Photophysics, photochemistry and thermal stability of diarylethene-containing benzothiazolium species

Accepted Manuscript Title: Photophysics, Photochemistry and Thermal Stability of Diarylethene-Containing Benzothiazolium Species Author: Morad M. El-Hendawy Tarek A. Fayed Mohamed K. Awad Niall J. English Safaa Eldin H. Etaiw Ahmed B. Zaki PII: DOI: Reference:

S1010-6030(15)00004-0 http://dx.doi.org/doi:10.1016/j.jphotochem.2014.12.015 JPC 9818

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

14-8-2014 3-11-2014 31-12-2014

Please cite this article as: Morad M.El-Hendawy, Tarek A.Fayed, Mohamed K.Awad, Niall J.English, Safaa Eldin H.Etaiw, Ahmed B.Zaki, Photophysics, Photochemistry and Thermal Stability of Diarylethene-Containing Benzothiazolium Species, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2014.12.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Photophysics, Photochemistry and Thermal Stability of DiaryletheneContaining BenzothiazoliumSpecies Morad M. El-Hendawy1,2,3,Tarek A. Fayed 1, Mohamed K. Awad*1, Niall J. English*2, SafaaEldin H. Etaiw,1 Ahmed B. Zaki1 1

The SFI Strategic Research Cluster in Solar Energy Conversion and the Centre for Synthesis and Chemical Biology, School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland 3

Chemistry Department, Faculty of Science, Kafrelsheikh University, Kafrelsheikh, Egypt

Corresponding authors:

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Prof. Mohamed K. Awad, E-mail: [email protected] Dr. Niall J. English, Email: [email protected]

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Highlights

Ring-size and solvent effects on photophysics, photo- and thermal-isomerisation of

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

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2

Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt

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diaylethene-containing benzothiazolium moieties

Effect of N-methylation on photo-properties neutral dyes.



Investigation of mechanism of cis-trans thermal isomerisation reaction.

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A



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photophysics,

photochemistry and

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The

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Abstract

benzothiazolium)ethene

thermal

iodides (aryl:phenyl,

stability of

1-naphthyl,

four

1-aryl-2-(N-methyl-2-

9-phenanthryl and9-anthryl)

were

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studied.Although the absorption spectra are found to behypsochromic-shifted, fluorescence spectra are bathochromic-shifted. The dipole moment in the relaxed excited state was found to be larger than that

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in the ground state.To investigate effects of N-methylation and the aryl-ring size, a detailed comparison was made between those in the present work (of charged compounds) and previous studies of their neutral analogues, with computed electron affinity and ionisation potentials serving to rationalise the experimentally observed bathchromic shifts in absorption and emission spectra. The kinetics of thermal isomerisation depend strongly on the nature of the aryl moiety and solvent; the larger the aryl ring, the slower the rate of isomerisation. The fastest isomerisation process was found 1

to take place in MeOH. The anthryl derivative did not isomerize either by light- or heat-exposure, due to high energy barriers of rotation around the ethenic bond. Based on the significant blue-shift of the Z-isomer absorption maximum relativeto that of the E-isomer, and the high percentage of Z-isomers in the photostationary state, these compounds may serve as potential promising candidates for optical data-storage applications. Keywords: Benzothiazolium; Isomerisation

1- Introduction Photo-induced Z-E isomerisation about double bonds has long been a subject of intense research,

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motivated by either pure-chemistry or more applied, industrial, perspectives. For instance, photoisomerisation about a carbon-carbon double bond in the rhodopsin chromophore is the initial

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trigger for light recognition in human eyes [1]. The photoisomerisation of urocanic acid acts as a ‘natural sunscreen’, protecting DNA from photo-damage [2]. Owing to photoisomerisation,

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diarylethenesare a popular example of photo-isomerizable molecules; they have found widespread

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application as sensitisers and other additives in the photographic industry [3-7],ion recognition [8],as optical-recording media in laser disks [9], as flexible dyes [10], laser dyes [11], and, indeed, as optical

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sensitisers in various other fields [12–15]. It has been reported that this class of dyes act as imaging

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agents for Alzheimer disease [16], as effective biological markers [17], and as promising candidate

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dye sensitisers in solar cells [18].Azastilbenes and their quaternary salt, stilbazolium, constitute good representative examplesofthe diarylethene family (cf. Fig. 1). Motivated by the desire to make further

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progress into potential modern technological applications, the synthesis of new photo-isomerisable

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molecules and characterisation of their photophysics, photochemistry and thermal stability in different environments are sine qua non.

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The Z↔E photoisomerisation of neutral azastilbene derivatives has been the subject of several

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studies [19-21]. The introduction of a positive charge (upon quaternisation) into them give rise to a new phenomenon, driven and controlled by heat, namely thermal Z→E isomerisation [20, 22-29]; this is the ‘dark’ counterpart of the E→Z photoisomerisation reaction (cf. Fig. 1). Quaternary salts of 1-

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alkyl-4-(-4’-R-styryl) quinolinium iodide (e.g., R = NO2, H or CH3) exhibit thermal Z→E isomerisation at room temperature [24-26], whilst neutral analogues do not [19]. For stilbazolium salts, the rate constant has been found to depend significantly on the polarity of the solvent and to a smaller extent on the substituent in the 4-position of the styrene ring, as well as on the nature of the anion [26].

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About two decades ago, we studied, in various solvents, the N-methylation of 1-(2-naphthyl)-2-(2benzothiazolyl)ethene and the kinetics of thermal Z→E isomerisation. Recently, we observed Nmethylation of 1-(9-phenanthryl)-2-(2-benzothiazolyl)ethene (9-PhBE) [21, 22], where the Nmethylated form(9-PhBEI, Fig. 2) back-isomerises thermally [26], whilst, intriguingly, the neutral analogue does not [22]. We also established the effect of N-methylation on the photophysics on 1-(9anthryl)-2-(2-benzothiazolyl)ethene (9-ABE) and its quaternary salt (9-ABEI, Fig. 2)[30];dual emission was observed for both compounds, which was found to originate from the involvement of locally-excited (LE) states and either twisted intramolecular charge transfer (TICT) or intramolecular exciplex formation in the excited state, respectively.Previously [22], we studied the effect of aryl ring

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size on the photophysics and photochemistry of styrylbenzothiazole (PBE) via systematic replacement of the phenyl ring of 1-phenyl-2-(2-benzothiazolyl)ethene with naphthyl (1-NBE) and phenanthryl (9-

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PhBE) rings. Increasing the size of aromatic rings served to increase the extent of ICT between the benzothiazolyl and aryl fragments. This was confirmed by red shifts of their various spectra, along

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with increases in their dipole moment and Stokes shift. The percentage of the Z-isomers due to photoisomerisation was found to be size-dependent for the given solvent and irradiation

investigators,

the

solvent

and

substituent

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other

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wavelength;this was found to be in the order PBE > 9-PhBE > 1-NBE [22]. In interesting work by effects

on

the

photophysics

ofstyryl-

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ethylbenzothiazolium derivatives, possessing different electron-withdrawing or electron-donating

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groups,have also been studied [31, 32]. The formation of aggregated structures was also observed at higher concentrations of the benzothiazolium bromides [31].

