Selective photodissociation of highly photoactive Bis-2-benzylidenemalononitrile in solution

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 18–23

Contents lists available at ScienceDirect

Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Selective photodissociation of highly photoactive Bis-2benzylidenemalononitrile in solution

T

⁎

Sumit Kumar Panjaa, , Suvajit Koleyb, Haddad Boumedienec a

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560012, Karnataka, India Department of Medicinal Chemistry, The University of Kansas, 1251 Wescoe Hall Drive, Lawrence, Kansas, 66045, United States c Department of Chemistry, Dr. Moulay Tahar University of Saida, Algeria b

A R T I C LE I N FO

A B S T R A C T

Keywords: Photodissociation Photoactive material Dipolar mechanism Benzylidene malononitrile analogue Density functional theory (DFT)

Photophysical study of highly photoactive bis-2-benzylidenemalononitrile (BBM) has been investigated in solution using UV–vis, fluorescence and NMR spectroscopic techniques. The seletive photodissociation can be attributed to the presence of highly polarized C]C bond, attached to dicyno groups (CN). Nevertheless, the observed photodissociation of BBM is highly solvent specific and temperature sensitive. The product A (2-(4formylbenzylidene)malononitrile), formed by the selective photodissociation of BBM under UV irradiation has been isolated and identified by NMR studies. The photodissociation followed the pseudo 1st order kinetics with a rate constant (k) 10−3-10-4.s-1. Moreover the mechanistic study of selective photodissociation of bis-2-benzylidenemalononitrile (BBM) has been investigated and found to follow the dipolar mechanism path during photochemical reaction.

1. Introduction Photochemical behaviours of organic stilbenes have been extensively studied as the model systems in solutions and biological systems for the better understanding of most fundamental and elementary physicochemical processes [1–4]. Photoinduced trans-cis isomerization about a C]C double bond is widely occured in various processes such as vision process, phototaxis etc [5,6]. On the other hand trans-stilbene is a very prevalent organic molecule, which shows numerous interesting photo-induced cis-trans isomerization about a C]C bond [7]. The solvent-induced photoisomerization of trans-stilbene revealed that the trans-cis photoisomerization occurred at lowest excited singlet state (S1) [8–10]. The photoisomerization of olefins in solution normally proceeds by the rotation of the double bond, because the reaction coordinate mainly involves the one bond twist (OBT) mechanism about the central bond [11–13]. The Hula-twist mechanism (HT), which comprises of concerted rotation about a double bond and an adjacent essential single bond (envisioned as equivalent to a 180° translocation of one CH unit) was proposed to account for preliminary understanding of photoisomerization in free volume restricted media [14–16]. It is also reported that photoionization of trans-stilbene occurs in solution to generate the trans-stilbene radical cation [17–19]. Generally, diradical isomerization of olefins (mainly photoisomerization) proceeds either via the π, π* singlet (S1) or triplet (T1) excited states, depending upon ⁎

the properties of the molecule and the experimental conditions [20,21]. The ionization mechanism still remains controversial with an intriguing fact that the rise of the radical cation shows a marked delay from the photoexcitation [22]. However, intersystem crossing (T1→ S0 + heat), photosensitization by singlet-singlet or triplet-triplet (TT) transfer can also lead to isomerization [7]. It is also reported that photoisomeriaztion may occur simply by the breaking of the double bond, either through a homolytic or a heterolytic process [7]. Arai et al. has reported that exciplex-induced isomerization played a key role in highly efficient photoisomerization of stilbene-cored derivatives [23,24]. Eyring has proposed that the trans-cis isomerization may happen either through the formation of a biradical (on a triplet surface) or in the ground state (S0) by rotation about the C]C bond [25]. An alternative cis/trans isomerization pathway for conjugated dienes was depicted by Buechele and co-workers which involves the formation of a transient intermediate through tautomerization [26]. The effect of substituents on trans-cis photoisomarization of stilbene analogues has been widely explored and throughly investigated for the better understanding of the mechanism of photoisomerization process [27–30]. Several theoretical and experimentat studies were also carried out for getting the valuable informations about the ground and excited state of C]C bond during photochemical process [31–35]. Particularly, the effect of donor-acceptor substituents of asymmetric benzylidenemalononitrile analogues on photoisomerization and formation of highly

Corresponding author. E-mail address: [email protected] (S. Kumar Panja).

