Hydrogen bonded dimers of ketocoumarin in the solid state and alcohol:water binary solvent: Fluorescence spectroscopy, crystal structure and DFT investigation

Article abstract of DOI:10.1039/c9nj01053j

The photophysical properties of 3,3′-carbonylbis(7-diethylamino)-2H-chromen-2-one (ketocoumarin) were investigated in the solid state, different solvents and an alcohol:water binary mixture using steady state, time-resolved fluorescence spectroscopy, XRD studies and DFT calculations. In neat solvents ketocoumarin (KC) shows a small Stokes shift as compared to other 7-diethylaminocoumarin (7-DEC) dyes and this is due to the π-π? electronic transitions involving the bridge keto group. In aqueous alcoholic solution, the fluorescence maximum of KC depends on the percentage of water and a new emission maximum at 550 nm with a large Stokes shift is observed at 90% water content. The solid state fluorescence and XRD crystal structure analysis reveals that the new decay component with a lifetime of 2.71 ns and new emission maximum in the alcohol:water mixture is due to the intermolecular hydrogen bonding [HB] interaction between KC molecules in the alcohol:water binary mixture. The XRD structure reveals the two modes of intermolecular CH?OC hydrogen bonding (HB) which lead to two dimeric forms of KC, D1 and D2. The emission wavelength dependent excitation spectrum in the solid state and alcohol:water binary mixture confirms the H-type nature of dimer D1 and the J-type nature of dimer D2. The time-resolved fluorescence studies indicate that the new decay component with a lifetime of 2.71 ns is due to dimer D1 and the new emission with the maximum at 550 nm is due to dimer D2. Density functional theory (DFT) analysis confirms that the KC dimers are stabilized by hydrogen bonding and other non-covalent interactions. The hydrogen bonded KC dimers are responsible for the anomalous fluorescence behavior of KC in the alcohol:water binary mixture and for the bright yellow fluorescence in the solid state.

Full text of DOI:10.1039/c9nj01053j

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: K. Ramamurthy,
E.J. P. Malar and S. Chellappan, New J. Chem., 2019, DOI: 10.1039/C9NJ01053J.
Volume 40 Number
1
January 2016 Pages 1–846
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Page 1 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
Hydrogen bonded dimers of ketocoumarin in solid state and alcohol: water binary solvent by
fluorescence spectroscopy, crystal structure and DFT investigation.
Kannan Ramamurthy, E.J. Padma Malar* and Chellappan Selvaraju*
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
National Centre for Ultrafast Processes, University of Madras, Chennai 600 113, Tamil Nadu,
India.
*
Corresponding author E-mail addresses: selvaraj24@hotmail.com (Chellappan Selvaraju)
Abstract:
The photophysical properties of 3,3’ Carbonylbis (7-diethylamino)-2H-chromen-2-one
Ketocoumarin) was investigated in solid state , different solvents and alcohol: water binary
(
mixture using steady state, time-resolved fluorescence spectroscopy, XRD studies and DFT
calculations. In neat solvents ketocoumarin (KC) shows small Stokes shift as compared to other
7
-diethylaminocoumarin dyes (7-DEC) and is due to the π-π* electronic transitions involving
the bridge keto group. In aqueous alcoholic solution, the fluorescence maximum of KC depends
on the percentage of water and a new emission at 550 nm with large Stokes shift is observed at
9
0% water content. The solid state fluorescence and XRD crystal structure analysis revels that
the new decay component with lifetime 2.71 ns and new emission maximum in alcohol: water
mixture is due to the intermolecular hydrogen bonding [HB] interaction between KC molecules
in alcohol: water binary mixture. XRD structure reveals that the two modes of intermolecular
CH…O=C hydrogen bonding (HB) which lead to two dimeric forms of KC, D1 and D2. The
emission wavelength dependent excitation spectrum in solid state and alcohol: water binary
mixture confirms the H-type nature of dimer D1 and J-type nature of dimer D2. The time-
resolved fluorescence studies indicates the new decay component with lifetime 2.71 ns is due the
dimer D1 and new emission with maximum at 550 nm is assigned to dimer D2. Density

New Journal of Chemistry
Page 2 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
functional theory (DFT) analysis confirms that the KC dimers are stabilized by hydrogen
bonding and other non-covalent interactions. The hydrogen bonded KC dimers are responsible
for the anomalous fluorescence behavior of KC in alcohol: water binary mixture and bright
yellow fluorescence from solid state.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
. Introduction
Coumarin and its derivatives are important class of organic heterocyclic systems and are
widely used in the different fields of science and technology. They can be found in natural and
synthetic drug molecules and exhibit wide variety of pharmacological activities such as
1
2
3
anticoagulant, anti-bacterial, anticancer and specific inhibitor activitities. Coumarins constitute
4
-7
the largest class of laser dyes in the “blue-green” region. 7-aminocoumarin is one of the well
documented candidate from this family due to its excellent photophysical and photochemical
properties such as large Stokes shift with environmental sensitivity,8-9 high fluorescence quantum
1
0
11
yield , reasonable solubility, excited state electron donor and acceptor capability , excited state
12
13
conformational change and photoionization . These properties make them useful in a wide
1
4
15
16
variety of applications such as fluorescent probes, sensors, optical brighteners, laser dyes,
1
7
18
solar energy collectors and dopants in organic light emitting diodes.
The unusual photophysical properties of 7-aminocoumarins in different solvent
environments have been reported recently and are due to i) presence of different conformations
and ii) intermolecular interactions.19-20 In non-polar solvents, 7-aminocoumarins adopt non-
planar conformation and the fast flip-flop motion of the 7-amino group introduces additional
non-radiative decay channel, which is absent in the planar conformation present in moderate to
1
9
highly polar solvents. In the case of 7- N,N-dialkylamino substituted coumarin dyes such as
Coumarin-481 and coumarin-152 in highly polar solvents, the planar fluorescent ICT

