A luminescence molecular switch via modulation of PET and ICT processes in DCM system

Article abstract of DOI:10.1007/s11426-016-0500-2

A novel versatile dicyanomethylene-4H-pyran (DCM) based derivative bearing ferrocenyl group (DCM-N-Fc) is designed as modulator to construct “off-on” logic operation. The optical properties of DCM-N-Fc are characterized by absorption and steady-state fluorescence technique, showing that the fluorescence from DCM chromophore via intramolecular charge transfer (ICT) is strongly quenched by photoinduced electron transfer (PET) process from ferrocene moiety. In contrast with the references (DCM-N and DCM-Fc), the fluorescence of DCM-N-Fc can be triggered by oxidizing ferrocenyl unit either chemically or electrochemically, exhibiting a characteristic emission modulation at around 610 nm with an electrofluorochromic behavior. Furthermore, the free energy and the fluorescence lifetime in the PET path verify the thermodynamic feasibility. Cyclic voltammetry, absorption spectroscopy, time-resolved fluorescence as well as DFT calculation have been used to elaborate the manipulation via both PET and ICT processes.

Full text of DOI:10.1007/s11426-016-0500-2

SCIENCE CHINA
Chemistry
doi: 10.1007/s11426-016-0500-2
• ARTICLES •
A luminescence molecular switch via modulation of PET and ICT
processes in DCM system
Jinfeng Yang1,2†, Meng Li3†, Lihua Kang2 & Weihong Zhu1*
1Shanghai Key Laboratory of Functional Materials Chemistry; Key Laboratory for Advanced Materials and Institute of Fine Chemicals; School
of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China
2Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan; College of Chemistry and Chemical Engineering,
Shihezi University, Shihezi 832000, China
3School of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, China
Received November 29, 2016; accepted December 30, 2016; published online March 6, 2017
A novel versatile dicyanomethylene-4H-pyran (DCM) based derivative bearing ferrocenyl group (DCM-N-Fc) is designed
as modulator to construct “off-on” logic operation. The optical properties of DCM-N-Fc are characterized by absorption
and steady-state fluorescence technique, showing that the fluorescence from DCM chromophore via intramolecular charge
transfer (ICT) is strongly quenched by photoinduced electron transfer (PET) process from ferrocene moiety. In contrast with
the references (DCM-N and DCM-Fc), the fluorescence of DCM-N-Fc can be triggered by oxidizing ferrocenyl unit either
chemically or electrochemically, exhibiting a characteristic emission modulation at around 610 nm with an electrofluorochromic
behavior. Furthermore, the free energy and the fluorescence lifetime in the PET path verify the thermodynamic feasibility. Cyclic
voltammetry, absorption spectroscopy, time-resolved fluorescence as well as DFT calculation have been used to elaborate the
manipulation via both PET and ICT processes.
dicyanomethylene-4H-pyran, luminescence, redox switch, ferrocene
Citation:
Yang J, Li M, Kang L, Zhu W. A luminescence molecular switch via modulation of PET and ICT processes in DCMsystem. Sci China Chem, 2017, 60:
doi: 10.1007/s11426-016-0500-2
1 Introduction
is one prospective application of fluorescent chemosen-
sors for sensing and information processing [16]. Among
these molecules, PET is often used to generate luminescent
chemosensors that feature fluorescence quenching or en-
hancement [17–19]. The PET process is particularly useful
in this regard since the signal, depending on the special
circumstances, is either an “on-off” or “off-on” type, which
results in well-defined sensing activity and logic operation
[20,21].
Ferrocene (Fc) derivatives are the most popular family of
redox-active moieties due to their redox stability and ver-
satility of chemical functionalization. Because of the elec-
trochemical properties in both low-lying triplet excited state
and an effective triplet quencher, ferrocene is known to be a
Fluorescent chemosensors predominantly attract remarkable
interest in various fields including biological imaging [1,2],
supramolecular chemistry [3–5], material science [6–9],
and redox-sensors [10,11]. Many chemosensors of practical
utility have been developed based on photoinduced electron
transfer (PET), intermolecular charge transfer (ICT), fluores-
cence resonance energy transfer (FRET), and excited-state
intramolecular proton transfer [12–15]. Molecular switch
†These authors contributed equally to this work.
*Corresponding author (email: whzhu@ecust.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2017
chem.scichina.com link.springer.com