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Bearing in mind this photochemical activity and previous studies of similar molecular systems as

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discussed above, we seek to study in the present work the photophysics and photo-/thermo-chemical aspects of back E↔Z isomerisation, motivated by the tantalising possibility of realising temperature-

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controlled photochromic end-applications. In particular, important goals are the investigation of the effect of replacing the phenyl ring by larger polycyclic aromatic rings, as well as the nature of solvent

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on spectral characteristics and rate of photo- and thermal- isomerisation. We also use Density Functional Theory (DFT) to study the mechanism of thermal isomerisation. Finally, and importantly,

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we report a concise comparison between the photophysics, photo-and thermal reactivities of the tested (charged) compounds and their neutral analogues.

2- Experimental and computational details The molecular structure and abbreviations of the compounds under scrutiny are presented in Fig. 2. The 1-aryl-2-(N-methyl-2-benzothiazolium)ethene iodides were obtained simply by heating equimolar 3

quantities of the neutral analogues [22]with methyl iodide in ethanol for one hour under reflux. The products were filtered and purified by recrystallisation twice from dry ethanol. Following this, the structure was confirmed by elemental analysis, IR and UV–Vis spectral measurements. The melting points of PBEI, 1-NBEI,9-PhBEI, and 9-ABEI are 208-210, 222-226, and 181–183, and 87-89 °C, respectively. All synthesised compounds were found to be E-isomers. Spectroscopic-grade MeOH, EtOH, CH3CN, CH2Cl2, CHCl3 and Et-Glycol (Aldrich or Merck) were used, while water was double-distilled. All solvents were non-fluorescent within the range of fluorescence measurements. Steady-state electronic absorption spectra were recorded on a Shimadzu UV-3101 PC spectrophotometer using 1.0 cm-matched silica cells, while steady-state fluorescence

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spectra were recorded using a Perkin-Elmer LS 50B scanning spectrofluorometer.Typical concentrations of 2 ×10−5 M were used for the measurement of the investigated compounds.Although

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the fluorescence quantum yields (Φf) were determined at room temperature relative to quinine bisulphate in 0.1 N H2SO4 (Φf = 0.515)[33], with standard-yield approach of Meech and Phillips [34],

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because of the difficulty of comparing the fluorescence quantum yield with two compounds (9-PhBEI and 9-ABEI) exhibiting dual fluorescence [21 30], a straightforward Φf comparison is very

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problematic; therefore, Φfvalues are not reported here.The experimentaluncertainty was ±10%. The

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photoisomerisation quantum yields as well as the composition of the photostationary states (PSS) were

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calculated as described previously [20], with experimental error of ±10%. The light intensity was

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determined using ferrioxalateactinometry [35]. For the inferred activation-energy parameters from experimental rate data, the experimental error was roughly ±10%, determined from a number of

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independent measurements for selected dyes.

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The thermal ZE isomerisation of these dyes were tracked spectrophotometrically in solvents in which the Z-forms are thermally unstable. A solution of each dye was prepared freshly in the dark,

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and was irradiated subsequently at 365, 405 and 436 nm light (Mercury Lamp with an intensity of 4.3

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x 10-7 E s-1 cm-2, 30 W) for a sufficient period until the PSS was reached. At the PSS, a maximum population of the Z-isomers was generated. After switching off the irradiation, the sample was moved quickly to a temperature-controlled spectrophotometer cell holder where the Z-isomer

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started to revert back to the E-form in the dark at the desired temperature.This was confirmed by observing that the spectrum of the PSS reverted to the original (recorded for the fresh sample). This procedure was repeated at four different temperatures,i.e., 15, 20, 25 and 30◦C. All DFT calculations were performed using the Gaussian 09 suite [36]. Structural optimisations were performed using B3LYP/6-31+G(d,p) [37-39], andthe activation parameters of the thermal isomerisation reactions were calculated. To locate the transition states efficiently, the PM6 level of 4

semi-empirical treatment [40], followed by B3LYP,were used. Vibrational-frequency calculations were carried out in all cases to confirm the stationary points as either minima or first-order saddle points, and to obtain the activation parameters. The calculations have been performed under bulk solvent effects of methanol using the universal solvation model, SMD [41]. Further details on locating the transition states (TSs) are provided in Supporting Information [42], and Cartesian coordinates of the TSs are provided therein (cf. Table S1).

3- Results and discussion

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3.1 Spectroscopic characteristics The absorption and fluorescence spectra of 1-aryl-2-(N-methyl-2-benzothiazolium)ethene iodides

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(ArBEIs) were measured in H2O, MeOH, EtOH, MeCN, CH2Cl2, and CHCl3. The relevant

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spectroscopic data of PBEI and 1-NBEI are gathered in Table 1, while those of 9-PhBEI and 9-ABEI have already been published in refs. 21 and 30.The absorption spectra of ArBEIs in three different

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solvents, namely H2O, MeOH and CHCl3, are presented in Fig. 3. The spectra show a broad band,

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with maxima between 371 and 528 nm, usually in and around ~420 nm, depending on the solvent and the size of the aryl moiety. One absorption maximum for PBEI was observed at around 225 nm while

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in the case of 1-NBEI, two absorption bands were observed around 280 and 320 nm. More detailed

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information about the spectroscopic characteristics of 9-PhBEI and 9-ABEI have been reported previously [21, 30]. For the present spectra, although it could perhaps be argued (albeit somewhat

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implausibly) that this corresponds to a charge-transfer (CT) transition from the aryl to the

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benzothiazolium moiety, as could those of other absorption bands at shorter wavelengths for local transitions in the aryl or benzothiazolium fragment, these transitions are much more likely to originate

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from electron transfer from the iodide ion to the benzothiazolium ion in an intimate ion pair. This is because of the key rôle of the iodide counterion in promoting aggregation.In chloroform, the solutes

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are expected to exist exclusively as ion pairs; indeed, in their seminal study of charge transfer in pyridinium iodides, Kosoweret al selected chloroform as the solvent for this reason, and identified the

