Azomethine phthalimides fluorescent E→Z photoswitches

Journal Pre-proof Azomethine Phthalimides Fluorescent EnullZ Photoswitches Anton Gergiev, Dancho Yordanov, Deyan Dimov, Ivailo Zhivkov, Dimana Nazarova, Martin Weiter

PII:

S1010-6030(20)30242-2

DOI:

https://doi.org/10.1016/j.jphotochem.2020.112443

Reference:

JPC 112443

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

27 November 2019

Revised Date:

4 February 2020

Accepted Date:

8 February 2020

Please cite this article as: Gergiev A, Yordanov D, Dimov D, Zhivkov I, Nazarova D, Weiter M, Azomethine Phthalimides Fluorescent Esrarr;Z Photoswitches, Journal of Photochemistry & amp; Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112443

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.

Azomethine Phthalimides Fluorescent E→Z Photoswitches Anton Gergiev a,b*, Dancho Yordanov c , Deyan Dimov b, Ivailo Zhivkov b,d, Dimana Nazarova b, Martin Weiter d a

Department of Organic Chemistry, University of Chemical Technology and Metallurgy, 1756 Sofia, 8 St. Kliment Ohridski Blvd, Bulgaria. b

Department of Optical Metrology and Holography, Institute of Optical Materials and Technologies, Bulgarian Academy of Science, 1113 Sofia, 109 "Acad. G. Bonchev” Blvd., Bulgaria. c

of

Laboratory of Chemistry and Biophysics of Proteins and Enzyme, Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Science, 9 "Acad. G. Bonchev" Blvd., Sofia 1113, Bulgaria. Materials Research Centre, Faculty of Chemistry, Brno University of Technology, Purkyňova 118, 612 00 Brno, Czech Republic.

ro

d

a,*

-p

Corresponding author: Anton Georgiev, Department of Organic Chemistry, University of Chemical Technology and Metallurgy, 1756 Sofia, 8 “St. Kliment Ohridski” Blvd, Bulgaria, e-mail: [email protected]

Jo

ur na

lP

re

Graphical abstract

1

of ro -p re lP ur na

Highlights

Two azomethine phthalimides dyes have been synthesized.



The kinetics of E→Z photoswitching has been investigated.



Steady-state and time-resolved fluorescence E→Z photoswitching behavior have been studied.



The fluorescence quantum (Φfl) yield at photostationary state has been increased compared to the E-isomers.

Jo



Abstract 2

Herein, we report the synthesis and E→Z photoswitching behavior of two 4-substituted azomethine phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties (EAMP1 and EAMP2). These compounds represent newly synthesized and unstudied photoswitches with dual fluorescence properties as E-isomers and at photostationary state (PSS) depending on the solvent polarity. Steady-state fluorescence measurements were performed in various solvents and the results show strong sensitivity on the environmental polarity. The kinetics of E→Z photoswitching to PSS was studied in AcCN by visible light activation at 410 nm (EAMP1) and long wavelength UV-light activation at 350 nm (EAMP2). The quantitative and qualitative performance of the switching

of

behavior was evaluated by the degree of photoisomerization (R) and the rate constant (k). It was found for EAMP1 R = 6.95 %, k = 8.87×10-4 s-1 and EAMP2 R = 88.72 %, k = 4.00×10-4 s-1,

ro

respectively. The reason for the lower photoconversion of EAMP1 compared to the EAMP2 was analyzed through optimization of the molecular geometry of E- and Z-isomers in the ground state

-p

(S0) and first excited state (S1) by DFT/TD-DFT calculations with B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN. It was found that E-isomers in the S0 have nonplanar conformation, while the Z-isomer of EAMP1 prefers twisted conformation and the Z-isomer of EAMP2 T-shaped

re

conformation is energetically advantageous compared to the twisted one. The reason is the weak H…..π noncovalent interaction (NCI) between 4-(dimethylamino)phenyl moiety and phthalimide

lP

ring. Moreover, the Z-isomer of EAMP2 is unusual stable up to 600 min at room temperature in dark compared to the EAMP1, which undergoes full Z→E relaxation for less than 60 min at the same conditions. The Z→E relaxation of EAMP2 is achieved for 90 min at 60 oC. The fluorescence

ur na

E→Z switching behavior was studied by emission measurements in AcCN and 1,4-DOX as Eisomers and at PSS in room and liquid nitrogen (77 K) temperatures. In the polar and nonpolar solvents, red-shifted emissions with increased fluorescence quantum (Φfl) yields have been observed at PSS compared to the E-isomers. The molecular rotor behavior was studied in the binary mixture of glycerol:ethanol and the results show a sensitivity of the emission bands depending on the

Jo

environmental viscosity. Time-resolved fluorescence decay measurements were performed in AcCN and 1,4-DOX as E-isomers and at PSS to estimate the mechanism of fundamental fluorescence bands. We found that dyes at PSS have longer lifetime (τ) compared to the E-isomers, especially in less polar 1,4-DOX. Keywords: phthalimides, fluorescence photoswitches, E→Z photoisomerization, rate constant, molecular rotors.

3

1. Introduction Photoactive compounds containing isomerazable >X=Y< units (X and Y might be equal or different substituted atoms of C or N) can be reversibly switched on and off upon light irradiation. According to the isomerazable units, the materials are mainly classified as: (i) azobenzenes; (ii) azomethines; (iii) stilbenes, spiropyrans, and diarylethenes. Aromatic azomethines or Schiff bases undergo reversible trans (E)→cis (Z)→trans (E) photoisomerization cycle like azobenzenes [1][2][3]. Photoinduced changes of molecular geometry between E- and Z- switched states allow to control over the electron rearrangement of fluorophore units resulting in changed levels of frontier

of

molecular orbitals, which are responsible for the emission. The large review by D. Kim and S.Y. Park provides a very insightful systematization of multicolor fluorescence photoswitches [4]. They

ro

have classified them according to their fluorescence switching principles and behavior as: (i) colorcorrelated and, (ii) color-specific photoswitches. The first switching behavior is explained that one

-p

emission is switched off upon light irradiation and simultaneously the other emission is switched on correlatedly. The latter is that one emission is switched on and off upon light irradiation, while the other emission is unchanged. T. Fukaminato has published а critical review on the design and

re

synthesis of various types of photoswitchable fluorescent molecules with an application as singlemolecule optical memory and super-resolution fluorescence microscopy [5]. The nature of the

lP

fluorescence bands of E/Z photoswitching system is described by twisted intramolecular charge transfer (TICT), which is a fundamental action of fluorescent molecular rotors (FMR’s). Simplify description of the process is a rotation of the donor (D) via a single C-C bond around acceptor (A).

ur na

Typically D-A connection in the fluorophore consists of the push-pull π-conjugated bridge and the photophysical properties (absorption and emission) can be modified by the variation of electron donor and acceptor substituents and the length of the π-conjugated system [6]. Upon light excitation, the electrons transit from the ground state (S0) to the first excited state (S1) and locally excited (LE)

Jo

state with a partial D-A charge transfer (CT) is formed. In this state, the molecule is still planar that able to produces LE emission or following by a concomitant rotation of the D until twisted to the right angle concerning the A, which gives red-shifted TICT emission (Fig. 1). The TICT state is characterized by the total charge separation between D-A and conjugation is destroyed. The overcoming of the barrier transferring from LE to TICT state critically depends on the environmental polarity and microviscosity of the solvents, which are responsible for dual fluorescent properties of the fluorophore (Fig. 1) [7] [8] [9] [10] [11]. Many researchers have studied the TICT phenomenon long time ago related to its application as FMR’s [6][12][13]. A recent paper 4

by S. Sasaki and co-authors updates the mechanisms and nature of TICT in terms of applicability of the phenomena in device fabrications such as solar cells and OLED’s [7]. They have concluded that there are several shortcomings: (i) the extent of pretwisting required to form the TICT state is unclear; (ii) the data on the excited-state CT equilibria are still incomplete due to the difficulty of discerning excited-state species from fluorescence spectra; (iii) the influence and contribution of other transfer effects such as exciplexes remains unclear; (iv) various isomeric substitution effects

ur na

lP

re

-p

ro

of

have not yet been fully elucidated.

