Two Different Emissions of (2Z)-2-(4-Bromophenyl)-3-[4-(dimethylamino)phenyl]prop-2-enenitrile Due to Crystal Habit and Size: Synthesis, Optical, and Supramolecular Characterization

Article abstract of DOI:10.1021/acs.cgd.6b01670

Optical and X-ray diffraction characterizations of crystals I and II of (2Z)-2-(4-bromophenyl)-3-[4-(dimethylamino)phenyl]prop-2-enenitrile (Z-4-BrPhDMAPhACN) are reported. I and II belong to the same monoclinic space group and have nearly identical unit-cell dimensions [I: a = 11.0169(2); b = 6.02041(11); c = 21.8541(4); (β) 98.4082(18). II: a = 11.0475(6); b = 6.0273(3); c = 21.8533(11); (β) 98.315(5)]. Crystals I and II were formed under different conditions and have different crystal habits and colors: crystals I are yellow blocks, whereas crystals II are small, orange needles. I and II show absorption maxima (λ_abs) in solution at 404 nm and emission (λ_em) maxima at 501 nm (I) and at 502 nm (II). However, their emission maxima are different in the solid state: I shows an emission at 512 nm and a shoulder at 534 nm, whereas II exhibits a λ_em maximum at 582 nm. The differences in solid-state photoluminescence were attributed to variations in the crystal morphology and to the crystal habit and size. Both I and II were characterized by nuclear magnetic resonance, mass spectrometry, infrared spectroscopy, UV-vis spectroscopy, fluorescence, cyclic voltammetry, single-crystal and powder X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and density functional theory calculations.

Full text of DOI:10.1021/acs.cgd.6b01670

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Article
Two different emissions of (2Z)-2-(4-bromophenyl)-3-[4-
(dimethylamino)phenyl]prop-2-enenitrile due to crystal habit and
size: synthesis, optical and supramolecular characterization.
Judith Percino, Margarita Cerón, Perumal Venkatesan, Paulina Ceballos, Alejandro Bañuelos,
Oscar Rodríguez, Maxime A. Siegler, Fernando Robles, Victor M. Chapela, Guillermo
Soriano-Moro, Enrique Pérez-Gutiérrez, JOSE BONILLA-CRUZ, and Subbiah Thamotharan
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01670 • Publication Date (Web): 21 Feb 2017
Downloaded from http://pubs.acs.org on February 24, 2017
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Two different emissions of (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ
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ꢀ[4ꢀ(dimethylamino)phenyl]propꢀ2ꢀenenitrile due to
crystal habit and size: synthesis, optical and
supramolecular characterization
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Judith Percino, * Margarita Cerón, Perumal Venkatesan, Paulina Ceballos, Alejandro
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Bañuelos, Oscar Rodríguez, Maxime A. Siegler, Fernando Robles, Víctor M. Chapela,
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Guillermo SorianoꢀMoro, Enrique PérezꢀGutiérrez, José BonillaꢀCruz and Subbiah
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Thamotharan
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Lab. de Polímeros, Centro de Química, Instituto de Ciencias, Benemérita Universidad
Autónoma de Puebla, Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y San Claudio, Puebla,
Puebla, México, C.P. 72570.
2
Department of Chemistry, Johns Hopkins University, New Chemistry Building, 3400 N.
Charles St. Baltimore, MD, 21218, USA.
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CONACYT–Lab. de Polímeros, Centro de Química, Instituto de Ciencias, Benemérita
Universidad Autónoma de Puebla, Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y San
Claudio, Puebla, Puebla, México, C.P. 72570.
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Cento de Investigación en Materiales Avanzados S.C. (CIMAVꢀUnidad Monterrey), Av.
Alianza Norte 22, Autopista MonterreyꢀAeropuerto Km 10, PIIT, ApodacaꢀNuevo León,
México, C.P. 66600.
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Biomolecular Crystallography Laboratory, Department of Bioinformatics, School of Chemical
and Biotechnology, SASTRA University, Thanjavur, 613 401, India.
Corresponding author:
Judith Percino;
judith.percino@correo.buap.mx;
Centro de Química, Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla,
Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y San Claudio, C.P. 72570, Puebla, Puebla.
México. Phone: +(52) 222 2295500 x7285.
ABSTRACT Optical and Xꢀray diffraction characterizations of crystals I and II of (2Z)ꢀ2ꢀ(4ꢀ
bromophenyl)ꢀ3ꢀ(4ꢀ(dimethylamino)phenyl)propꢀ2ꢀenenitrile
(Zꢀ4ꢀBrPhDMAPhACN)
are
reported. I and II belong to the same monoclinic space group and have nearly identical unitꢀcell
dimensions [I: a = 11.0169(2); b = 6.02041(11); c = 21.8541(4); (β) 98.4082(18) and II: a =
1
1.0475(6); b = 6.0273(3); c = 21.8533(11); (β) 98.315(5)]. Crystals I and II were formed under
different conditions and have different crystal habits and colors: crystals I are yellow blocks,
whereas crystals II are small, orange needles. I and II show absorption maxima (λ ) in solution
abs
at 404 nm and emission (λ ) maxima at 501 nm (I) and at 502 nm (II). However, their emission
em
maxima are different in the solid state: I shows an emission at 512 nm and a shoulder at 534 nm,
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whereas II exhibits a λ maximum at 582 nm. The differences in solidꢀstate photoluminescence
em
were attributed to variations in the crystal morphology and to the crystal habit and size. Both I
and II were characterized by nuclear magnetic resonance, mass spectrometry, infrared
spectroscopy, UVꢀVis spectroscopy, fluorescence, cyclic voltammetry, singleꢀcrystal and powder
Xꢀray diffraction, differential Scanning calorimetry, scanning electron microscopy and density
functional theory calculations.
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Two different emissions of (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ
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ꢀ[4ꢀ(dimethylamino)phenyl]propꢀ2ꢀenenitrile due to
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crystal habit and size: synthesis, optical and
supramolecular characterization
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Judith Percino, * Margarita Cerón, Perumal Venkatesan, Paulina Ceballos, Alejandro
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Bañuelos, Oscar Rodríguez, Maxime A. Siegler, Fernando Robles, Víctor M. Chapela,
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Guillermo SorianoꢀMoro, Enrique PérezꢀGutierrez, José BonillaꢀCruz and Subbiah
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Thamotharan
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Lab. de Polímeros, Centro de Química, Instituto de Ciencias, Benemérita Universidad
Autónoma de Puebla, Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y San Claudio, Puebla,
Puebla, México, C.P. 72570.
