A new dithienylethene dimer with terminal tertiary amine redox centers: Electrochemical, UV–vis–NIR spectral and electronic transfer charges induced by a stepwise photochromic process

Article abstract of DOI:10.1016/j.dyepig.2016.09.029

Photochromic diarylethene dimers (R-DTE-C[tbnd]C[sbnd]C[tbnd]C-DTE-R, R?=?benzene, 1oo; R?=?triphenylamine, 2oo; DTE?=?dithienylethene) with remarkable stepwise photochromic process were synthesized successfully. Dimer 2oo, whose two terminal redox-active

Full text of DOI:10.1016/j.dyepig.2016.09.029

Accepted Manuscript
A new dithienylethene dimer with terminal tertiary amine redox centers:
Electrochemical, UV-vis-NIR spectral and electronic transfer charges induced by a
stepwise photochromic process
Daobin Zhang, Congbin Fan, Chunhong Zheng, Shouzhi Pu
PII:
S0143-7208(16)30718-5
10.1016/j.dyepig.2016.09.029
DYPI 5481
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 16 June 2016
Revised Date: 16 August 2016
Accepted Date: 12 September 2016
Please cite this article as: Zhang D, Fan C, Zheng C, Pu S, A new dithienylethene dimer with
terminal tertiary amine redox centers: Electrochemical, UV-vis-NIR spectral and electronic transfer
charges induced by a stepwise photochromic process, Dyes and Pigments (2016), doi: 10.1016/
j.dyepig.2016.09.029.
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Graphical Abstract
A new Dithienylethene Dimer with Terminal Tertiary Amine
Redox Centers: Electrochemical, UV-vis-NIR Spectral and
Electronic Transfer Charges Induced by a Stepwise
Photochromic Process
Daobin Zhang, Congbin Fan, Chunhong Zheng, Shouzhi Pu*
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal
University, Nanchang, Jiangxi 330013, PR China
*Corresponding author: E-mail address: pushouzhi@tsinghua.org.cn; Tel./Fax: +86-791-83831996
A new diarylethene dimer, with triphenylamine as redox centers, separated by 1,3-butadiynyl has
been designed and prepared (see scheme). The dimer exhibited stepwise photochromic reactions
by extending irradiation time with UV light, thus successfully achieve photoswitchable charge
delocalization and two terminals electronic communication.

ACCEPTED MANUSCRIPT
A new Dithienylethene Dimer with Terminal Tertiary Amine Redox
Centers: Electrochemical, UV-vis-NIR Spectral and Electronic
Transfer Charges Induced by a Stepwise Photochromic Process
Daobin Zhang, Congbin Fan, Chunhong Zheng, Shouzhi Pu*
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang,
Jiangxi 330013, PR China
*Corresponding author: E-mail address: pushouzhi@tsinghua.org.cn; Tel./Fax: +86-791-83831996
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A new Dithienylethene Dimer with Terminal Tertiary Amine Redox
Centers: Electrochemical, UV-vis-NIR Spectral and Electronic
Transfer Charges Induced by a Stepwise Photochromic Process
Daobin Zhang, Congbin Fan, Chunhong Zheng, Shouzhi Pu*
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang,
Jiangxi 330013, PR China
*Corresponding author: E-mail address: pushouzhi@tsinghua.org.cn; Tel./Fax: +86-791-83831996
ABSTRACT
Photochromic diarylethene dimersꢀR-DTE-C≡C-C≡C-DTE-RꢁR = benzene, 1oo; R = triphenylamine, 2oo; DTE
= dithienylethene) with remarkable stepwise photochromic process were synthesized successfully. Dimer 2oo,
whose two terminal redox-active triphenylamine centers are separated by two photochromic DTE-acetylide
moieties, could accomplish photoswitchable charge delocalization between the N···N electro-active centers. Upon
UV-light irradiation, due to the progressive increase of π-conjugated systems as the order of 2oo → 2co → 2cc, the
electronic interaction of two N-centers in 2oo is increasingly facilitated following the stepwise photocyclization.
Furthermore, for monocationic species [2oo]+/[2co]+/[2cc]+, their absorption strength was gradually increased
during the conversion of [2oo]+→[2co]+→[2cc]+, which demonstrated that the extent of charge delocalization along
the molecular backbones was progressively enhanced during the stepwise photocyclization. The lower electronic
density of reactive carbon atoms during the oxidation of N0 to N+ could result more difficult photocyclization
reaction to produce oxidized species 2oo2+/2co2+. Additionally, compound 2 exhibited eight switchable states
through stepwise photochromic and reversible redox processes.
Keyword: Dithienylethene; Stepwise Photochromism; Electronic transfer; Mixed-valence system.