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two lowest-energy electronic transitions as CT bands for transfer from I- to N-MePy+, withthe exact correspondence of separation between the CT bands to two energy states of the iodine atom supporting that assignment [43]. In the case of the present spectra (cf. Fig. 3), their similarity in all solvents suggests a common origin for the spectroscopic transition – likely to be electron transfer from the iodide to the pyridinium ion.The present ArBEIs exhibit negative solvatochromic behaviour: specifically, on going fromCHCl3 to H2O, the maxima of the CT-bands of PBEI, 1-NBEI, 9-PhBEI 5

and 9-ABEI blue-shift by 14, 34, 42 and 55 nm, respectively. Such behaviour reflects a decrease in the dipole moment upon photo-excitation to the Frank-Condon state. Dyes with similar structures have been reported to exhibit this behaviour [20, 32, 44-46].On the contrary, the emission maxima of ArBEIswerered-shifted with increasing the solvent polarity (cf.Fig. 4). The spectral shift ranges from 7 to 33 nm, on going from CHCl3 to water, depending on the size of the aryl moiety. This isdue to the increase in dipole moment of the relaxed excited singlet state,relative to that of the ground state. From inspection of Figs. 3 and 4, it is evident that the emission spectra of PBEI, 1-NBEI and 9-ABEI are ‘mirror images’ of their corresponding absorption spectra;this similarity arises from electronic excitation not altering greatly the nuclear geometry. Hence, the spacing of the vibrational energy

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levels of the excited state is similar to that of the ground state. However, the shape of the fluorescence spectrum of 9-PhBEI deviates from those of the absorption spectrum, indicating a different geometric

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arrangement of nuclei in the excited and ground states.Dual emission from 9-PhBEI has been observed and presented in ref. [21].Also, dual emission was observed for ABEI as shown in Fig. 4. One

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fluorescence band around 400 nm (upon excitation ~365 nm) originates from the involvement of locally-excited (LE) states, whilst the other, separated relative red-shifted band around 500 nm (upon

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excitation,~430 nm) originates from the intramolecular exciplex formation in the excited state; for

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further details, see ref. [30].The absorption and the locally excited band of the fluorescence spectra of

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the ABEI dye are structured with a vibronic progression characteristic of the anthrylchromophore

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[30].The localisation of excitation energy at the anthrylchromophorearises as a result of the non-planar geometry.

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The difference in energy between the absorbed and emitted maxima is known as the Stokes shift

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(e.g., expressed in nanometres), which serves as a measure to the change in the magnitude of a chromophore’s dipole moment following excitation; these are listed in Table 1 and Table S2.

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Evidently, the Stokes shift increases with solvent polarity. The remarkably large magnitudes(ranging from 63 up to 132 nm) of the Stokes shifts are attributed to the charge-transfer nature of the long-

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wavelength transition and indicates that the relaxed excited state dipole moment is larger than that of the ground state.The Stokes shift is ring-size dependent,i.e.,in general, the largerthe hydrocarbon

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moiety’s size in a dye, the larger the Stokes shift. The fluorescence excitation spectra of the respective emission bands of the investigated compounds

were measured in various solvents.The maxima of the excitation spectra are not particularly dependent on the nature of the solvent (cf. Table 1).The excitation spectra of PBEI, 1-NBEI and 9-ABEI [30] do not display any changes in their shapes, except for changes of intensity at different emission wavelengths. They resemble each other closely in the lower energy absorption band. This reflects that 6

there is only one species in their ground states. However, 9-PhBEI exhibited two distinct excitation spectra [21] at f = 520 and 600 nm, due to the formation of ground-state J-aggregate from monomer molecules.Also, the oscillator strength is not affected greatly by the nature of solvent, but it was observed to decrease as the size of aryl subunit increases.

3.2 Photochemical activity Except

for

9-ABEI,

the

photochemical

E-Zisomerisations

of

1-aryl-2-(N-methyl-2-

benzothiazolium)ethene iodides were carried out at room temperature only in MeOH and H2O due to the fast backward thermal ZEisomerisation of the irradiated solution in the other solvents such as

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EtOH and MeCN. This was confirmed by the finding that the PSS absorption spectra reverted completely back to the original spectra at rates depending on the solvent and temperature, as reported

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previously[20, 21], indicating that the thermal Z-E reaction competes with photoisomerisation.

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The photochemical parameters of the investigated compounds are gathered in Table 2. The results depend on ring size, solvent characteristics and excitation wavelength.As a representative example, the

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photochemical parameters of 1-NBEI in MeOH at λirr = 405 nmare described in some detail. Changes

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characteristic to EZ isomerism were observed on irradiating a methanol solution of this compound

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as illustrated in Fig. 5a. These changes were a reduction in the intensity of the characteristic E-band

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accompanied by a small hypsochromic shift (from 417 to 404 nm), and an increase in the intensity of the absorption in the cissoid region of the spectrum [47]. Moreover, the spectrum of the final isomeric

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mixture at PSS indicates a very high degree of conversion of E-isomer, Fig. 5a. Similar behavior was

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observed for the other tested dyes (cf. Fig. S1). The difficulty in monitoring the photoisomerisation of 9-PhBEI in water is attributed to extensive

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aggregation even with very dilute concentrations, as reported previously [21]. Three pieces of spectral evidence suggest the uniformity of the photo-isomeric reaction: (i) firstly,the intensity of the

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fluorescence spectrum of 1-NBEI in MeOH diminishes at the PSS, upon irradiation at 436 nm, without appearance of new bands, as illustrated in Fig. 5b -similar behavior was observed in the other dyes,so

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the presence of the non-fluorescent Z-form is most probablyresponsible for the reduction of the emission intensity; (ii) secondly, the clean interconversion of E-1-NBEI in MeOH during irradiation at 405 nm to the Z-form, without any side reaction, is indicated by the presence of two isosbestic points at 333 and 275 nm in the UV-Vis spectra, as shown in Fig. 5a; (iii) thirdly, agraphical plot between the change of absorbance of long-wavelength maximum and that of the maximum in the cissoid region (A/A diagram) during the period of irradiation exhibitsa linear relationship for 1-NBEI (cf. Fig. 5c). In

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any event, the linear relationship in Fig. 5c would be expected: the isosbestic points in Fig.5a establish that this is a two-component closed system. Fig. 5ddepicts the time-dependence of the absorption of 1-NBEI in water at their maxima during the irradiation period. For example, upon irradiation at 405 nm, the absorbance at the maximum for NBEI decreases gradually as the irradiation time proceeds until reaching a constant value confirming the achievement of PSS.Fig. 6aillustrates the calculated absorption spectra according to Fischer’s method [48] of the Z-isomer of 1-NBEI in MeOH, as well as those of the corresponding E-isomer. As can be seen, the absorption maximum of the photoproduct is displaced towards shorter wavelengths than that of the E-isomer.PBEI and 9-PhBEI showed similar behavior(cf. Fig. S2 [42]).

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The percentage of Z-isomer of 1-NBEI at its PSS is increased from 85 % in MeOH to 99.3 % in H2O upon irradiation at 405 nm. Also, it is found that the larger the ring size of the aryl moiety, the higher

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the percentage of Z-isomer. For example, the replacement of phenyl ring by naphthyl or phenanthryl

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ring, the % Z increases from 79.1 % to 85 and 89 % in MeOH, respectively, upon irradiation at 405 nm. In addition, the effect of irradiation wavelength on % Z of ArBEIs is clear. In MeOH, the shorter

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the irradiation wavelength is the lower the % Z at the PSS. Fig. 6b shows an example for the growth of

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% Z with increasing irradiation time until reaching a steady value at the PSS. The high percentage of composition of Z-isomer at the PSS, coupled with the blue shift of the Z-isomer absorption maximum

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vis-à-vis the E-isomer, suggests that these compounds can serve as candidates for data-storage

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applications.