2
Department of Chemistry, Johns Hopkins University, New Chemistry Building, 3400 N.
Charles St. Baltimore, MD, 21218, USA.
3
CONACYT–Lab. de Polímeros, Centro de Química, Instituto de Ciencias, Benemérita
Universidad Autónoma de Puebla, Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y San
Claudio, Puebla, Puebla, México, C.P. 72570.
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Cento de Investigación en Materiales Avanzados S.C. (CIMAVꢀUnidad Monterrey), Av.
Alianza Norte 22, Autopista MonterreyꢀAeropuerto Km 10, PIIT, ApodacaꢀNuevo León,
México, C.P. 66600.
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Biomolecular Crystallography Laboratory, Department of Bioinformatics, School of Chemical
and Biotechnology, SASTRA University, Thanjavur, 613 401, India.
Corresponding author:
Judith Percino
Centro de Química, Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla,
Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y San Claudio, C.P. 72570, Puebla, Puebla.
México.
judith.percino@correo.buap.mx
ABSTRACT
Optical and Xꢀray diffraction characterizations of crystals I and II of (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ3ꢀ
(4ꢀ(dimethylamino)phenyl)propꢀ2ꢀenenitrile (Zꢀ4ꢀBrPhDMAPhACN) are reported. I and II
belong to the same monoclinic space group and have nearly identical unitꢀcell dimensions [I: a =
1
1.0169(2); b = 6.02041(11); c = 21.8541(4); (β) 98.4082(18) and II: a = 11.0475(6); b =
.0273(3); c = 21.8533(11); (β) 98.315(5)]. Crystals I and II were formed under different
6
conditions and have different crystal habits and colors: crystals I are yellow blocks, whereas
crystals II are small, orange needles. I and II show absorption maxima (λ ) in solution at 404
abs
nm and emission (λ ) maxima at 501 nm (I) and at 502 nm (II). However, their emission
em
maxima are different in the solid state: I shows an emission at 512 nm and a shoulder at 534 nm,
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whereas II exhibits a λ maximum at 582 nm. The differences in solidꢀstate photoluminescence
em
were attributed to variations in the crystal morphology and to the crystal habit and size. Both I
and II were characterized by nuclear magnetic resonance, mass spectrometry, infrared
spectroscopy, UVꢀVis spectroscopy, fluorescence, cyclic voltammetry, singleꢀcrystal and powder
Xꢀray diffraction, differential Scanning calorimetry (DSC), scanning electron microscopy (SEM)
and density functional theory calculations.
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KEYWORDS: nanocrystalsꢀemission, crystal habit, singleꢀcrystal Xꢀray diffraction, DSC, SEM,
bromophenylacetonitrile, PIXEL, Hirschfeld Surface.
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INTRODUCTION
It is well known that different crystalline forms of the same substance can exhibit markedly
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different physical properties, leading to a wide array of applications. Crystals or aggregates
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based on the same molecule are an active research topic in modern solidꢀstate chemistry because
the structureꢀproperty relations of materials could contribute to the design of new materials. For
example, singleꢀcrystal Xꢀray diffraction and thermal analysis characterizations of 1,4ꢀ
distyrylbenzene with different substituents showed a correlation between the substituents and
structural properties due to the presence of two polymorphs obtained under different
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crystallization conditions. Differences in the photophysical properties of the two crystals were
attributed to differences in the stacking modes of the chromophore and intermolecular electronic
interactions in the crystals. Therefore, the effect of selfꢀassembly and electronic interactions of
molecules on the properties of a molecular solid is a fundamental and crucial aspect of material
science. Recently, highly luminescent dicyanodistyrylbenzene crystals that exhibit an interesting
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mechanochromism have been reported. The behavior was explained by the presence of weak
…
…
local dipole interactions, C–H π interactions, and C–H N hydrogen bonding in the molecular
stacking assembly. The dicyanodistyrylbenzene polymorphs can be differentiated by the
different modes of local dipole coupling, which cause a substantial alternation of the π–π overlap
and excited state delocalization. These effects result in different colored fluorescence emissions
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and highly efficient deep/nearꢀinfrared emissions. In contrast, solidꢀstate lighting (SSL) using
organic chromophores has attracted much attention due to the potential applications of the latter
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in devices such as lightꢀemitting diodes,
photovoltaic devices,
and sensors.
Tuning
and controlling the emission wavelength of an organic material is crucial to identify the
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20,21
appropriate application.
The optical properties of different dyes in the solid state depend
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strongly on the molecular structure and intermolecular interactions.
Recently, organic chromophores that exhibit quenching of fluorescence in the solid state have
been reported, and this phenomenon is termed aggregationꢀcaused quenching (ACQ). In contrast,
when the emission of fluorescence depends on the presence of the solid state, the process is
termed aggregationꢀinduced emission (AIE) or aggregationꢀinduced emission enhancement
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(AIEE).
These organic materials may also exhibit no or weak fluorescence in solution,
whereas the aggregated particles (obtained by adding the solution into the poorly solubilizing
solvent) exhibit relatively intense fluorescence upon UV irradiation. Examples of such materials
are (dimethylamino)acrylonitrile derivatives, specifically Zꢀ2ꢀ(phenyl)ꢀ3ꢀ(4ꢀ
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dimethylaminophenyl)acrylonitrile and αꢀcyanostilbene. A variety of AIE compounds have
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been created and their fluorescence properties have been investigated.
In most cases,
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AIE is explained by the restriction of intramolecular rotation.
Herein, we report the synthesis, crystal structures, and optical characterization (absorption and
emission) of acrylonitrile containing the dimethylamine substituent as an electron donor, and
bromide and ꢀCN as electron acceptors. The title compound, (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ3ꢀ[4ꢀ
(
dimethylamino)phenyl)]propꢀ2enenitrile (Zꢀ4ꢀBrPhDMAPhACN), is completely characterized
using various techniques. The investigation reveals differences in the characteristic emission,
such as the enhancement of fluorescence in both a solvent and the solid state as well as the
photophysical properties. The data for the title compound are compared with those of the closely
related and analogous (2Z)ꢀ3ꢀ[4ꢀ(dimethylamino)phenyl]ꢀ2ꢀphenylpropꢀ2ꢀenenitrile (A). Zꢀ4ꢀ
BrPhDMAPhACN exhibits emission in solution and in the crystal form, depending on the habit
and size of the crystal. Additionally, Hirshfeld surfaces were calculated to analyze the
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intermolecular interactions in a wholeꢀmolecule approach so the structure–property relationships
could be established unambiguously.
