Photodissociation and kinetics of bis-2-benzylidenemalononitrile (BBM) in different micelles environment

Article abstract of DOI:10.1016/j.jphotochem.2020.112744

In present work, photodissociation kinetics of selective C[dbnd]C bond in BBM is investigated in different micelles concentration. In polar solvents, rate of photodissociation (k) is relatively lower compared to H₂O solvent under similar condit

Full text of DOI:10.1016/j.jphotochem.2020.112744

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Photodissociation and Kinetics of Bis-2-benzylidenemalononitrile (BBM)
in Different Micelles Environment
Sumit Kumar Panja
PII:
S1010-6030(20)30543-8
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112744
JPC 112744
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Journal of Photochemistry & Photobiology, A: Chemistry
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Accepted Date:
28 April 2020
29 June 2020
2 July 2020
Please cite this article as: Panja SK, Photodissociation and Kinetics of
Bis-2-benzylidenemalononitrile (BBM) in Different Micelles Environment, Journal of
Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112744
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Photodissociation and Kinetics of Bis-2-benzylidenemalononitrile (BBM) in
Different Micelles Environment
Sumit Kumar Panja,
Department of Chemistry, Uka Tarsadia University, Maliba Campus, Gopal Vidyanagar,
Bardoli, Mahuva Road, Surat-394350, Gujrat, India
Graphical Abstract
Highlights
 Photodissociation kinetics of selective C=C bond in BBM is investigated in different
micelle concentration.
 It is clear that ionic environment (in presence of KCl) does play important role on
photodissociation kinetics but micelle concentration plays significant role on
photodissociation kinetics.
 In micelle, hydrophobic region plays important role on enhancing rate of photodissociation.
 It can be stated that rate of photodissociation in hydrophobic environment (Specially CMC
condition) is higher compared to polar protic and aprotic solvents.

Abstract: In present work, photodissociation kinetics of selective C=C bond in BBM is
investigated in different micelles concentration. In polar solvents, rate of photodissociation (k) is
relatively lower compared to H2O solvent under similar condition. The rate of photodissociation
(k) is enhanced with increase of aqueous CTAB concentration and found to be highly sensitive to
aqueous CTAB concentration. It is clear that micelle concentration and hydrophobic region plays
significant role on enhancing rate of photodissociation. Crucial role of hydrophobic region in
micelle has been discussed and confirmed by experimental results.
KEYWORDS: Photodissociation; Micelle; Cetyl Trimethyl Ammonium Bromide (CTAB); Critical
Micelle Concentration (CMC); Solute-Solvent Interaction.
Introduction
Photochemistry of organic molecules has been studied extensively as model system for
understanding fundamental and elementary physicochemical process in solution and biological
system.[1-3] The π-conjugated organic molecules have been intensively developed as an
attractive platform for the design and fabrication of a wide range of nano- and microstructures
for use in organic optoelectronics.[4] Photoinduced cis-trans isomerization of C=C bond is
widely observed by external light stimulus and is responsible for various photochemical process
like vision process, phototaxis, control of germination, and photoinduced ion pumping.[5-6]
Popular trans-stilbene organic molecule shows interesting photo-induced cis-trans isomerization
about a C=C bond.[7-8] The structural dynamics and solvent-induced photoisomerization of

trans-stilbene have been investigated to understand fundamental mechanism of cis-trans
photoisomerization.[9-11] Previously, it was reported that photoionization of trans-stilbene
occurs in solution via radical cation.[12-13] The ionization mechanism still remains controversial
with an intriguing fact that the rise of the radical cation shows a marked delay from the
photoexcitation.[7] Two most popular photoisomerization mechanisms of stilbene analogues in
solution have been reported: a). One bond twist (OBT) mechanism, [14-16] b) Hula-twist (HT)
mechanism. [17-19]
Photoisomerization may occur simply a homolytic or a heterolytic breaking of the C=C bond in
solution.[8] Partially double bond character of C=C bond is observed by resonance and facilitates
the interconversion. Exciplex-induced isomerization plays a key role in highly efficient
photoisomerization of stilbene-cored dendrimer.[20-21] Eyring et al. had proposed two
alternatives: isomerization either through the formation of a biradical (on a triplet surface) or in
the ground state (S0) by rotation about the C=C bond.[22] Buechele et al. suggested the
formation of a transient intermediate through tautomerization during trans-cis isomerization for
conjugated dienes.[23]
Recently, effect of substituents and solvent on photochemistry of stilbene analogues has been
widely investigated for understanding the mechanism of photoisomerization process.[23-27]
Stilbene and azobenzene analogues represent mostly studied cases of photoisomerization by both
experimentally and theoretically.[28-31] Role of ground and excited state of molecule during
photochemical process has been investigated widely by theoretical studies.[32-35] The state,
geometry dependent and the shape of the S1(ππ*) potential energy curve (PEC) along the C=C
torsion are important for better understanding the photoisomerization process of stilbene
analogues. [36] Photocleavage of stilbene is observed in presence of oxygen. [37] Mechanistic