https://doi.org/10.1016/j.jphotochem.2019.01.034 Received 20 November 2018; Received in revised form 24 January 2019; Accepted 30 January 2019 Available online 10 February 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 18–23

S. Kumar Panja, et al.

2.4. Raman spectroscopy FT-RAMAN spectra were acquired by means of a Vertex 70-RAM II Bruker FT-RAMAN spectrometer with a Nd:YAG laser (yttrium aluminium garnetcrystal doped with triply ionized neodymium) having a wavelength of 1064 nm and a maximum power of 1.5 W. The measurement accessory is pre-aligned; only the Z-axis of the scattered light is adjusted to set the sample in the appropriate position regarding the focal point. The RAM II spectrometer is equipped with a liquid-nitrogen cooled Ge detector and FT-RAMAN spectra (200-3500 cm−1) were collected with 1 cm−1 nominal resolution by co-adding 128 scans for each spectrum at room temperature.

Chart 1. Structural Formula and Abbreviation.

polar twisted intramolecular charge-transfer species (TICT) in the excited state has been investigated widely [36–39]. Nevertheless, the solvent and substituent effects on light-induced rotations of C]C bond of asymmetric benzylidenemalononitrile analogues are also well reconnoitred [40,41]. The solvent and substituent effect on selective photodissociation of C]C bond of benzylidenemalononitrile analogues have not been investigated yet. So, in present work, we depicted the solvent dependent photochemical behaviour of C]C bond in bis-2-benzylidenemalononitrile molecules (BBM, Chart 1 ). Effects of solute-solvent interactions on selective photodissociation of BBM are also studied and discussed for better understanding of the mechanism and kinetics of this chemical process.

2.5. UV–vis spectroscopy Electronic absorption spectrum was measured on Varian-UV–vis spectrophotometer (Carry-100-Bio) in various solvents. 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-100Bio) equipped with a Peltier series II thermostatic cell holder. 2.6. Quantum chemical calculation

2. Experimental section

DFT calculation using B3LYP functional with 6-311++G (d, p) basis set is employed to calculate the optimized geometry of BBM molecule. The electronic absorption spectra is calculated using the time-dependent density functional theory (TD-DFT) method at B3LYP/ 6-311++G(d,p) using the polarisable continuum model (PCM) and ACN as solvent. The Gaussian 09 suite of programs was used for all the computations, and the GaussView 5.1 program was used to visualize the structures [43].

2.1. Synthesis of Bis-2-benzylidenemalononitrile (BBM) Synthesis of BBM was done by 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 (monitored by TLC), solid product was collected by the filtration and washed with water and ethanol for obtaining pure product.

3. Results and discussion 3.1. UV–vis studies of BBM in different solvents

2.1.1. Characterization of BBM State: Solid; Colour: White; Melting Point: 205 °C; FTIR (KBr, cm−1): 2937, 2228, 1555, 1565, 1495; 1H-NMR (300 MHz, CDCl3): δ (ppm) 8.04 (d, J = 7.0 Hz, 4 H), 7.81 (s, 2 H); 13C-NMR (125 MHz, CDCl3): δ (ppm) 160.3, 135.8, 131.3, 114.3, 113.2, 85.2; ESIMASS: [M+H] 231.2.

Initially, electronic absorption spectra is measured using UV–vis spectroscopy in different solvents (Fig. 1) for better understanding the solvent effect on electronic properties of the BBM molecule. The UV–vis spectra of BBM does not exhibit a bathochromic shift with the increase in the polarity of the solvents except H2O. The intramolecular charge transfer (ICT) band of BBM is appeared at 369 nm and π→π* transition is observed at 347 nm in polar solvents. In H2O, the spectral pattern is quite interesting and different from other polar solvents. For BBM, ICT band is appeared at 400 nm and the π→π* transition is observed at 371 nm seperately with a tailing part which extended up to 900 nm in H2O (Fig. 1). From solvent dependent UV–vis spectra, it is evident that