EXPERIMENTAL SECTION
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Singleꢀcrystal Xꢀray diffraction (SCXRD). Xꢀray intensity data were collected for I and II at
110(2) K on a SuperNova diffractometer (equipped with an Atlas detector) with CuKα radiation
(mirror optics, λ = 1.5418 Å). The program CrysAlisPro (Version 1.171.36.24 Agilent
Technologies, 2012) was used to determine the unit cell dimensions. Analytical numerical
absorption correction based on a multifaceted crystal model was applied, using CrysAlisPro. The
temperature during data collection was controlled using a Cryojet system (manufactured by
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Oxford Instruments). The crystal structures of I and II were solved by the SHELXS program.
All nonꢀhydrogen atoms were placed in idealized geometrical positions and constrained to ride
on their parent atoms, with U (H) = 1.2U (C). The methyl hydrogen atoms were also
iso
eq
constrained to an ideal geometry with U (H) = 1.5 (C) but were allowed to rotate about the Nꢀ
iso
eq
C bonds.
Fourier transform infrared spectrometry (FTꢀIR). The IR spectra of the products were
recorded from neat solids on a Vertex model 70 Bruker 750 FTꢀIR spectrophotometer with ATR
ꢀ1
ꢀ1
(
Attenuated Total Reflection). The spectra were collected from 4000 to 400 cm , with a 4 cm
spectral resolution.
1
Proton nuclear magnetic resonance ( HꢀNMR). Spectra were recorded using a Bruker 500ꢀ
MHz NMR spectrometer. The solvents CDCl , DMSOꢀd and THFꢀd were used.
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Mass spectrometry. EI spectra were acquired using a Mass Spectrometer Jeol MStation 700ꢀ
D.
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Absorbance and emission (UVꢀVis and PL). Absorbance spectra were measured using a
SD2000 (Ocean Optics) spectrometer equipped with a pulse Xenon light source Pꢀ2 (Ocean
optics for the UV region (220ꢀ270 nm)), and a Cary 300 (Agilent) spectrometer equipped with a
deuterium and halogen lamp. For measurements (190ꢀ900 nm) in solution, the solvents used
were of spectroscopic grade and were subjected to preliminary checks to ensure the absence of
absorbing or fluorescent impurities within the scanned spectral ranges. For powder samples, the
absorption spectra were measured with KBr pellets using a DT 1000 CE light source (Analytical
Instrument Systems, Inc.). Emission spectra (PL) were acquired from a QEꢀProꢀFL (Ocean
Optics) equipped with a laser diode excitation source at a wavelength of 405 nm. The quantum
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yields (Ф ) of the three compounds (I, II and A) were determined as described by de Mello,
f
with slight modifications. In our fluorescence spectra measurements, the powdered samples
(
prepared form crystals) were placed into quartz tubes instead of films. Alq (Trisꢀ(8ꢀ
3
hydroxyquinolinato)aluminium was used as reference in the quantum yield (Ф ) measurements.
f
Powder Xꢀray diffraction (PXRD). Powder samples were characterized by powder Xꢀray
diffraction analysis using a Panalytical Empyrean diffractometer (Cu Kα= 1.54178 Å). The
samples were scanned over the 2θ range of 5 to 100°.
Scanning Electron Microscopy (SEM). The morphology of the crystals was examined using
a scanning electron microscope (Nova NanoSEM 200, FEI) operating at 5 kV. The samples were
mounted on a slide glass under vacuum in an argon atmosphere prior to observation.
Differential Scanning Calorimetry (DSC). DSC thermograms of the samples (1ꢀ2 mg) were
recorded by a thermal analysis system (TA Instruments DSC Q200). The measurements were
performed in aluminumꢀpierced lid pans (Tzero) at a heating rate of 10 °C/min under a nitrogen
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atmosphere (100 mL/min) over the range of 25–195 °C. The samples were cooled at the same
rate of 10 °C/min from 195 °C to 25 °C.
Films deposition. Organic films were deposited onto glass substrates. The substrates were
cleaned and treated with oxygen plasma for 5 min before deposition. Then, they were transferred
to an evaporation chamber, and 10 mg of the fluorescent compound was evaporated onto the
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substrates at a ratio of 0.5 A/s under a pressure of 10 Torr. The samples were monitored by a
Keyence VHXꢀ5000 optical microscope with 500X magnification. The fluorescence
measurements were conducted using a Mineral Light UVꢀLamp with an excitation at 350 nm and
a detector consisting of an Ocean Optics spectrometer coupled to an optic fiber.
Cyclic Voltammetry (CV). Measurements were carried out with a potentiostat
(PGSTAT128N), using a threeꢀelectrode cell assembly comprising Ag/Ag⁺ in a 3 M solution of
KCl as a reference electrode, a platinum wire (φ=0.2 mm) as the working electrode, a 0.1 M
solution of tetrabutylammonium hexafluorophosphate (TBAFP ) CH₂Cl₂ as a supporting
6
electrolyte and a platinum wire as the counter electrode. Ferrocene was used as an external
standard. The scanning rate was 50 mV/s.
Computational details. Density functional theory (DFT) calculations were performed using
41
the Gaussian 09 program package. The crystal structure of each respective compound was
taken as the initial model for gasꢀphase geometric optimization at the M06ꢀ2X/ccꢀpVTZ level of
theory. The vibrational frequencies were calculated at the same level of theory to assess the
stationary point. No negative frequency was observed for the optimized models. For the optical
part, calculations of properties were performed by timeꢀdependent DFT (TDDFT) with the timeꢀ
42
dependent KohnꢀSham formalism.
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Hirshfeld surface (HS) analysis and PIXEL energy calculation. The Hirshfeld surface and
4
3
twoꢀdimensional fingerprint (FP) plots were generated using Crystal Explorer 3.1, as described
4
4
previously. The lattice energies and intermolecular interaction energies were evaluated using
the PIXEL method. The distances involving hydrogen atoms were moved to their neutron values
0
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0
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9
0
4
3,44
before the calculation.
The electron density of the molecule was obtained at the MP2/6ꢀ
3
1G** level of theory using the Gaussian 09 program.
SYNTHESIS AND CRYSTALLIZATION
Chemicals. 4ꢀBromophenylacetonitrile, 4ꢀ(dimethylamino)benzaldehyde, Nꢀmethylaminoꢀ
ethanol and CuI were purchased from Aldrich Chemical Co. (Mexico). KOH and K CO were
2
3
purchased from J.T. Baker. DMSO and ethanol were acquired from Fermont. The reagents and
solvents were used as received.