Fig. 1. Mechanism of the LE and TICT emissions according to the Jablonski diagram. The D-A interaction via –CH=N− group in the excited state is performed by rotation of aromatic

Jo

substituents, where the π→π* (S0→S2) and n→π* (S0→S1) transitions have different contributions. This is essential to their emissions behavior to enhance the quantum yield the π→π* energy must be reduced at the expense of n→π* transitions [14]. In general, the fluorescent properties of 4substituted phthalimides with amino or dialkylamino groups are widely studied [15] [16] [17] [18] [19]. They show remarkable Stoke shifts upon different solvent environments and are used as fluorescent probes in cell biology, protein labeling as well as fluorescent polymers for flexible OLED’s devices. For example, 4-aminophthalimide (4AP) having electron-donor amino group, 5

exhibited large quantum yield in THF (Φfl = 0.70), DCM (Φfl = 0.76) and acetone (Φfl = 0.68), while replacing the amino group with dimethylamino one to 4-dimethylaminophthalimide (DAP) exhibits very weak fluorescence. The reason is the large torsion at the bond between the dimethylamino group and the phthalimide resulting in TICT emission [14]. R. Orita and co-workers have studied 4-substituted phthalimides with five- and six-membered alicyclic electron-donating amino groups [14]. Based on DFT calculations and lifetime measurements, they have concluded that the alicyclic amino group produces planar intramolecular charge transfer with improved quantum yields, especially when the dispersion in polymer matrices such as PMMA occurs (Φfl =

of

0.99). Several examples in the literature could be found for the investigation of fluorescence photoswitching properties of phthalimide and naphthalimide dyes [20] [21] [22]. The paper from P.

ro

Panchenko describes the E/Z fluorescence photoswitching performance of styrylnaphthalimides by TICT [21]. They have found that the TICT of some dyes is stabilized in polar media ensuring rather

-p

high fluorescence and long wavelength emission near to 620 nm. The E/Z photochemistry of the aromatic azomethines has been studied by many researchers in terms of their reversible optical memory [23][24][25][26][26]. The tuning of light induced isomerization is achieved by ortho- and

re

para-substitution of phenyl rings with electron-donating and electron-accepting groups [23]. The motivation of our study for the design and synthesis of two novel 4-substituted azomethine

lP

phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties was to investigate their bidirectional photoswitching behavior. We have investigated the kinetics of their E/Z photoconversion by Vis-light activation and long wavelength UV-light, as well as back Z/E thermal

ur na

relaxation. The fundamental fluorescence bands of E-isomers and at the photostationary state (PSS) were studied by steady-state and time-resolved spectroscopy in AcCN and 1,4-DOX. In the switched state, the chromophores have shown increased fluorescence quantum yield.

Jo

Experimental

2.1.Materials Used The phthalimide, 9-anthracenecarboxaldehyde, 4-(Dimethylamino)benzaldehyde, ethyl bromide, and triethylamine were purchased from Sigma-Aldrich and used without further purification. The nitric acid, sulphuric acid, potassium carbonate, DMF, and MeOH were delivered from local supplier Valerus Ltd. For spectroscopy measurements, all solvents were spectroscopy grade purity. 2.2.Synthesis of the N-ethyl-4-arylazomethine phthalimides 6

The synthetic pathway of the azomethine dyes is presented on Scheme 1. In order to synthesis 1 (4nitrophthalimide), 2 (4-nitro-N-ethylphtalimide) and 3 (4-amino-N-ethylphthalimide) we have followed the methodology of early published paper [27]. The 1H- and 13C-NMR spectra of 3 used

ro

of

as an intermediate compound to Schiff bases synthesis are presented in the Supporting information.

Scheme 1. Synthetic pathway of the EAMP’s.

-p

2.3.General procedure for condensation reaction between 4-amino-N-ethylphthalimide and aromatic aldehydes to azomethines The N-ethyl-4-aminophthalimide (2.628 mmol, 1.0 equiv) and aromatic aldehydes (2.628 mmol, 1.0

re

equiv) were suspended in dry MeOH (25 mL). Subsequently, 0.5 mL of Et3N was added and the mixture was heated to reflux without stirring for 7 hours. The reaction was controlled by TLC

lP

(CHCl3 : EtOAc = 2:3). After cooling to room temperature the reaction mixture was concentrated by evaporation and then stored in refrigerator to precipitate, then filtered off and washed with cold MeOH:H2O (1:1). The Schiff bases were recrystallized by ethanol to obtain pure products. The 1H-

ur na

and 13C-NMR spectra are presented in the Supporting information.

2.3.1. Synthesis of the (E)-5-((anthracen-9-ylmethylene)amino)-2-ethylisoindoline-1,3-dione, (EAMP1) The compound was synthesized from 0.5 g (2.628 mmol) 4-amino-N-ethylphthalimide and 0.541 g

Jo

(2.628 mmol) 9-anthracenecarboxaldehyde, yield 71 % as yellow-orange solids, m.p. = 147-149 ºC. ATR-FTIR cm-1: 3048 (ν Ar-H), as 2972, s 2934 (ν -CH3 and >CH2), s 1762, as 1701 (ν >C=O), 1623 (ν –CH=N-), 1580,1519 (ν Ar C=C), as 1449, s 1390 (δ -CH3 and >CH2), 1345 (C-N-C stretching vibration of imide ring). 1H-NMR (DMSO-d6) ppm: 9.85 (1H, s); 8.92-8.83 (3H, t, J = 10 Hz), 8.19 (2H, d, J = 9 Hz), 7.93-7.79 (3H, d, J = 14 Hz), 7.67-7.59 (4H, d, J = 11 Hz), 3.65-3.62 (2H, q, J = 3 Hz), 1.21-1.19 (3H, t, J = 5 Hz).

13

C-NMR (DMSO-d6) ppm: 167.93, 167.86, 157.75,

7

133.91, 132.17, 131.25, 130.83, 129.52, 129.03, 128.26, 127.68, 126.48, 126.16, 125.37, 124.68, 115.92, 32.94, 14.19. MS (ESI+) calculated for C25H18N2O2, [M+]: 378.1368; found: 378.3634. 2.3.1. Synthesis of the dione, (EAMP2)

(E)-5-((4-(dimethylamino)benzylidene)amino)-2-ethylisoindoline-1,3-

The compound was synthesized from 0.5 g (2.628 mmol) 4-amino-N-ethylphthalimide and 0.392 g (2.628 mmol) 4-(Dimethylamino)benzaldehyde, yield 78 % as red solids, m.p. = 163-165 ºC. ATRFTIR cm-1: 3065 (ν Ar-H), as 2978, s 2939 (ν -CH3 and >CH2), s 1766, as 1701 (ν >C=O), 1643 (ν –CH=N-), 1593, 1551 (ν Ar C=C), as 1463, s 1396 (δ -CH3 and >CH2), 1348 (C-N-C stretching

of

vibration of imide ring). 1H-NMR (DMSO-d6) ppm: 9.66 (1H, s), 7.69-7.67 (2H, d, J = 7 Hz), 7.477.45 (1H, d, J = 8 Hz), 6.91-6.90 (1H, d, J = 4 Hz), 6.79-6.76 (3H, m), 3.52-3.48 (2H, q, J = 6 Hz), 13

C-NMR (DMSO-d6) ppm: 190.36, 168.56, 168.27,

ro

3.04 (6H, s), 1.11-1.09 (3H, t, J = 7 Hz).

155.33, 154.65, 135.01, 132.00, 125.24, 124.96, 117.17, 116.94, 111.52, 107.40, 40.22, 30.33,

-p

14.32. MS (ESI+) calculated for C19H19N3O2, [M+]: 321.1477; found 321.2817. 2.4. Quantum Chemical Calculations

re

The molecular geometry and electronic structure optimization in the ground state (S0) of the studied Schiff bases were performed by GAUSSIAN 09D.01 software package using density-functional

lP

theory (DFT) with B3LYP exchange-correlation hybrid functional combined with the standard 631+G(d,p) basis set both in vacuo and using solvation in AcCN as E- and Z- isomers [28]. The interactions between the molecules and the solvent were evaluated on the same basis set by the

ur na

polarizable continuum model using the integral equation formalism variant (IEFPCM). Natural Bonding Orbital (NBO) and charges distribution were computed after optimization of geometry on the same basis set in vacuo. The molecular geometry optimization in the first excited (S1) state as E- and Z-isomers were computed by TD-DFT using the same basis set the in AcCN solvent. The frequency analysis was made at the same level of theory to characterize the stationary points on the