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1. Introduction
Photo- or electro-driven switches with a reversible switch between two different isomers are receiving
increasing attentions in the development of optical memory, optoelectronics, logic gates and switching devices over
the last few years [1-8]. Compared to most other systems, photochromic fulgides [9], dihydropyrenes [10], and
dithienylethene [11] (DTE) derivatives often offer enhanced thermal stability of the two interconverting isomers
and heightened fatigue resistance. In addition, more and more studies are focused on the dithienylethene derivatives
due to their feasible synthesis, convenient functionalization, multistep and multiple photochromism of
multidithienylethene units [3-5, 12-16]. Upon UV-light irradiation, the π conjugation of the ring-closed isomer of
DTE unit was significantly enhanced due to the C-C bond formation, accompanied with obvious enhancement of
electronic delocalization [17-26]. Encouraged by this strategy, a large number of various complexes with two
inorganic redox-active centers (electro-active iron-, or ruthenium-containing organometallic termini) bridged with
the DTE moiety were designed and synthesized. It is demonstrated that their redox splitting becomes much more
pronounced in the ring-closed form compared to that in ring-opened species. Therefore, the metal-metal electronic
communication can be successfully modulated through alternative UV and visible-light irradiation on the
photoswitchable DTE moiety [18-26]. On the other hand, in organic mixed-valence system, the most popular
redox-active centers are para-subsitituted triarylamines because of the excellent stability of their radical cation
forms and easily synthetic, resulted from relatively recent developments in palladium-catalyzed N-C coupling
reactions [27]. When two triarylamine redox centers are substituted to the photoswitchable core DTE, in the open
forms of monocations, the odd electron is localized on one branch of the another symmetrical systems attributed as
Robin-Day class I mixed-valence system, nevertheless, it is delocalized over the entire molecule in the ring-closed
forms ascribed as Robin-Day class II or III mixed-valence system [17a].
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Recently, the combined systems involve two or more photochromic switches are continuously obtaining
attention because they can exhibit multicolors and multistates upon irradiation with appropriate wavelengths of
light, which possess particular potential to achieve multifrequency optical memories and data storage [12a, 16]. In
general, for a molecule with two different DTE moieties simultaneous, stepwise or selective photochromic
processes could be achieved and result in four (oo, co, oc, and cc) photoswitchable isomers with appropriate light
irradiation [12, 28-30]. However, upon irradiation with UV light, stepwise photocyclization is usually unattainable
for that oo species with two identical DTE moieties, due to the facile energy transfer from ring-open DTE to
adjacent ring-closed moiety in co or oc that prohibits further photocyclization into fully ring-closed form (cc) [16,
31-32]. It is demonstrated that incorporation of two identical DTE-acetylide moieties into metal-coordinated
systems (Ru [13c, 13d, 33], Pt [13b, 14], or Au [13a]) is one of the feasible approaches to achieve the stepwise
photochromic reactions oo
oc
cc, finally obtaining all three photochromic isomers (oo, oc, and cc).
Up to present, although dithienylethene dimer, trimer, and tetramer composed of two or more DTE moieties
connected with ethynylene bonds do not display stepwise photochromic processes in a certain UV-light irradiation
time [15], we believe that dithienylethene dimer may possess stepwise photochromic properties through changing
electron configuration of bridged spacers or DTE moieties, or extending the irradiation time [28b]. Therefore, in
this study, two DTE dimers (1oo with phenyl terminal group and 2oo with triarylamine terminal group) with two
dithienylethene moieties with 1,3-butadiene as bridged group were designed and synthesized. Upon irradiation with
UV-light, both of dimers 1oo and 2oo exhibited stepwise photochromic processes, and stepwise photocyclization
reactions of compound 2oo showed more complete than that of compound 1oo (content of cc form: 2cc, 71.9%; 1cc,
33.3%), probably because the triarylamine groups partially overcame their intramolecular energy transfer. Different
from other stepwise photochromic molecule with two or more redox centers [13d, 33], dimer 2 afforded only eight
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switchable states through its stepwise photochromic and electrochromic processes due to its partially oxidative
cycloreversion reactions. Based on the advantage of the stepwise photochromic ability in dimer 2, for
mixed-valence monocationic species [2oo]+/[2co]+/[2cc]+, charge delocalization in mixed-valence systems was
progressive enhanced following the conversion of [2oo]+ꢂ[2co]+ꢂ[2cc]+. The studies on electrochemistry,
UV-vis-NIR spectroscopy, and theoretical computation (TD-DFT, time-dependent density functional theory)
implied that the electronic communication between two triarylamine centers could be well mediated through
stepwise photochromic reactions.
<Scheme 1>
2. Experiments
2.1 General Procedures and Materials.
All the operations were performed under a dry argon atmosphere using Schlenk techniques and a vacuum-line
system. Solvents were dried, distilled, and degassed before use, except those for UV-vis-near-IR spectral
measurements were of spectroscopic grade. The reagents phenylboronic acid, 4-(diphenylamino)benzeneboronic
acid, bis(triphenylphosphine)palladium (II) chloride (Pd(PPh3)2Cl2) and copper (I) iodide (CuI) were commercially
available. L1o and L2o were prepared according to the literature procedures [34].
