is usually more
stable in the excited state than these locally excited conformers
Introduction
Intra or intermolecular proton transfer reactions are one of the
most fundamental and important processes in chemistry and
biology. Excited state intramolecular proton transfer (ESIPT)
was first observed in salicylic acid, as reported by Weller.
Since then, a large number of dyes that present ESIPT have been
developed and investigated. Among them, the benzazoles
should be highlighted due to the versatility of the benzazolic
ring functionalisation as well as their high thermal and
(N*) and it is the specie responsible for shifting the fluorescence
emission band to longer wavelengths. After excited state
deactivation, the initial enol form is regenerated without any
1
,2
1
8
photochemical change. In many cases, the proton transfer is
1
8-25
an ultrafast process that occurs at the sub-picosecond scale
even at low temperatures.
3
-5
4
The existence of additional conformers, other than the enol-
cis can be directly related to the polarity of the medium. The
ESIPT dynamics can compete with solvent polarisation effects.
In polar protic solvents, for example, intermolecular hydrogen
bonding with the solvent can originate the enol-open
conformer (Fig. 1, right) inhibiting ESIPT. Thus, the emission
spectra show a band originated from the locally excited
conformer (so-called normal emission), at shorter wavelengths,
3
photochemical stability. These characteristics, combined with
large Stokes shift, allow envisaging several applications for
these compounds in the optical sensing and emissive materials
domains.4
-16
Fig. 1 presents the typical ESIPT process for a benzazole dye.
The process starts with UV light absorption by the enol-cis
conformer (N, left), which in nonpolar or aprotic media is more
stable than the enol-open form (N, right).
excitation, the locally excited enol-cis conformer is quickly
3
,26
with
a
small Stokes shift (Fig. 1, top).
However,
1
7,18
After vertical
intermolecular hydrogen bonding with the solvent can play a
role of a “proton-relay”, where the solvent molecules assist
2
,3
ESIPT.
a. Grupo de Pesquisa em Fotoquímica Orgânica Aplicada, Universidade Federal do
Rio Grande do Sul - Instituto de Química, Avenida Bento Gonçalves 9500. CEP
9
1501-970 Porto Alegre-RS, Brazil. Fax: +55 51 33087204; Tel: +55 51 33087206;
E-mail: fabiano.rodembusch@ufrgs.br
Grupo de Química Teórica e Computacional, Universidade Federal do Rio Grande
do Sul - Instituto de Química, Avenida Bento Gonçalves, 9500, CP 15003, CEP
b.
9
1501-970 Porto Alegre-RS, Brazil; E-mail: paulo@iq.ufrgs.br.
Electronic Supplementary Information (ESI) available: Details for the spectroscopic
characterization of the dyes, as well as additional photophysical and computational
data can be found in the ESI. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2019
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
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ARTICLE
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effects afforded by the solvent. EDGs conjugatVeiedw AwrtiictlehOntlhinee
phenolic hydroxyl group can be strategic ll
D
a
O
I
y
: 1
i
0
n
.1
s
0
e
3
r
9
t
/
e
C
d
8Cin
P
t
0
o
51
t
86K
h
e
benzazolic core to allow proton transfer coupled to electron
transfer. Although the azolic nitrogen presents a negligible
effect on electron transfer, once the proton is transferred, the
generated anion becomes a strong EDG that can transfer charge
3
to the EWG via π conjugation.
Fig. 2 Chemical structure of the push-pull benzazoles 3a (X=NH) and 3b (X=O).
Carbon atoms are indicated by red numbers. Abbreviations: EWG, electron-
withdrawing group; EDG, electron-donor group.
Fig. 1 Förster cycle for the ESIPT process for the 2-(2'-hydroxyphenyl)benzoxazole
skeleton. The diagram presents the enol-cis (left), keto tautomer (middle) and
enol-open in polar protic solvent (methanol) (right). N and K are related to the
Based on the interesting photophysical properties of these
normal (enol) and tautomeric (keto) form, respectively. (Adapted from reference compounds, this paper reports the synthesis, photophysics and
3
).
theoretical study of two new benzazole derivatives that
simultaneously contain an EDG and EWG at positions 4' and 6,
Photoinduced proton transfer can also be affected by the respectively (Fig. 2). In order to better understand the influence
intramolecular charge transfer (ICT). In this case, the of ICT states on ESIPT, time-dependent density functional
3
9
competition with the solvent can lead to longer emission theory (TD-DFT) was used and two functionals, PBE1PBE and
4
0
wavelengths related to both ESIPT and ICT separately (in a CAM-B3LYP, were evaluated against the resolution-identity
stepwise manner) or coupled (simultaneous manner). The second-order approximate coupled-cluster (RI-CC2) method
presence of a charge transfer state can be responsible for the and experimental results. The report on the push-pull effects
strong solvatochromism observed in the fluorescence emission and the solvent-dependent transition of ICT and the ESIPT
2
7,28
spectra of benzazole dyes.
Furthermore, the nature and emissive states can provide key insights into the intricate
position of the substituent groups attached to the benzazole environmentally controlled interplay between electronic
core can strongly affect the ICT and ESIPT dynamics, modulating driving force (ICT) and molecular geometrical stabilities (ESIPT).
the ESIPT rate ratio.2 Different substituent groups give rise to Our findings could be used to better design new proton-transfer
different conjugations with the benzazole ring, changing the dyes.
9,30
2
1
electronic distribution of the excited state and, consequently,
3
1,32
their spectroscopic properties.
Depending on the position of
the electron-donor (EDG) or electron-acceptor (EWG) groups Experimental
attached to the phenyl rings (Fig. 2), a proton transfer can either
follow ESIPT or precede it. However, if these events occur at
Materials and methods
about the same rates, the distinction between both
mechanisms is not possible in most of the cases and they are
considered as coupled. Several studies deal with the influence
All reagents and solvents were purchased from commercial
suppliers and used without further purification. The solvents
were dried and distilled before use based on procedures
of the functional groups attached to the benzazole core in
41
detailed in the literature. Spectroscopic grade solvents were
ESIPT.2
3,24,33,34
Generally, benzazole derivatives that contain
used for fluorescence and ultraviolet-visible spectrum (UV-Vis)
measurements. Elemental analyses were performed with a
Perkin-Elmer model 2400. Melting points were measured with
a Gehaka PF 1000 apparatus and are uncorrected. Infrared
spectra were recorded on a Mattson Galaxy Series FT-IR3000
and EWG at position 6 (see Fig. 2) show ESIPT followed by ICT
2
7,30,35,36
from the keto tautomer.
On the other hand, in
compounds that present an EDG at the same position, ICT
occurs prior to ESIPT.2
4,34,37,38
If the position of the substituent
group changes, ESIPT and ICT dynamics are affected. For
example, changing the –NH
position hastens ICT and disfavours ESIPT due to intermolecular resonance (NMR) spectra were performed on a VARIAN INOVA
1
13
substituent from the 6- to the 5- model 3020 or Varian 640-IR FTIR. H and C nuclear magnetic
2
2
,3
interactions with the solvent.
YH300 using tetramethylsilane (TMS) as the internal standard
and dimethyl sulfoxide (DMSO-d ) (Sigma-Aldrich) as the
In compounds that contain an EDG at the 4' position (see Fig.
), the charge transfer must occur prior to proton transfer,
6
2
solvent at room temperature. The chemical shifts () are
reported in parts per million (ppm) relative to TMS (0.00 ppm),
and the coupling constants (J) are reported in hertz (Hz). High
resolution mass spectrometry was recorded with electrospray
ionization (HRMS-ESI) data in the positive mode using a Q-TOF-
MS Bruker Impact II. UV-Vis absorption spectra were obtained
with a Shimadzu UV-2450 spectrophotometer. Steady state
because the acidity of the hydroxyl group is significantly
reduced. In this case, ESIPT is still subject to a barrier induced
by the solvent polarity, which can control the dynamics of the
process. Increasing solvent polarity should stabilise the charge
3
,30
transfer states rather than induce ESIPT process.
Thus, the
ICT rate is mainly determined by solvent polarity. Therefore,
compounds where ESIPT occurs before charge transfer are of
great interest, since they must be independent of the relaxation fluorescence spectra were measured with a Shimadzu
2
| Phys. Chem. Chem. Phys., 2019, 00, 1-3
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
PCCP
ARTICLE
spectrofluorometer (model RF-5301PC). All experiments were
performed at room temperature (25°C). The quantum yield of DMSO-d
fluorescence (QY) was measured at 25°C using spectroscopic 129.7, 121.6 117.9, 108.2, 107.0, 99.0, 97.4. FTIR (KBr, =cm )
J
o
=8.7 Hz); 6.18 (d, 1H, H3’, J
m
=2.1 Hz). 13C NMR (75 MHz,
View Article Online
DOI: 10.1039/C8CP05186K
6
,  ppm) 168.1, 160.9, 156.3, 148.2, 146.5, 143.9,
-1
grade solvents within solutions with absorbance intensity lower 3481, 3383, 3233, 1636, 1439, 3123, 1502. Anal. calcd. for
−
1
than 0.05 (optical dilute methodology). Quinine sulfate (Riedel)
C
13
H
9
N
3
O
4
(271.23 g·mol ): C 57.57; H 3.34; N 15.49. Found: C
4
2
+
2
in 1 N H SO
4
was used as the QY standard.