2.2. Sample preparation of BBM in solution For studying kinetics of BBM in solution by UV–vis spectroscopy, ˜10−5(M) of BBM is taken in solvents during experiment. Solution of BBM is kept under UV irradiation at 365 nm for 1 min before UV–vis measurement in solvents. Similarly, for studying kinetics of BBM in solution by fluorescence spectroscopy, ˜10−6(M) of BBM is taken in solvents during experiment. This solution of BBM is kept under UV irradiation at 365 nm for 1 min before fluorescence measurement in solvents. Commercially available 4 W power of UV lamp @ 365 nm is exposed for 1 min on the sample for spectroscopic measurement. For NMR experiment of photodissociated products, photochemical reaction is performed taking 10−2 (M) solution of BBM in H2O under UV irradiation @ 365 nm for 5 min. From reaction mixture, crude products are extracted by ethylacetate and further used for 1H and 13CNMR in CDCl3 solvent. 2.3. Infrared spectroscopy For the study of the vibrational properties, Fourier Transform Infrared (FTIR) spectra was acquired on a Shimadzu FTIR-8900 spectrophotometer and recorded in the 4000-400 cm−1 region.

Fig. 1. Normalized UV–vis spectra of BBM in different solvents at 25 °C. 19

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 18–23

S. Kumar Panja, et al.

Fig. 4. Time dependent UV–vis spectra of BBM in H2O at 45 °C (External UV irradiation at 365 nm). Fig. 2. Time dependent UV–vis spectra of BBM in H2O at 25 °C (External UV irradiation at 365 nm for 1 min before and after UV–vis measurement).

(v/v)), is entirely different in nature from pure H2O solvent (Figs. 3 and 4) at similar reaction condtion. The tail part which is observed in UV–vis spectra of BBM in H2O, is completely disappeared in case of UV–vis spectra of BBM in binary solvent (ACN: H2O = 1:1 (v/v)) at 25 °C. The absorption band at 348 nm of BBM in binary solvent (ACN: H2O = 1:1 (v/v)) is decreasing with time and shifted to 315 nm during photodissociation process at 25 °C. During photochemical process at 25 °C, two isobestic points are observed at 317 nm and 243 nm in binary solvent (ACN: H2O = 1:1 (v/v)). Above experimental results in H2O and binary solvent (ACN: H2O = 1:1 (v/v)) indicate that nature of spectral pattern is affected by the nature of solvents during photodissociation process. For this photodissociation process, 1st order kinetic model is considered and rate constant (k) is found to ˜10−3 s-1 in different protic solvents and binary solvent (ACN:H2O = 1:1 (v/v)) at 25 °C (Table 1). The pseudo 1st order rate constant (k) is calculated during photochemical process by monitoring the decrease of absorption band at 403 nm with time in H2O at 25 °C and found to be 2.4 × 10−3.s-1. Similarly, the pseudo 1st order rate constant (k) is calculated by monitoring the decrease of absorption band at 348 nm with time in binary solvent (ACN-H2O) and found to be 6.2 × 10-4.s-1 at 25 °C. Further, the rate constant (k) is found to be 4.4 × 10-4.s-1 for EtOH and 1.2 × 10-4.s1 for 2-Pro−OH respectively monitoring the decresing absorbance at 348 nm (Table 1). The rate constant (k) in polar protic solvents is presented in Table 1 and ESI-Figs. 1-4.

the specific solute-solvent interaction is not observed in polar protic solvents (Fig. 1). The tailing part in UV–vis spectra is clearly indicating the solvent specific strong solute-solvent interaction between BBM and H2O. The aborption maxima (λmax) of BBM in different solvents is tabulated in ESI-Table 1. 3.2. Effect of solvents on UV–vis studies at room temperature For investigating the nature of photodissociation reaction and solute-solvent interaction, time dependent UV–vis spectra of BBM is carried out in different solvents. During time dependent UV–vis studies, the corresponding UV–vis spectra of BBM in polar aprotic solvents is not changed (under UV irradiation at 365 nm for 1 min before and after UV–vis measurement) but significant spectral change is observed in prolar protic solvents (ESI-Figs. 1–4) under same condition. This obsrevation indicates that the photodissociation process is occurred during UV–vis spectral measurement in polar protic solvents (Fig. 2 and ESI-Figs. 1–4). Decrease of spectral intensity with blue shift of the ICT absorption band (Figs. 2 and 3 and ESI-Figs. 1–4) is observed in polar protic solvents. From the time dependent UV–vis spectra of BBM in polar protic solvents, interestingly, the change of UV–vis spectral pattern in H2O with two isobestic points (observed at 598 nm and 283 nm) is different from other polar protic solvents (ESI-Figure 1–4). The observed unique charactristic spectral change in H2O is due to the presence of strong solute-H2O interaction (Fig. 2) during photodissociation process. Further, in binary solvent (ACN: H2O = 1:1 (v/v)), photodissociation process of BBM is also monitored at room temperature and it is observed that the spectral pattern in binary solvent (ACN: H2O = 1:1