2.1.1 1oo. A mixture of L1o (234 mg, 0.50 mmol), Pd(PPh3)2Cl2 (18 mg, 0.025 mmol) and CuI (10 mg, 0.05
mmol) was added into Et3N (20 mL). The mixture was heated with stirring at 50 ꢃ for 12 h under atmosphere.
The solvent was removed under vacuum. The residue was purified through a silica gel column chromatography
1
using petroleum ether as eluent. Yield: 78%. H NMR (CDCl3, 400 MHz),
δ (ppm): 7.54-7.52 (m, 4H), 7.41-7.37
(m, 4H), 7.34-7.31 (m, 4H), 7.23 (s, 2H), 1.95-1.94 (d, 12H, J = 2.40 Hz). 13C NMR (CDCl3, 100 MHz),
δ
(ppm):144.2, 141.6, 140.3, 133.2, 132.1, 128.0, 127.0, 124.6, 124.4, 124.3, 121.1, 119.0, 77.4 (-CꢄC), 74.9 (-Cꢄ
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C), 13.5 (-CH3), 13.4 (-CH3). IR (KBr, υ, cm-1): 2146 m (C≡C). ESI-MS: m/z: 934 [M]+. Anal. calcd. for
C46H26F12S4, C, 59.10; H, 2.80; Found: C, 59.57; H, 2.73.
2.1.2 2oo. The synthetic procedure of this compound was the same as that of 1oo using L2o instead of L1o.
Yield:75 %. 1H NMR (CDCl3, 400 MHz),
(s, 4H), 7.25 (s, 2H), 7.12-7.10 (d, 10H, J = 8.28 Hz), 7.07-7.05 (d, 8H, J = 8.20 Hz), 1.96-1.93 (d, 12H, J =11.80
Hz). 13C NMR (CDCl3, 100 MHz),
δ (ppm): 7.39-7.37 (d, 4H, J = 8.52 Hz), 7.33 (s, 2H), 7.29 (s, 2H), 7.27
δ
(ppm): 147.8, 147.3, 145.2, 142.5, 134.3, 129.4, 126.9, 126.4, 125.4, 124.6,
123.4, 123.3, 121.1, 119.9, 78.3 (-CꢄC), 76.0 (-CꢄC), 14.6 (-CH3), 14.5 (-CH3). IR (KBr, υ, cm-1): 2142 m
(C≡C). ESI-MS: m/z: 1269 [M]+. Anal. calcd. for C70H44F12N2S4, C, 66.24; H, 3.49; N, 2.21; Found: C, 66.38; H,
3.61; N, 2.33.
2.2 Physical Measurements
1H and 13C NMR spectra were performed on a Bruker Avance III (400M Hz) spectrometer with SiMe4 as the
internal reference. UV−vis absorption spectra were measured on a Perkin-Elmer Lambda 25 UV−vis
spectrophotometer. The UV−vis−NIR spectra were measured on a Perkin-Elmer Lambda 900 UV−vis−NIR
spectrometer. Infrared spectra (IR) were recorded on a Magna 750 FT-IR spectrophotometer with KBr pellets.
Elemental analyses (C, H and N) were carried out on a Perkin-Elmer model 240 C elemental analyzer. Electrospray
ionization mass spectrometry (ESI-MS) was recorded on a Finnigan DECAX-30000 LCQ mass spectrometer using
dichloromethane-methanol as mobile phases. UV light was produced using a ZF5 UV lamp (312 nm), and visible
light irradiation (672 nm) was carried out by using a LZG220 V 1kW tungsten lamp with cutoff filters. The
quantum yields were determined by comparing the reaction yields of the diarylethenes with that of
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene. The cyclic voltammogram (CV) and differential pulse
voltammogram (DPV) were measured using a potentiostat/galvanostat model 263A in dichloromethane solutions
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containing 0.1 M (n-Bu4N)(PF6) as the supporting electrolyte. CV was performed at a scan rate of 100 mV s-1 and
DPV at a rate of 20 mV s-1 with a pulse height of 40 mV. Platinum and glassy graphite were used as the counter and
working electrodes, respectively, and the potential was measured against Ag/AgCl reference electrode.
The crystal of 2oo was mounted on glass fibre and transferred to a Bruker SMART CCD area detector. The
crystallographic measurement for the crystal of 2oo was carried out using a Bruker SMART APEX2 CCD
diffractometer, σ scan, graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) under room temperature. The
structure was solved by direct methods and refined by full-matrix least-squares on F2 values using the program
SHELXS-97. All non-hydrogen atoms were subjected to anisotropic refinement. Hydrogen atoms were generated
theoretically and ridden on their parent atoms in the final refinement. For the full-matrix least-squares refinements
[I >2σ(I)], the unweighted and weighted agreement factors of R1 = ꢅ(Fo-Fc)/Fo and wR2 = [ꢅ(Fo2-Fc2)2/ꢅ
wFo4]1/2 were used. Further details on the crystal structures have been deposited in the Cambridge Crystallographic
Data Centre as supplemental publication CCDC 1446395 for 2oo. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033 or email:
deposit@ccdc.cam.ac.uk).