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Table 2 illustrates that the photoisomerisation quantum yields, EZ and ZE, are high for all dyes

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vis-à-vis their neutral analogues [22]. EZ and ZE are comparable in some cases, particularly for PBEI;at first glance, this may suggest the importance of a singlet mechanism [49], although triplet-

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state photoisomerisation remains quite possible [50]. In most cases, the EZ is higher than ZE; this may be attributed to solvent stabilisation of the Z-isomer, which discourages the formation of E-

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isomer from a ‘phantom’ state[51-53]. An exception was found for PBEI in water, where ZE is higher than EZ, particularly upon irradiation at 405 nm. This behavior can be explained by solvent

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stabilisation, where the E-isomer encounters substantial stabilisation through solvation, which reduces its tendency to isomerize into the Z-form. It is worth mentioning that the highest quantum yield (0.687) was observed in the case of E→Zisomerisation of 1-NBEI at 436 nm in water. On the other hand, upon irradiation at 405 nm, the partial rate constants of the forwardisomerisation, kEZ, follow the trend 1-NBEI > 9-PhBEI > PBEI in methanol,whilst those of the backward kZE reactionsadopt the 1-NBEI > PBEI > 9-PhBEI. This reflects the fact that 1-NBEI exhibits the fastest rotation of side moieties around the ethenic bond for E-Z interconversion, and, consequently, is the most kinetically 8

favourable.From a geometric viewpoint, the size of the benzothiazolium moiety is closerin size tothe naphthyl one than those of phenyl or phenanthryl. Accordingly, this geometric similarity serves to facilitate mechanical rotation.The photo-stationary state’s composition depends strongly on the irradiation wavelength, as seen previously in photo-reversible systems [22,49]; in other words, the shorter the irradiation wavelength, the lower the % Z in the photo-stationary state. Generally, the other parameters display a different dependence where shorter-wavelength irradiation induces rapid E↔Zinterconversion with a high quantum yield, with some exceptions in the case of 1-NBEI.

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3.3 Thermal Stability 3.3.1 Solvent effects on thermal ZE isomerisation

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Except for 9-ABEI, the investigated compounds exhibit thermal ZE isomerisation in MeCN, MeOH and EtOH. However, in other solvents, such as water, these dyes are thermally stable, even at

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higher temperatures over very long time (hour-scales). Accordingly, this study focuses on three

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solvents in which thermal ZE isomerisation takes place in a reasonable timeframe. Fig. 7a illustrates the change in absorbance during thermal ZE isomerisation of PBEI in EtOH at 25 C was followed

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at 374 nm. The appearance of an isosbestic point at 322 nm demonstrates the inter-conversion between

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Z- and E-isomers. Similar behaviour was found for other compounds. More details about the

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behaviour of 9-PhBEI dye have already published in ref. 21; details of the underlying methods for

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calculation of parameters are provided in the Supplementary Information [42]. The influence of the solvent polarity on the rate constant and the activation parameters of the thermal

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ZE isomerisation for the 1-NBEI dye shall be described in detail as a representative example. As can be seen from Table 3, the rate constant is almost doubled upon raising the temperature from 15 to

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30°C in MeOH (k = 0.88 and 1.54 min-1, respectively). In MeCN, the rate constant of thermal

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isomerisation increases by about three orders of magnitude, but in EtOH, it increases by about four. In general, the isomerisation rate constant for the studied compounds increases in the order

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MeOH>MeCN>EtOH, except in the case of PBEI (MeOHEtOHMeCN), (cf. Table S3[42]). As expected, a higher rate constant is associated with lower activation energy (cf. Table 3). Also, for 1NBEI, the activation energy increases from 26.9 to 69.1 kJ/mol,on going from MeOH to EtOH, respectively. This explains why the rate of thermal isomerisation is faster in MeOH than that in EtOH, and also shows the effect of the dielectric constant of the solvent in lowering the activation barrier in this type of reaction. However, the E# value in MeOH is much smaller than in MeCN, demonstrating the probability of forming hydrogen bond under the effect of temperaturewhich may facilitate thermal 9

isomerisation. The variation of the E# value takes place when the reactant is in a solvent environment which destabilises or stabilises the reactant ground state vis-à-vis that of the transition state in the respective solvents. Similar results were observed in the case of the other dyes, as illustrated in Table S3 [42]. It has been suggested that reaction-generated electric charges exhibit negative entropies of activation [48]. The observed negative entropies of activation in this study for thermal isomerisation (between 94.1 and -167 J/mol.K) indicate that the transition states have a greater degree of charge separation vis-à-vis the ground state. Because of their highly polar character, the transition state requires a greater degree of ordering of the solvent molecules than the ground state [54]. The negative entropy of

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activation in the case of 1-NBEI dye increases upon going from MeOH to MeCN. This suggests that the reaction is more highly ordered in polar protic solvents in comparison to their polar aprotic

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counterparts. On the other hand, the S value increases from MeOH to EtOH, reflecting the role of

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the dielectric constant of the solvent in controlling the ordering of the system in both the ground and transition states. Similar results are observed in the case of the remaining dyes, as illustrated in Table

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S3 [42].

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3.3.2 Effects of size of aryl moiety on the thermal ZE isomerisation

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The effect of the replacement of the phenyl moiety of the styrylbenzothiazolium dye by larger

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aromatic polycyclic ones on the rate of thermal ZE isomerisation was elucidated from the calculated data in MeOH (cf. Table 4). The kinetic data reveals that the increase in the size of aryl moiety retards

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the rate of ZE isomerisation. For example, the k values of the isomerisation in MeOH are 1.53,1.26

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and 0.90 min-1 for PBEI, 1-NBEI and 9-PhBEI, at 22.5C (the average experimental temperature), respectively.This can be interpreted on the basis of the rotation-energy barriers in these dyes,

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computed from the Arhenius equation (Supplementary Information, eqn. 2). Ongoing from PBEI to 9PhBEI, the rotational barrier increases from 25.8 to 47.1 kJ/mol. In conclusion, PBEI is the fastest

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interconverting molecule,as seen from the shortest half-life time of the isomerisation reaction,