57.72; H 3.49; N 14.62. HRMS-ESI [M+H] calcd for
+
[
C
13
H
10
N
3
O
4
] : 272.0671, found 272.0666.
Synthesis
2
-(4’-Aminophenyl)-6-nitrobenzimidazol (3c). Yield: 10%. FTIR
The ESIPT (3a-b) and non-ESIPT (3c-d) benzazoles were
-1
(KBr, =cm ) 3481, 3383, 3233, 1636, 1439, 3123, 1502 and
synthesised from an adapted synthetic methodology previously
1
3
2,43
1323. H NMR (300 MHz, DMSO-d
6
,  ppm) 10.89 (s, 1H, OH);
=1.8 Hz); 8.27 (dd, 1H, H5, J =2.1 Hz, J =8.4
Hz); 7.83 (d, 1H, H4, J =8.7 Hz); 7.68 (d, 1H, H6’, J =8.7 Hz); 6.38
s, 2H, NH ); 6.32 (d, 1H, H5’, J =2.1 Hz, J =8.7 Hz); 6.18 (d, 1H,
H3’, J =2.1 Hz). Anal. calcd. for C13
1.41; H 3.96; N 22.04. Found: C 60.89; H 3.77; N 21.70. HRMS-
described in the literature (Scheme 1).
8
.59 (d, 1H, H7, J
m
m
o
o
o
(
2
m
o
−1
m
10 4 2
H N O (254.24 g·mol ): C
6
+
+
ESI [M+H] calcd for [C13
11 4 2
H N O ] : 255.0882, found 255.0877.
Scheme 1 Synthetic route for obtaining benzazoles 3a-d.
2
-(4’-Aminophenyl)-6-nitrobenzoxazol (3d). Yield: 8%. FTIR
-1
(KBr, =cm ) 3468, 3358, 3328, 1640, 1436, 3115, 1515 and
The benzazole derivatives (3a-d) were synthesised by
reaction of 1.0 g (6.59 mmol) of the respective nitroaniline (1a-
b) with 1.0 g (6.59 mmol) of 4-aminosalicylic acid (2a) or 1.0 g
1
1
339. H NMR (300 MHz, DMSO-d
6
,  ppm) 8.54 (d, 1H, H7,
=2.1 Hz and J =8.7 Hz); 7.90 (dd,
=8.7 Hz); 7.80 (d, 1H, H4, J =8.7 Hz); 6.72 (dd,
H, H3’ and H5’, J =8.7 Hz); 6.29 (s, 2H, NH ). Anal. calcd. for
J
m
=2.1 Hz); 8.24 (dd, 1H, H5, J
H, H2’ and H6’, J
m
o
2
2
C
6
o
o
(7.29 mmol) of 4-aminobenzoic acid (2b) in 20 ml of
o
2
polyphosphoric acid (PPA). To better solubilise the reagents into
the solvent, the PPA was previously heated to 60 C under
−1
H
13 9
N
3
O
3
(255.23 g·mol ): C 61.18; H 3.55; N 16.46. Found: C
o
+
0.66; H 3.02; N 15.88. HRMS-ESI [M+H] calcd for
vigorous stirring. Then, the reaction mixture was heated under
+
[
C
13
H
10
N
3 3
O
] : 256.0722, found 256.0717.
o
o
an N
1
2
atmosphere at 60 C for 1 h, 120 C for 2h and, finally,
o
80 C for more 2 h. The reaction mixture was poured into
Theoretical calculations
crushed ice, with stirring, and neutralised with NaHCO
The dark precipitate obtained was filtered, washed with water
3 x 20 mL) and dried at room temperature. A previous
3
(10%).
Theoretical studies of ESIPT in large systems are
computationally expensive to employ high-level ab initio
calculations. Thus, due to its cost-benefit, many publications
adopt time-dependent density functional theory (TD-
(
purification was performed using a Soxhlet extractor with
acetone as the solvent. The solvent was removed under
reduced pressure, and the obtained solid was purified by
column chromatography on silica gel 60 (0.063-0.200 mm) using
8
,24,36-38,44-46
19,22,47,48
DFT).
Additionally, studies using RI-CC2,
4
9-52
multireference methods
also reported.
and semi-classical dynamics are
The ground electronic state and first
18,22,48,53
CH
2
Cl
2
/acetone (92:8) (for 3a), acetone (for 3b), CHCl
3
/acetone
excited state equilibrium geometries were obtained by
optimising the geometries using PBE1PBE and CAM-B3LYP
functionals together with cc-pVDZ basis set. Subsequently,
single-point TD-DFT calculations were performed using both
functionals but larger basis set (jun-cc-pVTZ) in order to obtain
the vertical absorption, vertical emission, and population
analysis. The jun- type basis set, named calendar basis set, is
recommended by Truhlar et al. as a good alternative to the
highly costly augmented (aug) basis sets because of its
(10:1) (for 3c) and CHCl
3
/acetone (20:1) (for 3d) as the eluents.
2
-(4’-Amino-2’-hydroxyphenyl)-6-nitrobenzimidazole (3a). Pale
1
orange solid. Yield: 28%. H NMR (300 MHz, DMSO-d
6
,  ppm)
1
3.07 (broad s, 1H, OH); 12.27 (broad s, 1H, NH); 8.35 (s, 1H,
H7); 8.08 (dd, 1H, H5, J =1.8 Hz, J =8.7 Hz); 7.72 (d, 1H, H6’,
=8.7 Hz); 7.65 (d, 1H, H4, J =8.7 Hz); 6.25 (d, 1H, H5’, J =1.5
Hz, J =8.4 Hz); 6.17 (d, 1H, H3’, J =1.5 Hz); 5.99 (s, 2H, NH
NMR (75 MHz, DMSO-d
1
4
m
o
J
o
o
m
1
3
o
m
2
). C
performance versus cost. The calendar basis set is constructed
,  ppm) 160.4, 154.0, 128.7, 118.3, by removing diffuse functions from the aug basis sets. All
54
6
-1
1
1
07.1, 100.8, 100.2. FTIR (KBr, =cm ) 3481, 3393, 3273, 3242, equilibrium geometries were confirmed by vibrational analysis,
650, 1422, 1520, 1334. Anal. calcd. for C13
(270.24 having no imaginary frequencies. Solvent effects were included
in all cases by the integral equations formalism of the
10 4 3
H N O
−1
g·mol ): C 57.78; H 3.73; N 20.73. Found: C 58.21; H 3.73; N
1
5
+
+
polarisable continuum model (IEF-PCM). The solvents
considered were 1,4-dioxane, dichloromethane, ethanol and
acetonitrile. The electrostatic potential surfaces in the ground
and excited states, atomic charges, and dipole moment were
obtained by population analysis using Charges from
2
0.01. HRMS-ESI [M+H] calcd for [C13
11 4 3
H N O ] : 271.0831,
found 271.0826.
2
-(4’-Amino-2’-hydroxyphenyl)-6-nitrobenzoxazole
(3b).
1
Orange solid. Yield: 36%. H NMR (300 MHz, DMSO-d
6
,  ppm)
20
Electrostatic Potential using a Grid-based method (ChelpG).
1
m
0.89 (s, 1H, OH); 8.59 (d, 1H, H7, J =1.8 Hz); 8.27 (dd, 1H, H5,
Relaxed scans for the proton transfer coordinate between enol
and keto tautomers were done at the first excited state
J
m
=2.1 Hz, J
o
=8.4 Hz); 7.83 (d, 1H, H4, J =8.7 Hz); 7.68 (d, 1H,
o
o 2
H6’, J =8.7 Hz); 6.38 (s, 2H, NH ); 6.32 (d, 1H, H5’, J
m
=2.1 Hz, potential energy surface for 3a and 3b. To further evaluate the
This journal is © The Royal Society of Chemistry 2019
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Page 4 of 13
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proton transfer in these compounds, relaxed scans for 2-2’-
hydroxyphenylbenzoxazole (HBO) in the S state were also
performed. For these calculations, the geometries were M ·cm ), f
Table 1 Relevant photophysical data of the UV-Vis absorptionVisepweAcrttrical,ewOhnelinree
4
1
abs is the absorption maxima (nm),  is the molar aDbOsoI:rp10ti.v1i0ty39c/oCe8ffCicPie0n5t18(160K
-
1
-1
0
e
is the calculated oscillator strength, k
e
is the calculated radiative
8
-1
0
rate constant (10 s ) and  is the calculated pure radiative lifetime (in ns).
optimised by increasing the OH bond distance by 0.05 Å for
each step using the CAM-B3LYP/cc-pVDZ level of theory and IEF-
PCM/1,4-dioxane as the implicit solvent. The calculations were
carried out using Gaussian 09 Revision D.01. RI-CC2 is suitable
0
0
Benzazole Solvent
1,4-Dioxane
Dichloromethane 373 1.18 0.20 1.47 6.82

abs

f
e
k
e
378 2.17 0.46 3.21 3.11
3
a
5
5,56
to provide benchmark results for TD-DFT calculations
and
Ethanol
Acetonitrile
380 1.68 0.34 2.36 4.24
377 1.70 0.34 2.43 4.12
386 3.36 0.63 4.25 2.35
was used for comparison with the results obtained by using
PBE1PBE and CAM-B3LYP functionals. RI-CC2 single point
calculations in the gas phase at the optimised TD-DFT
geometries were performed by employing the aug-cc-pVDZ
1
,4-Dioxane
Dichloromethane 383 5.26 0.91 6.20 1.61
3
b
Ethanol
Acetonitrile
394 3.61 0.74 4.74 2.11
386 2.15 0.42 2.81 3.56
370 1.88 0.32 2.31 4.33
370 1.71 0.34 2.46 4.07
374 2.63 0.48 3.46 2.89
376 3.00 0.61 4.34 2.31
5
7
basis using Turbomole 7.0 package.