2.3 Computation details
All the calculations were implemented by using Gaussian 09 program package [35]. The density functional
theory (DFT) [36] method with hybrid functional M06-2X [37] of Truhlar and Zhao was firstly used to optimize the
geometrical structures of compounds 1oo, 1co, 1oo, 2oo, 2co, 2cc, 2oo+, 2co+, 2cc+, 2oo2+, and 2co2+ in ground
states without considering symmetry restriction. Then, based on the optimized geometrical structures, 100 singlet
excited states were calculated to determine the vertical excitation energies by time-dependent DFT (TD-DFT)
method [38] at the same functional as that used in the geometrical optimization. In the calculation of excited states,
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the solvent effect in dichloromethane, acetone, and acetonitrilemedia was taken into account by performing the
self-consistent reaction field (SCRF) calculations by using the polarizable continuum model method (PCM) [39]. In
all these calculations, the atoms were described by all-electron basis set of 6-31G(p,d). Visualization of the
optimized structures, frontier molecular orbitals, and charge density difference (CDD) map were performed by
GaussView. Ros & Schuit method (C-squared population analysis method, SCPA) [40] was supported to analyze
the partition orbital composition by using Multiwfn 3.3.8 program [41]. The simulated UV-Vis absorption spectra
were performed by Gaussian function with the full width at half-maximum of the gaussian curves (FWHM) of 3000
cm-1.
<Scheme 2>
3. Results and Discussion
The initial precursor L1o and L2o were obtained according to the literature procedures [34]. Two dimer
molecules were prepared in high yield (78% for 1oo, 75% for 2oo) through the coupling reaction of their
responsive monomer (L1o for 1oo, L2o for 2oo) in the NEt3 solution under 50ꢃ exposed to air, with Pd(PPh3)2Cl2
1
and CuI as catalysts (Scheme 2). Products 1oo and 2oo were characterized by ESI-MS spectrometry, H and 13C
NMR spectroscopy. And the chemical structure of compound 2oo was confirmed by X-ray single-crystal diffraction
analysis (the crystal data was summarized in the Supplementary data).
When colorless L2o in CH2Cl2 solution was irradiated with UV light at 312 nm, the color turned into blue,
accompanied with a broad absorption band of ring-closed isomer L2c centered at 611 nm (Fig. S1) appeared. The
conversion percentage of L2o was >95% at the photostationary state (PSS). Compared with L2o, L1o had been
reported with a ring-closed absorption band centered at 584 nm [13a].
<Fig. 1>
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3.1 Photochromic studies
As shown in Fig. 1, upon irradiation at 312 nm, the absorption intensities at 290 and 348 nm of compound 1oo
were gradually decreased. Meanwhile, two new bands centered at 400 and 622 nm appeared progressively and the
colorless solution turned green in 2 min, indicating the ongoing photocyclization reaction of L1o into the singly
ring-closed species 1co (Scheme S1). When the solution of singly ring-closed species 1co was further irradiated
with UV light at 312 nm (ca. 450 min), the absorption intensity of the band centered at 622 nm was continually
increased and the absorption maximum progressively red-shifted to 646 nm attributed to the further
photocyclization reaction of 1co → 1cc (Scheme S1). As depicted in Fig. 1A, the maximum of ring-closed
absorption was red-shifted from the initial 622 nm to 646 nm, indicating that stepwise photocyclization reactions
occurred through 1oo → 1co → 1cc [13, 14, 33]. On the other hand, the green color of 1cc was bleached by
irradiation with the light of 672 nm, the absorption spectrum returned to the initial state of 1oo due to
corresponding stepwise cycloreversion reactions of 1cc →1co → 1oo.
As shown in Fig. 1B, the UV-Vis absorption of 2oo in CH2Cl2 was similar to that of dimer 1oo under the
irradiation at 312 nm. The progressive red-shift of the asymmetric visible absorption band with maximum at from
662 to 692 nm was ascribed to stepwise photocyclization reactions of 2oo → 2co → 2cc. Notably, the redshift of
the ring-closed absorption maximum (1, ca. 24 nm; 2, ca. 30 nm) during the stepwise conversion of oo →co → cc
was more significant than that in other metallic bridged or organic bridged compounds (ca. 14-20 nm) [13-14, 28].
Additionally, both of dimers 1oo and 2oo were very sensitive to UV irradiation and could rapidly convert to singly
ring-closed form (co) (1oo → 1co, ca. 2 min; 2oo → 2co, ca. 5 min). Under by further UV-light irradiation and
extended reaction time to overcome the partial intramolecular energy transfer from the excited ring-open unit to the
ring-closed unit, two ring-closed forms (cc) from co species (1, 7.5 h (1co → 1cc); 2, 9.5 h (2co → 2cc)) were also
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produced [15].
<Table 1>
<Fig. 2>
When 1oo and 2oo in CDCl3 were irradiated with UV light at 312 nm, the 1H NMR spectral signals were
changed corresponding to the occurrence of their photocyclization reactions, which further confirmed the
photochromic reactions of 1oo → 1co → 1cc (Fig. S5) and 2oo → 2co → 2cc, respectively.