Synthesis of (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ3ꢀ(4ꢀ[dimethylamino)phenyl]propꢀ2ꢀenenitrile [(Zꢀ
4
ꢀBrPhDMAPhACN)]. Zꢀ4ꢀBrPhDMAPhACN was synthesized as shown in Scheme 1. Briefly,
a solution of 4ꢀ(dimethylamino)benzaldehyde (1.0 equiv.) in 10 mL of ethanol, 4ꢀ
bromophenylacetonitrile (1.0 equiv.) and KOH (4.0 equiv.) was stirred at room temperature for
12 h. Upon completion of the reaction, a dark yellow precipitate was obtained, which was
washed with ethanol at 4 °C. The final product was recrystallized from a cyclohexane: ethyl
acetate mixture (1:2 v/v). The m.p. and yield of the product were found to be 192ꢀ194 °C and
85%, respectively.
NC
N(CH3)2
COH
CN
KOH
+
Br
Br
N(CH3)2
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Scheme 1. Reaction to obtain (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ3ꢀ(4ꢀ[dimethylamino)phenyl]propꢀ2ꢀ
enenitrile (Zꢀ4ꢀBrPhDMAPhACN)
Preparation of crystals I, I*, I** and II. Single crystals of I were obtained from cyclohexane
by slow evaporation. Briefly, 0.020 g of the title compound was dissolved in 25 mL of
cyclohexane, heated at 75 °C for 5 min and allowed to evaporate at room temperature (25 °C).
Crystals appeared after 18 h and were allowed to grow for another 12 h. The crystals were
filtered, washed with cyclohexane and used for further studies. The melting point of crystal I was
found to be 190ꢀ192 °C. We tried to replace the bromine atom in the title compound by an Nꢀ
methylaminoethanol group using the Ullmann coupling reaction. Unexpectedly, crystals of II
were identified. The experimental procedure is briefly summarized in the supplementary
information section (S1). Crystals II were obtained from two different solvents (DMSO and
DMF) without any other supporting reagents. Briefly, 0.02 g of the title compound was dissolved
in 25 mL of DMSO or DMF, heated at 110 °C for 4 h and allowed to evaporate slowly at room
temperature. The title compound also crystallized from DMF (I*) and DMSO (I**) solvents at
10
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47
48
49
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53
54
55
56
57
58
59
60
100 °C.
RESULTS AND DISCUSSION
The title compound, Zꢀ4ꢀBrPhDMAPhACN acrylonitrile, was synthesized as shown in
Scheme 1. Similar compounds have been synthesized using piperidine as a catalyst, as reported
earlier by our laboratory. Here, we used KOH as a catalyst for improving the reaction conditions
1
and yield. The synthesized compound was characterized by IR, HꢀNMR, EI mass spectrometry
and singleꢀcrystal Xꢀray diffraction studies. The title compound Zꢀ4ꢀBrPhDMAPhACN is
3
2
compared with an analogous compound A. Compound A exhibits weak fluorescence in
32
solution but also displays the AIE property. For the title compound, crystals (I, I*, I** and II)
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were obtained from different conditions. These crystals were extensively characterized by Hꢀ
NMR, using different deuterated solvents (such as CDCl , DMSOꢀd and THFꢀd ). The details
3
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8
of chemical shifts for I, I*, I** and II are presented in Table 1.
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Table 1. Chemical shifts (in ppm) of I, II, I*, and I** in different solvents and EI mass
spectrometry
e
N
d
c
N
c
e
b
a
a
d
H
f
b
Br
EI
(I/%), m/z
Chemical shifts δ (ppm) and coupling constants J (Hz)
(s, 327(64)
[M+],
26(100) base peak,
29(27) [M+2],
325(45), 203(19),
123(9), 18(25)
I
7
2
.88–7.86 (d, 7.55–7.50 (m, 4Hb,c, 7.40 (s, 6.74–6.72
(d, 3.09
3
3
Ha,
J = 10.0 Hz)
1Hf)
2Hd, J = 10.0 6He)
Hz)
CDCl3
J = 10.0 Hz)
I
7.94–7.92 (d, 7.65–7.59
(m,
6.83–6.81
(d, 3.09
(s,
2
Ha, 5Hb,c,f, J = 10.0 Hz)
2Hd, J = 10.0 6He)
Hz)
THF
J = 10 Hz)
I*
7.94–7.92 (d, 7.65–7.59
(m,
6.83–6.81
2Hd, J = 10.0 6He)
Hz)
(d, 3.09
(s,
(s,
(s,
2
Ha,
J = 10.0 Hz)
7.87–7.86 (d, 7.68–7.62 (m, 4Hb,c, 7.89 (s, 6.83–6.81
5Hb,c,f, J = 5.0, 10.0
Hz)
THF
I
(d, 3.39
2
Ha,
J = 5.0 Hz)
7.88–7.86 (d, 7.56–7.50 (m, 4Hb,c, 7.40 (s, 6.75–6.73
J = 5.0, 10.0 Hz)
1Hf)
2Hd, J = 10.0 6He)
Hz)
DMSOꢀ
d6
I**
(d, 3.09
2
Ha,
J = 10.0 Hz)
1Hf)
2Hd, J = 10.0 6He)
Hz)
CDCl3
J = 5.0, 10.0
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Hz)
7.87–7.86 (d, 7.67–7.62 (m, 4Hb,c, 7.89 (s, 6.83–6.61
I**
(d, 3.03
(s,
(s,
2
Ha,
J = 5.0 Hz)
7.89–7.87 (d, 7.56–7.51(m, 4Hb,c, 7.41 (s, 6.76–6.74
J = 5.0, 10.0 Hz)
1Hf)
2Hd, J = 10.0 6He)
DMSOꢀ
d6
Hz)
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II
(d, 3.10
2
Ha,
J = 10.0 Hz)
7.89ꢀ77 (m, 7.67–7.62 (m, 4Hb,c,
J = 5.0, 10.0 Hz)
1Hf)
2Hd, J = 5.0, 6He)
10.0 Hz)
CDCl3
327(64)
326(100) base peak,
29(27) [M+2],
(s, 325(44), 203(18),
[M+],
3
II
6.83–6.81
(d, 3.37
3
Ha,
J = 5.0, 10.0 Hz)
2Hd, J = 10.0 6He)
Hz)
123(9),
18(26)
115(4),
DMSOꢀ
d6
J = 5.0 Hz)
II
7.94–7.92,
(d, 2Ha,
7.65–7.59
(m,
6.82–6.81
(d, 3.09
(s,
5Hb,c,f, J = 10.0 Hz)
2Hd, J = 10.0 6He)
Hz)
THF
J = 10.0 Hz)
*
=
I treated with DMF for at 100°C, 24h (orange crystals); ** = I treated with DMSO at 100
°C, 24h (yellowꢀorange crystals)
1
The signals in the HꢀNMR spectra of I and II are presented in Table 1. The chemical shift of
the proton assigned to ꢀCHf=CNCꢀ appears as a singlet at 7.40 ppm for both I and II in CDCl3.