3.3. Temperature effect on photodissociation reaction Temperature effect on photodissociation process of BBM is also investigated in H2O and ACN-H2O solvents seperately. It is observed that the UV–vis spectral pattern of BBM in H2O is significantly changed from 25 °C to 55 °C during time dependent UV–vis studies (Figs. 2–4 and ESIFigs. 5–7). Interstingly the change of UV–vis spectra is not observed in H2O at 15 °C (ESI-Fig. 5), but significant change of UV–vis spectra is observed with increase in temperature. At higher temperature (> 45 °C), a prominent new absorption band is appeared at 283 nm with three distinct isobestic points at 288 nm and 241 nm in UV–vis spectra of BBM, indicates that the photochemical process is occurred at fast rate with higher amount of photodissociated product A (Fig. 4). The most interesting factor in this case is that the pseudo 1st order rate constant (k) in H2O solvent is decreased with increase in temperature (Table 2). Table 1 Rate constant (k) in protic polar solvents at 25 °C.

k (s−1)

Fig. 3. Time dependent UV–vis spectra of BBM in ACN: H2O (1:1 (v/v)), at 25 °C (External UV irradiation at 365 nm).

χ2

20

H2O

ACN-H2O

EtOH

2-Pro-OH

2.4 × 10−3 0.99

6.2 × 10−4 0.99

4.4 × 10−3 0.99

1.2 × 10−3 0.99

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 18–23

S. Kumar Panja, et al.

Table 2 rate constant (k) for BBM at different temperature.

a

Temperature

kobs in H2O @ 403 nm

kobs in ACN-H2O @ 348 nm

15 °C 25 °C 35 °C 45 °C 55 °C

a 2.42 × 10−3.s-1 2.80 × 10−3.s-1 1.85 × 10−3.s-1 1.53 × 10−3.s-1

4.26 × 10−4.s-1 6.21 × 10−4.s-1 15.80 × 10−4.s-1 26.63 × 10−4.s-1 34.61 × 10−4.s-1

Fig. 6. ORTEP diagram of BBM (CCDC No. 1,056,280) (ref. 44).

observation in ACN-H2O (1:1 (v/v)) solvents may be same emision spectra of BBM and photochemical product A at λex = 340 nm. From previously reported crystal structure of BBM, it is clearly observed that in solid state most stable conformer is S-trans (Fig. 6). [44] The intermolecular interactions and photophysical studies of BBM and its analogues were reported in previous reported work in solid state [43]. It is clear that the C]C bond of BBM is activated under external UV irradiation and takes part in photodissociation process (Fig. 8). In our presnt work, it is observed that selective photodissociation of BBM is highly feasibile and occurred at slower rate in specific polar protic solvents under the external exposer of UV radiation at 365 nm. In presence of dicyano group (acceptor), the C]C bond of BBM molecule becomes highly polarizable and becomes susceptible for the photodissociation, even at room temperature. Optimization of the geometry at ground state of BBM is carried out at B3LYP/6-311++g(d,p) level of theory and shown in Fig. 8. The molecular orbitals involved towards HOMO and LUMO of BBM at ground and excited state electronic properties, are investigated using the TD-DFT calculation at the B3LYP/6-311++G (d,p) level of theory. The HOMO-LUMO calculations are carried out using TD-DFT method at the B3LYP/6-311++G (d,p) level of theory using the polarisable continuum model (PCM) and ACN as solvent. From TD-DFT calculations, the higher wavelength absorption band corresponds to HOMO→LUMO transition is exclusively represents the ICT state and clearly indicates that the intramolecular charge transfer (ICT) process is taking place from benzene ring to dicyano group in BBM. This intramolecular charge transfer (ICT) state of the BBM molecule may results the partially double bond character of C]C bond in BBM. The calculated electronic absorption spectrum reproduces the experimental spectrum though the intensity of the S0→S1 transition (Fig. 7). The experimental and theoretical results are well matched and presented in Fig. 7. From above experimental UV–vis and fluoresence results, it is observed that photodissociation process occurs in H2O and binary ACNH2O solvent (1:1 (v/v)) (Fig. 8) under UV irradiation via the dipolar state ((δ+C … Cδ−).). The presence of dicyano group in BBM, enhanced the formation of dipolar state (δ+C … Cδ−) and become more stabilized by polar protic solvents. [45] In presence of H2O and UV irradiation, the dipolar state (δ+C …Cδ−) becomes highly active and favours the photodissociation reaction and forms photodissociated product A (Fig. 8). In presence of dicyano group, activation energy barrier of C]C bond in BBM may be lower and photodissociation process takes in H2O. From 1H-NMR studies, charactristic chemical shift of CH proton in CH = C(CN)2 is observed at 8.62 ppm and aromatic CH chemical shift is at 8.10 ppm in solution of pure BBM molecule (ESI- Figs. 14–15). After UV irradiation@365 nm on solution of BBM in H2O for 5 min (ESIFigs. 16–17), we observed new proton peak at 10.09 ppm, 8.66 ppm and 8.11 ppm with 8.62 ppm and 8.10 ppm. These new characteristic proton peaks are due to presence of CHO, H proton (CH = C(CN)2) and aromatic protons of new product A (2-(4-formylbenzylidene)malononitrile). These new proton peaks are confirmed by the formation of (2-(4-formylbenzylidene)malononitrile) from selective photodissociation reaction of BBM under UV irradiation in H2O. Further, the carbonyl group (C]O) is also observed at 193.0 ppm in 13C-NMR from product A (2-(4-formylbenzylidene)malononitrile) (ESI- Figs. 16–17). The product A (2-(4-formylbenzylidene)malononitrile) has also showed the characteristic mass at 182 (m/z) in GCMS spectra (ESI-Fig. 18).