As shown in Scheme 1, there were three states (2oo, 2co and 2cc) when 2oo in CDCl3 was irradiated with UV
light at 312 nm to the photostationary state (PSS). The 1H NMR spectral changes of 2oo with two DTE units in
CDCl3 upon irradiation of 312 nm were very unambiguous to elucidate the stepwise photochromic reaction of 2oo
→ 2co → 2cc. When 2oo in CDCl3 was irradiated with UV light at 312 nm, the 1H NMR spectral signals (Fig. 2A)
at 1.955 ppm and 1.925 ppm of the methyl 2a and methyl 2b were gradually weakened, whereas two new signals at
low field of 2.149 ppm (2a’) and 2.127 ppm (2b’) appeared due to the closed-isomer DTE unit of 2co. Meanwhile,
another two peaks at 1.964 ppm (2c’) and 1.925 ppm (2d’, overlap with 2b of 2oo) appeared and were increased
gradually, which could be ascribed to 1H NMR signals of open-isomer DTE unit of 2co due to the photocyclization
reaction of 2oo → 2co (Scheme 1). Followed the persistent of irradiation at 312 nm to the PSS, the signals at 2.149
ppm (2a’), 2.127 ppm (2b’), 1.964 ppm (2c’) and 1.925 ppm (2d’) were gradually decreased, and two new peaks at
2.152 ppm (2a’’) and 2.129 ppm (2b’’) were observed due to the further photocyclization of the second DTE unit to
produce the dually ring-closed form 2cc. As showed in Scheme 1 and Fig. 2, NMR changes of thienyl protons
demonstrated the process of stepwise ring-closed reaction of 2oo → 2co → 2cc as well. When the thienyl proton
signals of 2oo at 7.331 (H1o) and 7.291 ppm (H2o) were gradually reduced following the formation of 2co,
ring-closed thienyl protons in L2c were observed at 6.559 (H1c’) and 6.516 ppm (H2c’), and the thienyl protons of
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ring-opened L2o were observed at 7.335 (H1o’) and 7.296 ppm (H2o’) with a slight low-field shift. When the
conversion of 2co → 2cc occured, the thienyl protons signals of H1o’, H2o’, H1c’ and H2c’ were gradually
decreased, accompanied by two new signals appeared progressively at 6.562 (H1c’’, overlapped with H1c’) and
6.578 ppm (H2c’’).
Fig. 3 shows the contents of oo, co and cc forms of compound 1 and 2 against irradiation time based on the
1H NMR spectral changes. In both compounds 1 and 2, oo was first converted to co quickly and then to cc slowly
upon irradiation at 312 nm and the singly ring-closed intermediate co was accessed with ꢆ95 % of maximum
percentage. At the final product on PSS, according to the H signal integral ratio of methyl between co and cc, the
content of 1cc and 2cc occupied 33.3% and 71.9%, respectively.
According to NMR data, both compounds 1 and 2 displayed obviously stepwise photochromism, in which
singly ring-closed state (co) content processed ꢆ95 % rapidly. However, it required relatively long time to
accomplish dually ring-closed reaction to PSS (7.5 h for 1; 9.5 h for 2, 2×10-5 M) due to the intramolecular energy
transfer from the excited ring-open unit to the ring-closed unit. Particularly, the content of dually ring-closed specie
(cc) from compound 1 was only 33.3%.
<Fig. 3>
<Table 2>
3.2 Electrochemical studies
The electrochemical data of 2oo, 2co and 2cc versus Ag/AgCl in 0.1 M (n-Bu4N)(PF6)-dichloromethane
solutions were summarized in Table 2. Their electrochemical behaviors were obtained using cyclic and differential
pulse voltammetry through pretreatment consisted of UV irradiation at 312 nm in dichloromethane solution. The
plots of cyclic voltammogram (CV) and differential pulse voltammogram (DPV) in 0.1 M (n-Bu4N)
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(PF6)-dichloromethane solutions were shown in Fig. 4.
<Fig. 4>
0/+
Compound 2oo displayed a reversible wave at +1.05 V versus Ag/AgCl due to the N2
two-electron-oxidation process, illustrating that there is hardly electronic communication between these two
terminal triarylamine redox-active centers. When singly ring-closed species 2co was formed upon irradiation of 2oo
with UV light at 312 nm, the wave at +1.05 V gradually decreased, and a new quasi-reversible oxidation wave was
observed at +0.85 V (vs Ag/AgCl). The wave at +0.85 V might be due to the one electron oxidation process of
N-centered coupled with ring-closed DTE unit, which showed a significant cathodic shift compared to species 2oo.
Meanwhile, the residual reversible wave at +1.05 V could be ascribable to a mixture of the oxidations of
ring-closed DTE unit center and N-center connected with ring-open DTE unit. Although the potential difference
(ΔE1/2 = E1/2 – E1/2′) was 0.20 V, it could not imply that there was electronic communication between two
triarylamine centers in 2co. Because this splitting of the redox couple resulted from unsymmetrical structure of 2co.