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t1/2(determined as in Supplementary Information, eqn. 4), and the lowest rotation-energy barrier. The data in Table 4 reveal also that the entropies of activation become less negative on going from

the phenyl to phenanthryl derivative (from -167.0 to -94.1 J/mol. K), revealing that the transition stateof PBEI compound is more organised than that of 1-NBEI and 9-PhBEI derivatives. This may be attributed to reduction of the formal charge on the benzothiazolium moiety in the transition state as a result of increasing the donation ability of the aryl group as its size increases and consequently decreases the charge separation over the whole molecule. Actually, DFT supports this expectation 10

where the group charge on benzothiazolium moiety of PBEI, 1-NBEI and 9-PhBEI in their transition states are 0.117, -0.010 and -0.095 e, respectively. Otherwise, the dipole moment is a measure of charge separation in a molecule. As shown in Table 5, the dipole moment of transition state decreases as the aryl size increases, also supporting the above expectation. The largest activation energy for the slowest reaction (i.e., that of 9-PBEI, cf. Table 4) indicates that the reaction is enthalpy-controlled, within the reaction series. The variation in the rate may be caused by changes in either the enthalpy or entropy of activation, or both. Enthalpy and entropy of activation , the isokinetic relationship, where β is the isokinetic

are correlated by

temperature. When the experimental temperature T<β, the reaction rate is controlled mainly by the

PT

enthalpy change. The isokinetic relationship between Hand S of thermal isomerisation of the dyes in different solvents gives a straight line whose slope  is equal to 119 °C with a good correlation

RI

coefficient (r = 0.963; cf. Fig. 8). This result provides evidence that isomerisation in different solvents

SC

follows the same mechanism, i.e., the same rate-determining step [55]. It is evident that the isokinetic temperature is higher than the average of the experimental value (22.5 °C); therefore, the

U

isomerisation process is an enthalpy-controlled reaction [55]. At this temperature, it is expected that

N

the variation of aryl moiety and/or solvent has no influence on the rate of the thermal ZE isomerisation of the concerned dyes. From the isokinetic plot, it can be concluded that the

M

A

isomerisation reactions for all dyes proceeds via identical common mechanistic pathway. 3.3 Mechanism of Z→E thermal isomerisation

D

The structures for Z, TS and E forms of the investigated dyes were optimised by DFT in the gas

TE

phase, followed by optimisation in methanol. Fig. 9 shows those of PBEI as a representative example. The aryl moiety of the optimised Z-isomer is not on the same plane formed by the central double bond

EP

due to steric repulsion between the aryl moiety and the benzothiazole moiety. Lacking steric repulsion, the optimised E-isomer is almost planar. Selected optimised parameters for the Z-isomer and the

CC

transition state for PBEI, 1-NBEI and 9-PhBEI are shown in Table S4[42]. For any given substrate, upon going from Z to TS, shows (i) lengthening of C2=C3 bond, (ii) shortening of C1-C2 and C3-C4

A

bonds, (iii) substantial increase of the
11

Although DFT can reproduce qualitatively experimental trends of the activation parameters of the isomerisation process, as shown in Table 4, it is also evident that DFT overestimates them somewhat for all investigated compounds, except for those of PhBEI. B3LYP often tends to lead to systematic errors in estimating energies for a family of compounds, often with the disparities growing with molecule size (as suggested by Table 4) [57,58]. This is attributable predominately due to inadequacies in treating longer-range non-bonded attractiveinteractions (dispersion), and it is to be hoped that more sophisticated functionals with better treatment of this would lead to performance improvements in future.The computed values of activation energy and enthalpy for PhBEIreplicate trends observed experimentally, although are not in quantitative agreement per se. On the contrary, the

PT

E# andH#of PBEI or 1-NBEI isomerisation reaction are overestimated by about 15 and 17.5 kJ/mol, respectively. Fig. 9 shows the energy profile for the isomerisation pathway clarifying the internal

RI

rotation around the ethenic bond. It is clear that the thermal isomerisation of PBEI is the most kinetically favoured reaction, whilst that of 9-PhBEI is the most thermodynamically favourable. It is

SC

well known that the activation energy is governed by the energy of rotational transition state with respect to that of the Z-isomer [50,56]. The activation energy is affected primarily by two factors:

U

(a) The degree of electronic interaction between the benzothiazolium and aryl moieties across the

N

central ethenic double bond. This includes the steric hindrance and potential putative ICT from the

A

aryl ring to the benzothiazoliumcation.

D

and benzothiazolium) of the dye.

M

(b) The degree of the interaction between the solvent molecules and the three subsystems (aryl, bridge

The interplay of both factors leads to the following order of activation energy: PBEI < 1-NBEI < 9-

TE

PhBEI. Accordingly, the number of molecules reaching the rotational transition state and then the rate

EP

of thermal ZE isomerisation increases in the order PBEI  1-NBEI  9-PhBEI. 3.4 Electron affinities and ionisation potentials

CC

Table 6 and Fig. 10 demonstrate a comparison between the current N-methylated derivatives and their neutral analogous [21, 22, 30]. The extent of any potential, putative ICT between the two

A

moieties (aryl and benzothiazol(e)/ium) is determined by their ionisation potentials and electron affinity, as determined by HartreeFock/6-31(d) calculations. The electron affinity of benzothiazolium (-2.69 eV) in comparison to benzothiazole (2.76 eV) suggests that any putative ICT may be stronger under the effect of N-methylation, and the ionisation potential of the aryl moiety was found to decrease as its size increases, either in the neutral or charged case.However, this is a marginal difference in electron affinity, rendering aryl- benzothiazole ICT less likely, which is not inconsistent 12

with the notion that iodide anion-to-stilbazolium CT is more likely. The computed ionisation potentials of benzene, naphthalene, phenanthacene, anthracene were 8.98, 7.72, 7.59 and 6.90, respectively. This order may well serve to rationalise the experimentally observed bathochromic shifts in absorption and emission spectra.

4- Conclusions The ultimate thrust of the present work has been to demonstrate the effect of replacing the phenyl ring of the styrylbenzothiazolium dye by a larger polycyclic ring, as well as the solvent effect(if applicable) on its photophysics, photochemistry and thermal stability, motivated by real-world

PT

applications. Increasing the size of aromatic rings led tored-shifting of absorption and fluorescencespectra, along with dipole-moment increases and Stokes shifts.The computedelectron

RI

affinity and ionisation potentials help to rationalise the experimentally observed bathchromic shifts in absorption and emission spectra as the aromatic ring size increases. PBEI is the fastest thermally-

SC

interconverting amongst the investigated compounds, as it has the lowest rotation-energy barrier. 1NBEI is the fastest for photochemical interconversion, as the rotation around the bridge is the most

U

kinetically favourable, in this case.

N

The mechanism of thermal Z→E transformation was found to proceed via rotation around the ethenic

A

bridge by forming a ‘perpendicular’ transition state, although that for the anthryl derivative possesses

M

a high rotational barrier around the ethenic bond, rendering light- or heat-driven activation difficult. Based on the significant blue shift is the Z-isomer’s absorption maximum relative to that of the E-

D

isomer (~40-110 nm), and the high percentage of Z-isomers in the PSS, these compounds may serve as

TE

potential promising candidates for data-storage applications. The B3LYP functional reproduced

A

CC

values.