Jo

potential surface and to obtain thermodynamic parameters such as absolute enthalpy (H) and Gibbs free energy (G) at 298 K. 2.5. Photoisomerization Experiment The photoinduced E→Z isomerization of the studied Schiff bases was performed in AcCN solution at room temperature on custom-built optical set-up with high-resolution USB-650 Red Tide spectrometer (Ocean Optics) operating in the range of 200 - 1000 nm. As the light source was used, a pulsed xenon light source (PX-2) directed to pass through the dye solutions and to get directly to 8

the spectrometer through optical output fiber. Special care has been taken to eliminate all possible sources of stray light. The source of Visible (Vis) light was halogen lamp (power 10 W) equipped with bandpass filter at 410 nm of EAMP1 as well as long wavelength UV irradiation was broadband lamp with λmax = 350 nm (power 10 W) of EAMP2, perpendicular directed to the probe beam. The illumination time was set at 60 min (EAMP1) and 200 min (EAMP2) until the photostationary state (PSS) was reached. After exposure, the probes undergo Z→E relaxation in darkness for 60 min at room temperature (EAMP1) and 90 min thermal relaxation at 60 ºC ±1 ºC (EAMP2) using cuvette thermostat. The spectra were collected at every 1 or 4 min during the entire experiment by

of

integration time set at 100 ms and scans to average at 15. Before the experiments start, the solutions of the dyes were stored in the dark overnight.

ro

2.6. Spectral Measurements

The NMR spectra were recorded on a Bruker Avance II+ spectrometer operating with frequency 13

C in DMSO-d6. ATR-FTIR spectra of the compounds were

-p

600 MHz for 1H and 125 MHz for

recorded on a Bruker Tensor 27 FTIR spectrophotometer in the range of 4400-600 cm-1 with a

re

resolution of 2 cm-1 at room temperature. The external reflection diamond crystal has been used and the samples were scanned 128 times. The UV-VIS spectra were carried out on the Cary 5E- UV-

lP

VIS-NIR spectrophotometer. The steady-state fluorescence spectra were recorded via a FluoroLog 3-22 (HORIBA) spectrofluorometer in the range 200-800 nm with a resolution of 0.5 nm and double-grating monochromators by excitation wavelength set near to the absorption maxima of

ur na

dyes. The emissions at PSS were measured immediately after Vis light irradiation at 410 nm for EAMP1 and UV irradiation at 350 nm for EAMP2 in AcCN and 1,4-DOX. Absolute fluorescence quantum yields (Φfl) of the dyes were determined using calibrated integrated sphere Quanta-φ (HORIBA) as E-isomers and at PSS in AcCN, 1,4-DOX and glycerol. The low temperature (at liquid nitrogen ~ 77 K) emissions were measured in AcCN and 1,4-DOX as E-isomers and at PSS. The

Jo

cuvette sample was placed in Dewar and before the measurement, the sample was tempered for 30 min (Fig S8 in the supporting information). To investigate the viscosity impact on molecular rotors, the emissions of binary mixtures of glycerol:ethanol were carried out in different ratios. Timeresolved fluorescence decay was measured on the TBX Picosecond Photon Detection Module (HORIBA) using pulsed Nano-LED laser diodes with excitation wavelengths at 372 nm for EAMP1 as E-isomer and at PSS, 329 nm for EAMP2 as E-isomer and 360 nm for EAMP2 at PSS. The concentrations of the dyes were CM ~ 1×10-5. 9

3. Results and Discussion

of

3.1.Synthesis and molecular geometry optimization Two 4-substituted azomethine phthalimides containing anthracenyl and 4-(dimethylamino)phenyl

ro

π-donor moieties were synthesized via four-step reactions. The first step was classical nitration of phthalimide to 4-nitrophthalimide (1) and the second one was alkylation of NH imide by ethyl

-p

bromide in DMF in the presence of K2CO3 to N-ethyl-4-nitrophthalimide (2) [27]. The ethyl group was introduced to increases solubility. The third step was catalytic hydrogenation of −NO2 to –NH2

re

by Pd/C catalyst in MeOH to N-ethyl-4-aminophthalimide (3) and the final step was condensation of 3 with 9-anthracenecarboxaldehyde and 4-(Dimethylamino)benzaldehyde by refluxing in dry

lP

MeOH in the presence of Et3N (Scheme 1) to corresponding 4-substituted phthalimide Schiff bases (EAMP1 and EAMP2). The synthesized dyes are new and unstudied of their photochromic behavior, which will be promising for spatial and temporal control of fluorescence photoswitching, TICT-

ur na

active molecular rotors as well as in dynamic light modulation between two switched states. Before the spectral study of switching behavior, we focus our attention on the optimized molecular geometry of E- and Z-isomers in S0 and S1 states to understand the rearrangements of the aromatic substituents around –CH=N- group between both isomers related to their spectral properties. As Eisomers in the S0 state, the dyes are characterized by the nonplanar configuration of anthracenyl/4-

Jo

(Dimethylamino)phenyl moieties respect to the –CH=N− bond. A similar observation has been done by Y. Luo and coworkers carrying out a detailed investigation of a large number of Nbenzylideneaniline derivatives [23]. Depending on the aromatic π-donors nonplanarity is determined by C(27)-C(25)-C(19)-N(11) dihedral angle (φ): 29.19 º for EAMP1 and 2.16 º for EAMP2 (Fig. 2). It can be accepted that 4-(Dimethylamino)phenyl part of EAMP2 has a nearly planar configuration around –CH=N− bond. The reason for the bigger angle of EAMP1 is the repulsion between azomethine H(20) and H(35) of anthracene fused ring compared to the EAMP2. The other φ respect to the phthalimide ring via –CH=N− is C(19)-N(11)-C(1)-C(2), which is slightly 10

affected from the aromatic π-donors: 42.22 º (EAMP1) and 39.72 º (EAMP2) due to the orbital repulsion between azomethine H(20) and H(12) phthalimide. The aromatic moieties can act as rotors around TICT-active C(25)-C(19) and N(19)-C(1) bonds, which spectral properties further discussed in subsection 3.3. As can be expected, E-isomers ensure free rotation of D by twisted D-A charge transfer, which is strongly sensitive to the polarity and viscosity environments. To verify the rotation of substituents around the above-mentioned σ-bonds the calculations of S1 state indicate changing of the φ to 8.40 º and 17.01 º for EAMP1 and 0.01 º and 2.36 º for EAMP2, which suggest almost planar structure, especially for EAMP2 (Fig. 2). As E-isomer the φ around Me2N(35)-C(32) bond

of

remains unchanged. The Z-isomer of EAMP1 in the S0 state is characterized by the twisted conformation of anthracene ring regard to the phthalimide one via –CH=N- bond by the φ 63.94 º

ro

of C(27)-C(25)-C(19)-N(11) fragment. Such structure ensures face-to-face (sandwich) like πinteraction between aromatic rings enabling the charge transfer (CT) from anthracene (donor) to

-p

phthalimide (acceptor). Moreover, an insignificant change of φ to 66.89 ° is observed in the S1 state, which means the rotation of the anthracenyl rotor off due to the steric hindrance. In other words, the TICT emission will be blocked at the expense of LE one. The geometry of Z-isomer of EAMP2 has

re

a T-shaped conformation of 4-(Dimethylamino)phenyl ring in the S0 state regard to the phthalimide one via –CH=N− bond by the φ -5.66 º of C(27)-C(25)-C(19)-N(11) fragment. The shape of Z-

lP

isomer suggests that is possible to exist a weak H…..π noncovalent interaction (NCI). This makes Z-isomer relatively favorable and energetically preferable compared to the twisting structure. Such structure of Z-isomer restricts the rotation of 4-(dimethyamino)phenyl part, hence the TICT

ur na

emission will be decreased. After excitation to the S1 state, the φ is changed to 36.35 ° and the other specific rotation around Me2N(35)-C(32) dihedral angle from 0.06 ° (S0) to 9.75 ° (S1) confirm the assumption for the molecular rotor behavior of the dye as Z-isomer (Fig. 2). Photoinduced changes of the molecular geometry always cause rearrangement of the electron densities and specific configuration of Z-isomers of EAMP’s provides different photophysical properties as will be

Jo

discussed later about their photoswitching behavior. Similar systems of azoheteroarenes with high stability of Z-isomer have been studied by J. Calbo et.al. [29]. The authors have investigated several phenylazo pyrazoles and pyrrazolones of their photoswitching behavior and based on DFT calculations and experimental kinetic data, they have concluded that T-shaped conformation via – N=N- bond is preferable than the twisted one.