Page 3 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
(
Intramolecular charge transfer) is converted to non-fluorescent TICT (Twisted intramolecular
charge transfer) state and there is unusual decrease in fluorescence quantum yield and lifetime.20
Poonom et.al reported the formation of H-aggregate and H-dimer in the aqueous solution of
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
21
Coumarin-481 through steady state and time resolved fluorescence techniques. They also found
that the aggregates are emissive in nature. These aggregates are also formed in high polar
solvents such as ethanol and acetonitrile when increasing the concentration of the coumarin.
KC is an important member of coumarin dyes and has a carbonyl group attached to the 3-
position in the coumarin structure through a single bond. The coumarin-3-carboxylic acid and
2
2
coumarin-343 also belong to the family of ketocoumarin dyes. ketocoumarins have very high
singlet-triplet intersystem crossing quantum yield and serve as a new class of triplet sensitizer.23
The intersystem quantum yield increases on replacing the hydroxyl group by alkyl/aryl group in
coumarin -3-carboxylic acids. Xiaogang Liu et.al reported the origin of dye aggregation and
complex formation in 7-(diethylamino)-coumarin-3-carboxylic acid [DCCA] and in coumarin -
3
43.24 The formation of J-aggregates was understood from photophysical and TD-DFT
calculations. The formation of H-aggregates in cholesterol substituted ketocoumarin was
reported in pluronic surfactant based polymeric micelles.25 The intermolecular interaction
induced polymorphism and concentration dependent fluorescence emission of ketocoumarins
such as 7-(Diethylamino)coumarin-3-aldehyde [DCA] and DCCA was recently reported by Rong
Rong Cui et.al.26 Ketocoumarins are widely used for the sensitization of semiconductor
nanoparticles and studies related to aggregation are quite important for the construction of dye-
sensitized solar cells and other optoelectronic devices.27
Aggregate formation is governed by weak intermolecular interactions which are
noncovalent in nature. Hydrogen bonding (HB) interactions28,29 and London dispersion

New Journal of Chemistry
Page 4 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
30
interactions are predominant noncovalent interactions operating in large organic and biological
systems. HB interactions are electrostatic in origin whereas the dispersion interactions arise due
3
0
to induced dipole-induced dipole interactions. The dispersion interaction contributes to stability
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
in all systems, in principle and has a significant role in stabilizing weak van der Waals dimers.31-
3
4
Different studies show that the dispersion interactions play a major role in stabilizing sterically
crowded inorganic, organic and biological systems, guest–host complexes etc.35 In large
molecular systems, the DFT-D3 method developed by Grimme based on the atom-pair wise
potential is a convenient tool to study the dispersion interaction.36
In the present study the photophysical properties of KC was studied and the role of
aggregation was investigated. This KC has very small Stokes shift and low fluorescence quantum
yield in both polar and non-polar solvents (Scheme-1). In non-polar solvents, KC shows strong
2
3
triplet-triplet absorption with triplet lifetime of several microseconds. KC acts as a heavy-atom
3
7
free triplet sensitizer used for variety of applications such as photocatalysis , photodynamic
3
8
therapy and triplet-triplet annihilation up conversion. The two photon absorption cross-section
39
of KC two to three orders of magnitude larger than that of the coumarin dyes. Numerous
studies report the application of KC as photosensitizer or photoinitiators for one and two photon
polymerization. KC based OLED’s and electroluminescent materials have been reported
40
recently. In polar solvents, the triplet yield decreases with decrease in the fluorescence quantum
yield. The unusual photophysical properties of KC such as small Stocks shift and low
fluorescence quantum yield in polar and non-polar solvents are not understood fully. We have
investigated the photophysical properties of KC in solid state and alcohol: water binary mixture
using steady state, time-resolved fluorescence spectroscopy. In alcohol: water binary mixture,
KC shows new fluorescence maximum with large Stokes shift. This observation is similar to the

Page 5 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
emission behavior from the single crystal of KC. The origin of anomalous bright yellow
emission in solid state and alcohol: water binary mixture is explored using x-ray diffraction and
DFT studies.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
N
O
O
N
Scheme 1. Structure of KC
2
. Experimental section
Materials
4-Diethylaminosalicyaldehyde and dimethyl 1,3-acetonedicarboxylate were purchased
from Sigma Aldrich. Piperidine were purchased from spectrochem. All the solvents used in this
study are HPLC quality obtained from Fisher Scientific. The spectroscopic grade solvents like
1,4 dioxane, toluene, tetrahydrofuran (THF), chloroform (CHCl ), 1,2-dichloromethane (DCM),
3
methanol (MeoH), ethanol (EtoH), n-propanol, acetonitrile (ACN), N,N-dimethylformamide
(
DMF), dimethyl sulfoxide (DMSO) and water (H O) were used after proper distillation as
2
needed.
General techniques
1
13
The H and C NMR spectra of KC compound were recorded on a Bruker- Avance at
4
00 MHz and 100 MHz NMR instruments using CDCl and DMSO-d as solvents containing
3
6
tetramethylsilane (TMS) as an internal standard. Absorption spectra of the KC were recorded
using an Agilent 8453 UV-visible diode array spectrophotometer. The fluorescence spectral
measurements were carried out using Fluoromax-4 spectrophotometer (Horiba Jobin Yvon). The
fluorescence quantum yield of KC was calculated using a single point method. The coumarin-

New Journal of Chemistry
Page 6 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
53 in ethanol was used as reference. The quantum yield of coumarin-153 is 0.544 at 422 nm
4
1
excitation . The quantum yield of KC was calculated using the following equation
2
퐼 퐴 휂
푠
푟
푥
푠
훷 = φ 푥 푥
푓
r
2
퐼 퐴
푟
푠 휂
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
푟
where φ refers to the quantum yield of KC and I, η and A is the measured integrated emission
f
intensity (area under the curve), refractive index and absorption at 422nm respectively. The
subscript r and s refers to the reference coumarin-153 and KC respectively. An excitation slit
width of 4 nm and emission slit width of 2 nm was used to record the fluorescence spectra in the
wavelength range of 440-700 nm. The refractive index of the methanol: water mixture was
obtained from the literature and used in the quantum yield calculation.42
Time-resolved fluorescence decays were obtained by the time correlated single-photon
counting (TCSPC) technique with microchannel plate photomultiplier tube (Hamamatsu,
R3809U) as detector and femtosecond laser as an excitation source. The 425 nm output from the
mode-locked femtosecond laser (Tsunami, Spectra physics) was used as the excitation source.
The instrument response function for TCSPC system is ∼50 ps. The data analysis was carried out
by the software provided by IBH (DAS-6), which is based on a deconvolution technique using
nonlinear least-squares methods. Femtosecond fluorescence decays were recorded using
1
3
fluorescence up-conversion technique as reported elsewhere. The fluorescence decays were
fitted using a Gaussian shape for the excitation pulse with FWHM of 200 fs. The femtosecond
fluorescence decay was analyzed using IGOR software. All the fluorescence lifetime
measurements were carried out for at least three times and error in the fitted lifetime values is
well within 5%.