Photochemical process is not observed.

Similarly, effect of temperature on photochemical process of BBM in ACN-H2O (1:1 (v/v)), is also investigated at different temperatures. It is observed that UV–vis spectral pattern of BBM in ACN-H2O (1:1 (v/v)), is also changed significantly from 15 °C to 55 °C, during time dependent UV–vis studies (ESI-Figure 8–11). The pseudo 1st order rate constant (k) of BBM in ACN-H2O increases with increase in temperature (Table 2). Further, rate constants (k) in H2O and ACN-H2O systems are also observed at different temperature and presented in Table 2. The results clearly indicate that the photochemical process is influenced by the nature of solvents due to solvent specific solute-solvent interactions. The rate constant (k), is decreased in H2O, but increased in binary ACNH2O with the rising of temperature. This observation can be explained on the basis of solubility of the product A (2-(4-formylbenzylidene) malononitrile) during the photodissociation process. The product A (2(4-formylbenzylidene)malononitrile) might has the greater solublity in ACN-H2O binary system compared to pure H2O. 3.4. Fluorescence studies: effect of solvents Fluorescence spectra of BBM is measured in different solvents and it is observed that the fluorescence spctra is not influenced much by solvents and their polarity. Fluorescence result is presented in ESITable 1 and ESI-Fig. 12. It is observed that fluorescence spectral pattern of BBM is similar to previously reported fluorescence results. [44] The photochemical process of BBM is also monitored by time dependent fluorescence spectra in H2O and H2O-ACN solvents seperately at room temperature. For studying kinetics of BBM in solution by fluorescence spectroscopy, ˜10−6 (M) of BBM is taken in solvents during experiment. This solution of BBM is kept under UV irradiation at 365 nm for 1 min before and after fluorescence measurement in solvents. Commercially available 4 W power of UV lamp @ 365 nm is exposed for 1 min on the sample for time dependent spectroscopic measurement. From time dependent fluorescence results, the fluorescence intensity is decreasing with time in H2O solvent at λex = 360 nm (Fig. 5). But interesting factor is that fluorescence intensity in ACN-H2O (1:1 (v/v)) solvents is unaltered with time at λex = 340 nm (ESI-Fig. 13). The unique

Fig. 5. Time dependent fluorescence spectra of BBM in H2O (λex = 360 nm) (Exposer of external UV Irradiation at 365 nm). 21