11

of ro -p re lP

ur na

Fig.2. Optimized molecular geometry of the EAMP1 (A) and EAMP2 (B) as E- and Z-isomers in the ground state S0 and first excited state S1 by DFT/TD-DFT /B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN. The change of dihedral angels (φ) around –CH=N– and –N(Me)2 from S0 to S1 are depicted in the diagram. In general, the energy barrier of Z/E back relaxation of azomethines (70 kJ mol-1) is smaller than this of the azo compounds (95 kJ mol-1) [23]. One of the advantages of the low barrier for the relaxation process is the opportunity for rapid control of the reversible geometrical changes in light-

Jo

driven molecular devices. Thermodynamic parameters provide information about the relative energy between two switched states via photoinduced changes of the geometry. Table 1 summarizes the calculated ΔEE→Z, ΔμE→Z, ΔHE→Z, ΔGE→Z and ΔSE→Z of EAMP’s, where the photoinduced E/Z conversion is the difference between final state (f) Z-isomers (products of the photochemical reaction) and initial state (i) E isomers (reactants). As can be seen, EAMP1 has lower ΔGE→Z and ΔμE→Z values compared to the EAMP2. The calculated ΔS describes the order of the macroscopic system and the probability of photochemical reactions. EAMP1 is characterized by a negative value, while EAMP2 has a positive one. The negative change of the entropy for E/Z isomerization means 12

that the photoconversion is favored by the kinetic factor, as opposed to positive change that indicates a thermodynamically controlled process. Table 1. Theoretically calculated total electronic energy E (RB3LYP), dipole moment μ and thermodynamic parameters of the EAMP1 and EAMP2 as E- and Z- isomers by DFT/ B3LYP/631+G(d,p) level of theory using IEFPCM in AcCN.

E-EAMP2 Z-EAMP2

*ΔS =

H [kcal mol-1]

6.89 4.19

-767806.52

-767565.08 -1.03

5.85

-659061.56

ΔH−ΔG 𝑇

3.2.Kinetics study of E/Z photoswitching behavior

*ΔS [cal K-1 mol-1]

4.71

-2.06

-767610.10 -658877.89

5.60 -658823.81

ΔG [kcal mol-1]

-767614.82

-658829.42 -1.79

10.22

G [kcal mol-1]

4.10 -767560.97

12.01 5.43

-659056.13

ΔH [kcal mol-1]

of

Z-EAMP1

-767810.71

Δμ [D]

5.22

1.29

-658872.67

ro

E-EAMP1

μ [D]

-p

Compounds

ΔE [kcal mol-1]

E (RB3LYP) [kcal mol-1]

re

The E/Z photoswitching performance of the dyes was studied in AcCN by a different wavelength of activation depending on the absorption maxima of π→π* transitions. EAMP1 shows maximal

lP

response to a Vis-light at 410 nm, while EAMP2 was activated by long wavelength UV-light at 350 nm (Fig. 3). The PSS of EAMP1 was reached within 60 min (Fig.3A1) of the main absorption band at λmax = 409 nm with a relatively small response. EAMP2 was switched to PSS after 140 min

ur na

illumination with a large response of π→π* transition band at λmax1 = 337 nm and small changes of weak n→π* transition band as a shoulder at λmax2 = 392 nm (Fig.3B1). New blue-shifted bands have appeared at λmax1 = 300 nm (π→π*) and λmax2 = 365 nm (n→π*), which characterize the features of Z-isomer. The degree of E/Z photoisomerization, R at PSS, was determined according to Eq. 1 and photoisomerization kinetics data, rate constant k were fitted by Eq’s 2 (E/Z) and 3 (Z/E) [30]

Jo

[31][32][33]:

𝑅=

𝐴0 −𝐴∞ 𝐴0

ln(

𝐴0 −𝐴∞

ln(

𝐴∞ −𝐴0

𝐴𝑡 −𝐴∞ 𝐴∞ −𝐴𝑡

× 100 (1)

) = 𝑘. 𝑡

(2)

) = 𝑘. 𝑡

(3)

13

where A0 is the initial absorbance, At is the absorbance at the moment t during the irradiation and A∞ represents absorbance at the PSS. The degree of E/Z photoisomerization of EAMP1 is considerably low (R = 6.95 %) with fast conversion (Table 2). In contrast, the EAMP2 is characterized by large photoisomerization (R = 88.72 %) and a two-fold lower speed of conversion compared to the EAMP1 (Table 2). Let‘s consider the back Z/E relaxation, where EAMP1 fully returned to E-isomer up to 60 min at room temperature in the dark, while EAMP2 at these conditions is stable for 600 min (Fig. 3A1 and 3B1inset graphics) that is unusual for Schiff bases. After rising temperature up to 60 oC the recovery of EAMP2 becomes up to 90 min. The kinetics of thermal Z/E

of

relaxation of EAMP1 is faster than the E/Z reaction (Table 2), which is typical for the Schiff bases. As we pointed out in the previous section the T-shaped conformation of EAMP2 is preferable

ro

resulting in H…..π NCI of Z-isomer at PSS (Fig. 2), which stabilize switched state at room temperature compared to the EAMP1. The Z/E relaxation is favorable at 60 oC, where the NCI’s are destroyed. From the degree of photoisomerization and kinetics data, the following conclusions can

-p

be drawn: (i) aromatic substituents at 4-position of phthalimide ring connected by –CH=N- have a strong influence on E/Z photoconversion and back thermal relaxation; (ii) anthracene fused ring of

re

EAMP1 is a π-donor leads to absorbance maximum in the visible range, where the kinetic factor dominates the E→Z→E photoisomerization cycle (Fig. 3A2), therefore can be referred to as a T-

lP

type photochromic switch; (iii) 4-(dimetylamino)phenyl donor moiety of EAMP2 stabilizes Zisomer by T-shaped conformation, therefore bidirectional switching is thermodynamically

Jo

ur na

controlled process (Fig. 3B2) and can be referred to as a P-type photochromic switch [34][35][36].

14

of ro -p re lP

Fig. 3. E→Z→E photoisomerization cycle in AcCN of: EAMP1 by Vis-light at 410 nm up to 60 min

ur na

(A1), dark relaxation at room temperature up to 60 min (inset graphic A1) and kinetic curves ΔA at 409 nm as a function of the time (A2); EAMP2 by UV-light at 350 nm up to 200 min (B1), dark relaxation at room temperature up to 600 min, thermal dark relaxation at 60 ºC up to 90 min (inset graphics B1) and kinetic curves ΔA at 337 nm as a function of the time (B2). The absorbance changes

are

presented

as

∆𝐴 = 𝐴0 − 𝐴𝑡

Jo

(𝐴0 𝑖𝑠 𝑡ℎ𝑒 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑎𝑛𝑑 𝐴𝑡 𝑖𝑠 𝑡ℎ𝑒 𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑚𝑜𝑚𝑒𝑛𝑡, 𝑡) as a function of the time of illumination and dark relaxation.

15

Table 2. Experimentally determined values of the photoisomerization degree (R), rate constants (k) and the half-life of reaction (t1/2) of E/Z photoisomerization and Z/E thermal relaxation of the EAMP’s dyes in AcCN (The graphics are presented on Fig.S9 in the Supporting information). Compounds R [%] 6.95 E→Z EAMP1 6.86 Z→E EAMP1 88.72 E→Z EAMP2 88.63 Z→E EMAP2 *Determined at room temperature **Determined at 60 ºC

k [s-1] *8.87 ± 0.07×10-4 *1.14 ± 0.12×10-3 *4.00 ± 0.04×10-4 **1.06 ± 0.26×10-3

t1/2 [s] 781.4 608 1732.8 653.9

of

The analysis of TD-DFT spectrum of the E-isomer of EAMP1 shows that vertical π→π* (S0→S2) transition has a major contribution, while n→π* (S0→S1) transition has minor one (Fig.S10 in the

ro

Supporting information). The experimental spectrum of EAMP1 in AcCN is characterized by broadband at λmax = 409 nm, where both transitions are overlapped. Upon excitation, strong of

frontier

molecular

orbitals

and

mixture

of

HOMO→LUMO

and

-p

overlapping

HOMO→LUMO+1electron transitions between anthracene and phthalimide rings are observed

re

(Fig. 4). As a result, there is no effectively charge separation, which leads to a low photoisomerization degree of EAMP1. As, well known the photoisomerization of such system

lP

becomes when the electrons populated S2 excited state and after relaxation to the S1 switched to the ground state of Z-isomer. The back process is happened by excitation to the S1 state and relaxation to the ground state of E-isomer or by thermal relaxation [37]. The calculated spectrum of pure Z-

ur na

isomer is characterized by a blue shifting and decreasing oscillator strength of π→π* transition, while for n→π* transition slightly increases. In the switched state upon excitation, the separation of HOMO→LUMO+1 orbitals leads to CT from anthracene fused ring to the phthalimide one (Fig. 4), which is essential to fluorescence behavior as we discussed later. The TD-DFT spectrum of EAMP2 as E-isomer displays a major contribution of π→π* (S0→S2) transition attributed to the