2
Yang et al. Sci China Chem
good donor for luminescent PET sensors [22]. Therefore, fer-
rocene-based derivatives combined with functional unit have
obtained much interest in subjects of anion recognition, ag-
gregation behavior, thermal analysis and antitumor activity
[23–26].
strated, the reference compound DCM-Fc shows fluorescent
quenching from ferrocenyl termini acting as donor in the neu-
tral form with a low-lying ICT. Compared with the references
(DCM-N and DCM-Fc), the fluorescence of DCM-N-Fc can
be triggered by oxidizing ferrocenyl unit either chemically
or electrochemically, showing a characteristic emission band
around 610 nm. Furthermore, the free energy and the fluo-
rescence lifetime in the PET path verify the thermodynamic
feasibility.
Design of electro-active molecular switches, especially
incorporating fluorophore, is based on functional integration
containing donor and acceptor moieties. As known, elec-
tron donor-π-conjugate-electron acceptor (D-π-A) system
is highly sensitive to changes in the external environment
[27–29]. Moreover, increasing π-electron system is distinct
gateway to near-infrared (NIR) fluorescence [30–33]. At-
tributing to its ICT, dicyanomethylene-4H-pyran (DCM),
with a typical D-π-A system, is also one of the most en-
couraged organic fluorophores. In addition, DCM-based
derivatives show very excellent spectroscopic properties,
namely a broad absorption, long red emission wavelength,
large Stocks-shift and corresponding high fluorescent quan-
tum yield [33,34]. Thus, DCM-based derivatives have found
widespread applications such as non-linear optic materials
[35], photovoltalic materials [36],solar cells [37], logic gates
[38–40], fluorescent film sensors [41] and in vivo imaging
[42,43]. Generally, the OFF-ON or ON-OFF fluorescence
response of molecular switch is based on two different
approaches [44–48]. To optimize function, it is possible to
design functional integration within a single molecule, and
mainly based on the fluorescent signaling mechanisms of
PET and ICT.
2 Experimental
2.1 Materials and apparatus
Toluene, dichloromethane (CH2Cl2), chloroform (CHCl3), pe-
troleum ether and acetic acid were purchased from Chinese
Chemical Reagent Shanghai Titan Scientific Co., Ltd. All
solvents were of analytical grade.
Nuclear magnetic resonance hydrogen (1H NMR) and
nuclear magnetic resonance carbon (13C NMR) spectra in
CDCl3 were measured on a Brucker AV-400 spectrometer
(Germany) with tetramethylsilane (TMS) as an internal
standard.
Electrospray-ionization high-resolution mass
spectra were measured on a Waters LCT Premier XE instru-
ment (USA) under standard conditions. Ultraviolet-visible
(UV-Vis) spectra were obtained by using a Varian Cary
500 spectrophotometer (USA) (1 cm quarts cell) at 25 °C.
Fluorescence spectra were recorded on a Varian Cary Eclipse
fluorescence spectrophotometer (USA) (1 cm quarts cell) at
25 °C, and all fluorescence spectra were uncorrected by the
photo-multiplier tube response. The width of the slit was 5
nm. TLC analyses were performed on silica gel plates and
column chromatography was conducted over silica gel (mesh
200–300), both of which were obtained from the Qingdao
Ocean Chemicals (China).
In this paper, we construct a novel DCM-based compound
with D1-π-A-π-D2 system (DCM-N-Fc, Scheme 1). On the
one hand, enlightened by the PET process of ferrocene-in-
corporated derivatives resulting in the fluorescence quench-
ing [49–51], the ferrocene moiety as one donor is strategi-
cally conjugated with DCM fluorophore. On the other hand,
the substituent effect on DCM chromophore is so significant
that the dimethylamino group relating to ICT character is cho-
sen as another effective electron donor [52,53]. As demon-
The electrochemical characterizations of DCM-N-Fc and
Scheme 1 Synthetic route of DCM-N-Fc, and reference compounds DCM-N and DCM-Fc. Reaction conditions: i) ferrocenecarboxaldehyde, piperidine,
toluene, acetic acid, yield 15%; ii) dimethylaminobenzaldehyde, piperidine, toluene, acetic acid, yield 46%; iii) dimethylaminobenzaldehyde, piperidine,
toluene, acetic acid, yield 35% (color online).