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 18–23

S. Kumar Panja, et al.

Fig. 7. Optimized structure and HOMO-LUMO diagram of BBM from DFT calculation.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.01. 034. References [1] B.-K. An, J. Gierschner, S.Y. Park, π-Conjugated cyanostilbene derivatives: a unique self-assembly motif for molecular nanostructures with enhanced emission and transport, Acc. Chem. Res. 45 (2012) 544–554. [2] Y. Zhang, J. Sun, G. Bian, Y. Chen, M. Ouyang, B. Hua, C. Zhang, Cyanostilbenbased derivatives: mechanical stimuli-responsive luminophors with aggregationinduced emission enhancement, Photochem. Photobiol. Sci. 11 (2012) 1414–1421. [3] J. Deng, J. Tang, Y. Xu, L. Liu, Y. Wang, Z. Xie, Y. Ma, Cyano-substituted oligo(pphenylene vinylene) single-crystal with balanced hole and electron injection and transport for ambipolar field-effect transistors, Phys. Chem. Chem. Phys. 17 (2015) 3421–3425. [4] M. Shimizu, Y. Takeda, M. Higashi, T. Hiyama, 1,4-Bis(alkenyl)-2,5-dipiperidinobenzenes: minimal fluorophores exhibiting highly efficient emission in the solid state, Angew. Chem. Int. Ed. 48 (2009) 3653–3656. [5] T. Sumi, T. Kaburagi, M. Morimoto, K. Une, H. Sotome, S. Ito, H. Miyasaka, M. Irie, Fluorescent photochromic diarylethene that turns on with visible light, Org. Lett. 17 (2015) 4802–4805. [6] M.A. van der Horst, K.J. Hellingwerf, Photoreceptor proteins, “Star actors of modern times”: a review of the functional dynamics in the structure of representative members of six different photoreceptor families, Acc. Chem. Res. 37 (2004) 13–20. [7] C. Dugave, L. Demange, Cis-trans isomerization of organic molecules and biomolecules: implications and applications, Chem. Rev. 103 (2003) 2475–2532. [8] K. Nakazawa, M. Hishida, S. Nagatomo, Y. Yamamura, K. Saito, Photoinduced bilayer-to-Nonbilayer phase transition of POPE by photoisomerization of added stilbene molecules, Langmuir 32 (2016) 7647–7653. [9] B. Carlotti, A. Cesaretti, P.L. Gentili, A. Marrocchi, F. Elisei, A. Spalletti, A two excited state model to explain the peculiar photobehaviour of a flexible quadrupolar D–π–D anthracene derivative, Phys. Chem. Chem. Phys. 18 (2016) 23389–23399. [10] S. Wang, S. Schatz, M.C. Stuhldreier, H. Bohnke, J. Wiese, C. Schroder, T. Raeker, B. Hartke, J.K. Keppler, K. Schwarz, F. Renth, F. Temps, Ultrafast dynamics of UVexcited trans- and cis-ferulic acid in aqueous solutions, Phys. Chem. Chem. Phys. 19 (2017) 30683–30694. [11] J. Saltiel, Perdeuteriostilbene. The triplet and singlet paths for stilbene photoisomerization, J. Am. Chem. Soc. 90 (1968) 6394–6400. [12] J. Saltiel, S. Gupta, Photochemistry of the Stilbenes in Methanol. Trapping the Common Phantom Singlet State, J. Phys. Chem. A 122 (2018) 6089–6099. [13] J. Saltiel, M.A. Bremer, S. Laohhasurayotin, T.S.R. Krishna, Photoisomerization of cis,cis- and cis,trans-1,4-Di-o-tolyl-1,3-butadiene in glassy media at 77 K: one-bondtwist and bicycle-pedal mechanisms, Angew. Chem. Int. Ed. 47 (2008) 1237–1240. [14] C. Redwood, V.K.R. Kumar, S. Hutchinson, F.B. Mallory, C.W. Mallory, R.J. Clark, O. Dmitrenko, J. Saltiel, Photoisomerization of cis-1,2-di(1-Methyl-2-naphthyl) ethene at 77 K in glassy media, Photochem. Photobiol. 91 (2015) 607–615. [15] R.S.H. Liu, D. Mead, A.E. Asato, Photochemistry of polyenes. 23. Application of the H. T. -n mechanism of photoisomerization to the photocycles of bacteriorhodopsin.

Fig. 8. Propose photodissociation pathway of BBM in H2O solvent.

From above experimental results, it may be stated that the C]C bond is highly polarizable in presence of cyanide groups and shows partial double bond character (δ+C … Cδ−). This partial double bond (δ+C … Cδ−) may interact with H2O and formed Product A (Fig. 8).