Jo

HOMO→LUMO+1 and n→π* (S0→S1) related to the HOMO→LUMO (Fig.S10 in the Supporting information and Fig. 4). The HOMO level mainly occupied the 4-(dimethylamino)phenyl ring, while LUMO and LUMO+1 mainly occupied the phthalimide one, which leads to a separation of the charges and a reasonable degree of photoisomerization. The experimental spectrum in AcCN before isomerization is characterized by the bands at λmax1 = 337 nm and shoulder at λmax2 = 392 nm. The calculated spectrum of the Z-isomer displays decreasing oscillator strength and blue shifts of the π→π* transition, resulting in an increased contribution of the n→π* transition. The

16

HOMO→LUMO and HOMO→LUMO+1 indicate the separation of the CT from donor 4-

re

-p

ro

of

(dimethylamino)phenyl ring to acceptor phthalimide one.

lP

Fig.4. Isodensity surface plots of the frontier molecular orbitals for E- and Z-isomers of EAMP’s dyes calculated by DFT/B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN. After UV-irradiation to PSS, quantitative (or near-quantitative) photoswitching to the Z-isomer can

ur na

be determined by the area of minimum absorbance at a wavelength where the E-isomer absorbed and Z-rich PSS can be quantified by UV-VIS spectroscopy. The pure Z-isomer spectrum was extracted using the E-isomer spectrum and the quantified PSS. PSS ratio was assigned by interpolating between the E- and Z-isomer spectra using the following equation (4) [29]: 𝐴𝑃𝑆𝑆 − 𝐴𝑍

Jo

𝑓𝑃𝑆𝑆 =

𝐴𝐸 −𝐴𝑍

(4)

where fPSS is the fraction of E-isomer present in the PSS, APSS, AE, and AZ are the absorbances at the π→π* transitions at the PSS, E- and Z-isomers, respectively. The spectrum of the pure Z-isomer was estimated based on the PSS containing highest concentration of Z-isomer. Initially, the fraction of E-isomer presented in this PSS was estimated from the absorbance at the E-isomer absorption maximum. Using this estimation, the spectrum of the authentic E-isomer was subtracted from that of the PSS, using equation (5) [29]: 17

𝐴𝑍 =

𝐴𝑃𝑆𝑆 − 𝐴𝐸 ×𝑓 1−𝑓

(5)

where f is the fraction of E-isomer present in the PSS, and A is the corresponding absorbance at the PSS, E-spectrum. Fig. 5. shows the spectrum of pure Z-isomer of EAMP2 for 11.28 % E-isomer in PSS. The features of pure Z-isomer obtained by interpolation between the experimental spectra of the E-isomer and at PSS show a reasonable fit with TD-DFT spectrum (Fig.S10 in the supporting

lP

re

-p

ro

of

information).

ur na

Fig.5. Determination of pure Z-spectrum of EAMP2 in AcCN.

3.3.Fluorescence E→Z photoswitching behavior and molecular rotors The broad absorption bands of EAMP1 in five different solvents (1,4-DOX, THF, MeOH, glycerol, and AcCN) are characterized by overlapped π→π* (S0→S2) and n→π* (S0→S1) transitions (Fig.6A1). Less polar 1,4-DOX and viscous glycerol increase the energy of electron transitions

Jo

compared to the red-shifted absorption in polar protic EtOH and more polar aprotic solvents (THF and AcCN). Red-shifted absorption in the EtOH and THF (λmax = 416 nm) are associated with specific solvent-solute interactions by intermolecular hydrogen bonding of EtOH with phthalimide ring and better solvation by solvent-solute dipole interactions in THF related to the excited state electronic structure of anthracene fused ring. EAMP2 displays opposite behavior compared to the EAMP1 due to the 4-(Dimethylamino)phenyl moiety, where the increase of solvents polarity shows slightly blue-shifting of the absorption (Fig. 6B1). Less polar 1,4-DOX indicates that dipole moment 18

decreases during the electronic transitions and Franck-Condon excited state is more stabilized by nonpolar solvation, which is the reason for relative higher red-shifting compared to the AcCN and THF [38]. In contrast, the steady-state emission spectra of EAMP1 display a strong sensitivity of the environmental polarity and viscosity, where the EtOH red-shifted emission with a large Stokes shift is observed (Fig.6A2 and Table 3). The reason is in the excited state dye molecules form specific solute-solvent hydrogen bonding with the polar protic solvents, like MeOH or EtOH (Fig. 7A), which produces large polarization with D-A CT followed by TICT emission [7] [8][18]. Unusual emission band in 1,4-DOX (λem = 587 nm) near 119 nm red-shifted from the emissions in

of

THF and AcCN (λem = 468 and 476 nm) is observed. Generally, the structure of the studied dyes can be represented as D−CH=N−A, where A is constant acceptor substituent (phthalimide ring) and

ro

D is variated donors. Consequently, the spectral behavior will be determined by changing the electronic structure of the D - anthracene fused and 4-(Dimethylamino)phenyl rings. It is well known

-p

that the dual fluorescence of FMR’s is due to the red-shifted TICT emission, which is favored in polar (protic or aprotic) solvents compared to the blue-shifted LE emission favorable in nonpolar ones [6][10]. EAMP2 shows similar behavior like EAMP1 by red-shifted emissions in the EtOH

re

and 1,4-DOX (λem = 588 and 579 nm) and blue-shifted emissions in the THF and AcCN (λem = 554 and 561 nm) (Fig.6B2 and Table 3). The TICT and LE emissions strongly depend on the viscosity

lP

environment, where the increase of viscosity, the rotation is reduced resulting in increased LE emission [6]. EAMP1 in the glycerol displays red-shifted emission (λem = 555 nm) compared to the THF and AcCN. On the other hand EAMP2 has the lowest energy emission (λem = 612 nm) in

ur na

glycerol compared to the all used solvents. What is the reason for this unspecific dual fluorescence behavior of the dyes? A reasonable explanation can be given by the electronic structure of –CH=N− bond and n-π interaction with the phthalimide ring. Based on the second order perturbation theory analysis by resonance stabilization energy E(2), as E-isomers the dyes have considerable π-π and nπ conjugation between donor aromatic rings and acceptor phthalimide one via –CH=N- bond (Table

Jo

S1 in the Supporting information). The calculated NBO charges of E-isomers presented in Fig.7B by summing of the atomic charges of the donor and acceptor parts are associated with the resonance stabilization energy E(2) of both isomers. As E-isomers, significant D−A π-π interaction via – CH=N− is observed, while as Z-isomers, the conjugation is decreased due to the twisted conformation resulting in reduced π-donor strength of aromatic substituents and increased acceptor character of phthalimide ring, which enhances the n→π* conjugation (contribution) of lone pair (LP) of 4-substituted nitrogen (Fig. 7C). Upon illumination of the dyes to their PSS depending on 19

the polarity of the solvent, the fluorescence quantum yield (Φfl) considerably increases. In the polar AcCN, the dyes show Φfl = 62.92 % (EAMP1) and Φfl = 57.91 % (EAMP2), compared to the low yields of E-isomers around 10 % (Table 3). The Φfl in less polar 1,4-DOX also increases at PSS to 40.06 % (EAMP1) and 75.88 % (EAMP2) compared to the E-isomers (Table 3). The reason for the increased Φfl confirms the statement that reducing the π→π* (S0→S2) transitions of the Z-isomers

ur na

lP

re

-p

ro

of

enhances the contribution of n→π* (S0→S1) transitions.

Jo

Fig.6. Experimentally measured UV-VIS and steady-state fluorescence spectra of EAMP1 (A1 and A2) and EAMP2 (B1 and B2) in various solvents. The excitation wavelength was set near to the absorption maxima.

20

of ro -p

lP

re

Fig. 7. (A) Specific dye-alcohol interactions with imide carbonyl oxygen by intermolecular hydrogen bonding. (B) NBO charges of the E- and Z-isomers of EAMP dyes calculated at DFT/ B3LYP/631+G(d,p) level of theory in vacuo. Charge transfer on the molecular backbone is presented by summing the atomic charges of donor and acceptor part of the molecules. (C) Resonance energy of n→π conjugation of the 4-substituted phthalimide nitrogen as E- and Z-isomers of the dyes.

ur na

Table 3. Summarized results of absorption wavelengths (λabs), emission wavelengths (λem) and Stokes shifts (Δῡ) of the E-isomers and at PSS in different polarity solvents in room temperature (RT). The emissions of the E-isomers and at PSS in AcCN and 1,4-DOX were measured at liquid nitrogen (LN), ~ 77 K. The fluorescence quantum yield (Φ) of E-isomers and at PSS was measured in 1,4DOX, AcCN and glycerol. Lifetime measurements (τ), radiative (kr) and non-radiative (knr) decay rate constants were determined in 1,4-DOX and AcCN of E-isomers and at PSS.