1
,4-Dioxane
Acetonitrile
,4-Dioxane
Acetonitrile
3
c
1
3
d
Results and discussion
UV-Vis absorption
hydrogen bonding.58 When compared to their analogues
benzazolic molecules without the EWG (nitro) and EDG (amino)
groups, the so-called HBI and HBO, both compounds present
redshifted bands (around 60 nm) in dichloromethane, as
The normalised absorption spectra of benzazoles 3a and 3b are
shown in Fig. 3, and the relevant data calculated from UV-Vis
absorption spectroscopy are presented in Table 1.
5
9
expected. The simultaneous presence of the amino group in
the 4' position and the nitro group at the 6 position lead to the
extension of the chromophore through the benzazolic core and,
consequently, to a pronounced bathochromic shift in the
absorption spectra. It is worth mentioning that the
chromophore extension is not possible when the EWG and EDG
3
0
groups are attached at different positions.
In order to better understand the photophysical behavior in
the ground state of the studied compounds 3a and 3b, the non-
ESIPT benzazoles (3c and 3d) were also evaluated via UV-Vis
absorption spectroscopy in solution using 1,4-dioxane and
acetonitrile as solvents (Fig. 3 and Table 1). These compounds
present similar chemical structures to 3a and 3b but without the
hydroxyl group at position 2' and show absorption maxima
located around 370 nm and 375 nm, respectively with absent
solvatochromic effect, indicating that the auxochrome group
hydroxyl plays an important role on the  conjugation of these
compounds, as expected.
From the UV-Vis absorption spectra, oscillator strengths (f
e
),
0
radiative rate constants for emission (k
e
), which is the
Fig. 3 Normalised UV-Vis absorption spectra in solution for benzazoles (a) 3a, (b)
-5
probability of emission of photons per time unit, as well as pure
radiative lifetimes ( ) were calculated. The first one, fe, was
3
b and (c) non-ESIPT benzazoles 3c-d at 10 M. For comparison, the respective
benzazoles without the EWG and EDG groups, 2-2’-hydroxyphenyl)benzimidazole,
HBI) and 2-2’-hydroxyphenylbenzoxazole (HBO), are also presented in
dichloromethane.
0
(
calculated by applying the Strickler-Berg equation (Equation
6
0
1
).
푓푒 = 4.32 × 10 ―9∫휀(푣)푑푣,
The push-pull benzazole 3a presented absorption maxima
located around 375 nm. A small solvatochromic effect (abs
(1)
7
where the integral is the area under the absorption spectra
nm) was observed, indicating an almost absent of charge
transfer character in the ground state. Compound 3b presented
absorption maxima located around 385 nm and a slightly higher
solvatochromic effect (abs 11 nm). In all the solvents, the
oxygenated derivative 3b presented a red shifted absorption
compared to the nitrogenated analogue 3a. The observed
redshift values for the absorption maxima in ethanol are
probably due to specific solvent-solute interaction, such as
-
curves from plotting the molar absorptivity coefficient  M
1
-1
-1
0
·
cm  against wavenumber 푣 in (cm ). k
a similar way, using Equation (2).
e
can be obtained in
0
-
9 2
푘 ≈ 2.88푥10 푣 ∫휀(푣)푑푣,
(2)
푒
0
where 푣
0
is the wavenumber corresponding to the maximum
0
absorption wavelength. The pure radiative lifetime,  , is
0
61
defined as 1/k
e
.
4
-1
-1
The high values obtained for  (10 M ·cm , Table 1), as
0
e
well as the calculated radiative rate constant (k ) can be related
4
| Phys. Chem. Chem. Phys., 2019, 00, 1-3
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Page 5 of 13
Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
PCCP
ARTICLE
1
to * transitions for he studied benzazoles 3a-d. The f
e
values Table 2 Relevant photophysical data of the fluorescence emission spectra,
View Article Online
1
-1
obtained from the experiments also corroborate with * where em is the emission maxima (nm), ST is theDOStIo: k1e0s.1s0h3i9ft/C(c8mCP)0a5n1d86KF
electronic dipole-allowed transitions since they are comparable
to the expected ones for the * transitions (f
calculated  values indicate that after absorption of radiation,
is the fluorescence QY.
1
-3
62
e
10 1). The
Benzazole
Solvent

497
444
443
449/563
505
ST
6335
4288
3742

F
em
0
1,4-Dioxane
Dichloromethane
Ethanol
0.009
0.007
0.008
these benzazoles populate the same excited state.
3
a
b
Acetonitrile
4254/8764 0.005
Fluorescence emission
1
,4-Dioxane
6105
8343
6001
0.100
0.018
0.073
The normalised fluorescence emission spectra of compounds 3a
and 3b are presented in Fig. 4. The emission curves were
obtained by exciting the compounds at the respective
absorption maxima wavelength. The relevant data from the
fluorescence emission spectroscopy are summarised in Table 2.
Dichloromethane
Ethanol
564
516
3
Acetonitrile
434/571
505
424/576
504
414/556
2865/8393 0.010
7226 0.027
3443/9666 0.001
6896 0.059
2441/8610 0.001
1
,4-Dioxane
Acetonitrile
,4-Dioxane
Acetonitrile
3c
1
3
d
medium polarity. It is worth mentioning that the parent
compound 3c show a quite similar photophysics in 1,4-dioxane,
with a main emission band located above 500 nm, but a FWHM
below 90 nm (Fig. 4). Since, this compound is not able to ESIPT
due to the absence of the hydroxyl moiety, and the half
maximum (FWHM) of the emission band is significantly smaller
than those observed for 3a in the same solvent, its believed that
the second proposition, ICT coupled to ESIPT process, are
probably taking place in the excited state for this compound in
such solvent.
It is therefore difficult to assign from the experimental
results, the emitting species definitively in this case, as already
6
3
observed for different proton-transfer compounds. Although
HBO presented emission maxima in the same range of 3a (Fig. 4
top, dashed line) it is worth mentioning that different
photophysical behaviour takes place for these two compounds.
Fig. 4 Normalised emission spectra in solution for benzazoles (a) 3a, (b) 3b and (c)
non-ESIPT benzazoles 3c-d at 10- M. For comparison, the respective benzazoles It is well known that HBO presents ESIPT, and therefore its
5
without the EWG and EDG groups, 2-2’-hydroxyphenyl)benzimidazole, (HBI) and
emission is attributed to the keto tautomer, with a Stokes shift
2
-2’-hydroxyphenylbenzoxazole (HBO), are also presented in dichloromethane.
-1 5
greater than 9000 cm .
Compound 3b presented emission maxima above 500 nm,
redshifted compared to 3a. Despite the emission value in
ethanol, which was strongly affected by the solvent-
fluorophore hydrogen bonding, 3b presents a redshift on the
maxima location for the long-wavelength emission as solvent
polarity increased. In 1,4-dioxane, due to the maxima location
and Stokes shift value, it seems that the same explanation
proposed to 3a can be used for this compound. In acetonitrile,
compound 3a presented dual fluorescence emission. A main
emission band located at 449 nm and a less intense redshifted
band located at 563 nm, a quite similar behaviour to that
observed for the non-ESIPT analogue 3d (Fig. 4). In this way, it
is believed that the dual fluorescence emission can related to
locally excited (short wavelength) and ICT (long wavelength)
character. Compound 3b also presents in acetonitrile a dual
fluorescence emission with maxima located at 434 nm and 571
nm. The character of the long-wavelength was properly
answered also regarding the 3d, that is not able to ESIPT, where
dual fluorescence emission was clearly observed located at 414
nm and 556 nm. This result strong indicates that in this solvent,
Benzazole 3a presented emission maxima located in the
blue-green region (441-497 nm). However, unexpectedly, a
redshift higher than 50 nm was observed using 1,4-dioxane. In
this solvent, considering the full width at the half maximum
FWHM) of the emission band (FWHM=119.0 in 1,4-dioxane and
2.6 in dichloromethane) and the location of the observed
shoulder (450 nm), we could assign the emission peak to: (i)
both locally excited (LE) and ICT contributions or (ii) locally
excited (LE) and ICT coupled to ESIPT process. In both
propositions, the locally excited (LE) emission band can be
related to the observed shoulder around 450 nm, since there is
no influence of the solvent polarity on this location (see Fig.