The signals at 7.88–7.86 ppm correspond to 2H
, which shows an integration corresponding to four protons; the signals at 6.74–6.72 ppm
correspond to 2H ; and the methyl groups (H ) appear at 3.09 ppm. Additionally, the results of
the fragmentation patterns (see S1) give the following ion abundance (I/%) vs m/z, for I: 327(64)
a; the signals at 7.55–7.50 ppm are assigned to
Hb,c
d
e
[
[
M+], 326(100) base peak, 329 (27) [M+2], 325(45), 203(19), 123(9), 18(25), for II: 327(64)
M+], 326(100) base peak, 329(27) [M+2], 325(44), 203(18), 123(9), 115(4), 18(26), Table 1.
4
5
Crystals of I and II are acquired in polar and nonpolar aprotic solvents. The chemical shift of
the proton Hf is at 7.89 ppm in DMSOꢀd , and it overlaps with the two protons assigned as Hb
6
and Hc yielding a multiplet at 7.67–7.62 ppm in THFꢀd . The protons Ha, Hb and Hc display
8
shifts from 7–8 ppm to a low field, whereas the chemical shift of methyl groups does not
undergo a strong change. Intramolecular hydrogen bonds affect the proton Hf by their
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environment, shifting it downfield. Intramolecular hydrogen bonds do not have such an effect.
1
Therefore, monitoring the structural changes of Zꢀ4ꢀBrPhDMAPhACN and A by HꢀNMR at
4
6ꢀ48
room temperature reveals the presence of small signals that may indicate E–Z isomerization.
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For A, the signal from the H proton of the acrylonitrile ꢀCNC=CHꢀ is found to be overlapped
with the signals from the protons in the ortho position on the aromatic ring attached to –CCN=.
However, in DMSOꢀd and THF, the proton chemical shifts are appreciable, but the protons on
6
the double bond move to the deshielding zone, appearing at 7.82 ppm in DMSOꢀd and as a
6
singlet signal at 7.61 ppm in THF. This behavior is important because it could give an indication
of the change in dipole moment (µ) upon photoluminescence and quenching in solution
49
conditions.
ꢀ1
The Fourier transform infrared (FTꢀIR) spectrum shows important signals at 2205 cm due to
ꢀ1
the –CN group, displaying a strong conjugation at 1611 cm due to the double bond and at 1580
ꢀ1
cm due to the νC=C assigned to the conjugated aryl group. The δCꢀH for the triꢀsubstituted
ꢀ1
double bond is observed at 810 cm for A. The IR spectra show only small differences between
ꢀ1
ꢀ1
the crystals; the absorption band occurring at 816 cm in I and 818 cm in II is assigned to δCꢀ
H of the trisubstituted double bond ꢀC=CCN. This value indicates an effect of the Br atom in the
ꢀ1
para position. The corresponding peak is slightly shifted in I and II compared to A (at 810 cm ).
ꢀ1
The intensity of the peak at 2360 cm in I is decreased as compared to II. For the functional
ꢀ1
group νC≡N attached to the alkene double bond, a peak is observed at 2205 cm for both
crystals I and II. This signal has a strong intensity, sharp absorption, and a lower frequency than
50
those attributed to conjugation with double bonds. The δ and δ bands for –CH are observed
as
s
3
ꢀ1
at 1443 and 1375 cm , respectively. For the aromatic rings, the νC=C absorptions occur at 1580
ꢀ1
50
and 815 cm , in accordance with the reported values. The typical overtone/combination bands
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ꢀ1
ꢀ1
appear at 1994 cm and 1806 cm and indicate the expected ring substitution patterns in I and
ꢀ
1
II. In the region of 1335–1250 cm , no variations are observed for I and II (see S1). The IR
spectra of A and crystals I and II show variations in the intensity of bands in the range ~550ꢀ850
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48
49
50
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52
53
54
55
56
57
58
59
60
ꢀ1
cm . Vibrations due to the motions of C ꢀH of the ring are more intense in A, whereas in I and
Ar
II, similar bands are not seen or are weak in intensity. This may indicate the presence of bromine
on the aromatic ring. In contrast, the CꢀBr stretch of the pꢀbromophenylꢀ moiety cannot be
assigned clearly because CꢀH fingerprints appear in this range. Additionally, the intensities of
δCꢀH in both aromatic rings are changed. This finding suggests that these vibrations are more
ꢀ1
intense when Br is attached on the phenyl group. νCꢀC is assigned at 1067 cm for A and at
ꢀ1
1
072 cm for I and II, being shifted to a higher frequency and giving stronger signals for I and
II.
Singleꢀcrystal Xꢀray Crystallography. Structural determination by singleꢀcrystal Xꢀray
diffraction reveals that both crystals are ordered and identical (i.e., characterized by the same
unitꢀcell parameters, Z number, crystal system, and space group). The main difference is that the
title compound crystallizes in two different crystal habits (I: yellow blocks and II: orange
needles, Figure 1). The crystal data and structure refinement details for crystals of I and II are
3
2
summarized in Table 2. The molecular structure of compound A is comparable to those of I
and II, although the phenyl group formed by the C(12)ꢀC(17)ꢀC(13)ꢀC(14)ꢀC(15)ꢀC(16) atoms is
substituted with a bromine atom. The electronic density is distributed almost homogenously over
the whole ring. The C(12)ꢀC(17) bond lengths in I and II are elongated (1.401 Å and 1.413 Å for
II) compared with A. This elongation possibly indicates that the distribution of charge density is
located anti to the electronꢀwithdrawing –CN for all structures and that the overall values are
51
slightly different from the standard value of 1.380 Å for Car=Car in a phenyl ring. The C(10)ꢀ
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C(12) bond length values (Csp –Car) for a conjugated double bond in (I), (II), and (A) are
slightly larger than the typical value (1.470 Å). This could be an effect of the strongly electronꢀ
withdrawing –CN group. In general, the degree of πꢀelectron delocalization is identical in I and
0
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II due to the identical values within standard uncertainties (see Table 3 and S1). As shown in
Figure 2, the structures I, II and A superimpose well, and the maximum root mean square
deviation (rmsd: 0.095 Å) is observed for II and A. It is worth noting that the overall
conformations and crystal packing of I and II are nearly identical. Therefore, the changes in
fluorescence emission observed in I and II are not an effect of the crystal packing. It is of
interest to mention that the C(2)ꢀN(1) bond length in II is significantly shorter than that in I.