Jo

Compounds

E-EAMP1

Solvents

1,4-DOX 1,4-DOX at 77 K THF EtOH Glycerol AcCN AcCN at 77 K 1,4-DOX

λabs [nm] 372

λem [nm] 587

Δῡ [cm-1] 9845

Φ [%] 12.53

-

622

-

-

416 416 389 409

468 607 555 476

2670 7563 7689 3441

31.06 13.70

-

637

-

-

374

587

9845

40.06

*

 [ns] 23.4 -

kr [s-1] 5.3×106 -

knr [s-1] 3.7×107 -

20.5 -

6.6×106 -

4.2×107 -

24.1

1.6×107

2.4×107

21

PSS-EAMP2

-

-

383 411

571 481

8596 3540

42.45 62.92

-

632

-

-

342

579

11968

8.59

-

593

-

-

331 340 354 337

554 588 612 561

12160 12404 11908 11848

23.47 7.36

-

614

-

-

368

570

9630

75.88

-

593

-

-

370 365

614 589

10740 10419

33.20 57.91

-

614

-

-

Determined by ∆ῡ = 107 × (𝜆

1

−

1

re

*

-

-

-

24.1 -

2.6×107 -

1.5×107 -

34.43 -

2.4 ×106 -

2.6×107 -

27.1 -

2.7×106 -

3.4×107 -

52.9 -

of

E-EAMP2

588

1.4×107 -

4.5×106 -

23.7 -

2.4×107 -

1.7×107 -

ro

PSS-EAMP1

-

-p

1,4-DOX at 77 K THF EtOH Glycerol AcCN AcCN at 77 K 1,4-DOX 1,4-DOX at 77 K THF EtOH Glycerol AcCN AcCN at 77 K 1,4-DOX 1,4-DOX at 77 K THF EtOH Glycerol AcCN AcCN at 77 K

𝑎𝑏𝑠

𝜆𝑒𝑚

)

lP

In order to support the assumption for the described above unspecific dual fluorescence behavior and their D-A interactions as E- and Z-isomers (at PSS), we have conducted an investigation by steady-state fluorescence spectroscopy at room temperature (RT) and liquid nitrogen (LN ~ 77 K)

ur na

as well as by lifetime measurements. The increased emission intensity of EAMP1 in AcCN at PSS in RT compared to the E-isomer is observed due to reducing the rotation of the anthracene ring and enhanced the n→π* contribution (Fig. 8A1). At LN huge red-shifting emission (λem ~ 637 nm) of both isomers indicates minor vibrational relaxations and major HOMO→LUMO transition related to the n→π* (S0→S1) excited state. This can be confirmed by comparison of excitation spectra at

Jo

RT and LN, which are given on the Fig S11 in the Supporting information. The behaviour in 1,4DOX at RT displays slightly increased emission intensity at PSS and blue-shifted emission at PSS at LN, which matches with this in RT (λem ~ 588 nm). This means that the n→π* (S0→S1) of LE excited state is preferable compared to the TICT state (Fig. 8B1 and Fig S11 in the Supporting information). The behaviour of EAMP2 is similar with EAMP1, except the considerable red-shifting emission at PSS (λem ~ 589 nm) in AcCN at RT compared to the E-isomer (λem ~ 561 nm) due to polar D-A interaction of 4-(Dimethylamino)phenyl moiety and good solvation of n→π* (S0→S1) excited state (Fig. 8A2 and Fig S11 in the Supporting information). 22

of ro -p re lP

ur na

Fig. 8. Steady-state fluorescence emissions of E- and Z- isomers at PSS in room temperature (RT) and liquid nitrogen (LN) ~ 77 K : A1) EAMP1 in AcCN, A2) EAMP2 in AcCN, B1) EAMP1 in 1,4DOX and B2) EAMP2 in 1,4-DOX. The E→Z photoisomerization was performed by Vis-light at 410 nm up to 60 min (EAMP1) and UV-light at 350 nm up to 200 min (EAMP2) until PSS and the samples ware immediately subjected to measurements. Time-resolved fluorescence decay experiment provides specific information for the excited state stability (lifetime) related to the D-A interaction. Depending on the donor strength of substituents

Jo

in the excited state and nature of the solvents the CT between D and A is characterized by different lifetime. The obtained results show good fit with a triple exponential approach as E-isomers and at PSS in AcCN and 1,4-DOX (Fig. 9 and Table 3). The values of the radiative (kr) and non-radiative (knr) decay rate constants were estimated (Eq’s 6.1 and 6.2) using the quantum yields (Φ) and lifetimes (τ) based on the previously reported approaches [8][39]. 𝑘𝑟 =

Ф



6.1

23

𝑘𝑛𝑟 =

100 − Ф

6.2



The average lifetime of EAMP2 is longer than EAMP1 before isomerization in both solvents and the results could be attributed to the observed reduced emission of EAMP1 due to non-radiative de-excitation process. At PSS EAMP1 increases its lifetime especially in less polar 1,4-DOX due to the better contribution of n→π* transition and reduced TICT state (Fig. 9A1 and 9B1), which confirms by increased fluorescent de-excitation. The longest stability of the excited state of EAMP2 at PSS is observed in 1,4-DOX (Fig. 9A2 and 9B2) and could be assigned to NCI of T-shaped

of

conformation and intensive CT between the reduced distance of D-A interaction. In summary, the extended lifetimes at PSS of both compounds in the polar and nonpolar solvents are associated with reducing the π→π* (S0→S2) transitions and enhance the contribution of n→π* (S0→S2) ones, where

Jo

ur na

lP

re

-p

ro

the Φfl and fluorescence de-excitation are considerably increased.

Fig.9. Time-resolved fluorescence decay measurements: A1) and A2) Intensity decay of E-isomers and at PSS of EAMP1 and EAMP2 in AcCN; B1) and B2) Intensity decay of E-isomers and at PSS of EAMP1 and EAMP2 in 1,4-DOX: The excitation light sources were 372 (for EAMP1 as E-isomer 24

and at PSS), 329 and 360 nm (for EAMP2 as E-isomer and at PSS) pulsed Nano-LED laser diodes. The fitting was performed by triple exponential decay function. In order to investigate the molecular rotors’ behavior, the binary mixtures of glycerol and ethanol in different ratios as E-isomers were investigated (Fig. 10). The results show that by increasing the viscosity the emission gradually enhances, which means decreases of TICT state and enhances the LE emission due to reducing the rotation hence vibrational or non-radiative relaxation. At PSS in glycerol, EAMP1 has two-fold increased red-shifted emission due to break off the rotation related to the LE state with increased Φfl = 42.45 % (Fig’s 10A1 and 11 and Table 3). EAMP2 at PSS

of

indicates weak increasing the emission due to the T-shaped conformation and specific D-A NCI

Jo

ur na

lP

re

-p

ro

related to increased Φfl = 33.20 %, where break off is not fully completed.

Fig. 10. Steady-state fluorescence spectra of EAMP’s in binary mixtures of glycerol and ethanol in different ratios as E-isomers (A1 and B1). The emissions in glycerol at PSS were of EAMP1 (A1) and EMAP2 (B1). The functional dependences of the emission intensities as E-isomers by increasing viscosity are presented for EAMP1 (A2) and EMAP2 (B2).

25

Based on the experimental and theoretical study of E/Z photoswitching performance of the dyes and its dual fluorescence in different polarity solvents the mechanism of their action can be suggested. As E-isomers, the rotation of the donors around –CH=N− is characterized by reduced emission in AcCN and 1,4-DOX with relative low quantum yield (around 10 %), this mode is assigned as rotation on (Fig. 11). After photoisomerization to the Z-isomers, the rotation off due to the steric hindrance of donor substituents with the phthalimide ring. Anthracene fused rings of EAMP1 in the switched mode unable to rotation and its quantum yield considerable increases in AcCN, 1,4-DOX, and viscous glycerol. In contrast, 4-(Dimethylamino)phenyl moiety of EAMP2 is characterized by

of

T-shaped conformation and reduced distance between D-A CT. The specific NCI retains the donor fixed around the phthalimide ring and the rotation is reduced. In terms of the steady-state and time-

ro

resolved fluorescence measurements the origin of fundamental bands is associated with excitation to LE state. Depending on the nature of solvents and aromatic substituents the CT D-A interaction

-p

is a key factor for the emissions. As E-isomers, the distance between D-A is larger compared to the Z-isomers and this is an important factor for the TICT emissions. Moreover, the photoinduced changes of the molecular geometry always cause the electron rearrangement followed by changing

re

the levels of the frontier molecular orbital. From a practical point of view, the bidirectional photoswitches allow controlling the emission between the two isomers. The applications of this

lP

phenomenon can be realized in the optical memory storage, OLED’s and high-resolution microscopy. The major problem of our dyes is low tuning of the n→π* (S0→S1) transitions between both switched state (especially for EAMP1), which is essential for their performance. Our

ur na

measurements show that photoswitch EAMP2 containing 4-(dimethylamino)phenyl ring allows better tuning capability compared to EAMP1, which is promising for the aforementioned

Jo

applications.