S12). The emission maximum can be related to an ICT character,
observed by the redshift from 500 nm up to 550 nm (see Fig.
S12) increasing the medium polarity. However, the second
explanation, ICT coupled to ESIPT process, cannot be discarded
since the main emission band, despite the observed redshift,
presents a decrease on its intensity increasing the
(
6
3
b also presents a locally excited (LE) and ICT contributions.
Lippert-Mataga correlation and NLO properties
This journal is © The Royal Society of Chemistry 2019
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Page 6 of 13
ARTICLE
PCCP
In order to better correlate the observed differences on the analogue (3a). On the other hand, in the excited state, and
View Article Online
DOI: 10.1039/C8CP05186K
calculated Stokes shift with ICT or ESIPT in 3a and 3b, the taking the longer emission wavelengths above 500 nm into
6
4
solvent polarity plot function was obtained (Fig. 5). Here, the account (see Fig. S12), it was observed
a positive
differences in the dipole moment between the excited and solvatochromism. The fluorescence maxima against f in the
2
ground states were calculated by applying the Lippert-Mataga 0.020.20 range presented a linear correlation (R =0.99 and
6
4
relation (Equation 3):
0.98 for 3a and 3b, respectively), a finding which corroborates
∆휇 푒2 푔
the occurrence of an ICT mechanism in the excited state, as
2
∆푣
푠푡 = ℎ푐푎3 ∆푓 + 푐표푛푠푡푎푛푡
(3)
2,24,27,36
already observed for similar structures.
Additionally, in
more polar media, with an solvent polarity function greater
than 0.20, the emission maxima location no longer redshifted
but maintained the large Stokes shift. A possible explanation
could be that the polarity medium seems no longer to increase
the stabilization of the compounds, reaching a limit.
In this equation, h is the Planck's constant, c is the speed of
light, a is the Onsager cavity radius and ∆휇푒푔 is the difference in
2
the dipole moment between the excited and ground states for
the studied compounds. In the Lippert-Mataga relation, the
solvent polarity function (f) can be obtained from Equation
5
8,60
Nonlinear optics (NLO) is concerned with how the
electromagnetic field of a light wave interacts with the
(4),
푛2 ― 1
2푛2 + 1
)
(
휀 ― 1)
(
)
6
6
∆푓 = (
―
(4) electromagnetic fields of matter and other light waves. Sohn
2휀 + 1)
(
et al. affirms that the interaction of light with a NLO material
where the dielectric constant () and the refractive index (n) for will causes changes in its properties where the next photon will
a mixture of solvents can be calculated by the following experience a structure with different properties. In these
equations:58
materials, strong interactions can disturb the frequency, phase,
polarisation, or path of the incident light. In this sense, it is
worth investigating the NLO properties of 3a and 3b in order to
understand how matter, and specifically the electronic charge
휀
푛
= 푓 ∙ 휀 + 푓 ∙ 휀
(5)
(6)
푚푖푥
2
퐴
퐴
2
퐵
퐵
2
= 푓 푛 + 푓 푛
푚푖푥
퐴
퐴
퐵
퐵
6
6
In Equations (5) and (6), f
A
B
and f are the volumetric fractions of density in matter, interacts with light. It was obtained NLO
6
7,68
the two solvents. A linear plot of the absorbance, fluorescence properties by applying the solvatochromic method;
emission or Stokes shift versus the solvent polarity function compounds 3a and 3b presented high first hyperpolarisabilities
indicates the internal charge transfer character. By applying () achieved by Hyper-Rayleigh Scattering.
6
5
32,43
In this
the Lippert-Mataga correlation, there was no clear evidence for methodology, the first polarisability (xx) and first order
charge transfer in either compound in the ground state (Fig. 5a). hyperpolarisability (xxx) were determined from the ground
The original spectra can be seen in Fig. S11.
state data, as summarised in Tables 3 and 4. The linear
3
polarisability xx (in cm ) was calculated according to Equation
6
9
(7):
2
휇
푔푒
훼푥푥 = 2퐸푔푒
,
(7)
where Ege is the energy difference from the ground to excited
7
0-72
state charge transfer transition, in erg.
is the experimental transition dipole moment given by Equation
8), where 푓 is the oscillator strength and 푣 is absorption
In this equation, 휇푔푒
(
-1 69
wavenumber, in cm .
푓
휇푔푒
=
.7016 × 10 ―7
(8)
4
푣
Compound 3a presented higher values to the experimental
transition dipole moment (ge) if compared to the analogue 3b
(Table 3). Further, both compounds showed similar values for
the energies of ground to excited state charge transfer
transition (Ege). The linear polarisability xx values ranged from
.61x10- to 2.89x10-23 cm . Compound 3a presented lower xx
23
3
0
values compared to 3b, as already observed for the
benzimidazole derivative when compared to benzoxazole
3
2,43
one.
In this sense, it is believed that in the presence of an
external electromagnetic field, 3b will produce a more intense
displacement of the electron density away from the nucleus, a
phenomenon that would result in a charge separation (induced
Fig. 5 Solvent effects on the spectral position of 3a and 3b taking the (a)
absorption, (b) emission and (c) Stokes shift into account.
6
6
dipole).
Even though the variation is small, compound 3b was more
destabilised in more polar environments than the nitrogen
6
| Phys. Chem. Chem. Phys., 2019, 00, 1-3
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
PCCP
ARTICLE
Table 3 Table of constants, where ge is the experimental transition dipole Table 4 Table of constants, where a is the cavity radius within Onsager’s
View Article Online
-
8
DOI: 10.1039/C P05186K
moment (in Debye), Ege is the energy of ground to excited state charge model (in 10 cm), eg is the difference between the ground ate and
8Cst
-
12
-23
3
transfer transition (10 erg) and xx is the linear polarizability (10 cm ).
excited state dipole moments (in Debye) and xxx is the first
-30
5
-1
hyperpolarisability (in 10 cm ·esu )
.
Benzazole
Solvent
ge
E
ge
xx
1
,4-Dioxane
6.08
4.02
5.25
5.26
7.21
8.64
7.86
5.86
5.26
5.33
5.23
5.27
5.15
5.17
5.04
5.15
1.41
0.61
1.05
1.05
2.02
2.89
2.45
1.34
Benzazole
aa
eg

xxx
xureab
Dichloromethane
Ethanol
Acetonitrile
3
a
b
3
a
b
4.51
4.49
1.98
2.15
12.0
31.4
32
83
1
,4-Dioxane
3
Dichloromethane
Ethanol
Acetonitrile
3
aOnsager cavity radius was obtained in the gas phase using CAM-
Urea: xxx= 0.38x10-30 cm
B3LYP/jun-cc-pVTZ level of theory.
b
5·esu-1
72
.
It was applied Equation (9) in order to compute first order Usually, the first hyperpolarisability mainly depends on
7
2
hyperpolarisability (xxx). The obtained values are summarised conjugation path length and strength of electron donor and
7
1
in Table 4.
electron acceptor group. In this way, although 3a and 3b have
훽푥푥푥
2
휇
푔푒
a strong EDG (NO
the ones obtained for urea are probably tailored not only by
where 휇푔푒 is the transition dipole moment (in esu·cm), 퐸푔푒 is molecular conjugation of the benzazolic ring but also by the
2
), the obtained values for
compared with
훽푥푥푥 = 6퐸2 ∆휇 ,
(9)
푒푔
푔푒
2
the energy of the transition from the ground to the excited state relatively weak EDG (NH ).
charge transfer (in erg), and ∆휇푒푔 is the difference between the
7
2
excited and ground state dipole moments (in esu·cm). This last Theoretical calculations
parameter can be obtained from the McRae polarity function
(
The keto and enol conformers were optimized in the ground
and first excited state using DFT and TD-DFT, respectively. Two
different exchange-correlation functionals were used, the
hybrid PBE1PBE, which is reported to give good results for
Equation 10):7
2-74
2
푛2 ― 1
푛2 + 2
2
∆휇푒푔 휀 ― 1
ῡ푎푏푠 ― ῡ푒푚
=
―
+
(
훿푎푏푠 + 훿푒푚
)
.
(10)
ℎ푐푎3 휀 + 2
(
)
7
6-78
-1
benzazole derivatives,
and CAM-B3LYP, which includes
In this equation ῡ푎푏푠 ― ῡ푒푚 is the Stokes shift (in cm ), 훿푎푏푠
-
1
long-range corrections, in order to deal with excited states
and 훿푒푚 are the differences in the vibrational energy (in cm ) of
the molecule in the ground and excited states for absorption
and emission, respectively, h is the Planck’s constant (in erg·s),
c is the speed of light in a vacuum (in cm·s ), ε is the relative
dielectric constant and 푛 is the static refractive index of the
2
3
which present an ICT character.
All the conformers found for 3a and 3b in the ground state
are presented in Figs. S14 and S15, respectively. The
correspondent energies are summarised in Tables S1 and S2.