Figure 1. Crystal habits of I and II and ORTEP diagram of (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ3ꢀ[4ꢀ
(
dimethylamino)phenyl]propꢀ2ꢀenenitrile, showing the displacement ellipsoids drawn at a 50%
probability level.
Table 2. Crystal data and refinement parameters for (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ3ꢀ[4ꢀ
(
dimethylamino)phenyl]ꢀpropꢀ2ꢀenenitrile
I
II
Empirical formula
Crystal system
Color, Habit
Formula weight
Space group
T(K)
C H Br N
Monoclinic
Yellow, block
327.22
P 2 /c
C H Br N
1
7
15
2
17 15
2
Monoclinic
orange, needle
327.22
P 2 /c
110(2)
1
1
110(2)
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a( Å)
b (Å)
c (Å)
γ (°);
V (Å )
Z
11.0169(2)
6.02041(11)
21.8541(4)
98.4082(18)
1433.92(5)
4
11.0475(6)
6.0273(3)
21.8533(11)
98.315(5)
1439.84(13)
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3
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ꢀ3
Dc(g cm )
1.516
1.510
F (000)
664
3.817
1.54178
0.30 × 0.23 × 0.21
143.8
664
3.802
1.54178
0.25 × 0.18 × 0.09
143.8
10017
3497 (including the
overlapped
reflections from the
second
component), 1947
4.51
9.34
0.776
0.89 and −0.45
ꢀ1
ꢀ
(mm )
λ (Å)
Crystal size (mm )
θmax(°)
3
2
N
9545
2809, 2652
Nunique, N(I>2.0 σ(I))
twin
R1
wR2
goodnessꢀofꢀfit
2.20
5.66
1.051
Largest diff peak and hole 0.29 and −0.36
ꢀ
3
(
e Å )
Table 3. Selected bond lengths (Å) and torsion angles (°) in I and II of (2Z)ꢀ2ꢀ(4ꢀbromophenyl)ꢀ
3
ꢀ[4ꢀ(dimethylamino)phenyl]propꢀ2ꢀenenitrile
Bond lengths
C(1)ꢀN(1)
I
II
A
1.448(2)
1.443(6)
1.447(3)
C(2)ꢀN(1)
C(3)ꢀN(1)
C(6)ꢀC(9)
C(10)ꢀC(11)
C(10)ꢀC(12)
C(12)ꢀC(17)
C(11)ꢀN(2)
C(15)ꢀBr(1)
Torsion angle (°)
C(8)ꢀC(3)ꢀN(1)ꢀC(1)
C(4)ꢀC(3)ꢀN(1)ꢀC(1)
C(8)ꢀC(3)ꢀN(1)ꢀC(2)
C(4)ꢀC(3)ꢀN(1)ꢀC(2)
1.4500(19)
1.366(2)
1.447(2)
1.439(2)
1.484(2)
1.401(2)
1.149(2)
1.8961(15)
1.409(6)
1.369(5)
1.462(5)
1.436(7)
1.491(6)
1.413(6)
1.145(7)
1.904(4)
1.454(3)
1.371(3)
1.454(3)
1.441(3)
1.496(3)
1.393(3)
1.151(3)
ꢀ176.94(14)
2.7(2)
ꢀ5.8(2)
ꢀ176.7(5)
2.5(8)
ꢀ5.2(8)
ꢀ6.0(3)
174.6(2)
ꢀ170.1(2)
10.5(3)
173.89(14)
174.0(5)
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2
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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
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4
4
4
4
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5
5
5
5
5
6
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1
2
3
4
5
6
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2
3
4
5
6
7
8
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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
Figure 2. Structural superimposition of I (red), II (green), and A (blue).
Structural overlay studies were carried out to identify the conformational changes in the
structures of crystal I, II and their parent compound (A). Common atoms other than the N,Nꢀ
dimethyl group and Br atom were used for the structural superposition. The root mean square
deviation (rmsd) between I and II is 0.011 Å. The rmsd between I and A is 0.094 Å, and that
between II and A is 0.095 Å. It is worth noting that the structures I, II and their parent structure
(A) are well superimposable, apart from the difference observed in the hydrogen atoms of the
N,Nꢀdimethyl group (Figure 2). No other significant difference is observed in the structures of l,
II and A.
Usually, the arrangement of the molecular structure in a crystal is influenced by the
5
2
intermolecular strain experienced by each molecule. A molecular structure can experience three
different types of strain: angular strain, torsional strain and steric strain. Due to the interactions
and repulsions between different molecules in the packing, the contact distances between
molecules can be affected by any, all, or none of these three strains. In the present report, the
torsion angles of I and II are identical within the standard uncertainties (Table 3 and S1),
indicating that the crystal packing of I and II is the same. Therefore, the differences between the
fluorescence emission spectra of I and II are not related to crystal packing, as described in other
reports, Figure 3(a). In contrast, it can be seen that the C(2)ꢀN(1) bond length of II is
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significantly shorter than that of I owing to ꢀCH . This is an indication of the light twist present
3
in the (CH ) Nꢀ moiety, Figure 3(b). When a part of a molecule is able to rotate around a sigma
3 2
bond, different levels of torsional strain are generated and vary with the placements of the
substituents relative to each other. This leads to different conformations of molecules, which
affect the πꢀπ interactions in the solid and thus affect the optical properties. Figure 3(b) depicts
the overlap of both molecular structures I and II. The torsion angles for the molecules in I and II
remain unchanged, and consequently, there is no change in the molecular conformation. The
intermolecular potentials UNI (Unified pairꢀpotential parameters), attributed to molecular forces,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
53
54ꢀ57
were calculated using the Mercury program as an approach toward the total lattice energies.
The value of ꢀ146.20 kJ/mol was obtained for A, 161.04 kJ/mol for I, and 161.39 kJ/mol for II.
Additionally, values of the strongest interactions were obtained by calculating the packing
energy from intermolecular potentials.
(b)
(c)
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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
53
Figure 3. Crystal packing diagrams of I (gray) and II (green) and overlap of both (grayꢀgreen)
(
a) molecular structure overlap of I and II (b), and perspective view of the structures showing the
…
58
strong π π interactions (c).