26

of ro -p re lP

ur na

Fig. 11. Suggested mechanism of the fundamental fluorescence bands and rotation modes of the Eand Z-isomers of EAMP’s based on the steady-state and lifetime measurements.

4. Conclusions

We present a systematic theoretical and experimental E/Z photoswitching study of two 4-substituted

Jo

azomethine phthalimide dyes. EAMP1 is characterized by twisted conformation as Z-isomer, while for the EAMP2 T-shaped conformation is preferred resulting in weak H…..π noncovalent interactions (NCI’s) between 4-(dimethylamino)phenyl moiety and phthalimide ring. The Z-isomer of EAMP2 at room temperature in the dark is stable up to 600 min compared to the EAMP1 at the same conditions, which undergoes full Z→E relaxation for less than 60 min. The back thermal relaxation to E-isomer of EAMP2 is achieved up to 90 min at 60 ºC. From the experimental data following conclusions can be drawn: (i) aromatic substituents at 4-position of phthalimide ring connected by –CH=N- have a strong influence on E/Z photoconversion and back thermal relaxation; 27

(ii) Anthracene fused ring of EAMP1 kinetic factor dominates the E→Z→E photoisomerization cycle, therefore can be denoted as T-type photochromic switch; (iii) 4-(Dimethylamino)phenyl donor moiety of EAMP2 stabilizes Z-isomer by T-shaped conformation, therefore the bidirectional switching is a thermodynamically controlled process and can be referred to as P-type photochromic switch. Steady-state fluorescence measurements in various solvents show strong sensitivity of the emission on the solvent polarity and viscosity. It was found that the dual fluorescence character of the dyes depends on the electronic structure of the donor substituents. EAMP1 with anthracene fused rings

of

show red-shifted emission in less polar 1,4-DOX compared to the AcCN. Upon illumination to PSS the fluorescence quantum yields (Φfl) considerable increases in AcCN and1,4-DOX. The reason for

ro

the increased Φfl is the reduced π→π* (S0→S2) transitions of the Z-isomers and enhanced contribution of n→π* (S0→S1) transitions. The molecular rotor behaviour was investigated in binary

-p

mixtures of glycerol and ethanol in different ratios as E-isomers The results show that increasing the viscosity the emission gradually enhances, which means decreases of TICT state and enhances the LE emission due to reducing the rotation hence vibrational or non-radiative relaxation. Time-

re

resolved fluorescence decay experiment provides specific information for the excited state stability related to the D-A interaction. Before isomerization in both solvents, the average lifetime of EAMP2

lP

is longer than EAMP1. The results could be attributed to the observed reduced emission of EAMP1 and high rate of non-radiative de-excitation. At PSS EAMP1 improves its lifetime especially in less polar 1,4-DOX due to the better contribution of n→π* transition and the reduced TICT state. After

ur na

photoisomerization to Z-isomers, the rotation off due to the steric hindrance of donor substituents with phthalimide ring resulting in increased LE emission. The extended lifetimes at PSS of both compounds in the polar and nonpolar solvents are associated with reducing the π→π* (S0→S2) transitions and enhance the contribution of n→π* (S0→S2) ones, where the Φfl considerable

Jo

increases.

Author statement

Anton Georgiev: formulated the idea, synthesized the compounds, done the DFT calculations, done the spectral measurements and written the paper Dancho Yourdanov: done the spectral measurements and discussed on the results Deayn Dimov: done the spectral measurements and discussed on the results 28

Ivaylo Zhivkov: help for the life-time measurements and read the final version of manuscript. Dimana Nazarova: discussed on the experiment and check the final paper. Martin Weiter: check the final version of the paper and provided the helpful discussion.

Conflict of Interest The authors declared that the article content has no conflicts of interest.

of

Acknowledgements

This work was financially supported by the Bulgarian National Scientific Fund project ДН 08/10

ro

of the Ministry of Education and Science, Bulgaria. The authors also gratefully acknowledge the Czech Science Foundation project No. GA19-22783S. Computational resources were provided by

-p

the CESNET LM2015042 and the CERIT Scientific Cloud LM2015085, provided under the programme "Projects of Large Research, Development, and Innovations Infrastructures". Research

re

equipment of distributed research infrastructure INFRAMAT (part of Bulgarian National roadmap for research infrastructures) supported by Bulgarian Ministry of Education and Science under

Jo

ur na

lP

contract D01-284/17.12.2019 was used in this investigation.

29

References B.L. Feringa, W.R. Browne, N.R. Branda, S. Bronco, F. Ciardelli, A. Credi, F. Diederich, J.H. van Esch, L. Fabbrizzi, J. Frey, E. Friedrichs, R. Gomes, T. Gushiken, K. Ichimura, J. de Jong, W. Knoll, V. Lemieux, P. Liljeroth, M. Maesri, C.R. Mendonca, V.I. Minkin, S.J. van der Molen, O.N. Oliveira Jr., A.J. Parola, O. Pieroni, F. Pina, A. Pucci, F.M. Raymo, D.S. dos Santos Jr., Z. Sekkat, J.P. Sauvage, T. Seki, M. Semerano, A.P. de Silva, F.C. Simmel, C. Tock, H. Tian, M. Tomasulo, D. Trauner, T. Ubukata, T.P. Vance, M. Venturi, M. Volgraf, Q.-C. Wang, B. Wannalerse, C.C. Warford, M.E.S. West, Y. Yokoyama, V. Zucolotto, Molecular Switches, 2011. doi:10.1002/9783527634408.

[2]

J. Boelke, S. Hecht, Designing Molecular Photoswitches for Soft Materials Applications, Adv. Opt. Mater. 7 (2019) 1900404. doi:10.1002/adom.201900404.

[3]

S. Crespi, N.A. Simeth, B. König, Heteroaryl azo dyes as molecular photoswitches, Nat. Rev. Chem. 3 (2019) 133–146. doi:10.1038/s41570-019-0074-6.

[4]

D. Kim, S.Y. Park, Multicolor Fluorescence Photoswitching: Color-Correlated versus Color-Specific Switching, Adv. Opt. Mater. 6 (2018) 1–30. doi:10.1002/adom.201800678.

[5]

T. Fukaminato, Single-molecule fluorescence photoswitching: Design and synthesis of photoswitchable fluorescent molecules, J. Photochem. Photobiol. C Photochem. Rev. 12 (2011) 177–208. doi:10.1016/j.jphotochemrev.2011.08.006.

[6]

S.-C. Lee, J. Heo, H.C. Woo, J.-A. Lee, Y.H. Seo, C.-L. Lee, S. Kim, O.-P. Kwon, Fluorescent Molecular Rotors for Viscosity Sensors, Chem. - A Eur. J. 24 (2018) 13692– 13692. doi:10.1002/chem.201803969.

[7]

S. Sasaki, G.P.C. Drummen, G.I. Konishi, Recent advances in twisted intramolecular charge transfer (TICT) fluorescence and related phenomena in materials chemistry, J. Mater. Chem. C. 4 (2016) 2731–2743. doi:10.1039/c5tc03933a.

[8]

M.N.B. Bernard Valeur, Molecular Fluorescence: Principles and Applications, Second Edi, Wiley‐ VCH Verlag GmbH & Co. KGaA, 2012. doi:10.1002/9783527650002.