4. Conclusion In conclusion, photodissociation of selective C]C bond in BBM is investigated in solution and found to be highly solvent specific and temperature sensitive in nature. The presence of dicyano group (acceptor) in BBM makes the C]C bond highly polarizable, photoactive and susceptible for photodissociation process even at room temperature. In presence of dicyano group, the formation of dipolar state is enhanced and stabilized by polar protic solvents and followed dipolar mechanism. The product A (2-(4-formylbenzylidene)malononitrile) is formed exclusively and identified from photochemical reaction under UV irradiation in H2O and H2O –ACN solvents. Kinetic study of this photodissociation process was also carried out and found to follow pseudo 1st order mechanism with a rate constant (k) 10−3-10-4.s-1.

Notes The authors declare no competing financial interest

Acknowledgment SKP acknowledges to Prof. Puspendu K. Das for his suggestion and SERB for N-PDF post-doc fellowship (Scheme: N-PDF/2016/000041) at Department of Inorganic and Physical Chemistry (IPC), Indian Institute of Science (IISc), Bangalore-560012, India. 22

Journal of Photochemistry & Photobiology A: Chemistry 375 (2019) 18–23

S. Kumar Panja, et al.

A model study, J. Am. Chem. Soc 107 (1985) 6609–6614. [16] R.S.H. Liu, G.S. Hammond, The case of medium-dependent dual mechanisms for photoisomerization: one-bond-flip and Hula-Twist, Proc. Natl. Acad. Sci. U.S.A. 97 (2000) 11153–11158. [17] K. Iwata, R. Ozawa, H. Hamaguchi, Analysis of the solvent- and temperature-dependent raman spectral changes of S1 trans-stilbene and the mechanism of the trans to cis isomerization: dynamic polarization model of vibrational dephasing and the CC double-bond rotation, J. Phys. Chem. A 106 (2002) 3614–3620. [18] T. Majima, M. Fukui, A. Ishida, S. Takamuku, Stilbene radical anions in the excited doublet state, J. Phys. Chem. 100 (1996) 8913–8919. [19] M. Quick, A.L. Dobryakov, I.N. Ioffe, A.A. Granovsky, S.A. Kovalenko, N.P. Ernsting, Perpendicular state of an electronically excited stilbene: observation by femtosecond-stimulated raman spectroscopy, J. Phys. Chem. Lett. 7 (2016) 4047–4052. [20] E. Garcia-Exposito, R. Gonzales-Moreno, M. Martin-Vila, E. Muray, J. Rife, J.L. Bourdelande, V. Branchadell, R.M. Ortuno, On the Z–E photoisomerization of chiral 2-pentenoate esters: stationary irradiations, laser-flash photolysis studies, and theoretical calculations, J. Org. Chem. 65 (2000) 6958–6965. [21] F.D. Lewis, D.K. Howard, S.W. Barancyk, J.D. Oxman, Lewis acid catalysis of photochemical reactions. 5. Selective isomerization of conjugated butenoic and dienoic esters, J. Am. Chem. Soc. 108 (1986) 3016–3023. [22] W. Cai, H. Fan, D. Ding, Y. Zhang, W. Wang, Synthesis of Z-alkenes via visible light promoted photocatalytic E - Z isomerization under metal-free conditions, Chem. Commun. (Camb.) 53 (2017) 12918–12921. [23] J. Hayakawa, A. Momotake, R. Nagahatay, T. Arai, Controlling the hydrophobic properties of water-soluble stilbene dendrimers, Chem. Lett. 32 (2003) 1008–1009. [24] T. Takizawa, T. Arai, Remote controlled intramolecular exciplex formation and enhanced photoisomerization in stilbene-cored poly(benzyl ether) dendrimers with alkoxycarbonyl surface functional groups, Chem. Lett. 40 (2011) 1124–1126. [25] J.L. Magee, W. Shand, H. Eyring, Non-adiabatic reactions. Rotation about the double bond, J. Am. Chem. Soc. 63 (1941) 677–688. [26] J.L. Buechele, E. Weitz, F.D. Lewis, Laser-induced infrared multiphoton isomerization reactions of 2,4- and 1,3-hexadienes, J. Phys. Chem. 88 (1984) 868–876. [27] M. Quick, A.L. Dobryakov, I.N. Ioffe, F. Berndt, R. Mahrwald, N.P. Ernsting, S.A. Kovalenko, Rotamer-specific photoisomerization of difluorostilbenes from transient absorption and transient raman spectroscopy, J. Phys. Chem. B 122 (2018) 1049–1059. [28] I.N. Ioffe, M. Quick, M.T. Quick, A.L. Dobryakov, C. Richter, A.A. Granovsky, F. Berndt, R. Mahrwald, N.P. Ernsting, S.A. Kovalenko, Tuning stilbene photochemistry by fluorination: state reordering leads to sudden polarization near the Franck–condon region, J. Am. Chem. Soc. 139 (2017) 15265–15274. [29] S.M. Bilal, S. Kayal, K.S. Sanju, Y. Adithya Lakshmanna, Femtosecond time-resolved