Upon further irradiation of singly ring-closed species 2co with UV light at 312 nm, the waves at +0.85 V was
gradually reduced but did not completely disappear because the ring-closed species 2co had 28.1% percentage at
PSS, whereas two new sequential reversible wave at +0.76 and +0.87 V occurred progressively due to stepwise
one-electron-oxidation processes of two triarylamine centers. The potential differencesΔE1/2 between the two
waves was 0.11 V and the comproportionation constant Kc (Kc = exp(
Δ
E1/2/25.69)) was 4.19 ×104, clearly
suggesting that there was a significant mutual interaction between the two redox-active triarylamine centers in the
dually ring-closed isomer 2cc. The other two waves at +1.07 V and + 1.38 V occurred progressively due to the
sequential oxidation of two ring-closed DTEs in 2cc. It is noticeable that the potential difference
ΔE1/2 of stepwise
oxidation of two identical DTEs in 2cc is 0.31 V, corresponding to the comproportionation constant Kc of 1.18
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×105, indicating the significant electronic communication between the two ring-closed DTE units through the
0/+
1,3-butadiynyl group. Compared to none redox splitting in 2oo (N2 two-electron-oxidation process at +1.05 V),
there was two one-electron-oxidation processes in 2co occurred at +1.05 V and +0.85 V for open unit L2o and
close unit L2c respectively, then redox splitting became significant (+0.87 V and +0.76 V for sequential
one-electron-oxidation processes, ꢀE1/2 = 0.11 V) in 2cc finally. These results implied that electronic delocalization
exhibited progressive enhancement with stepwise ring-close of DTEs unit upon the irradiation at UV light.
3.3 Spectroscopic studies on oxidized species
It is well known that the electronic distribution along the molecular backbones could influence the
photochromic properties of diarylethene derivatives, such as oxidative cyclization and oxidative cycloreversion [3a,
33, 42-44]. The optical properties of 2oo and 2cc upon oxidation by Ce(IV)(NH4)2(NO3)6 (CAN) were also
investigated by means of UV/vis/NIR spectroscopy in dichloromethane solution. As shown at Fig. 5A, when
compound 2oo was readily oxidized to [2oo]2+ in dichloromethane, a new NIR band at ca. 943 nm was observed
due to π → π* transitions from DTE to the triarylamine center, which was consistent with the oxidization behavior
of species [L2o]+ obtained by adding one equivalent CAN to L2o dichloromethane solution (Fig. S2). When the
solution of [2oo]2+ was under UV light irradiation at 312 nm, with gradual decrease of the NIR band at ca. 943 nm,
a new low-energy band centered at 633 nm occurred progressively due to photocyclization reaction at one of the
two DTE units to produce singly ring-closed species [2co]2+. Upon further irradiation of the solution at 312 nm,
both the intensity and maximum of ring-closed absorption band at 633 nm did not display any changes, illustrating
that dually ring-closed species [2cc]2+ could not be generated. On the other hand, when 2cc was completely
oxidized in CAN-acetonitrile solution, the dually ring-closed absorption band at 692 nm was progressively
blue-shifted to 633 nm with decreasing intensity. Upon the irradiation with UV light, the band at 633 nm was
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gradually increased without any shift, which was consistent with irradiation changes of ring-open isomer [2oo]2+
(Fig. 5). When L2c was stepwise oxidized into [L2c]+ by titrating with CAN-acetonitrile solution, the ring-closed
absorption band was gradually decreased without disappear, which was progressively enhanced as continuous
irradiation with UV-light (Fig. S3). These results suggested that 2cc did undergo a partially oxidative
cycloreversion reaction, and only generated one ring-closed reaction to produce [2co]2+ because of the
electronic-deficiency of oxidized species [2oo]2+ [45].
<Fig. 5>
<Fig. 6>
To further determine the degree of charge delocalization following the stepwise photocyclization reactions, the
changes of UV-vis-NIR absorption spectrum of monocationic species [2oo]+, [2co]+ and [2cc]+, acquired through
chemical oxidation with corresponding solutions of neutral compounds upon the addition of CAN (one equiv),
were detailedly studied. As depicted in Fig. 6A, when the 2cc in dichloromethane was stepwise oxidized upon the
addition of CAN to form [2cc]+, with a gradual decrease and blue-shift of the ring-closed absorption, the band
centered at 1020 nm grew progressively possibly due to N0-center → N+-center IVCT absorption, which exhibited
an obvious reduction with the addition of two equivalents of CAN (Fig. 5B). Moreover, the solvent-dependent
behavior of the NIR band centered at 1020 nm of [2cc]+ further assigned the NIR absorption band as IVCT
character (Fig. S4) [46]. The experimentally estimated half-height bandwidth (∆ν1/2) of the NIR band is 1873 cm-1,
1/2
which is some narrower than the values calculated (∆ν1/2 = (2310νmax
Meanwhile, combining with electrochemical studies (
)
= 4780 cm-1) by Hush model theory.