Page 7 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
X-ray structure analysis and refinement
Single crystal X-ray diffraction data of KC were collected from Bruker AXS Kappa
Apex2 CCD diffractometer at room temperature, (SAIF-IIT Madras) using graphite
monochromated Mo Kα (λ=0.71073 Å) radiation. For data reduction SAINT/XPREP program
was used and for cell refinement APEX2/SAINT program was used. The structures were
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
resolved by direct methods and refined by full-matrix least-squares on F (SHELXL97). The
positions of all the non-hydrogen atoms were refined anisotropically. All the hydrogen atoms
were placed at ideal positions and included in the refinement. The Hirshfeld surfaces represented
4
3
by dnorm, shape index and 2-D fingerprint plots were calculated using Crystal Explorer 3.1. For
each point on the Hirshfeld surface, two parameters are defined. d is the distance from the point
e
to the nearest nucleus external to the surface and d is the distance from the point to the nearest
i
nucleus internal to the surface. The normalized contact distance, dnorm, based on both d and d ,
e
i
4
4
and the vdW radius of the atom, is given by the equation:
vdW
vdW
d ri
d r
e - e
i -
+
dnorm
=
…1
vdW
vdW
r
i
r
e
Here, ri Vdw and re Vdw are the internal and external van der Waals radii of the atom. The
combination of de and d in the form of a 2-D fingerprint plot provides summary of
i
intermolecular contacts in the crystal.
Computational method
The molecular and electronic structures of KC in different geometrical arrangements
were studied in the gas-phase by performing complete structural optimization using the density
functional theory (DFT) methods B3LYP.45,46 M052X and BP86.
47
48,49
The relative energies of
the low-energy structures of KC by the above DFT methods were compared. Intermolecular
interaction energies (IE) between two KC molecules were computed at two different dimer

New Journal of Chemistry
Page 8 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
structures D1 and D2, which were characterized from the XRD structure of the single crystal.
The stability of the dimer was examined by single-point energy calculations using the DFT
3
50-52
that are suitable to study noncovalent interactions.53-56
methods M052X and M062X-D3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Benchmark analysis of Goerigk and Grimme reveals that the M062X-D3 method yield reliable
predictions of the dispersion energy in varied types of systems. The IE is computed from the
total energy in the dimer (Etotal (dimer)) and the total energy in the monomer (Etotal (monomer))
using the equation:
IE = Etotal(dimer) –2Etotal(monomer).
The B3LYP and M052X calculations were performed by the Gaussian 03 software.57
5
8
Pople’s extended basis set including polarization and diffuse functions , 6-31+G**, was used in
the B3LYP and M052X study. The BP86 and M062X-D3 calculations were carried out by the
ORCA4.0.1 software59,60 making use of the extended triple-zeta basis set def2-TZVP and
auxiliary coloumb fitting def2/J and RI-JK basis sets.61,62
3. Results
3
.1 Stationary absorption and emission spectral studies of KC in different solvents:
The steady state absorption and fluorescence spectra of KC were recorded in different
solvents. The absorption maximum of KC in different solvents was complied in Table 1. The
UV–Vis absorption spectra of KC in different solvents is shown in (Fig. 1a). On increasing
solvent polarity, the absorption maximum of KC is found to be red shifted. KC has non-bonding
electron,  electrons and electron donor and acceptor groups. The long wavelength electronic


absorption may be due to i) n    and 3) ICT transition. The effect of the solvent on the
absorption spectra confirms that the long wavelength absorption of KC is not due to

n  transition. The molar extinction coefficient of KC was determined in methanol and non-

Page 9 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
5
5
-1
-1
polar toluene solvent and was found to be 0.911x10 and 1.014x10 M cm respectively. The
higher molar extinction coefficient value further indicates the * nature of the long
wavelength absorption band.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The fluorescence spectra of KC in different solvents is shown in (Fig. 1b). The
fluorescence maximum is shifted to red on increasing the solvent polarity. The fluorescence
maximum and Stokes shift of KC in different solvents is given in Table 1. The Stokes shift is
-1
-1
independent of solvent polarity and was found to be in the range of 900 cm to 1300 cm . The
smaller stokes shift observed in KC confirms that the emission is not due to ICT excited state. In
KC, the nature of the electronic transition is completely different from the other 7-
aminocoumarin dyes due to the presence of bridge carbonyl group. The steady state absorption
and fluorescence studies confirm that long wavelength absorption and fluorescence of KC is due
to the * electronic transition in all the solvents studied.
Non polar
polar
Non-polar
polar
(b)
(a)
1.0
0.8
0.6
0.4
1
.0
Dioxane
Toluene
EA
THF
DCM
n-propanol
EtoH
MeoH
DMF
ACN
DMSO
Dioxane
Toluene
EA
THF
DCM
n-propanol
EtoH
MeoH
DMF
ACN
DMSO
0.8
0
0
0
0
.6
.4
.2
.0
0.2
0.0
4
50
500
550
600
650
350
400
450
500
550
Wavelength(nm)
Wavelength (nm)
Fig. 1 a) Absorption Spectra of KC in different solvents. b) Emission Spectra of KC in different
solvents.