-1
7
9
Similar to the non-substituted benzazoles HBO and HBI, in the
gas phase or in low polarity solvents the most stable
solvent. Thus, Equation (10) can be rewritten as Equation (11):
푦 = 푚푥 + 푐
(11)
conformation in the ground state was the so-called enol-cis (Ecis
Fig. 6).
;
where:
푦 = ῡ푎푏푠 ― ῡ푒푚푠
휀 ― 1 푛2 ― 1
푥 =
―
(
푛2 + 2
)
휀 + 2
2
∆휇 2푒 푔
푚 =
and
ℎ푐푎3
푐 = 훿푎푏푠 + 훿푒푚
.
Fig. 6 Most stable structures for 3a (left) and 3b (right) in the ground state, using
CAM-B3LYP/cc-pVDZ and PCM/1,4-dioxane. The bond lengths are presented in
In this study, it was obtained the Onsager cavity radius (a) in
the gas phase using the CAM-B3LYP/jun-cc-pVTZ theory level. In Ångstroms and the dihedral angles in degrees.
Equation (11), it was used Onsager radii 0.5 Å smaller than those
7
5
In this conformation, benzazole and phenyl rings are in the
same plane and there is a hydrogen bonding between OH N
calculated, as recommended in the literature (5.01 and 4.99 Å
for 3a and 3b, respectively). The differences between the
ground state and excited state dipole moments (eg) in
different solvents were calculated using the slope and Onsager
radii of the molecules in their solvent environments. Once
again, compound 3b presented higher values (2.15 D) for eg
compared to the benzimidazole derivative 3a (1.98 D).
Moreover, the values for first hyperpolarisability obtained using
the solvatochromic method indicated that 3a and 3b present
values of 훽푥푥푥 32 to 83 times greater than urea.

atoms. In the ground state, the keto tautomers were broadly
-1
disfavoured by, at least, 10 kcal·mol . They could be obtained
only by imposing constraints in geometry optimisation. The
main change in the geometries of the ground states was
observed in the NH
structure, with the hydrogens in the same plane of the phenol
ring, the remaining structures have a pyramidalised NH , with
2
group. While Ecis presents a planar
2
out-of-the-plane hydrogens (Figs. S13 and S14). Another
remarkable difference is the change in the hydrogen bonding
This journal is © The Royal Society of Chemistry 2019
Phys. Chem. Chem. Phys, 2019, 00, 1-3 | 7
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Page 8 of 13
ARTICLE
PCCP
length from the ground to the excited state. For Ecis in the S
CAM-B3LYP gave a longer bond length than PBE1PBE and the almost degenerated in ethanol and acetDoOnIi:tr10il.e10. 3O9n/C8thCeP0o5t1h86eKr
opposite was observed for the keto tautomer. hand, the CAM-B3LYP functional suggests that the enol form is
1
,
in low polarity solvents, and the two tautomerViicewfAorrtimclesOnalirnee
The vertical excitation energies and oscillator strengths more stable in all the solvents. For 3b, only in 1,4-dioxane (using
calculated for structures 3a and 3b are presented in Tables 5 PBE1PBE) the keto form is the most stable. Yet, the energy
and 6, respectively. TD-DFT vertical excitation energies were in differences between the keto and enol forms are larger in 3b
good agreement with experimental ones. Consistent with than in 3a. A common characteristic for these molecules using
previous works,8
0-81
PBE1PBE energies were lower than both functionals is that solvents with higher dielectric constant
experimental values, while CAM-B3LYP energies were higher should disfavour the ESIPT mechanism, a finding in agreement
8
2
than the experimental ones. On the other hand, the results with previous work. It is worth mentioning that compounds 3a
provided by both functionals for the vertical emission energies and 3b presents in 1,4-dioxane and acetonitrile similar
were substantially different, mainly for the keto tautomers.
excitations and transition orbitals (see ESI). Due to the
Another remarkable difference was that, while CAM-B3LYP significant difference between CAM-B3LYP and PBE1PBE
provided excitation energies for enol and keto tautomers close results, we decided to benchmark the TD-DFT functionals for
to each other, this difference was much larger (around 200 nm) these molecules. The vertical energies were compared toh
when PBE1PBE was used. For 3a, CAM-B3LYP gave results closer those obtained using RI-CC2 (Table 7). These calculations were
to the experimental ones. However, comparing the done in the gas phase by using the geometries obtained with
experimental emission energies with the calculated ones, it is PBE1PBE and CAM-B3LYP. RI-CC2 offers a computationally
not easy to identify which functional provides a better result. efficient route to recover electron correlation and has been
2
1,48,78
The experimental data show higher emission energies for 3b used as a reference value for ESIPT calculations.
compared to 3a. However, the energy differences between keto the TD-DFT excitation energies, it was observed that, in general,
and enol conformers in the S state (E*), show that ESIPT is CAM-B3LYP provided results in better agreement with RI-CC2
more disfavoured for compound 3b than for 3a. Negative values calculations. This fact seems to indicate an ICT character for the S
Comparing
1
1
indicated that the keto tautomer is more stable than the enol state since it is well known that ICT states are better described using
tautomer. From these values, there is an apparent different long-range corrected functionals.
trend when using PBE1PBE and CAM-B3LYP functionals. Using
the first one, the 3a keto form is most stable
Table 5 Absorption maxima (λabs) and emission wavelengths (λem), in nm, respective of oscillator forces (f), and energy differences between the keto and enol
forms (ΔE*, in eV), in the corresponding solvents, calculated for 3a. The energies were calculated using the jun-cc-pVTZ basis set.
PBE1PBE
CAM-B3LYP
SOLVENT
λ
abs (nm)
404
422
426
426
475
478
477
477
f
λ
em (nm)
465
f
ΔE*(eV)
λ
abs (nm)
342
349
351
351
381
379
378
377
f
λ
em (nm)
391
f
ΔE*(eV)
1
,4-Dioxane
Dichloromethane
Ethanol
Acetonitrile
,4-Dioxane
0.688
0.633
0.602
0.595
0.299
0.315
0.309
0.306
0.559
0.715
0.757
0.765
0.103
0.217
0.262
0.271
−
−
−
1.038
0.994
0.960
0.951
0.764
0.776
0.758
0.752
1.172
1.341
1.383
1.390
0.612
0.898
0.966
0.978
−
−
−
507
520
523
678
647
641
641
427
439
441
430
441
446
447
E
cis
−
−
1
-0.20
-0.03
0.01
0.02
0.12
0.20
0.21
0.21
Dichloromethane
Ethanol
K
cis
Acetonitrile
ΔE = Eketo - Eenol ; 1 eV = 23.0609 kcal·mol-1
*
Table 6 Absorption maxima (λabs) and emission wavelengths (λem), in nm, respective of oscillator forces (f) and energy differences between the keto and enol
forms (ΔE*, in eV), in the corresponding solvents, calculated for 3b. The energies were calculated using the jun-cc-pVTZ basis set.
PBE1PBE
CAM-B3LYP
SOLVENT
λ
abs (nm)
403
420
423
424
454
455
454
454
f
λ
em (nm)
459
f
ΔE* (eV)
λ
abs (nm)
342
350
351
351
371
371
370
369
f
λ
em (nm)
387
f
ΔE*(eV)
1
,4-Dioxane
Dichloromethane
Ethanol
Acetonitrile
,4-Dioxane
Dichloromethane
Ethanol
Acetonitrile
0.754
0.711
0.682
0.676
0.288
0.329
0.329
0.328
0.591
0.777
0.827
0.837
0.059
0.144
0.196
0.207
−
−
−
1.117
1.095
1.067
1.060
0.781
0.840
0.835
0.832
1.263
1.448
1.494
1.502
0.497
0.857
0,.59
0.977
−
−
−
496
508
510
677
636
626
625
422
433
435
429
430
432
433
E
cis
−
−
1
-0.04
0.15
0.19
0.20
0.30
0.41
0.42
0.43
K
cis
*
ΔE = Eketo - Eenol ; 1 eV = 23.0609 kcal·mol-1
Table 7 Energy differences (in eV) between the ground state enol-cis and
keto-cis (Ek-E) and enol-cis and keto-trans (Etk-E), vertical absorption
e
energies at ground state geometry (Evert (S
0
) and vertical emission energies
8
| Phys. Chem. Chem. Phys., 2019, 00, 1-3
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
PCCP
ARTICLE
.8 kcal·mol , respectively. Moreover, PBE1PBE calculations
from enol-S
calculated in the gas phase.
1
(Evert (S
1
e) and from keto-S
1
(Evert (S
1
k)). The energies were
-1
8
View Article Online
-1
0.1039/C8CP05186K
indicated barriers of 1.7 and 4.6 kcal·mDOoIl: 1 for 3a and 3b,
respectively. These results are an additional indication that
ESIPT is disfavoured in the studied molecules, when compared
to HBO. The specific substituent groups give a push-pull
character for the molecules, which increases the possibility of
ICT. Since the inclusion of these substituents affects the charge
distribution of the proton donor and acceptor, it is interesting
to evaluate not only the orbitals involved in the transition but
also the electronic density over the atoms.