studies and photodegradation of 2,5-dialkyloxyl-substituted para-phenylenevinylene oligomers
by singlet oxygen is also reported. [38]
From computational and experimental evidences, evolution of a charge transfer state populated
directly upon photoexcitation and non-radiative deactivation to the ground state are affected by
the polarity of the solvents, play important role on photoisomerization and photochemistry. [39-
40]
It is reported that the cyano-substituted group has been found to open a non-radiative decay
mechanism related to the isomerization of the double bond via a crossing (conical intersection)
of electronic states with distinct nature via the conical intersect (CI). [40] Recently, our group
has reported the photodissociation kinetic of bis-2-benzylidenemalononitrile (BBM) in solutions
and role of cyano group and solvents. [41] Further, role of solvents and photodissociation of
C=C bond in bis-2-benzylidenemalononitrile (BBM) are not clearly understood.
In present work, we have investigated photochemistry and photodissociation of C=C bond in
photoactive bis-2-benzylidenemalononitrile molecules (BBM, Chart 1). Solvent and environment
dependency of photodissociation have been investigated throughout work. Interesting
observations are obtained from our present work.
Experimental Section
UV-Vis Spectroscopy
Electronic absorption spectrum is measured on Varian-UV-Visible spectrophotometer (Carry-
100-Bio) in various solvents and different aqueous CTAB solution. A 1 cm quartz sample cell at
the rate of 0.5 C/min during the heating and cooling scans using Cary 100-Bio UV-Vis
spectrophotometer (Varian Carry-100-Bio) equipped with a Peltier series II thermostatic cell
holder.

Synthesis of Bis-2-benzylidenemalononitrile (BBM)
Synthetic procedure of BBM was reported in our previously work. [42] In brief, a mixture of
terephthaldehyde (1.0 mmol), manolonitrile (2.01 mmol) and MP(DNP) (0.05 mmol) in round
bottom flask containing ethanol was stirred at room temperature for appropriate time. After
completion of the reaction, which was monitored by TLC, the solid product was collected by
filtration, washed with water and ethanol for obtaining pure product.
Characterization of BBM: State: Solid; Colour: White; Melting Point: 205 °C; FTIR (KBr, cm-
1): 2938, 2228, 1556, 1565, 1494; 1H-NMR (300MHz, CDCl3): δ (ppm) 8.04 (d, J = 7.0 Hz, 4H),
7.81 (s, 2H); 13C-NMR (125 MHz, CDCl3): δ (ppm) 160.3, 135.8, 131.3, 114.3, 113.2, 85.2;
ESIMASS: [M+H] 231.2.
Sample Preparation
Time dependent UV-Vis spectra of BBM is measured under UV irradiation at 365 nm for 1
minute before and after during UV-Vis measurement in solvents. Commercially available 4-watt
power of UV lamp @ 365 nm is exposed for 1 minute on the sample. The concentration of BBM
is ~10-5(M) in solvents during experiment.
Chart 1: Structural Formula and Abbreviation
Results and discussion