[30] [31] [32]

[33] [34] [35] [36] [37]

[38] [39]

[40]

[41]

[42]

[43] [44]

[45]

23

raman spectroscopy reveals structural evidence for meta effect in Stilbenols, J. Phys. Chem. A 122 (2018) 4601–4608. F. Wang, S. Chatterjee, Dominant Carbons in trans- and cis-Resveratrol Isomerization, J. Phys. Chem. B 121 (2017) 4745–4755. N. Minezawa, M.S. Gordon, Photoisomerization of stilbene: a spin-flip density functional theory approach, J. Phys. Chem. A 115 (2011) 7901–7911. Y. Lei, L. Yu, B. Zhou, C. Zhu, Z. Wen, S.H. Lin, Landscapes of four-enantiomer conical intersections for photoisomerization of stilbene: CASSCF calculation, J. Phys. Chem. A 118 (2014) 9021–9031. Z. Lan, Y. Lu, O. Weingart, W. Thiel, Nonadiabatic decay dynamics of a benzylidene malononitrile, J. Phys. Chem. A 116 (2012) 1510–1518. I.N. Ioffe, A.A. Granovsky, Photoisomerization of stilbene: the detailed XMCQDPT2 treatment, J. Chem. Theory Comput. 9 (2013) 4973–4990. J. Breffke, B.W. Williams, M. Maroncelli, The photophysics of three naphthylmethylene malononitriles, J. Phys. Chem. B 119 (2015) 9254–9267. K.B. Jørgensen, Photochemical oxidative cyclisation of stilbenes and stilbenoids-the mallory-reaction, Molecules 15 (2010) 4334–4358. S. Wiedbrauk, B. Maerz, E. Samoylova, A. Reiner, F. Trommer, P. Mayer, W. Zinth, H. Dube, Twisted hemithioindigo photoswitches: solvent polarity determines the type of light-induced rotations, J. Am. Chem. Soc. 138 (2016) 12219–12227. A. Gerwien, P. Mayer, H. Dube, A photon-only molecular motor with reverse temperature dependent efficiency, J. Am. Chem. Soc. 140 (2018) 16442–16445. S. Wiedbrauk, B. Maerz, E. Samoylova, P. Mayer, W. Zinth, H. Dube, Ingredients to TICT formation in donor substituted hemithioindigo, J. Phys. Chem. Lett. 8 (2017) 1585–1592. S. Yang, K. Han, Effects of solvent dielectric constant and viscosity on two rotational relaxation paths of excited 9-(Dicyanovinyl) julolidine, J. Phys. Chem. A 120 (2016) 4961–4965. S. Arzhantsev, K.A. Zachariasse, M. Maroncelli, Photophysics of trans-4(Dimethylamino)-4-cyanostilbene and its use as a solvation probe, J. Phys. Chem. A 110 (2006) 3454–3470. S.K. Panja, N. Dwivedi, S. Saha, First report of the application of simple molecular complexes as organo-catalysts for Knoevenagel condensation, RSC Adv. 5 (2015) 65526–65531. M.J. Frisch, et al., Gaussian 09, revision E.01, Gaussian, Inc., Wallingford, CT, 2009. J.N. Zhang, H. Kang, N. Li, S.M. Zhou, H.M. Sun, S.W. Yin, N. Zhao, B.Z. Tang, Organic solid fluorophores regulated by subtle structure modification: color tunable and aggregation-induced emission, Chem. Sci. 8 (2017) 577–582. J. Saltiel, S. Gupta, Photochemistry of the stilbenes in methanol. Trapping the common phantom singlet state, J. Phys. Chem. A 122 (2018) 6089–6099.