Regarding the solvent effect, the calculations showed a
small influence of solvent polarity on absorption energies, a
finding in agreement with the experimental results. A more
significant effect was observed for the emission energies, where
a redshift can be seen for the enol and keto forms using the
CAM-B3LYP functional. In this case, the solvent polarity effect
was larger for the enol than to the keto tautomer. This implies
Exp.c
-
-
RICC2
CAMB3LYP
0.13*
0.21*
3.81
3.63*
3.45
3.17*
2.86
2.88*
PBE1PBE
-0.20*
-0.16*
3.29

E
k-E (S
1
)

E
tk-E (S
1
)
3
3
3
3
2
.70a
e)

E
E
E
vert (S
vert (S
vert (S
0
1
1
b
.70
.39
.49
.73
3.07*
2.85
3.28*
3
a
b
a


e)
k)
b
a
2.66*
1.74
1.83*
-0.04
0.07
2.49*
2.49*
b
2
.56

E
k-E
0.30
-
-

E
tk-E
0.29
3.81
3.62*
3.48
3.21*
2.85
.69a
3
3
3
3
2
2
3.30
3.07*
2.87



E
E
E
vert (S
vert (S
vert (S
0
1
1
e)
e)
k)
3.21*
2.45*
b
a
b
a
b
.69
.45
.55
.69
.57
3
2.70*
1.72
1.83*
2.45*
2.89*
Calculated in 1,4-dioxane; geometry obtained with CAM-B3LYP/cc-pVDZ; bgeometry
a
*
1 1
that S for the enol is more polar than S for the keto form (see
c
obtained with PBE1PBE/cc-pVDZ and experiment performed in 1,4-dioxane.
Table 8). Therefore, polar environments stabilise the enol
tautomer more than the keto tautomer, and shift the emission
spectra to higher wavelengths. For this reason, the energy
To further analyse the possibility of ESIPT in 3a and 3b, the
potential energy curves for the proton-transfer in the S state
1
were also computed compared to the curves obtained for HBO,
a well-known example of ESIPT (Fig. 7). These curves were
computed by keeping the OH bond distance in a range of
values that increased in steps of 0.05Å and optimising the
remaining coordinates without constraints. CAM-B3LYP and
PBE1PBE provided different barriers for the proton transfer, as
well as present inversion on the relative stability between both
tautomers. The calculated barriers for HBO are in agreement
with the ones calculated by Roohi et al. using PBE1PBE/6-
1
difference between the tautomers in the S state is larger in
more polar solvents, and emission from the keto tautomer
should be more disfavoured. Consequently, is expected that the
fluorescence spectra shows a double emission in non-polar
solvents, where keto tautomers are more easily reached.
Interestingly, PBE1PBE showed a different trend (blueshift) for
the keto form with the increase of the solvent polarity.
Fig. 8 shows the molecular orbitals involved in the
transitions from the ground to the first excited state. The first
excitation is predominantly from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular
orbital (LUMO) and it has a ππ* character.
6,83
23
3
3
11++G(2d,p)7
1G(d,p).
and Alarcos et al. using CAM-B3LYP/6-
Fig. 8 Molecular orbitals calculated for 3a and 3b using CAM-B3LYP.
Fig. 7 Potential energy curves in the first excited state, with different OH bond
lengths, for HBO, 3a and 3b computed with TD-DFT.
It can be noticed that the heteroatom in the benzazole unit
has a negligible effect. For 3a and 3b, HOMO is delocalised over
Using CAM-B3LYP, for HBO the keto tautomer is clearly almost the entire molecule, excluding the nitro group.
-1
more stable (3 kcal·mol ) and there the barrier for the ESIPT is Significant electron density is located over the nitrogen of the
-1
very small (less than 2 kcal·mol ). Using PBE1PBE both, the amino group and over the hydroxyl group. The LUMO is mainly
energy difference between the two tautomers and the energy delocalised over the fused rings and nitro group. This indicates
1
barrier, decreased. In contrast, CAM-B3LYP calculations for 3a that excitation to the S state should transfer electron density
and 3b show that the enol tautomer was the most stable. Yet, from the side that contains the amino group, the donor, to the
the energy barriers for the proton transfer are equal to 5.5 and
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Page 10 of 13
View Article Online
DOI: 10.1039/C8CP05186K
PCCP
ARTICLE
Table 8 Partial ChelpG charges (in a.u.) and dipole momenta (, in D) calculated using 1,4-dioxane as solvent for HBO, 3a (X=NH) and 3b (X=O) using CAM-
B3LYP and PBE1PBE (in italic) and the jun-cc-pVDZ basis set.
8'
7'
H
O
4
7
3
2' 3'
9
8
N
5
9'
NH2
2
1'
4'
1
O
1
1
N
0
X
1
6
6
'
5'
O
12
Structure
μ
N
3
O
7’
H
8’
X
1
N
9’
N
10
O
11
O
12
HBO (S
HBO (S
HBO (S
0
1
1
)
2.6
-0.508
-0.479
-0.545
-0.526
-0.466
-0.459
-0.562
-0.533
-0.528
-0.484
-0.467
-0.473
-0.533
-0.503
-0.489
-0.442
-0.491
-0.510
-0.567
-0.536
-0.528
-0.499
-0.655
-0.630
-0.592
-0.558
-0.551
-0.509
-0.638
-0.581
-0.569
-0.537
-0.539
-0.487
-0.637
-0.577
0.345
0.337
0.373
0.373
0.420
0.427
0.365
0.355
0.368
0.357
0.434
0.440
0.352
0.342
0.358
0.340
0.447
0.454
-0.290
-0.264
-0.303
-0.276
-0.275
-0.253
-0.598
-0.574
-0.583
-0.544
-0.504
-0.522
-0.287
-0.254
-0.300
-0.265
-0.262
-0.239
2
.5
1.6
.4
3.7
.7
8.6
.2
20.8
8.0
16.3
4.4
7.8
.5
19.6
7.8
12.7
5.5
-E)
-K)
1
2
3
3
3
3
3
3
a (S
a (S
a (S
0
1
1
)
-0.794
-0.752
-0.898
-0.805
-0.769
-0.761
-0.798
-0.760
-0.899
-0.789
-0.779
-0.903
0.739
0.703
0.661
0.597
0.672
0.585
0.740
0.705
0.690
0.617
0.699
0.594
-0.457
-0.444
-0.565
-0.580
-0.557
-0.584
-0.454
-0.440
-0.556
-0.579
-0.519
-0.581
-0.472
-0.457
-0.570
-0.596
-0.557
-0.598
-0.463
-0.449
-0.558
-0.589
-0.519
-0.591
9
-E)
-K)
2
2
b (S
b (S
b (S
0
)
8
1
-E)
-K)
2
1
2
Fig. 9 Electrostatic potential surfaces for HBO, 3a and 3b obtained using CAM-
B3LYP.
side containing the nitro group, the acceptor. The spatial
separation of HOMO and LUMO orbitals is characteristic of
push-pull molecules and indicates a charge transfer from one
side to the other (see Fig. 9). These results are consistent with
the charge distribution and with the fact that ICT should
overarch ESIPT in both 3a and 3b.
Phototautomerism is associated with changes in electronic
densities and to a certain amount of charge transferred
between the acceptor and donor atoms. Further, charge
transfer states are characterised by changes in the electronic
density from one side of the molecule to the other. Therefore,
it is interesting to analyse the redistribution of electronic
charges after the excitation. Table 8 shows the charge
distribution and dipole moment for HBO, 3a and 3b in S
0
-enol,
S
1
-enol and S -keto. From S - to S -enol, the negative charge
1
0
1
over O7’ decreased for all three molecules, but for HBO the
proton H8’ became much more positive than for 3a and 3b,
highlighting the increase in the proton donor acidity mainly for
the former molecule. Additionally, the electronic density over
3
the N atom in HBO increase and enhance its basicity, as
8
4
expected. As a result, a fast proton transfer takes place from
the donor to the acceptor atom along the reaction coordinate
in the S
density on the N
1
. On the other hand, for 3a and 3b, the electronic
atom decreases after excitation and relaxation
3
to the enol form, indicating a decrease in the basicity of the
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
PCCP
ARTICLE
acceptor atom and, therefore, disfavouring the ESIPT. This fact
can indicate that the electronic density is shifted to other atoms
in 3a and 3b. Indeed, going from the S - to S -enol, there was a
large increase (0.1 a.u.) in the negative charge over O11 and
12, a change that emphasizes the increase of electronic density
View Article Online
DOI: 10.1039/C8CP05186K
0
1
O
shifted by the ICT process.
The changes in electronic density can be easily observed in
the electrostatic potential surfaces plotted in Fig. 9. In HBO, the
electronic density is mainly localised on the donor group and
slightly spreads out over the benzene rings after excitation.