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 packings of I and II are illustrated in Figure 3(a), which shows a typical herringbone
arrangement. The crystal packing is identical for both structures. The herringbone packing mode
features a coꢀfacial πꢀstacking motif that is important for favorable electronic transfer properties,
with the principal difference being that of molecule A. As illustrated in Figure 3(c), the
molecules in I and II show a tilted faceꢀtoꢀface arrangement for the pꢀbromophenyl, with a
centroidꢀcentroid distance of 3.710 Å and a shift distance of 1.244 Å for I and with
corresponding values of 3.722 Å and 1.256 Å for II (these distances were calculated using
58
…
OLEX 2 ). These are typical values of aromatic π π interaction centroid distances (i.e. > 3.65 Å
5
9
and offsets of 1ꢀ6ꢀ1ꢀ8 Å).
PIXEL Calculation and Hirshfeld surface (HS) analysis. PIXEL analysis. The overall
lattice energies for I, II and A crystals and the intermolecular interaction energies for various
pairs extracted from respective crystal structures have been evaluated using the PIXEL
6
0ꢀ62
method.
The overall lattice energies for these three crystals are presented in Table 4. The
ꢀ
lattice energy is nearly the same for both I and II. It is worth noting that crystal A is ~2 kcal mol
1
less stable than are crystals I and II. The intermolecular interaction energies for different pairs
in I, II and A are listed in Table 5. This result clearly shows that the intermolecular C–H···π and
π···π interactions are contribute significantly to the crystal packing. As shown in Figure 4,
dimerꢀI is formed by intermolecular C–H···π and π···π interactions in I and II. The interaction
ꢀ1
energy for this dimer is –10.8 kcal mol . The second most significant contribution to the crystal
packing of I and II comes from the combination of intermolecular C–H···N and C–H···π
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ꢀ1
interactions. The interaction energy for this dimer is –8.5 kcal mol . The methyl groups of the
dimethylamino moiety participate in intermolecular C–H···N and C–H···π interactions. Dimerꢀ
III is stabilized by the methyl···methyl (C1–H1C···C2–H2C) interaction, whose interaction
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢀ1
energy is calculated to be –5.0 kcal mol . DimerꢀIV is formed by π··· π stacking interaction, and
it is found to have the same energetic strength as that of DimerꢀIII. The least contributing motif
(
dimerꢀV) in I and II is formed by an intermolecular C– H···N interaction.
−
1
Table 4. Lattice energies (kcal mol ) partitioned into Coulombic, polarization, dispersion and
repulsion terms for I, II and A
Crystal
Ecoul
ꢀ4.35
ꢀ4.42
ꢀ3.27
Epol
EDisp
Erep
Etot
I
ꢀ5.14 ꢀ41.42
ꢀ5.09 ꢀ40.73
ꢀ5.80 ꢀ36.11
14.75
13.65
11.78
ꢀ36.14
ꢀ36.59
ꢀ34.54
II
A
Tot
Table 5. Interaction energies (E ) partitioned into Coulombic (ECoul), polarization (EPol),
−
1
dispersion (EDisp) and repulsion (E ) contributions (in kcal mol ) for various molecular pairs in
rep
I, II and A
Geometry(Å, °)
HS
Label
a
Important
interactions
Dimer
Distance
E
coul
E
pol
E
Disp
E
rep
E
tot
Symmetry
D–
H···A
D···A
H···A
Crystal I
C8–H8···Cg2
C7–H7···Cg2
C17–H17···Cg1
C16–H16···Cg1
C7–H7···C10
C7–H7···C12
C16–H16···C3
C16–H16···C8
C1–H1A···Cg1
C1–H1B···N2
C1–H1A···C6
C1–H1A···C7
C1–H1A···C8
C4–H4···C3
2.839
3.571
3.039
3.183
2.839
2.897
2.651
2.894
2.562
2.657
2.840
2.696
2.800
2.829
2.048
3.635
3.991
3.669
3.731
3.618
3.535
3.415
3.754
3.554
3.587
3.610
3.663
3.881
3.704
2.987
130.4
105.0
117.6
112.3
128.8
117.9
127.1
136.4
152.0
143.6
128.1
148.5
175.6
137.8
156.7
–
x, 1/2+y,
1
5.318
–3.8
–1.8 –15.0 9.8
–10.8
1/2–z
1
2
1
–x,
2
3
11.936
6.020
–4.7
–0.6
–1.6 –8.7
–1.4 –7.5
6.5
4.4
–8.5
–5.0
–1/2+y,
1
/2–z
x, –1+y, z
C1–H1C···H2C–C2
3
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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–z
–
1
x, 2–y,
4
6.760
8.540
–0.9
–0.9
–0.6 –9.6
–1.3 –4.6
6.1
3.6
–4.9
–3.3
Cg2···Cg2
3.710
3.653
x, 1–y,
–z
4
5
C14–H14···N2
2.531
159.0
Crystal II
C8–H8···Cg2
C7–H7···Cg2
C17–H17···Cg1
C16–H16···Cg1
C16–H16···C3
C7–H7···C10
C7–H7···C12
C1–H1A···Cg1
C1–H1B···N2
C1–H1A···C7
C1–H1A···C8
C4–H4···C3
2.844
3.570
3.037
3.204
2.667
2.832
2.894
2.588
2.574
2.827
2.844
2.816
2.153
3.642
3.995
3.677
3.753
3.437
3.622
3.544
3.565
3.604
3.682
3.883
3.702
3.031
130.6
105.3
118.4
112.4
127.6
129.8
118.7
149.7
158.6
135.8
160.9
139.0
136.4
–
1
x, 1/2+y,
/2–z
1
5.329
–3.7
–1.8 –14.8 9.5
–10.8
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
5
1–x,
–1/2+y,
1
2
11.962
–4.6
–1.6 –8.6
6.3
–8.6
/2–z
3
4
6.027
6.750
–0.5
–0.8
–1.2 –7.1
–0.6 –9.5
3.7
5.9
–5.1
–5.0
x, –1+y, z
–x, 2–y,
1
–
1
C1–H1C···H2C–C2
3
4
Cg2···Cg2
3.722
3.567
–z
x, 1–y,
–z
5
8.535
–0.9
–1.3 –4.6
3.5
–3.3
C14–H14···N2
2.531
159.6
Compound A
C2–H2A···Cg1
C1–H1C···Cg1
C2–H2A···C6
C2–H2A···C7
C8–H8···Cg2
C17–H17···Cg1
C7–H7···C10
C16···C3
2.604
3.723
2.811
2.782
3.298
2.936
2.856
3.638
4.650
3.594
3.692
4.099
3.565
3.536
3.375
3.181
3.321
3.738
3.597
159.3
144.8
129.1
135.0
131.7
117.3
120.9
–x, –y,
1
1
2
8.320
–4.5
–1.4 –8.1
5.4
–8.6
/2+z
1
–
–x, –y,
1/2+z
4.805
–2.5
–1.6 –12.7 8.8
–8.0
1
6
C16–H16···H1A–C1 2.230
145.3
121.6
176.0
151.4
3
4
6.235
9.599
–0.7
–1.0
–1.3 –8.3
–0.7 –4.1
5.3
3.1
–5.1
–2.7
x, y, –1+z
1/2+x,
/2 ꢀy, z
C17–H17···N2
C14–H14···N2
C14–H14···C11
2.622
2.657
2.609
ꢀ
1
a
Centroid-to-centroid distance of molecular pair; Cg1 is the centroids of C3 – C8 ring and Cg2 is the centroids of C12 – C17 ring
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Figure 4. Selected molecular pairs extracted from the crystal structures of I and II along with
their interaction energies (see Table 5).