Δ
E1/2 = 0.11 V, Kc = 4.19 ×104‐for compound 2cc), a
moderate coupling existed between two N-centers and the monocationic [2cc]+ behaved as a class II mixed-valence
compound. Similarly as [L2o]+, compound [2oo]+ displayed an intense NIR band centered at 943 nm, which may
14

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be ascribed mainly to DTE/C≡C unit → N+ LLCT states. It is noticeable that compounds [2co]+ and [2cc]+
exhibited broad NIR band centered at ca.998 nm and 1020 nm, respectively (Fig. 6B), and the intensity of the NIR
band in [2cc]+ was nearly double that in [2co]+. These results confirmed that that the degree of charge
delocalization along the molecular backbone was progressively enhanced during stepwise photocyclizaiton
processes of [2oo]+→[2co]+→[2cc]+.
3.4 Theoretical Studies
In order to get a deeper insight into the electronic structures and transition, and spectroscopic properties of the
eight ring-open/closed isomers, time-dependent density functional theory (DFT/TD-DFT) was employed for
computational studies.
For 1oo, both the HOMO and LUMO are predominantly resident on DTE units and 1,3-butadiene moiety with
slight population on phenyl group (4.1% for HOMO, 0.1% for LUMO). The low-energy absorption results from
DTE and IL, and mixed with some DTE →1,3-butadiene LLCT transitions. For 1co, both the HOMO and LUMO
are mostly populated on the ring-closed DTE units (81.6% for HOMO, 76.7% for LUMO) and much less on
1,3-butadiene (10.0% for HOMO, 12.8% for LUMO), thus the ring-closure absorption of 1co is ascribed to the IL
transition of the ring-closed DTE. Similarly as 1oo, the HOMO and LUMO of 1cc is also delocalized along DTE
(80.7% for HOMO, 80.8% for LUMO) and 1, 3-butadiene (14.9% for HOMO and LUMO). The dually ring-closure
absorption of 1cc arises from IL transition of the ring-closed DTE and 1,3-butadiene states. Notably, the calculated
energy gap between HOMO and LUMO progressively decreases following the conversion of 1oo → 1co → 1cc
(5.7 eV for E1oo, 4.1 eV for E1co, 3.7 eV for E1cc, respectively), which is in conformity with the measured
low-energy absorption bands experimentally (348 nm for 1oo, 622 nm for 1co, 646 nm for 1cc, respectively).
The HOMO of dimer 2oo is mainly distributed on triarylamine centers (86.7%) and DTE units (13.2%), and
15

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the LUMO is predominantly focused on DTE units (73.0%) and 1, 3-butadiene (26.9%). The low-energy absorption
in the range of λ = 260-400 nm for HOMO + n (n= 0, 1, 2) → LUMO + n (n = 1, 2, 5, 9, 10) is typical of DTE and
triarylamine centers IL, mixed with triarylamine centers → DTE states. Both the HOMO and LUMO of 2co is
essentially populated on the ring-closed DTE unit (54.5 %, 74.2 %, respectively). Therefore, the ring-closure
absorption of 2co from the HOMO ꢀ LUMO is attributed to the IL transition of the ring-closed DTE and
triarylamine center connected with ring-closed DTE. Although LUMO + 1 and LUMO +4 of 2co are resident on
the ring-opened DTE unit, only LUMO +1 is phase-matching with bonding character and favorable for further
photocyclization. The contribution of two UV absorption transitions involving LUMO + 1 (HOMO → LUMO + 1,
S5 and S7) are 24% totally, thus explaining the corresponding conversion yield of 2co → 2cc (71.9% at the PSS).
For 2cc, the HOMO is distributed along ring-closed DTE moieties with minor population at triarylamine centers
(30.2 %). In contrast, the LUMO is primarily focused on two ring-closed DTE moieties and less contribution on
triarylamine centers (6.56%). On the other hand, the transition energy of HOMO - LUMO follows E2oo (4.9 eV) ꢁ
E2co (3.9 eV) ꢁ E2cc (3.5 eV), coinciding with a gradual red shift of ring-closure absorption measured
experimentally.