New Journal of Chemistry
Page 10 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1 Photophysical data of KC in different solvents.
εa
Δν (cm-1)
Solvents
λab (nm)
447
λem (nm)
468
Φf
1
,4 Dioxane
2.25
2.35
6.02
7.58
20.0
24.55
32.7
36.7
37.5
46.58
1004
1130
1070
1174
1695
1496
1258
1366
1473
1251
0.0017
0.015
0.016
0.024
0.01
Toluene
EA
449
473
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
448
471
THF
449
474
n-ProH
EtoH
455
493
456
487
0.006
0.004
0.0056
0.0053
0.0054
MeoH
DMF
ACN
458
486
461
492
457
490
DMSO
466
495
3
.2 Steady-state absorption and emission spectra of KC in alcohol: water binary mixture:
The absorption and fluorescence spectra of KC were investigated in methanol and
methanol: water binary mixtures to understand the nature of the ground and excited state. The
absorption and fluorescence maximum of KC in methanol: water binary mixture is presented in
Table 2. The effect of water addition on the absorption and fluorescence spectrum of KC in
methanol is shown in (Fig. 2a and 2b). Upto 70% addition of water to methanol, the absorption
maximum of KC shift to longer wavelength from 458 nm to 480 nm without noticeable change
in absorption. Further addition of water, the absorption maximum of KC was blue shifted with
increase in full width half maximum and decrease in the absorption. The absorption maximum of
KC in 90% methanol: water mixture was found to be at 469 nm.

Page 11 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
The fluorescence maximum of KC in neat methanol was observed at 486 nm. On addition
of water to methanol upto 70%, the fluorescence intensity was quenched with red shift in the
fluorescence maximum. The fluorescence maximum of KC at 70% methanol: water is found to
be at 502. Further increase in water percentage, the KC fluorescence maximum shifted towards
red with increase in fluorescence intensity. The fluorescence maximum of KC in 90% water is
found to be at 551 nm. At higher water content, the KC shows new emission maximum with
large Stokes shift compared to methanol. The KC fluorescence with small Stokes shift in
methanol is assigned to π →π*excited state. The new fluorescence maximum with large Stokes
shift of KC at higher water content may be due to the ICT state induced by the specific solvent
effect such as hydrogen bonding/aggregation of hydrophobic KC in aqueous environment. The
steady state photophysical properties of KC in methanol: water mixtures confirm general solvent
effect upto 70% H O and specific solvent effect beyond 70% H O. The fluorescence quantum
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
yield of KC in methanol was found to be 0.0040. On increasing the water percentage upto 70%
H O, the fluorescence quantum yield decreases to 0.0013. Further increasing water percentage
2
to 80% and 90%, the fluorescence quantum yield value increases to 0.0054. The new emission
maximum with high quantum yield may be due to specific solvent effect between the KC and
solvent or hydrophobicity induced intermolecular interactions such as dimer/aggregate formation
,
brings back the ICT absorption and emission in KC, similar to other 7-DEC dyes.

New Journal of Chemistry
Page 12 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
to 70% H O
2
0
0
0
0
0
.4
.3
.2
.1
.0
6
5
5
5
(b)
1
.2x10
(a)
0%
to
0%
7
90% H O
2
9.0x10
8
0% H O
2
80% H O
2
6.0x10
3.0x10
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
90% H O
2
0.0
475
525
575
625
675
3
75
425
475
525
575
Wavelength (nm)
Wavelength (nm)
Fig. 2 Absorption (a) and Emission (b) spectra of KC in methanol and methanol: water mixtures.
-6
exc 460 nm
.
The concentration of KC is 4.4 X10 M.
The photophysical properties of KC was studied in ethanol: water and propanol: water
binary mixture. The absorption and fluorescence spectra of KC in ethanol: water and propanol
water: binary mixture was shown in the Fig. S3 and S4 (ESI†) respectively. Absorption and
fluorescence spectra of KC in ethanol : water mixture and propanol: water mixture were similar
to methanol: water mixture. The absorption and fluorescence maximum of KC in ethanol: water
and propanol: water mixture is presented in Table S1 and S2 (ESI†). Upto 70% addition of
water to ethanol and propanol results in the shift in the absorption and fluorescence maximum to
longer wavelength with decrease in the fluorescence intensity and beyond 70% results in
decrease in the absorbance with a new fluorescence emission with maximum at 545 nm, similar
to methanol: water mixture. The intensity of new emission was found to be more in methanol :
water mixture when compared to ethanol and propanol: water mixture. On increasing alcohol
chain length the new emission intensity decreases. Methanol is more hydrophilic solvent
compared to ethanol and n-propanol. The decrease in the new emission intensity on increasing

Page 13 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
hydrophobicity of alcohols, indicate the role of environment hydrophobicity on the new emission
intensity. This is further confirmed from the photophysical studies of KC in THF :water binary
mixture. The steady state photophysical properties of KC in methanol: water mixture confirms
the operation of general solvent effect upto 30:70 (v/v) methanol: water mixture and operation of
specific solvent effect beyond 30:70 (v/v) methanol: water mixture.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 2 The absorption and emission properties of KC in methanol: water binary mixture.
-
1
%
H O
2
λab(nm)
λem (nm)
Δν(cm )
Φf
0
458
486
1258
0.0040
10
464
468
473
475
478
480
479
477
469
493
497
499
500
500
502
502
546
551
1268
1247
1101
1052
920
0.0031
0.0025
0.0020
0.0019
0.0014
0.0016
0.0013
0.0014
0.0054
2
3
0
0
40
5
6
7
8
9
0
0
0
0
0
913
956
2649
3173
3.3 Steady-state absorption and emission spectra of KC in THF: water binary mixture.
The origin of new emission observed in KC at higher water may be due to any one of the
following i) aggregation ii) excited state reaction and iii) conformational change. The
absorption and fluorescence spectrum of KC was recorded in THF: water mixtures to
understand the origin of new emission of KC in methanol: water mixture. THF: water mixture is