However, after excitation in 3a and 3b, the electronic density is
2 2
clearly shifted from the NH to the NO side, strongly
indicating that ICT takes place. The changes in the electronic
density are also reflected in the dipole momentum () of the
Fig. 10 Energy profiles in the first excited state for 3a (left) and 3b (right) in 1,4-
dioxane calculated using CAM-B3LYP/jun-cc-pVDZ. The inserts show the
molecules and they can help to assign the character of the geometries that correspond to the transition states and the N-H and O-H distances
(
0
in Å). Energies were calculated in relation to the S -enol optimized structure,
transitions. In HBO,  undergoes a small variation in the excited
state, indicating a LE character, in contrast to 3a and 3b. There
is a large increase in the dipole moment when transitioning
except the values in parenthesis, which represent the energy difference between
the states connected by the dashed line and the transition state energy.
0 1
from S to S (particularly large for the enol tautomer), which is
The situation is different for 3b. For this compound, the
vertical absorption does not provide enough energy to the
molecules to overcome the barrier for the proton transfer since
the keto tautomer has even higher energy than FC transition.
Therefore, the higher wavelengths experimentally observed for
one more indicator of charge transfer states. The large increase
in the dipole moment implies that polar solvents can stabilise
ICT states more than nonpolar solvents by decreasing the
5
8
energy gap between S
1
and S
0
.
Consequently, there is a
redshift in the wavelengths in the emission spectra. This
observation agrees with the calculated emission wavelengths
presented in Tables 5 and 6.
3
b when compared to 3a were not reproduced in the
calculations and the reason for this discrepancy remains
unclear. Additionally, according to calculations, changing the
heteroatom from N to O does not impact considerably the
emission energy, in contrast to the experimental results. A
possible explanation for both effects could be associated to
specific interactions with the solvent molecules. Our current
research will explore these directions.
Additional calculations using explicit solvent (molecules of
acetonitrile plus implicit solvation) clearly show the effect of
this solvent in the emission spectra (for the plots of spectra and
molecular orbitals see supporting information). While the LE
band (around 300 nm) remains basically unchanged with the
addition of acetonitrile molecules, the ICT band is clearly
redshifted (from 440 nm to 470 nm). This is a strong
evidence for ICT.
It is worth noting that the PBE1PBE functional provided the
wrong trend for keto emission, where λem decreased with
increasing solvent polarity. This observation highlights the fact
that the PBE1PBE functional does not describe charge transfer
3
0
states properly. Our results agree with Hsieh et al., that show
that when a large charge separation in the molecule is present,
ICT should occur prior to ESIPT. In this case, with the increase of
the solvent polarity, the emission wavelength should present a
bathochromic effect. Additionally, solvent polarisation effects
can be more easily observed since there is a significant
difference in the dipole moment between both conformers in
the excited state.30
While most of the emission energies calculated for 3a using
CAM-B3LYP were fully in agreement with experimental results,
this concurrence was not observed for 1,4-dioxane. A possible
explanation can be rationalised in terms of the energy profiles Conclusions
given in Fig. 10. Although the enol form is the most stable one,
The synthesised compounds 3a and 3b presented in solution
the energy difference between the two tautomers is very small
0.12 eV). In addition, the energy barrier for the proton transfer
absorption maxima in the UVA-visible region due to spin and
(
1
symmetry allowed electronic * with no clear evidence for
is equal to the relaxation energy for the Franck-Condon state.
Therefore, after the light absorption, the system still has
enough energy to overcome the barrier and populate the keto
form to some extent. This fact implies that the abnormally
larger wavelength observed experimentally for 3a in 1,4-
dioxane could be associated with the emission from keto
tautomer. This supposition supports the mechanism of ICT
coupled to ESIPT for 3a in 1,4-dioxane, while the emission in the
remaining solvents is attributed to an ICT.
charge transfer in either compound in the ground state. In
general, two states, LE and ICT could be observed for
compounds 3a and 3b, which were tailored by solvent polarity,
leading to fluorescence emission in the violet-blue-green
region. Taking the longer-wavelength emission in the into
account, experimentally this work has also established that: (a)
relatively large Stokes shifts were observed for 3a and 3b, (b)
compound 3b was more polar in the excited state than 3a, (c)
excited state deactivation seemed to be ascribed to ICT,
observed from the Lippert-Mataga correlation in the f range
of 0.02-0.20 and (d) at higher f the medium polarity seems to
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Ph yP sl ei c aa sl eC hd eo mni os ttr ya dC jhu es mt mi c aa lr Pg hi ny ss ics
Page 12 of 13
ARTICLE
PCCP
reach a limit of stabilisation. In summary, experimentally, 18 A. J. A. Aquino, F. Plasser, M. Barbatti and H. LisVciehwsAkratic,leCOronalinte.
Chem. Acta, 2009, 1, 105.
9 O. K. Abou-Zied, R. Jimenez, E. H. Thompson, D. P. Millar and
F. E. Romesberg, J. Phys. Chem. A, 2002, 106, 3665.
0 W. Frey and T. Elsaesser, Chem. Phys. Lett., 1992, 189, 565.
1 M. Barbatti, A. J. A. Aquino, H. Lischka, C. Schriever, S.
Lochbrunner and E. Riedle, Phys. Chem. Chem. Phys., 2009,
DOI: 10.1039/C8CP05186K
charge-transfer emissions seems to be responsible prior to
ESIPT, to the long-emission wavelengths in these compounds.
To further investigate the possibility of ESIPT or ICT, we
performed TD-DFT calculations. They also indicate that ICT
should take place prior to ESIPT. ESIPT is largely disfavoured
mainly in polar solvents. The emission wavelengths were
sensitive to the solvent polarity, in agreement with the
occurrence of ICT. For 3a in 1,4-dioxane, the small energy
difference between the tautomeric forms added to the small
energy barriers, allow the emission from the keto.
1
2
2
1
1, 1406.
2
2 C. Schriever, M. Barbatti, K. Stock, A. J. A. Aquino, D. Tunega,
S. Lochbrunner, E. Riedle, R. de Vivie-Riedle and H. Lischka,
Chem. Phys., 2008, 347, 446.
23 N. Alarcos, M. Gutiérrez, M. Liras, F. Sánchez, M. Moreno and
A. Douhal. Phys. Chem. Chem. Phys. 2015, 17, 14569.
4 N. Alarcos, M. Gutierrez, M. Liras, F. Sánchez and A. Douhal,
Phys. Chem. Chem. Phys., 2015, 17, 16257.
2
2
Acknowledgements
5 S. Pijeau, D. Foster and E.G. Hohenstein, J. Phys. Chem. A
The authors would like to thank FAPERGS (17/2551-
2
017, 121, 4595.
0
000968-1), CNPq and Coordenação de Aperfeiçoamento de
Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001 for
the financial support. Theoretical calculations
2
2
6 T. C. Swinney and D. F. Kelley, J. Chem. Phys., 1993, 99, 211.
7 J. Seo, S. Kim and S. Y. Park, J. Am. Chem. Soc., 2004, 126,
11154.
were carried out with the support of Centro Nacional de 28 V. I. Tomin, A. P. Demchenko and P. T. Chou, J. Photochem.
Supercomputação (CESUP), from the Universidade Federal do
Rio Grande do Sul and National Laboratory of Scientific
Computation (LNCC-Santos Dumond).
Photobiol. C Photochem. Rev., 2015, 22, 1.
9 P. Chowdhury, S. Panja, A. Chatterjee, P. Bhattacharya and S.
Chakravorti, J. Photochem. Photobiol. A Chem., 2005, 170,
2
1
31.
3
3
3
3
0 C. C. Hsieh, Y. M. Cheng, C. J. Hsu, K. Y. Chen and P. T. Chou, J.
Phys. Chem. A, 2008, 112, 8323.
1 F. S. Rodembusch, F. P. Leusin, L. F. Campo and V. Stefani, J.
Lumin., 2007, 126, 728.
2 S. Hillebrand, M. Segala, T. Buckup, R. R. B. Correia, F.
Horowitz and V. Stefani, Chem. Phys., 2001, 273, 1.
3 J. Seo, S. Kim, S. Park and S. Y. Park, Bull. Korean Chem. Soc.,
Notes and references
1
2
3
A. Z. Weller, Electrochem., 1956, 42, 1144.
H. K. Sinha and S. K. Dogra, Chem. Phys., 1986, 12, 337.
A. P. Demchenko, K. C. Tang and P. T. Chou, Chem. Soc. Rev.,
2
013, 42, 1379.
4
5
J. E. Kwon and S. Y. Park, Adv. Mater., 2011, 23, 3625.
F. S. Santos, E. Ramasamy, V. Ramamurthy and F. S.
Rodembusch, J. Photochem. Photobiol. A Chem., 2016, 317,
2
005, 26, 1706.
3
3
4 C. Li, Y. Yang, C. Ma and Y. Liu, RSC Adv., 2016, 6, 5134.
5 C. H. Kim, P. Jaehun, J. Seo, S. Y. Park and T. Joo, J. Phys. Chem.
A, 2010, 114, 5618.
6 M. Gutierrez, N. Alarcos, M. Liras, F. Sanchez and A. Douhal, J.
Phys. Chem. B, 2015, 119, 552.