EP

successfully the experimental trends in activation parameters, albeit with somewhat overestimated

13

References [1] R. A. Mathies, S. W. Lin, J. B. Ames, W. T. Pollard, Annu. Rev. Biophys. Chem., 20 (1991) 491512. [2] B. Li, K. M. Hanson, J. D. Simon, J. Phys. Chem. A, 101 (1997) 969-972. [3] J. C. Banerji, A. K. Mandal, B. K. Banerje, Dyes Pigm.,3 (1982) 273-280. [4] B. N. Jha, J. C. Banerji, Dyes Pigm.,6 (1985) 213-226. [5] B. N. Jha, R. K. Jha, J. C. Banerji, Dyes Pigm., 7 (1986) 133-152. [6] K. Heilbron, B. Walter, J. Chem. Soc., 127 (1925) 690-696. [7] Y. Nakazawa, Y. Nakamura, T. Sueyoshi, A. Sato, DE2147586 (Fuji Photo Film Co. Ltd;

PT

1972)Available at: http://www.google.com/patents/EP0611109A1?cl=en (accessed 23 Jun 2014) [8] Thomas, K. J.; Thomas, K. G.; Kumar, T. K. M.; Das, S.; George, M. V. Proc. Ind. Acad. Sci.

RI

(Chem. Sci.) 1994, 106, 1375.

[9] L. Qun, B-X Peng, Photograph. Sci. Photochem., 12 (1994) 150-165.

SC

[10] M. S. Antonious, Spectrochim. ActaA Mol. Biomol. Spectr.,53 (1997) 317-324. [11] J-L Yang, GaodengXuexiaoHuaxueXuebao, 3 (1990) 286.

U

[12] J. H. H. Keller, US Patent 3888668 (1975).

N

[13] H. Berneth, F.K. Bruder, W. Haese, R.Hagen, K. Hassenrueck, S. Kostromine, P. Landenberger,

A

R. Oser, T. Sommermann, J.W. Stawitz, T. Bieringer, WO2002089128 A1 (2002).

M

Available at: http://www.google.st/patents/WO2002089128A1?cl=en (accessed 23 Jun 2014) [14] J. R. Manhardt, N. H. Nashua, US Patent 3630733 (1971).

D

[15] H. Tsuhahara, Japanese Patent 7405467 (1975).

(2012) 25-33

TE

[16] K. Cisek, J.R. Jensen, N.S. Honson, K.N. Schafer, G.L. Cooper, J. Kuret, Biophys. Chem. 170

EP

[17] V. Kovalska, K. Volkova, M. Losytskyy, O. Tolmachev, A. Balanda, S. Yarmoluk, Spectrochim. Acta A.65 (2006) 271 -277.

CC

[18] Z-S. Wang, F-Y. Li, C-H. Huang, Chem. Commun., (2000) 2063–2064.

A

[19] U. Mazzucato, Pure Appl. Chem., 54 (1982) 1705-1721 and references therein. [20] T.A. Fayed, S. El-Din H. Etaiw, Monash. Chem,130 (1999) 1319-1333. [21] T.A. Fayed, S.E.H. Etaiw, M.K. Awad, M.M. El-Hendawy,J. Photochem. Photobiol. A, 222(1) (2011) 276-282. [22] M.K. Awad, M.M. El-Hendawy, T.A. Fayed, S.E.H. Etaiw, N J. English, Photochem. Photobiol. Sci.12(7) (2013) 1220-1231. [23] H. Görner, D. Schulte-Frohlinde, Chem. Phys. Lett., 101 (1983)79-85. 14

[24] H. Güsten, D. Schulte-Frohlinde, Tetrahedron Lett.,11 (1970)3567-3570. [25] D. Schulte-Frohlinde, H. Güsten, Liebigs Ann. Chem.,749 (1971)49-55. [26] H. Güsten, D. Schulte-Frohlinde, Z. Naturforsch., 34b (1979)1556- 1566. [27] H. Güsten, D. Schulte-Frohlinde, Chem. Ber.104 (1971) 402-406. [28] K. Takagi, K. Aoshima, Y. Sawaki, H. Iwamura, J. Am. Chem. Soc.107(1985) 47-52. [29] S. T. Abdel-Halim, M. H. Abdel-Kader, U.E. Steiner, J. Phys.Chem., 92 (1988) 4324-4328. [30] S. H. Etaiw, M. K. Awad, T. A. Fayed, M. M. El-Hendawy, J. Mol. Struct., 919 (2009) 12–20. [31] A. Gaplovsky, J. Donovalova, P. Magdolen, S. Toma, P. Zahradnik, Spectrochim.Acta A,58

PT

(2002) 363–371.

[33] W. H. Melhuish, J. Phys. Chem.65 (1961) 229-235.

SC

[34] S. R. Meech, D. Phillips, J. Photochem.23 (1983) 193-217.

RI

[32] J. Kabatc, B. Jedrzejewska, P. Orlinski, J. Paczkowski, Spectrochim.Acta A,62 (2005) 115–125.

[35] S. L. Murov, I. Carmichael, G. L. Hug, Handbook of Photochemistry, 2nded, Marcel Dekker, New

U

York, 1993.

N

[36] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.

A

Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M.

M

Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.

D

Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E.

TE

Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi M. Cossi, N. Rega, J. M. Millam, M. Klene, J.

EP

E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.

CC

Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2010.

A

[37] A. D. Becke, J. Chem. Phys.,98 (1993) 5648-5652. [38] C. Lee, W. Yang, Parr, R. G. Phys. Rev. B, 1988, 37, 785-789. [39] P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem.,98 (1994) 1162311627. [40] J. J. P. Stewart, J. Mol. Model., 13 (2007) 1173-213. [41] A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B, 113 (2009) 6378-6396. 15

[42] See Electronic Supp. Info. Accompanying this article [43] E.M. Kosower, J.A.Skorcz, W.M. Schwartz, Jr., J.W. Patton, J. Am. Chem. Soc.,82 (1960) 21882191 [44] Ph. Hébert, G. Baldacchino, Th. Gustavsson, J. C. Mialocq, J. Photochem. Photobiol. A:Chem., 84 (1994)45-55. [45] A. Mishra, G. B. Behera, M. M. G. Krishna, N. Periasamy, J. Lumin., 92 (2001)175-188. [46] E. Lippert, W. Lüder, F. Moll, Spectrochim. Acta A.,10 (1959) 858–869 [47] D. Jansone, M. Fleisher, G. Andreeva, L. Leite, J. Popelis, E. Lukevics, Chem. Heterocycl.

PT

Comp.,39 (2003) 1584-1590.

RI

[48] E. Fischer, J. Phys. Chem.,71(1967) 3704–3706.

[49] G. Baratoci, F. Masetti, U. Mazzucato, A. Spalletti, G. Orlandi, G. Poggi, J. Chem. Soc. Faraday

SC

Trans.,84 (1988) 385-399.