New Journal of Chemistry
Page 14 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
63
well known for promoting aggregation induced emission. The absorption and fluorescence
spectrum of KC in THF: water mixture is shown in (Fig 3a and 3b). On increasing water
percentage, both the absorption and fluorescence maxima of KC are red shifted. At higher
water content the absorption spectrum of KC is red-shifted more as compared to the
fluorescence spectrum. The absorption and fluorescence maxima of KC in THF and THF:water
mixture is shown in Table 3. On addition of water to THF upto 90% (v/v), the fluorescence
intensity was quenched with red shift in the fluorescence maximum. The fluorescence
maximum of KC at 90% THF: water is found to be at 496 nm. Further increase in water
percentage results in the precipitation of KC with new emission maximum at 531 nm. The re-
precipitation of KC was observed from 92% water:THF and results in the formation of needle
like crystals. Due to the precipitation, the absorbance of KC decreases drastically at high water
content (above 92%). In alcohols the new emission with large Stokes Shift was observed from
the water content greater than 70% and there is no re-precipitation of KC was observed upto
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
9
0% water. The decrease in the fluorescence intensity on the addition of water is due to
additional non-radiative relaxation such as the non-emissive TICT state formation in the excited
state.64

Page 15 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
0
0
0
0
0
.20
.15
.10
.05
.00
0
% to 90% H O
6
1.2x10
5
5
4
4
4
4
90%
H
2
O
O
O
O
O
2
92% H2
94% H2
96% H2
(
a)
3.5x10 (b)
1.0x10
8.0x10
6.0x10
98% H
2
6
2.8x10
4.0x10
.0x10
.0
0
to
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
5
00
550
600
6
Wavelength (nm)
98% H O
2
1
7
.1x10
.4x10
.0x10
2
6
5
0.0
4
00
450
500
550
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 3 Absorption (a) and Emission (b) spectra of KC in THF and THF:water mixtures. Insert
shows the fluorescence spectrum of KC in 90, 92, 94, 96 and 98 % of water in THF. exc 450 nm.
-6
The concentration of KC is 1.811 X10 M.
Table 3 The absorption and emission properties of KC in THF: Water binary mixture.
-
1
%
0
H O
λab(nm)
449
455
457
460
463
464
468
472
477
481
474
475
λem (nm)
471
Δν (cm )
1040
1231
1220
1120
1106
1059
959
2
10
482
2
3
4
0
0
0
484
485
488
50
488
6
7
0
0
490
491
819
80
494
721
9
9
9
0
2
4
496
628
500
1097
2149
529

New Journal of Chemistry
Page 16 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
9
6
475
475
530
530
2184
2184
98
3
.4 Time-resolved fluorescence studies
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fluorescence decay of KC was recorded in methanol: water mixture using time
correlated single photon counting technique. The time resolved fluorescence decay of KC in
methanol: water mixture was monitored at 490 nm and 550 nm by exciting at 425 nm. In both
methanol and methanol water mixture, the fluorescence decay curves of KC dye do not obey
single exponential fit, but they are satisfactorily fitted by the biexponential and triexponential
function according to the equation
풕
–풕 흉ퟐ) ― ― ― ퟏ
푰(푻) = 푩 풆풙풑
(
– 흉ퟏ) + 푩 풆풙풑
(
ퟏ
ퟐ
풕
풕
–풕 흉ퟑ) ― ― ― ퟐ
푰(푻) = 푩 풆풙풑
(
– 흉ퟏ) + 푩 풆풙풑
(
– 흉ퟐ) + 푩 풆풙풑
(
ퟏ
ퟐ
ퟑ
where B , B and B are the pre-exponential factors,   and  are fluorescence lifetimes. The
1
2
3
1,
2
3
fractional intensity contribution of each decay component in the multiexponential decay towards
the total steady state intensity is given by the relative amplitude and is calculated from the pre-
exponential factor and decay time as follows
퐵푖 휏푖
퐴푖 =
∑ 퐵 휏
푗 푗
푗
The typical fluorescence decay of KC in different percentages of methanol: water mixture
is shown in (Fig. 4a and 4b). The fluorescence lifetime data of KC in different percentage of
water in methanol are compiled in Table 4. The fluorescence decay of KC in methanol shows bi-

Page 17 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
exponential nature with one short and long component. In neat methanol, the decay time of the
short component was found to be around 85 ps (35.52%), whereas the long component decay
time was found to be 1.4 ns (64.48%), which are found be independent of emission wavelength.
On increasing water percentage in methanol, decay time of both short and long component
decreases for decays monitored at both 490 and 550 nm. A new decay component with decay
time of 2.71 ns was observed from 30% water addition to methanol. The relative amplitude of
the new decay component observed at 490 nm increases with increasing water percentage in
methanol. The lifetime of the new decay component does not vary because in the decay analysis
the decay time of this decay component is fixed. In the free fitting this decay component is
varying from 2.4 -2.9 ns and this variation influence the relative amplitudes of the short decay
components particularly in the decay monitored at 550 nm at higher water content.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The decay time τ and it relative amplitude remains constant at higher water percentage
2
(> 70%) for the fluorescence decay curves monitored at 490 nm. There is a noticeable difference
in the amplitude of the decay component τ and τ at higher water percentage for fluorescence
2
3
decay of KC monitored at the new emission maximum (550 nm). As the water percentage
increases above 70% the relative amplitude of τ increases with decrease in the relative
2
amplitude of the τ decay component. This indicates that the new emission maximum observed at
3
5
2
50 nm at higher water percentage is not due to the new decay component τ with decay time
3
.71 ns. The higher relative contribution of τ decay component at 550 nm confirms that the new
2
emission maximum is due to the presence of this decay component. At 90% water, the
fluorescence decay monitored at 490 nm and 550 nm is tri-exponential with decay time constants
τ 45 ps and τ 300 ps and τ = 2.71 ns. At water content >70%, the decay component τ is
1
2
3
2
constant with higher contribution for intensity monitored at 550 nm and confirms that the decay