7 C. Li, C. Ma, D. Li and Y. Liu, J. Lumin., 2016, 172, 29.
8 N. Alarcos, M. Gutierrez, M. Liras, F. Sanchez and A. Douhal,
Photochem. Photobiol. Sci., 2015, 14, 1306.
1
75.
6
7
8
9
1
1
F. S. Santos, N. G. Medeiros, R. F. Affeldt, R. D. Duarte, S.
Moura and F. S. Rodembusch, New J. Chem., 2016, 40, 2785.
R. Wang, D. Liu, K. Xu and J. Li, J. Photochem. Photobiol. A
Chem., 2009, 205, 61.
P. O. Hubin, A. D. Laurent, D. P. Vercauteren and D.
Jacquemin, Phys. Chem. Chem. Phys., 2014, 16, 25288.
N. Jiang, C. Yang, X. Dong, X. Sun, D. Zhang and C. Liu, Org.
Biomol. Chem., 2014, 12, 5250.
0 J. Zhao, S. Ji, Y. Chen, H. Guo and P. Yang, Phys. Chem. Chem.
Phys., 2012, 14, 8803.
1 V. A. Corbellini, M. L. Scroferneker, M. Carissimi, F. S.
Rodembusch and V. Stefani, J. Photochem. Photobiol. B Biol.,
3
3
3
3
4
4
4
4
4
4
4
4
4
9 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996,
7
7, 3865.
0 T. Yanai, D. P. Tew and N. C. Handy, Chem. Phys. Lett., 2004,
93, 51.
3
1 W. L. F. Armarego. In Purification of Laboratory Chemicals, 5th
ed.; Cornwall, UK: Elsevier Academic Press; 2003.
2 A. Chekalyuk, V. Fadeev, G. Georgiev, T. Kalkanjiev and Z.
Nickolov, Spectrosc. Lett., 1982, 15, 355.
3 F. S. Rodembusch, T. Buckup, M. Segala, L. Tavares, R. R. B.
Correia and V. Stefani, Chem. Phys., 2004, 305, 115.
4 S. Hammes-Schiffer and A. A. Stuchebrukhov, Chem. Rev.,
2
010, 99, 162.
1
1
1
1
1
1
2 K. Dhanunjayarao, V. Mukundam and K. Venkatasubbaiah,
Sens. Actuators B, 2016, 232, 175.
3 H. M. Lv, D. H. Yuan, W. Liu, Y. Chen, C. T. Au and S. F. Yin,
Sens. Actuators B, 2016, 233, 173.
4 A. Maity, F. Ali, H. Agarwalla, B. Anothumakkool and A. Das,
Chem. Commun., 2015, 51, 2130.
5 U. N. Yadav, H. S. Kumbhar, S. S. Deshpande, S. K. Sahoob and
G. S. Shankarling, RSC Adv., 2015, 5, 42971.
6 M. Luo, S. Wang, M. Wang, S. Huang, C. Li, L. Che and X. Ma,
Dyes Pigm., 2016, 132, 48.
7 Turro, N. J. In. Modern Molecular Photochemistry; University
Science Books: Sausalito, 1991; pp. 628.
2
010, 110, 6939.
5 S. Luber, K. Adamczyk, E. T. J. Nibbering and V. S. Batista, J.
Phys. Chem. A, 2013, 117, 5269.
6 R. S. Iglesias, L. F. Campo, F. S. Rodembusch and V. Stefani, Int.
J. Quantum Chem., 2004, 108, 2334.
7 A. J. A. Aquino, H. Lischka and C. Hättig, J. Phys. Chem., 2005,
1
09, 3201.
8 N. Kungwan, F. Plasser, A. J. A. Aquino, M. Barbatti, P.
Wolschmann and H. Lischka, Phys. Chem. Chem. Phys., 2012,
1
4, 9016.
1
2 | Phys. Chem. Chem. Phys., 2019, 00, 1-3
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PCCP
ARTICLE
4
5
5
5
9 L. Wang, Y. Zhang, Y. Li, H. He and J. Zhang, J. Photochem. 78 O. Louant, B. Champagne and V. Liégeois, J. PhViyesw.ACrthicelemOn.liAne,
Photobiol. A Chem., 2014, 294, 14. 2018, 22, 972.
0 W. Yang and X. Chen, Phys. Chem. Chem. Phys., 2014, 16, 79 R. S. Iglesias, P. F. B. Gonçalves and P. R. Livotto, Chem. Phys.
242. Lett., 2000, 327, 23.
1 J. M. Ortiz-Sánchez, R. Gelabert, M. Moreno and J. M. Lluch, J. 80 A. Laurent, D. Jacquemin, Int. J. Quantum Chem., 2013, 113,
Chem. Phys., 2008, 129, 214308. 2019.
DOI: 10.1039/C8CP05186K
4
2 A. Fernández-Ramos, E. Martínez-Núñes, S. A. Vázquez, M. A. 81 C. Adamo and D. Jacquemin, Chem. Soc. Rev., 2013, 42, 845.
Ríos, C. M. Estévez, M. Merchán and L. Serrano-Andrés, J. 82 J. Hao and Y. Yang. Org. Chem. Front. 2018, 5, 2234.
Phys. Chem. A, 2007, 111, 5907.
3 Plasser, F. In: Dynamics simulation of excited state
83 H. Roohi, F. Hejazi, N. Mohtamedifar and M. Jahantab,
5
Spectrochim. Acta Part A, 2014, 118, 228.
intramolecular proton transfer; Master Thesis, Zentrum für 84 C. Li, Y. Yang, C. Ma and Y. Liu, RSC Adv. , 2016, 6, 5134.
Molekulare Biologie, Universität Wien: Viena, 2009.
4 E. Papajak, J. Zheng, H. R. Leverentz and D. G. Truhlar, J.
Chem. Theory Comput., 2011, 7, 3027.
5 M. Barbatti, A. J. A. Aquino, H. Lischka, C. Schriver, S.
Lochbrunner and E. Riedle, Phys. Chem. Chem. Phys., 2009,
5
5
1
1, 1406.
5
5
6 A. Köhn and C. Hättig, J. Chem. Phys., 2003, 119, 5021.
7 TURBOMOLE V7.0 2015, a development of University of
Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989-
2
007, TURBOMOLE GmbH, since 2007; available from
http://www.turbomole.com.
5
5
8 J. R. Lakowicz, In: Principles of Fluorescence Spectroscopy,
Springer, New York, 3rd edn, 2006, ch. 6, pp. 205–235.
9 P. S. Kalsi, In: Spectroscopy of Organic Compounds. New Age
International Pvt Ltd Publishers; 6th edn., 2016.
6
6
0 S. J. Strickler and R. A. Berg, J. Phys. Chem., 1962, 37, 814.
1 N. J. Turro, J. C. Scaiano and V. Ramamurthy, In: Principles of
Molecular Photochemistry: An Introduction. University
Science Books; 1st edn., 2008.
6
6
2 G. Calzaferri and R. Rytz, J. Phys. Chem., 1995, 9, 12141.
3 S. M. Ormson, R. G. Brown, F. Vollmer and W. Rettig, J.
Photochem. Photobiol. A Chem., 1994, 81, 65.
6
6
6
4 S. A. El-Daly, A. M. Asiri and K. A. Alamry, J. Fluoresc., 2014,
2
4, 1307.
5 E. Lippert, W. Lüder, F. Moll, W. Nägele, H. Boos, H. Prigge and
I. Seinbold-Blankenstein, Angew. Chem., 1961, 73, 695.
6 S. R. Marder, G. D. Stucky and J. E. Sohn (Editors). In: Materials
for Nonlinear Optics. Chapter 1. Linear and Nonlinear
Polarizability. Publisher: American Chemical Society, Vol. 455,
pp2-30, 1991.
6
7 M. S. Paley, J. M. Harris, H. Looser, J. C. Baumert, G. C.
Bjorklund, D. Jundt and R. Twieg, J. Org. Chem., 1989, 54,
3
774.
6
6
7
8 H. C. Song, Y. W. Chen, X. L. Zheng and B. N. Ying, Spectrochim.
Acta A, 2001, 57, 1717.
9 F. Momicchioli, G. Ponterini and D. Vanossi, J. Phys. Chem. A,
2
008, 112, 11861.
0 C. Reichardt, Solvent Effects in Organic Chemistry, Verlag
Chemie: Weinheim, 1979.
7
7
1 A. Kawski, Z. Naturforsch A 2002, 57, 255.
2 S. K. Lanke and N. Sekar, J. Photochem. Photobiol. A Chem.,
2
016, 321, 63.
7
7
3 E. G. McRae, J. Phys. Chem., 1957, 61, 562.
4 S. Bruni, E. Cariati, F. Cariati, F. Porta, S. Quici and D. Roberto,
Spectrochim. Acta A, 2001, 57, 1417.
7
7
5 http://gaussian.com/volume/, accessed in July 23, 2018
6 H. Roohi, N. Mohtamedifar and F. Hejazi, Chem. Phys. 2014,
4
44, 66.
7
7 L. Wilbraham, M. Savarese, N. Rega, C. Adamo and I. Ciofini, J.
Phys. Chem. B, 2015, 119, 2459
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