Photodissociation kinetics of selective C=C bond in BBM is investigated in different micelle
concentration at room temperature. Influence of hydrophobic and hydrophilic factors on rate of
photodissociation (k) has also investigated.
Time Dependent UV-Visible Studies at Different Aqueous CTAB Micelles Concentration.
Time dependent UV-Vis study of BBM is carried out in different solvents for understanding the
effect of solvents on photodissociation via solute-solvent interaction. In polar aprotic solvents,
UV-Vis spectra of BBM is not changed under UV irradiation for 1 minute during time dependent
UV-Vis studies but significant spectral variation is observed in polar protic solvents under same
condition (ESI- Figure 1). So, photodissociation process is occurred during time dependent UV-
Vis spectral measurement in protic polar solvents.
Interestingly, time dependent UV-Vis spectra and its spectral pattern in H2O are different from
other polar protic solvents and indicates special solute-H2O interaction during photodissociation
process. In time dependent UV-Vis spectra in H2O, two isosbestic points are observed at 598 nm
and 283 nm respectively during photodissociation reaction under UV irradiation (ESI-Figure 2).
The kinetic of this photodissociation reaction is fitted with pseudo 1st order kinetic model. The 1st
order kinetic model for photochemical process is considered and rate constant (k) is found to
~10-3 s-1 in different protic solvents and binary solvent at 25 C. The pseudo 1st order rate
constant (k) is calculated monitoring at 403 nm with time in H2O and found to be 2.4x10-3s-1.
Similarly, pseudo 1st order rate constant (k) is calculated monitoring at 348 nm with time in
binary solvent (ACN-H2O) and found to be 6.2x10-4s-1 (ESI-Figure 3). Further, different
alcoholic solvents, the rate constant (k) was also significantly different and found to be 4.4x10-4s-
1 for EtOH and 1.2x10-4s-1 for 2-Pro-OH respectively.[41]

Generally, critical micelle concentration (CMC) of CTAB in H2O is found to be 0.9 mM at room
temperature. So, above 0.9 mM of aqueous CTAB solution, different micelle concentration of
aqueous CTAB solution is formed. To investigate the rate of photodissociation under UV
irradiation, time dependent UV-Vis study is carried out in different aqueous CTAB solution at
room temperature. From time dependent UV-Vis spectra in 2.5 mM aqueous CTAB solution, one
isosbestic point is observed at 304 nm during photodissociation reaction under UV irradiation
(Figure 1). In UV-Vis spectra, longer wavelength region is not affected significantly (500-900
nm). The kinetic of this photodissociation reaction in 2.5 mM aqueous CTAB solution is fitted
with pseudo 1st order kinetic model and rate constant (k) is found to be 1.1x10-3 s-1 at 25 C.
Figure 1. Time dependent UV-Vis spectra of BBM in 2.5 mM aqueous CTAB solution (External
UV Irradiation at 365 nm for 1 minute during UV-Vis measurement)
Similarly, one isosbestic point is observed at 295 nm during photodissociation reaction under UV
irradiation in 5 mM aqueous CTAB solution from time dependent UV-Vis spectra (Figure 2). In
UV-Vis spectra, longer wavelength region is affected significantly (350-900 nm). The kinetic of

this photodissociation reaction in 2.5 mM aqueous CTAB solution is fitted with pseudo 1st order
kinetic model and rate constant (k) is found to be 2.1x10-3 s-1.
Figure 2. Time dependent UV-Vis spectra of BBM in 5 mM aqueous CTAB solution (External
UV Irradiation at 365 nm for 1 minute during UV-Vis measurement)
Further, one isosbestic point at 295 nm is observed under UV irradiation in 10 mM aqueous
CTAB solution from time dependent UV-Vis spectra (Figure 3). In UV-Vis spectra, longer
wavelength region is disappeared (350-900 nm) with time and affected significantly. The kinetic
of this photodissociation reaction in 10 mM aqueous CTAB solution is fitted with pseudo 1st
order kinetic model and rate constant (k) is found to be 5.6x10-3 s-1. With increase of aqueous
CTAB concentration, value of specific rate constant (k) increases two-fold when concentration
of aqueous CTAB solution is doubled.

Figure 3. Time dependent UV-Vis spectra of BBM in 10 mM aqueous CTAB solution (External
UV Irradiation at 365 nm for 1 minute during UV-Vis measurement)
In 50 mM aqueous CTAB solution, four isosbestic points are observed at 405, 348, 277 and 240
nm respectively under UV irradiation from time dependent UV-Vis spectra (Figure 4). In UV-
Vis spectra, longer wavelength region is disappeared (350-900 nm) with time. The kinetic of this
photodissociation reaction in 50 mM aqueous CTAB solution is fitted with pseudo 1st order
kinetic model and rate constant (k) is found to be 10x10-3 s-1.