New Journal of Chemistry
Page 18 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
component τ corresponds to the new emission maximum. The time-resolved fluorescence
2
results clearly suggest that there are three emitting species present in the aqueous solution of KC.
4
3
2
1
0
4
10
1
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
fw (Vol%)
fw (Vol%)
(a)
0
10
Monitored at 490 nm
b) Monitored at 550 nm
(
0
1
2
0
0
20
30
40
50
3
2
1
0
10
10
10
10
10
30
40
50
60
70
80
60
10
10
10
70
80
90
90
IRF
IRF
4
8
12
16
4
8
12
16
Time (ns)
Time (ns)
Fig. 4 Fluorescence decays of KC in different percentage of methanol: water mixture.
The fluorescence decay of KC was recorded in THF: water mixture using time correlated
single photon counting technique. The time resolved fluorescence decay of KC in THF: water
mixture was monitored at 490 nm by exciting at 445 nm. The typical fluorescence decay of KC
in different percentages of THF: water mixture is shown in Fig. S5. The fluorescence lifetime
data of KC in different percentage of water in THF are compiled in Table S3. The fluorescence
decay of KC in THF is single-exponential with decay time of 2.01 ns. On addition of water to
THF, the fluorescence decay of KC is biexponential with a two decay times τ (130 -160 ps) and
1
τ2. The decay time and relative amplitude of the long component decreases with increasing water
percentage. In THF: water mixture, there is no formation of additional decay component τ (2.71
3
ns) when compared to alcohol: water mixture. The absence of new decay component in THF:
water mixture confirms the role of hydrophilicity of the medium and preferential solvation on

Page 19 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
intra and intermolecular interactions which effects the photophysical properties of KC. This
study also confirms that the decay component with lifetime 2.71 ns is not due to the solute-
solvent hydrogen bonding promoted intramolecular charge transfer state.65
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In neat methanol and in presence of water, the fluorescence decay of KC exhibits
ultrafast decay component close to the time resolution of the TCSPC setup. Investigation of the
ultrafast excited state dynamics of KC in femtosecond time domain in methanol: water binary
mixture provides information on the excited state dynamics of the KC. The femto up-conversion
fluorescence decay of KC in methanol: water mixture is recorded at 490 nm and 550 nm and the
ultrafast decay time constants are presented in the Table 5. Femtosecond fluorescence decay of
KC in methanol: water mixture is shown in (Fig. 5a and 5b). The femtosecond fluorescence
decay of KC shows bi-exponential emission decay behavior in methanol: water mixture at both
4
90 nm and 550 nm. In methanol, the ultrafast fluorescence decay of KC monitored at 490 nm
and 550 nm follows bi exponential with two ultrafast components τ =555 fs, τ =2 ps and τ =1.10
1
2
1
ps, τ =3 ps respectively. Addition of water to methanol, the ultrafast decay time constant τ
2
1
monitored at 490 nm decreases to157 fs at 90% water. At higher water content 70% and 90% of
water, the τ lifetime increases to 6ps. The absence of rise component in the ultrafast
2
fluorescence decay of KC monitored at the new emission maximum in methanol: water mixture
leads to the conclusion that the new emission maximum is not due to the ultrafast excited state
reactions. The new emission maximum of KC at higher water percentage is due to the ground
state phenomenon such as dimerization or conformational change or inter and intramolecular
interactions. The time-resolved fluorescence results clearly suggest that there are three emitting
species present in the aqueous solution of KC.

New Journal of Chemistry
Page 20 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 4 The fluorescence lifetime data of KC in methanol: water mixture
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
%
Emission wavelength (nm)
H O
2
4
90 nm
550 nm
τ 1a ps
τ 2a ns
τ ns
a
τ 1a ps
τ 2a ns
τ ns
3
a
3
(
A %)
(A %)
(A %)
(A %)
(A %)
(A %)
3
1
2
3
1
2
0
85 (35.52)
75 (45.03)
70 (52.52)
61 (47.09)
57 (49.44)
55 (46.15)
51 (48.14)
49 (50.85)
49 (50.56)
45 (46.43)
1.43 (64.48)
1.22 (54.94)
1.06 (47.48)
-
-
-
88 (33.94)
81 (47.10)
68 (59.20)
1.33 (66.10)
1.15 (52.90)
1.06 (40.80)
0.70 (23.01)
0.63 (21.63)
0.55 (15.62)
0.48 (14.28)
0.31 (12.95)
0.28(17.61)
0.30(49.45)
-
1
2
3
4
0
0
0
0
-
-
0.71 (26.64) 2.71 (26.27) 58 (54.58)
0.71 (24.25) 2.71 (26.31) 53 (57.38)
0.60 (20.81) 2.71 (33.04) 53 (55.26)
0.49 (15.78) 2.71 (36.08) 52 (58.84)
0.34 (11.38) 2.71 (37.77) 50 (55.62)
0.29 (12.42) 2.71 (37.03) 50 (55.56)
0.29(12.12) 2.71 (41.44) 65 (34.30)
2.71 (23.72)
2.71 (22.23)
2.71 (30.46)
2.71 (27.22)
2.71 (28.29)
2.71 (22.93)
2.71 (6.47)
50
60
7
8
9
0
0
0
a
Error ± 10% (reported for lifetime only).

Page 21 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
Table 5 Femtosecond fluorescence decay parameters of KC in methanol: water mixture .
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
490 nm
550 nm
%
2
τ 1a
H O (fs)
τ 2 a
(ps)
2.0
A %
A % τ 1a
τ 2 a
(ps)
A %
A %
2
1
2
1
(fs)
0
574
54.37
58.46
53.98
60.09
20.17
20.15
45.63
41.54
46.02
39.91
79.84
79.75
1105
3.0
68.04
64.27
44.94
51.74
7.66
31.95
1
0
0
0
0
0
550
360
253
182
157
2.0
2.3
910
560
440
300
1000
3.5
1.8
1.9
7.1
6.0
35.72
55.05
48.25
92.33
91.13
3
5
7
9
2.2
6.7
4.15
8.86
aError ± 10% (reported for lifetime only).
Monitored at 490nm
Monitored at 550nm ƒw (vol%)
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.0
0.8
.6
0.4
.2
0.0
(a)
(b)
ƒw (vol%)
0
0
10
30
50
70
10
30
50
70
90
0
9
0
0
4
6
8
10
4
6
8
10
Time (ps)
Time (ps)