U

[50] H. Görner, H.J. Kuhn, Adv. Photochem.19(1995) 1-117. J. Saltiel, J. Am. Chem. Soc. 89 (1967) 1036-1037.

[52]

J. Saltiel, J. Am. Chem. Soc. 90 (1968) 6394-6400.

[53]

J. Saltiel, J.L. Charlton, ‘Cis-Trans Isomerization of Olefins’. In ‘Rearrangements in Ground

M

A

N

[51]

and Excited States’; P. de Mayo (Ed.); Academic Press, New York, (1980); Vol III, pp 25-89

D

[54] A. F. Carey, J. R. Sundberg, "Advanced Organic Chemistry, Part A: Structure andMechanism",

TE

2nd ed. Plenum Press: New York, (1984).

EP

[55] E. Grunwald, S. Winstein, J. Am. Chem. Soc.,70 (1948) 846-854. [56] J. E. Leffler, J. Org. Chem., 20 (1955) 1202-1231.

CC

[57] M.N. Glukhovtsev, R.D. Bach, A. Pross and L. Radom, Chem. Phys. Lett., 260 (1996) 558-564.

A

[58] M.D. Wodrich, C. Corminboeuf and P. von RaguéSchleyer, Org Lett., 8 (2006) 3631-3634.

16

PBEI

amax (nm)

max(M-1cm-1)

f

ex (nm)

 f (nm)

CHCl3

385

34148

0.804

397

448

Stokes Shift (nm) 63

DCM

385

33860

0.766

398

467

82

EtOH

376

31561

0.760

394

456

80

MeCN

372

35489

0.871

395

458

86

MeOH

373

34579

0.868

392

457

84

H2O

371

31705

0.797

395

458

87

CHCl3

442

25111

0.667

398, 422, 441

519

77

DCM

439

25988

0.562

397, 422, 441

PT

Table 1. Spectroscopic data of PBEI and 1-NBEI in various solvents at 25 ºC

529

90

EtOH

419

23414

0.541

403, 425, 443

522

103

MeCN

415

23414

0.536

400, 423, 438

526

111

MeOH

417

24045

0.550

404, 424, 443

527

110

H2O

408

21374

0.557

526

118

U

SC

RI

1-NBEI

A

CC

EP

TE

D

M

A

N

402, 421, 437

17

Table 2.Photochemical data for EZphotoisomerisation of PBEI, 1-NBEI, and 9PhBEI in methanol and water. MeOH

0.27

2.5

0.34

0.25

0.33

0.12 a

2.1 1.5

1.94

1.53 0.56

% Z 79

90 92

405 a/436 nm

EZ

ZE

KEZ 10-4s-1

KZE 10-4s-1

%Z

EZ

ZE

KEZ 10-4s-1

KZE 10-4s-1

0.29

0.20

1.1

0.73

66.1

0.46

0.54

1.51

1.77

0.27 0.27

0.28 0.11

1.10 1.00

1.20 0.41

99.3 ___

0.38 ___

0.008

___

%Z

EZ

 ZEt

KEZ 10-4s-1

KZE 10-4s-1

74.1

0.22

0.46

0.25

0.52

0.57

0.02

0.71

0.02

___

___

___

___

99.4

___

7 0.69

0.02 2

___

___

SC

89

0.35

KZE 10-4s-1

refers to the irradiation wavelength that is used only for PBEI.Notes: % Z: the quantity of Z-isomers

at the PPS. ΦE→Z& ΦZ→E: the quantum yields of photo-isomerisation of the E → Z and Z → E

U

reactions. kE→Z&kZ→E: the partial rate constants for the forward and the backward reactions. The

N

isomerisation process of 9-PhBEI could not monitored in water due to the heavy aggregation of the dye molecules.To monitor photo-isomerisation, a mercury lamp with highly intense peaks at 365, 405 and

A

436 nm was used. The absorption maximum of PBEI lies around 385 nm, thus 365 and 405 nm lamp

M

excitation is appropriate: both frequencies match high-absorptivity regions around the maximum, but 436 nm will not be of benefit, as it matches the tail of PBEI’s spectrum. For 1-NBEI and 9-PhBEI,405

TE

D

and 436 nm match the high-absorptivity regions around their 365 nm maxima. See also Fig. 5.

EP

9-PhBEI

85

104 -1 s

CC

1-NBEI

77

ZE

A

PBEI

Z

EZ

365 a/405 nm

PT

KE

% Z

405 a/436 nm

RI

365a/405 nm

H2O

18

Table 3.Rate constant (k) of the ZE thermal isomerisation of 1-NBEI, activation parameters in different solvents

MeCN

t1/2 (min) 0.78 0.58 0.55 0.45 22.87 18.34 11.42 6.95 693.14 433.21 256.72 161.20

E# (kJ/mol)

H# (kJ/mol)

S# (J/mol. K)

26.9

24.4

-159.6

57.5

55.0

-82.3

69.1

66.7

-69.4

U

SC

EtOH

k (min-1) 0.8833 1.1911 1.2607 1.5421 0.0303 0.0378 0.0607 0.0998 0.0010 0.0016 0.0027 0.0043

PT

MeOH

T (°C) 15 20 25 30 15 20 25 30 15 20 25 30

RI

Solvent

k (min-1)

t1/2 (min)

A

PBEI

1.5262

0.45

24.4

(40.4)

22.0

(40.4)

-167.0

1-NBEI

1.2607

0.55

26.9

(40.8)

24.4

(40.9)

-159.6

9-PhBEI

0.77

47.4

(49.5)

45.0

(49.4)

-94.1

D

M

Compound

TE

N

Table 4.Rate constant (k) of the thermal ZE isomerisation of PBEI, 1-NBEI and 9PhBEI, activation parameters in MeOH at 22.5 °C. The B3LYP activation parameters are presented in brackets H# (kJ/mol)

S# (J/mol. K)

CC

EP

0.8951

E# (kJ/mol)

Table 5. The computed dipole moments (Debye) for Z, E and transition state (TS) forms of the investigated compounds at B3LYP/6-31+G(d,p) level TS

E

PBEI

12.05

13.58

9.04

1-NBEI

9.16

10.13

8.04

9-PhBEI

9.46

10.16

7.06

A

Z

19

Table 6.Comparison between the tested compounds and their neutral analogues Solvatochromism

Neutral compounds

Charged Compounds

Positive solvatochromism in both

Negative solvatochromism in

absorption and fluorescence spectra

absorption spectra and positive in the fluorescence spectra

FC and relaxed excited states are

FC state is less polar than

more polar than ground state

ground state but the relaxed

PT

Dipole moment

excited state is larger Relatively moderate

Stokes shift

Relatively moderate

Fluorescence quantum yield

low

%Z

Reached to 95%

E→Z

Reached to 0.138

SC

Reached to 99%

Dual emission in the case of 9-

ABE due to the emission from LE

PhBEI (from monomer and

and TICT [30]

aggregates [21]) & 9-ABEI (due

M

to the extra emission from intramolecular exciplex [30]) Aggregation

D TE

Photoreactivity of the anthryl

of 9-PhBEI in

water with remarked change in the colour [21]