Figure 4. Time dependent UV-Vis spectra of BBM in 50 mM aqueous CTAB solution (External
UV Irradiation at 365 nm for 1 minute during UV-Vis measurement)
The variation of rate constant (k) is plotted with different concentered aqueous CTAB solution in
Figure 5 and presented in Table 1. Interesting factor is that the rate constant (k) is not further
enhanced with increase of CTAB concentration up to 100 mM solution.
Table 1: Calculated rate constant (k) in different CTAB micelle concentration at 25 C
2.5 mM
@ CTAB
1.1x10-3
@ 403 nm @ 403 nm
0.99 0.99
5 mM
10 mM
@ CTAB
5.6x10-3
@ 315 nm
0.99
50 mM
100 mM
@ CTAB
10x10-3
2.5 mM
@ KCl
2.2x10-3
@ 403 nm
0.99
@ CTAB
@ CTAB
k (s-1)
2.1x10-3
10x10-3
@ 315 nm @ 315 nm
0.99 0.99
ퟐ
Change of rate constant (k) with variation aqueous CTAB concentration has been analyzed using
curve fitting method. We have fitted the experimental results with exponential growth profile and
found that the increasing rate of rate constant (k) with variation of aqueous CTAB concentration
is 0.09 with ퟐ = 0.98 (Figure 5). The specific rate constant (k) of photodissociation is observed

maxima at 25 mM CTAB solution. The rate constant (k) is increased at different concentration of
aqueous CTAB solution and higher compared to solvents at room temperature. But above
concentration of aqueous CTAB (> 50 mM), rate constant (k) is almost constant and found to be
10x10-3 s-1 (Figure 5).
Figure 5: Variation of rate constant (k) with different aqueous CTAB solution
To investigate the effect of ionic strength on photodissociation process, different concentrated
aqueous KCl solution is used. It is observed that concentration of aqueous KCl solution cannot
influence on rate of photodissociation at room temperature (ESI-Figure 4-5).
From above experimental results, it is clear that ionic environment (in presence of aqueous KCl)
does play important role on photodissociation kinetics but different micelle concentration plays
significant role on enhancement of photodissociation kinetics. In micelle, hydrophobic region
plays important role on enhancing rate of photodissociation. Considering above experimental
facts, we have proposed a mechanism of photodissociation process in micelle environment and
presented in Figure 6.

Figure 6: Mechanism of photodissociation process in micelle environment
In micelle environment, BBM molecules entered to hydrophobic cavity of micelle and reacts
with water photodissociation occurred. Photo-dissociated product is coming out from
hydrophobic cavity. At higher micelle concentration, large number of hydrophobic cavities
would be available and density of BBM molecule in hydrophobic cavity of a micelle would be
higher, results enhancement of photodissociation process.
Conclusion
In conclusion, photodissociation kinetics of selective C=C bond in BBM is investigated in
different micelle concentration. In polar protic and polar aprotic solvents, rate of
photodissociation (k) is significantly lower compared to H2O solvent at room temperature. But
interesting factor is that rate of photodissociation (k) is enhanced with increasing aqueous CTAB
concentration and found to be highly sensitive to aqueous CTAB concentration.
From experimental results, it is clear that ionic environment (in presence of aqueous KCl) does
play important role on photodissociation kinetics but different micelle concentration plays
significant role on photodissociation kinetics. In micelle, hydrophobic region plays important
role on enhancing rate of photodissociation. The rate of photodissociation is highest at 25 mM
aqueous CTAB solution and found to be 10x10-3s-1, which is five-fold higher than H2O (k =

2.0x10-3s-1) at room temperature. It can be stated that rate of photodissociation in hydrophobic
environment (specially CMC condition) is higher compared to polar protic and aprotic solvents.
ASSOCIATED CONTENT
Supporting Information
This material is available free of charge via the Internet at http://
AUTHOR INFORMATION
Corresponding Author
E-mail: sumitkpanja@gmail.com; sumit.panja@utu.ac.in
Department of Chemistry, Uka Tarsadia University, Maliba Campus, Gopal Vidyanagar,
Bardoli, Mahuva Road, Surat-394350, Gujrat, India
ORCID of SKP: orcid.org/0000-0001-5658-0452
1. Conflict of Interest
No conflict of interest exists.
Funding
No funding was received for this work.
ACKNOWLEDGMENT
SKP acknowledges Department of Chemistry, Uka Tarsadia University, Surat-394350, Gujrat,
India for providing infrastructure and instrument facilities.
Notes
The authors declare no competing financial interest
References

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