New Journal of Chemistry
Page 22 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Fig. 5 Femtosecond fluorescence decay profile of KC in different percentage of methanol:
water mixture.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
.5 Photophysical properties of the KC in the solid state
The solid absorption and fluorescence spectrum of KC was recorded and is shown in Fig.
6
. The absorption and emission maximum of KC in solid state was found to be at 462 and 549
nm respectively. The solid state absorption and fluorescence spectrum of KC is similar to
0% methanol: water mixture. The fluorescence spectrum of a fluorophore in dilute solution
9
reflect mainly its single molecular characteristics, while the fluorescence spectrum in solid
state results from the interaction of large numbers of molecules. This confirms that
intermolecular interactions between the KC molecules are responsible for the new emission
observed at 550 nm. Solid state KC possesses strong absorption in the UV-A region and
yellow emission under UV-A excitation (Fig. 6 (b)). To resolve all these ambiguities in the
fluorescence behavior of KC in the solid state and alcohol: water binary mixture, a detailed
investigation on the crystal structure, solid state packing and the active intermolecular
interactions in the crystalline state was carried out.
(a)
(b)
(c)
90% MeoH:H
2
O
1
0
0
0
0
.0
1
0
0
0
.0
.8
.6
.4
Solid
.8
.6
.4
.2
0.2
.0
0.0
3
0
00
400
500
600
Wavelength(nm)

Page 23 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
Fig. 6 (a) Normalized absorption and emission spectra of KC in 90% methanol: water mixture
and solid state. (b) Photographs of solid state fluorescence of KC under UV (365nm)
excitation. (c) Photographs of solid state KC under room light.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
.6 Crystal structure analysis of KC
The crystallization of KC in alcohols such as methanol, ethanol etc., did not yield single
crystals suitable for X-ray crystal analysis. Single crystals of KC for X-ray crystal analysis were
prepared by slow evaporation from CHCl solution. Single crystal of KC belongs to an
3
orthorhombic crystal system, space group Pcab with unit cell dimensions of a = 11.7204(5) Å,
α= 90°; b = 17.1714(8)Å, β= 90° and c = 28.6443(14)Å, γ= 90°. The crystal structure of KC is
given in (Fig. 7). The crystal data and experimental details are shown in Table S4 (ESI†). In
KC, two 7-N,N-diethylaminocoumarin units are connected by the bridge >C=O group at the 3-
position.The bridge carbonyl group is shared by the two coumarin units A and B. The KC
molecule is not planar as a whole. The two coumarin rings are planar and the angle between the
planes of the two coumarin rings is 55.31°. The XRD structure of KC shows that the orientation
of the donor diethyl amino group is different in the two coumarin units. In the coumarin unit A,
the two ethyl groups in the diethylamino substituent are in eclipsed arrangement. In the second
coumarin unit B two ethyl groups adopt staggered arrangement. In coumarin A the plane
containing the atoms C24-N1-C26 deviate from the plane of the coumarin ring by 1.26°.On the
other hand in coumarin ring B, the corresponding angle between the plane containing C20-N2-
C22 and plane of coumarin ring is 11.06°.
Hirshfeld surface analysis is an effective tool for exploring packing modes and
intermolecular interactions in KC crystals, as they provide a visual picture of intermolecular
interactions and of molecular shapes in a crystalline environment. Hirshfeld surfaces comprising

New Journal of Chemistry
Page 24 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
dnorm surface and fingerprint plots were generated and analysed for the title compound in order to
explore the packing modes and intermolecular interactions. The two dimensional fingerprint
plots from Hirshfeld surface analyses are shown Fig. S6 (ESI†). From the 2D-fingerprint plot
and crystal packing analysis of KC, it is very clear that the nonbonded H...H ( 33%) and O...H
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(26.3%) intermolecular interactions are the prime interactions in stabilizing the packing in the
solid state. This intermolecular contact is highlighted by conventional mapping of dnorm on
molecular Hirshfeld surfaces and is shown in Fig. S7 (ESI†). The red spots over the surface
indicates that the strong intermolecular interactions through hydrogen bonding.
The XRD structure reveals the presence of intermolecular C-H...O interactions between
two KC molecules. These HB interactions between the KC molecules in the crystal were
analyzed using mercury software. Two modes of HB interactions between two KC molecules are
identified from the crystal structure depending on the type of the hydrogen bond acceptor, C=O
as seen from Fig. 8. The ring carbonyl group present in the coumarin unit A, wherein the two
ethyl groups are eclipsed, and the bridge carbonyl group that connects the two coumarin units in
KC participate as intermolecular HB acceptors. The CH groups in the coumarin rings adjacent to
the bridge carbonyl group behave as HB donors. The details of the intermolecular HB
interactions between two KC molecules are given in Table 6. The HB interactions between the
acceptor C9=O4 and donor C-H groups are illustrated using the expansion contacts between four
KC molecules in the crystal packing and is shown in Fig. 8a. The carbonyl oxygen O4 in the
coumarin ring A of KC-1 forms HB with the C-H hydrogens, C3-H3 and C7-H7, present in the
coumarin ring A' of KC-2 molecule. Similarly the O4 atom of KC-2 establishes HB with the
C3-H3 and C7-H7 hydrogens of KC-3. This mode of HB interactions leads to dimer, trimer and
higher order aggregates. The higher order aggregates are formed in the supramolecular structure,

Page 25 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
which is possible only in the solid state. However, in aqueous solution this mode of HB
interaction leads to the dimer D1. The HB interactions between the bridge carbonyl acceptor and
the C-H donor groups are illustrated using the expansion contacts between two KC molecules in
the crystal packing and is shown in the Fig. 8b. The oxygen atom O5 present in the bridge
carbonyl of KC-1 molecule forms hydrogen bonds with C15-H15 and C16-H16 present in the
coumarin ring B' of KC-2 molecule. Similarly the O5 atom of KC-2 establishes the HB
interaction with the C15-H15 and C16-H16 of KC-1. The one to one hydrogen bonding
interaction between the O5 and C15-H15 and C16-H16 leads to the dimer D2. The two modes of
HB interactions described above leading to the dimers D1 and D2 are responsible for the
anomalous fluorescence from the solid state and aqueous solutions of KC.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 6 Hydrogen bond parameters and symmetry operations in XRD structure of KC
D-H...A
D-H(Å)
H...A(Å)
D...A(Å)
D-H...A(°)
C3-H3...O4i
0.93
0.93
0.93
0.93
2.41
2.39
2.43
2.78
3.207 (2)
3.192 (2)
3.296 (2)
3.564 (2)
144
144
156
141
C7-H7...O4 i
C15-H15...O5ii
C16 -H16...O5 ii
Symmetry codes : (i) -1/2+x, 1/2-y,-z, (ii) 1-x, -y, -z.
B
A