Following the conversion of 2oo → 2co → 2cc, the distribution of HOMO on triarylamine centers exhibits
progressively decrease (86.7% → 37.4% → 30.2%) with an increased population of DTE-acetylide moiety (13.3%
→ 62.6% → 69.8%), which implies that the first one-electron oxidation of 2co and 2cc occurs essentially at the
DTE-acetylide moiety instead of at the triarylamine center. Therefore, the first oxidation process moves from the
triarylamine center in 2oo to ring-closed DTE-acetylide in 2co and 2cc as stepwise photocyclization of one and two
DTE moieties. The NIR absorption of monocationic species [2oo]+, calculated at λ = 943 nm from the βHOMO →
βLUMO transitions, is thus ascribed to L2o → [L2o]+ LLCT state. In contrast, oxidation process of [2co]+ mainly
16

ACCEPTED MANUSCRIPT
occurs on DTE-acetylide. The NIR absorption from αHOMO-1 → αLUMO exhibits L2c IL and ring-open DTE →
ring-closed DTE IVCT character, and the NIR absorption from βHOMO-1 → βLUMO featrues triarylamine center
→ DTE unit LLCT state on ring-closure moiety. For [2cc]+, the NIR absorption from αHOMO-1 → αLUMO is
typical triarylamine center → DTE-acetylide LLCT, and NIR absorption from αHOMO→αLUMO+1 exhibits L2c
IL character. The above studies demonstrate that the first oxidation occur on the ring-closed DTE unit, followed by
that on triarylamine center bound to the ring-closed DTE unit for both 2co and 2cc. Therefore, strictly speaking, the
NIR bands of monocationic [2co]+ and [2cc]+ are ascribed to triarylamine center → DTE-acetylide LLCT instead of
N0 -center→ N+ -center IVCT. Additionally, in dicationic species [2co]2+, there is no molecular orbital favorable for
further photocyclization observed, which is consistent with measured ring-closure absorption without a dually
ring-closed reaction upon further UV irradiation.
4. Conclusions
Dimer 1oo and 2oo with 1,3-butadiynyl as a linker between two identical dithienylethene (DTE) moieties,
exhibited distinctly stepwise photochromic reactions, were synthesized and fully characterized. Photochromic
properties of compound 2oo and L2o with terminal triarylamine center could be modulated by reversible
oxidation/reduction of N(0)
N(I), which displayed oxidative cycloreversion reaction ascribed to the reduced
electronic density at DTE moieties upon the oxidation of N0 to N+. As revealed by electrochemical studies, redox
interactions between two triarylamine centers in 2oo had progressive enhancement as a result of the gradual
increase in π conjugation along the molecular backbone during the stepwise ring-closed conversion of 2oo → 2co
→ 2cc. NIR spectra studies indicated that, as the conversion of [2oo]+ → [2co]+ → [2cc]+ proceed, the NIR bands
of monocationic species [2oo]+/[2co]+/[2cc]+ exhibited gradual enhancement illustrating that the extent of charge
17

ACCEPTED MANUSCRIPT
delocalization was increasingly improved with stepwise photocyclization. These results demonstrated that the
photoswitchable charge delocalization and electronic interactions could be triumphantly accomplished by way of
stepwise photocyclization/photocycloreversion processes in a dimer system with redox-active terminals bridged
with two identical DTE moieties.
5. Acknowledgment
The authors are grateful for the financial support from the National Natural Science Foundation of China
(51373072, 21362013, 21363009), the Project of Jiangxi Advantage Sci-Tech Innovative Team (20142BCB24012),
the Project of Jiangxi Science and Technology Normal University Advantage Sci-Tech Innovative Team
(2015CXTD002), the Science Funds of Natural Science Foundation of Jiangxi Province (20142BAB203005).
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Figures & captions
Scheme 1. The eight states of compound 2 through photo- and electroswitchable processes.
Scheme 2. Synthetic routes of dimers 1oo and 2oo.
Fig. 1. UV-vis absorption spectral changes of 1oo (A) and 2oo (B) (2×10-5 M) in CH2Cl2 at 298 K upon alternate
irradiation at λ = 312 nm and 672 nm due to the conversion 1oo
1co
1cc and 2oo
2co
2cc;
absorbance changes of 1oo (C) at λ = 646 nm and 2oo (D) at λ = 692 nm upon alternate irradiation at λ = 312 nm
and 672 nm for five cycles.
Fig. 2. 1H NMR spectral changes of 2oo (A, methyl protons; B, thienyl protons) in CDCl3 upon irradiation at 312
nm to the photostationary state due to 2oo → 2co → 2cc conversion.
Fig. 3. Relative content changes of 1oo (A) and 2oo (B) under UV light irradiation at 312 nm to the PSS based on
1H NMR spectral studies in CDCl3 solution.
Fig. 4. Plots of cyclic voltammogram (A) and differential pulse voltammogram (B) of 2oo (black), 2co (red) and
2cc (green) in 0.1 M (n-Bu4N) (PF6)-dichloromethane solution upon irradiation at 312 nm.
Fig. 5. (A) Changes of the absorption spectrum of [2oo]2+ (2 × 10-5 M) oxidized with CAN-acetonitrile solution (1×
10-4 M) upon irradiation at 312 nm, showing the reactions of 2oo→[2oo]2+→[2co]2+. (B) Changes of the absorption
spectrum of [2cc]2+ (2 × 10-5 M) oxidized with CAN-acetonitrile solution (1× 10-4 M) upon irradiation at 312 nm,
showing the reactions of 2cc→[2co]2+→[2co]2+ (PSS).
Fig. 6. (A) UV-vis-NIR absorption spectra of stepwise oxidation of 2cc with the addition of 0.2 eq, 0.5 eq and 1.0
eq CAN-acetonitrile solution. (B) UV-vis-NIR absorption spectra of [2oo]+, [2co]+, [2cc]+, 2oo, 2co and 2cc in
dichloromethane (2 × 10-5 M).
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Scheme 1
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