No isomerisation

No isomerisation

A

CC

EP

derivative

Reached to 0.461

Dual emission in the case of 9-

A

behaviour

low

U

spectroscopic

Relatively high

N

Abnormal

Relatively strong

RI

ICT

20

Figure captions Fig. 1.Introduction of positive charge to azastilbene adapts it to convert thermally by heat. Fig. 2. The molecular structure of 1-aryl-2-(N-methyl-2-benzothiazolium)ethene iodides (ArBEIs) with their abbreviated names. Fig. 3.ArBEIs’electronic absorption spectra in different solvents (concentration ~2 ×10−5 M). Fig. 4. Normalised fluorescence spectra of PBEI, 1-NBEI and 9-PhBEI as well as those of 9-ABEI in different solvents.Concentrations were ~2 ×10−5 M. Fig. 5. (a)Electronic absorption spectra recorded during the photoisomerisation of 1-

PT

NBEIin MeOH at λirr = 405 nm as a function of irradiation time.

(b)Fluorescence spectra recorded during the irradiation of 1-NBEI in MeOH.

RI

(c)Demonstration of linear correlation between the monitored absorbance changes at

SC

the two maxima of 1-NBEI in MeOH, upon irradiation at 436 nm. (d)Plot of absorbance at λamax versus irradiation time of 1-NBEI in H2O

U

confirming the achievement of PSS.

Fig. 6. (a) Calculated electronic absorption spectra of E- and Z-isomers of 1-NBEI in

N

MeOH according to Fischer method [48]

A

(b)Growth of the percentage of Z-1-NBEI (Z%) in MeOH with the irradiation time.

M

Fig. 7(a)The thermal-back isomerisation of PBEI wasmonitored by recording its absorption spectra as function of timein EtOH at 25 °C.

D

(b) Absorbance-time curves for the thermal ZE isomerisation of PBEI in MeCN

TE

at the indicated temperatures. Fig. 8. Isokinetic relationship for the thermal Z→E isomerisation of PBEI, 1-NBEI and 9-

EP

PhBEI in EtOH, MeCN and MeOH (referred to as 1, 2 and 3, respectively). Fig. 9.Computed energy profile of rotational Z-to-E isomerisation pathway. The values

CC

refer to the relative energy change with respect to the reactants, where the value of energy of all Z-isomers is taken zero as a reference for other ones. Inset:

A

B3LYP/6-31+G(d,p) optimised structures for Z, E and rotational transition state (TS) forms of PBEI in methanol.

Fig.10. Effect of N-methylation on absorption (abs.) and fluorescence(flu.) spectra; the excitation wavelengths(λex) inducing fluoresenceare shown.The neural compounds are PBE, 1-NBE, 9-PhBE and 9-ABE, while their N-methylated forms are PBEI, 1-NBEI, 9-PhBEI and 9-ABEI. Concentrations were ~2 ×10−5 M.

21

N

Thermally isomerized N

Z-stilbazolium

PT

Introducing of

a positive charge

E-stilbazolium

RI

N

SC

NOT thermally isomerized

N

Z-azastilbene

U

E-azastilbene

N

Fig. 1

D

M

A

S

I

N

Ar CH3

EP

1 9 9

PBEI 1-NBEI

A

CC

Ar =

TE

1-aryl-2-(N-methyl-2-benzothiazolium)ethene iodides (ArBEIs)

9-PhBEI

Fig. 2

22

9-ABEI

Fig. 3

D

TE

EP

CC

A

A

M

N

U

SC RI PT

Fig. 4

24

D

TE

EP

CC

A

A

M

N

U

SC RI PT

irr = 405 nm

Absorbance

0.4

0.3

Time (min) 0 2 4 7 12 18 24 30 40 53

Fresh solution

0.2

0.1

333 nm

275 nm

MeOH

45

Fluorescence intensity (nm)

MeOH 0.5

irr = 436 nm

30

PSS solution 15

0.0 300

400

0

500

500

Wavelength (nm)

550

600

650

(a)

RI

(b)

SC

0.3

0.5

U

N

Absorbance

0.2

0.3

A

A417

1-NBEI, irr = 405 nm

irr = 436 nm; r = 1

0.4

PT

Wavelength (nm)

0.21

0.24

A305

40

80

Time (min)

EP

(d)

A

CC

Fig. 5.

0

TE

(c)

0.1

D

0.18

M

0.2

25

120

Pure E

MeOH

(a)

irr = 436 nm

12000

-1 -1

 (M L )

9000

Pure Z

PT

6000

0 360

420

480

SC

300

RI

3000

Wavelength (nm)

N

U

b

90

M

A

(b)

MeOH irr = 436 nm

A

CC

EP

30

TE

%Z

D

60

0 0

22

44

Time (nm)

Fig. 6.

26

66

N

Absorbance

I CH3 0.6

Time (min) 90 75 60 45 33 23 13 5 0

SC

0.3

322 nm

0.0 300

350

U

a 250

400

M

A

N

Wavelength (nm)

S 0.75

(b)

30° C

CH3 I

25° C

TE

20° C 15° C

a

0.45 0

15

30

45

Time (min)

A

CC

EP

Absorbance

D

N

0.60

PT

(a)

RI

S 0.9

Fig, 7

27

450

80

 =119 °C; r = 0.963

9-PhBEI3

9-PhBEI2

PBEI3

60

PBEI2 1-NBEI2

#

H (KJ/mol)

1-NBEI3

PT

9-PhBEI1

40

1-NBEI1 PBEI1 -150

-100

-50

#

SC

S (J/mol.K)

RI

20

A

CC

EP

TE

D

M

A

N

U

Fig. 8

28

0

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

Fig. 9

29

.

Abs. PBE Abs. PBEI Flu. PBE, ex 335 nm

0.8

Flu. PBEI, ex-375 nm

0.4

0.0 0.8

Flu. 1-NBEI, , ex=420 nm

0.0

0.4

Abs. 9-PhBE Abs. 9-PhBEI Flu. 9-PhBE, ex= 360 nm Flu. 9-PhBEI,, ex= 420 & 520 nm

SC

0.0

PT

0.8

RI

Absorbance

0.4

U

0.8

Flu. 9-ABE, ex= 365, 425 nm

N

0.4

Abs. 9-ABE Abs. 9-ABEI Flu. 9-ABE,ex = 340 nm

200

A

0.0 300

400

Fig. 10

A

CC

EP

TE

D

M

Wavelength (nm)

30

500

600

700

Fluorescence intensity (a.u.)

Abs. 1-NBE Abs. 1-NBEI Flu. 1-NBE, , ex =36o nm