New Journal of Chemistry
Page 26 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Fig. 7 Crystal structure of KC. Labelling of the atoms are shown.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
.
(b)
A’
B’
KC-2
KC-4
D2
B
KC-1
A
KC-2
D1
KC-1
Fig. 8 Two modes of intermolecular HB interactions observed from the XRD structure. (a)
Intermolecular HB formation between ring carbonyl group of coumarin A with ring CH in
Coumarin A' (D1). (b) intermolecular HB formation between bridge carbonyl group in KC-1with
ring CH in Coumarin B' in KC-2 and vice-versa (D2).
3.7 DFT study on the KC monomer

Page 27 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
The optimized structures I, II and III of KC in the gas-phase are shown in Fig. 9a. In I,
the N, N- diethyl groups in the coumarin units B1 and B2 exhibit staggered arrangement and this
structure is predicted to be the lowest energy structure. In the structure II, the N,N-diethyl groups
in the coumarin A are in eclipsed arrangement while they are staggered in coumarin B. This
structure (II) is about 0.7 kcal/mol higher in energy than that of I. It is seen that in the crystal,
KC monomer possesses the structure II as evidenced from its XRD structure (Fig. 7). The
relative orientations of both the coumarin units with reference to the bridge carbonyl group are
similar in the structures I and II as seen from the dihedral angle C8C10C11C12 of about -34°
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Fig. 9a). However in III, the coumarin B2 is rotated with reference to the C10-C11 bond that is
adjacent to the bridge carbonyl, though the N,N-diethyl groups adopt staggered arrangement as
observed in structure I. It is found that the structure III of KC is about 5 kcal/mol higher than the
structure I. Similar trend in the relative stability of the KC structures is predicted by the M052X3
and BP8648,49 methods Table S5 (ESI†). The C=O length for the bridge carbonyl bond and the
adjacent C-C length given in Table S6 (ESI†) show the values are similar in the structures I and
II. The values predicted by the M052X method for the C=O and C8-C10 in structure II (1.223
and 1.486 Å) are in good agreement with the XRD values of 1.220 and 1.481 Å, respectively. To
understand the nature of electronic transitions in KC, the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) of structures I, II and III were
examined and are depicted in (Fig. 9b). The HOMO and LUMO are π-molecular orbitals that are
delocalized over the two coumarin units in the two lowest energy structures I and II. Thus the
lowest energy excited states in these structures are generated by π –π* transition. However as
seen from Fig. 9b, the electronic structure differs in III, as the HOMO is delocalized over the
coumarin B2 while the LUMO is delocalized over the coumarin B1. It is observed that the

New Journal of Chemistry
Page 28 of 49
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
HOMO has significant contribution from the amino nitrogen in coumarin B2 and there is an
increase in the contribution of bridge carbonyl oxygen in the LUMO. This analysis demonstrates
that the change in the molecular geometry leads to significant change in the electronic structure
and thus the nature of electronic transition.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 9 (a) B3LYP/6-31+G** optimized geometries of KC structures I, II and III. The total
energies (TE) in hartree and relative energies in kcal/mol (inside parenthesis) are given.(b)
HOMO and LUMO in the KC predicted at B3LYP/6-31+G** optimized geometries I, II and III.
3
.8 Stabilization of the dimers of KC and their electronic structures
The XRD structure of the single crystal of KC (Fig. 7) reveals the existence of
intermolecular C-H…O HB interactions28,29 between the carbonyl oxygen atom in one of the KC

Page 29 of 49
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ01053J
1
2
3
4
5
6
7
8
9
molecules with the ring C-H in the neighbouring molecule. These HB interactions are arche
types of weak HBs and their role in the formation of supramolecular assemblies is well
2
8
recognized. Recent studies using accurate wave function methods predict a stability of ~1
kcal/mol due to each CH…O interaction at coupled cluster singles, doubles and perturbative
singles CCSD(T) level using complete basis set limit.66,67 The HB interactions in two dimer
arrangements D1 and D2 of KC which are characterised from the XRD structure are shown in
Fig. 8a and 8b. In the dimer D1 there are two bifurcated CH…O interactions between the ring
carbonyl oxygen of coumarin unit A' with the two CH hydrogens which are adjacent to the ring
carbonyl in coumarin unit A (Fig. 8a). These HBs have lengths 2.406 and 2.391 Å and angles
144.2° (Table 6). D2 possesses centre of symmetry and is formed by HBs between the bridge
carbonyl group in one KC with two ring CH in B (B') which are adjacent to bridge keto group
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(C15-H15 and C16-H16) in the second KC and viceversa. The HB parameters are listed in Table
6. Thus it is seen that there are two CH…OHBs in dimer D1 while D2 possesses 4 CH…O HBs.
In addition to these weak HBs, other noncovalent interactions53-56 also contribute to stability in
the dimers.
It is seen from Table 7 that the interaction energies in the dimers D1 and D2 are,
respectively, -10.38 and -5.80 kcal/mol at the M052X/6-31+G** level. The more reliable
M062X-D3/def2-TZVP calculations yield IEs of -11.59 and -6.39 kcal/mol, respectively, in D1
and D2. The above results demonstrate that the dimer D1 is more stable than D2 by ~5.2
kcal/mol at the M062X-D3 level. From the number of hydrogen bonds we estimate that D1 and
D2 possess HB energy of ~ -2 and -4 kcal/mol, respectively. The dispersion interaction energy as
shown in Table 8 is -3.48 and of -1.92 kcal/mol, respectively, in D1 and D2. Clearly the
dispersion interaction energy is more significant than the HB energy in the dimer D1. The