[9]

Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures, Chem. Rev. 103 (2003) 3899–4032. doi:10.1021/cr940745l.

ur na

lP

re

-p

ro

of

[1]

Jo

[10] S. Sasaki, K. Hattori, K. Igawa, G.I. Konishi, Directional control of π-conjugation enabled by distortion of the donor plane in diarylaminoanthracenes: A photophysical study, J. Phys. Chem. A. 119 (2015) 4898–4906. doi:10.1021/acs.jpca.5b03238. [11] T. Fukaminato, T. Sasaki, T. Kawai, N. Tamai, M. Irie, Digital photoswitching of fluorescence based on the photochromism of diarylethene derivatives at a single-molecule level, J. Am. Chem. Soc. 126 (2004) 14843–14849. doi:10.1021/ja047169n. [12] F. Zhou, J. Shao, Y. Yang, J. Zhao, H. Guo, X. Li, S. Ji, Z. Zhang, Molecular rotors as fluorescent viscosity sensors: Molecular design, polarity sensitivity, dipole moments changes, screening solvents, and deactivation channel of the excited states, European J. Org. Chem. 25 (2011) 4773–4787. doi:10.1002/ejoc.201100606. [13] H. Qian, M.E. Cousins, E.H. Horak, A. Wakefield, M.D. Liptak, I. Aprahamian, 30

Suppression of Kasha’s rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission, Nat. Chem. 9 (2017) 83–87. doi:10.1038/nchem.2612. [14] R. Orita, M. Franckevičius, A. Vyšniauskas, V. Gulbinas, H. Sugiyama, H. Uekusa, K. Kanosue, R. Ishige, S. Ando, Enhanced fluorescence of phthalimide compounds induced by the incorporation of electron-donating alicyclic amino groups, Phys. Chem. Chem. Phys. 20 (2018) 16033–16044. doi:10.1039/c8cp01999a. [15] G. Saroja, T. Soujanya, B. Ramachandram, A. Samanta, 4-Aminophthalimide Derivatives as Environment-Sensitive Probes, J. Fluoresc. (1998). doi:10.1023/A:1020536918438.

of

[16] D. Singha, D.K. Sahu, K. Sahu, Anomalous Spectral Modulation of 4-Aminophthalimide inside Acetonitrile/AOT/ n-Heptane Microemulsion: New Insights on Reverse Micelle to Bicontinuous Microemulsion Transition, J. Phys. Chem. B. 122 (2018) 6966–6974. doi:10.1021/acs.jpcb.8b03901.

ro

[17] A. Tan, E. Bozkurt, Y. Kara, Investigation of Solvent Effects on Photophysical Properties of New Aminophthalimide Derivatives-Based on Methanesulfonate, J. Fluoresc. 27 (2017) 981–992. doi:10.1007/s10895-017-2033-2.

re

-p

[18] E. Krystkowiak, K. Dobek, A. Maciejewski, Origin of the strong effect of protic solvents on the emission spectra, quantum yield of fluorescence and fluorescence lifetime of 4aminophthalimide. Role of hydrogen bonds in deactivation of S1-4-aminophthalimide, J. Photochem. Photobiol. A Chem. 184 (2006) 250–264. doi:10.1016/j.jphotochem.2006.04.022.

lP

[19] D. Majhi, S.K. Das, P.K. Sahu, S.M. Pratik, A. Kumar, M. Sarkar, Probing the aggregation behavior of 4-aminophthalimide and 4-(N, N-dimethyl) amino-N-methylphthalimide: A combined photophysical, crystallographic, microscopic and theoretical (DFT) study, Phys. Chem. Chem. Phys. 16 (2014) 18349–18359. doi:10.1039/c4cp01912a.

ur na

[20] S. Chen, X. Li, L. Song, A fluorescent photochromic diarylethene based on naphthalic anhydride with strong solvatochromism, RSC Adv. 7 (2017) 29854–29859. doi:10.1039/c7ra05157c. [21] P.A. Panchenko, A.N. Arkhipova, O.A. Fedorova, Y. V. Fedorov, M.A. Zakharko, D.E. Arkhipov, G. Jonusauskas, Controlling photophysics of styrylnaphthalimides through TICT, fluorescence and: E, Z -photoisomerization interplay, Phys. Chem. Chem. Phys. 19 (2017) 1244–1256. doi:10.1039/c6cp07255k.

Jo

[22] S.F. Yan, V.N. Belov, M.L. Bossi, S.W. Hell, Switchable fluorescent and solvatochromic molecular probes based on 4-amino-N-methylphthalimide and a photochromic diarylethene, European J. Org. Chem. 2008 (2008) 2531–2538. doi:10.1002/ejoc.200800125. [23] Y. Luo, M. Utecht, J. Dokić, S. Korchak, H.M. Vieth, R. Haag, P. Saalfrank, Cis-trans isomerisation of substituted aromatic imines: A comparative experimental and theoretical study, ChemPhysChem. 12 (2011) 2311–2321. doi:10.1002/cphc.201100179. [24] P.J. Coelho, M.C.R. Castro, M.M.M. Raposo, Reversible trans-cis photoisomerization of new pyrrolidene heterocyclic imines, J. Photochem. Photobiol. A Chem. 259 (2013) 59–65. doi:10.1016/j.jphotochem.2013.03.004. [25] M.E. Belowich, J.F. Stoddart, Dynamic imine chemistry, Chem. Soc. Rev. 41 (2012) 2003– 31

2024. doi:10.1039/c2cs15305j. [26] A.C. Pratt, The photochemistry of imines, Chem. Soc. Rev. 6 (1977) 63–81. doi:10.1039/CS9770600063. [27] Y. Zhan, X. Zhao, W. Wang, Synthesis of phthalimide disperse dyes and study on the interaction energy, Dye. Pigment. 146 (2017) 240–250. doi:10.1016/j.dyepig.2017.07.013. [28]

Gaussian 09, Revision 09D.01 , M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G., Gaussian 09, (n.d.).

of

[29] J. Calbo, C.E. Weston, A.J.P. White, H.S. Rzepa, J. Contreras-García, M.J. Fuchter, Tuning azoheteroarene photoswitch performance through heteroaryl design, J. Am. Chem. Soc. 139 (2017) 1261–1274. doi:10.1021/jacs.6b11626.

ro

[30] A. Airinei, N. Fifere, M. Homocianu, C. Gaina, V. Gaina, B.C. Simionescu, Optical properties of some new azo photoisomerizable bismaleimide derivatives, Int. J. Mol. Sci. 12 (2011) 6176–6193. doi:10.3390/ijms12096176.

-p

[31] M. Iftime, R. Ardeleanu, N. Fifere, A. Airinei, V. Cozan, M. Bruma, New copoly(ether sulfone)s containing azobenzene crown-ether and fluorene moieties, Dye. Pigment. 106 (2014) 111–120. doi:10.1016/j.dyepig.2014.03.004.

lP

re

[32] K. Bujak, H. Orlikowska, A. Sobolewska, E. Schab-Balcerzak, H. Janeczek, S. Bartkiewicz, J. Konieczkowska, Azobenzene vs azopyridine and matrix molar masses effect on photoinduced phenomena, Eur. Polym. J. 115 (2019) 173–184. doi:10.1016/j.eurpolymj.2019.03.028.

ur na

[33] K. Bujak, H. Orlikowska, J.G. Małecki, E. Schab-Balcerzak, S. Bartkiewicz, J. Bogucki, A. Sobolewska, J. Konieczkowska, Fast dark cis-trans isomerization of azopyridine derivatives in comparison to their azobenzene analogues: Experimental and computational study, Dye. Pigment. 160 (2019) 654–662. doi:10.1016/j.dyepig.2018.09.006. [34] D. Bléger, S. Hecht, Visible-Light-Activated Molecular Switches, Angew. Chemie Int. Ed. 54 (2015) 11338–11349. doi:10.1002/anie.201500628. [35] M. Kathan, S. Hecht, Photoswitchable molecules as key ingredients to drive systems away from the global thermodynamic minimum, Chem. Soc. Rev. 46 (2017) 5536–5550. doi:10.1039/c7cs00112f.

Jo

[36] M.M. Russew, S. Hecht, Photoswitches: From molecules to materials, Adv. Mater. 22 (2010) 3348–3360. doi:10.1002/adma.200904102. [37] M. V. Peters, R.S. Stoll, A. Kühn, S. Hecht, Photoswitching of basicity, Angew. Chemie Int. Ed. 47 (2008) 5968–5972. doi:10.1002/anie.200802050. [38] C. Retichard, Christian Reichardt Solvents and Solvent Effects in Organic Chemistry, 2003. doi:10.1016/B978-0-12-416677-6.00029-9. [39] K. Suhling, P.M.W. French, D. Phillips, Time-resolved fluorescence microscopy, Photochem. Photobiol. Sci. 4 (2005) 13–22. doi:10.1039/b412924p. 32

33

of

ro

-p

re

lP

ur na

Jo

34

of

ro

-p

re

lP

ur na

Jo