Hirshfeld surface analysis. The relative contributions of various intermolecular interactions
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11
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15
16
17
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19
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21
22
23
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25
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29
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
are quantified using the Hirshfeld surface (HS) diagram and decomposed twoꢀdimensional
6
3ꢀ66
fingerprint (FP) plot.
These plots are generated using the CrystalExplorer program, as
4
4
described previously. Figure 5 shows the HS diagrams for the crystal structures of I, II and A.
As noted earlier that the crystal packings are same for I and II. The decomposed two
dimensional fingerprint plots and the relative contributions of various intermolecular contacts
present in I, II and A are depicted in Figures 6 and 7, respectively. As expected, the relative
contributions of various intermolecular contacts in I and II are nearly identical. The result
suggests that the intermolecular H⋯H contacts have a major contribution (35.1%) to the crystal
packing of I and II, whereas this contact is slightly higher in A (45.5%). Similarly,
intermolecular C⋯H/ H⋯C contacts are higher in A (39.7 %) when compared to I (31.1%) and
II (31.3%). This reduction could be the presence of bromine atom at the para position of phenyl
ring. The H⋯Br contacts contribute 12.6% in I and 12.7% in II. On the HS diagrams, there is a
small difference observed on the surface of methyl groups. This little difference could be the
reason for the different charge transfer for the molecule and it could also be explained through
different optical properties of crystals I and II.
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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
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
Figure 5. Two different views of the Hirshfeld surfaces mapped with dnorm for crystal I, crystal
II and compound A. (see Table 5 for the numbering points).
Figure 6. Twoꢀdimensional fingerprint (FP) plots for various intermolecular interactions. The
relative contributions of various intermolecular interactions are indicated.
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Other
Br…Br
C…Br
C…C
Relative Contribution (%)
A II I
H…Br
N…H
C…H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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29
30
31
32
33
34
35
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37
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H…H
0
10
20
30
40
50
Figure 7. The relative contributions (%) of various intermolecular contacts present in the
structures I, II and A.
Optical Characterization in Solution and Solid State. The crystals of I are yellow in color,
whereas those of II are orange. These crystals were characterized by UVꢀVis spectroscopy in
CHCl solvent and the results indicate that there is a slight variation in the absorption spectra for
3
I, II and A (λmax at 403nm for crystal I and at 405 nm for crystal II; Figure 8(a). The absorption
maxima are assigned to the typical πꢀπ* transition of a conjugated double bond. However,–CN is
superior as an electronꢀwithdrawing group. In a related compound, 4ꢀN,Nꢀ
dimethylaminophenylstilbene, λmax is found to be at 347 nm in hexane and at 351 nm in
6
7
acetonitrile. A second absorption is observed at approximately 254 nm for A (259 nm for I and
60 nm for II), which corresponds to the n→π* transition due to the amine moiety. A small and
2
invariant absorption band is observed at 328 nm, which may be assigned to the –CN group in I,
II and A. It is of interest to note that the absorbance values in solution do not explain the color
difference between I and II.
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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
II, 405 nm
, 403nm
503
I
II
I
1
1
5000
2000
3
0000
A
, 396 nm
20000
9
000
5
02
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
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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
59 nm
260 nm
254 nm
6000
1
0000
0
3
000
0
328 nm
250
300
350
400
450
500
550
450
500
550
600
650
700
wavelength (nm)
wavelength (nm)
(a)
(b)
512
582
I
404
1
0
.0
.8
II 1.0
0.8
0.6
0.4
0.2
0.6
0.4
0
0
.2
.0
0.0
3
00
400
500
600
700
800
wavelength (nm)
(
c)
Figure 8. Absorbance spectra of I, II and A in CHCl (a), emission spectra of I and II in CHCl3
3
(
b), and the absorption and emission of I and II in the solid state (in KBr) (c).
The fluorescence emission spectra of I and II in solution are displayed in Figure 8(b). From
the figure, it can be seen that the PL emission maxima for I and II are observed at 502 nm and
03 nm (green wavelength), respectively. This clearly indicates a strong redshift, with a Stokes
shift (ꢁλst) value of 98 nm. No quenching phenomenon is exhibited, as reported for compound
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A . This clearly indicates that the bromine atom may have a possible effect on the structure.
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Crystal Growth & Design
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The absorption and emission spectra of I and II in the solid state are shown in Figure 8(c). The
results indicate that the absorbance is nearly the same for both I and II in the solid state. It is
interesting to see that there is a change in the emission spectra. Crystal I shows a maximum of
λ at 512 nm, with a shoulder at 534 nm, whereas II displays a λ at 582 nm. Thus, the Stokes
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shift (ꢁλst) value is found to be 108 nm for I and 178 nm for II. Moreover, the crystal I has a
quantum yield (Φf) that is markedly different from that of the crystal II; the former is 27.5%,
and the latter is 6.6%.
The above observations underscore the role of the molecular packing in I and II on their
optical properties. Several compounds have been reported to exhibit a strong fluorescence
dependent on the aggregation state (AIE). However, the effect of molecular packing on the
emission wavelength is an interesting phenomenon that is distinct from the effect of molecular
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conformation that normally affects the emission.
To verify the observed fluorescence behavior, which depends on the solid state and
morphology of the title compound and is not caused by the presence of any impurity, orange
crystals of I* were obtained from the solvent DMF, as mentioned in the experimental section.
The comparative fluorescence spectra of I, I* and II are shown in Figure 9. It can be seen that
the fluorescence changes from green to yellow to orange as a result of the solvent treatment.
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Previous reports have suggested that these optical characteristics may be caused by changes in
molecular conformation, but this is not the cause for the optical properties of I and II.
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