Yang et al. Sci China Chem
3
reference ferrocene (Fc-H), DCM-Fc and DCM-N were
performed by means of cyclic voltammetry (CV) in THF
(2.5×10−5 mol L−1) as a solvent (S2), using a three-elec-
trode cell consisting of Pt as a working electrode, saturated
Ag/AgCl as a reference electrode (DCM-N-Fc, vs. SCE,
ESCE−EAg/AgCl=0.06 V), and platinum wire as the auxil-
iary electrode with a scan rate of 100 mV s−1 with TBAP
(n-Bu4NPF6Br, supporting electrolyte).
105.27, 111.99, 113.00, 115.65, 116.14, 122.39, 129.64,
138.42, 138.93, 151.84, 156.18, 158.39, 159.50. HRMS
(TOF-ESI+) m/z: Calcd. for C30H25FeN3O: 499.1347; Found:
499.1350. FT-IR (KBr, cm−1): 3418.75, 2203.58 (C≡N),
1455.80, 1044.59, 946.25, 827.19.
3 Results and discussion
The synthetic routes for DCM-N-Fc is depicted in Scheme 1.
The compounds are characterized by 1H NMR, 13C NMR, IR
and UV-Vis spectroscopy as well as HRMS (see Section 2).
In all cases, the mass spectra show the signals expected for
the proposed structure. Moreover, The IR spectra of these
compounds exist mainly the bands due to the characteristic
bands either the ferrocene ring or the aromatic ring. Apart
from these, mild strong characteristic bands at about 2203
cm−1 are stretching bands of –C≡N for the malononitrile moi-
2.2 Synthesis of DCM-N-Fc
Ferrocenecarboxaldehyde was synthesized by the established
literature procedure [54]. Intermediate of 1 is prepared by
known methods [13,40].
2.2.1 Synthesis of compound DCM-Fc
2,6-Dimethyl-4-(dicyanomethylene)-4H-pyran 1 (344.4 mg,
2 mmol) and ferrocenecarboxaldehyde (556.4 mg, 2.6 mmol)
were taken in 20 mL anhydrous toluene under argon, and
then both piperidine (0.2 mL) and acetic acid (0.4 mL) were
added. The contents were refluxed, and monitored by TLC
(dichloromethane:petroleum ether). After reaction (10 h),
the reaction mixture was cooled and the solvent was removed
under reduced pressure. Subsequently, the crude product
was purified by neutral aluminium oxide gel chromatography
with dichloromethane/petroleum ether (1:2) to afford the
brown solid. Crystallization from chloroform and petroleum
1
ety. In the H NMR spectra, the presence of a singlet at δ
1
6.52–6.72 ppm in the H NMR spectrum were attributed to
the hydrogens of pyran ring. The presence of (CH=) protons
resonance at δ 6.38 ppm demonstrated the successful synthe-
sis of DCM-N-Fc. Furthermore, the characteristic coupling
constant (J=15.8 Hz) of alkene protons is indicative of the
predominant trans isomer.
As shown in Figure 1(a), DCM-N is a typical donor-π-ac-
ceptor structure with a broad absorption band resulting
from ICT that located at 461 nm (ε=5.90×104 L mol−1
cm−1). However, DCM-Fc exhibits an extremely faint broad
absorption band due to the influence of ferrocene which
located at 521 nm (ε=0.70×104 L mol−1 cm−1) and 401 nm
(ε=2.60×104 L mol−1 cm−1), respectively. DCM-Fc shows
obvious bathochromic shift owing to the conjugating effect
from electron-donating group of ferrocene. As a derivative
of DCM, the main absorption band of DCM-N at 461 nm
corresponding to the DCM chromophore is split into two
bands, but not obvious in DCM-N-Fc. The additional ab-
sorption band at 486 nm of DCM-N-Fc (ε=4.10×104 L mol−1
cm−1) can be ascribed to an ICT process which significantly
red shifted due to the conjugation between two donor either
ferrocene or dimethylamino and the acceptor DCM subunits.
While, the other band (431 nm, ε=3.30×104 mol−1 cm−1)
is mainly due to the pyran centered S0-S1 transition, blue
shifted (30 nm) compared to DCM-N. Furthermore, the band
at 350–430 nm may possess some ferrocene character with
additional contributions from π-π* (350 nm) and mental cen-
tered d-d transtions (430 nm) in DCM-N-Fc. Subsequently,
Figure 1(b) displays the fluorescence spectra of DCM-N.
DCM-N emits light at λmax 584 nm with the intrinsic ICT
characteristic features of DCM chromophore, while DCM-Fc
and DCM-N-Fc does not exhibit any emission. This behavior
is ascribed to an efficient PET from ferrocene (acting as a
donor) to the DCM moiety (acting as an acceptor).
1
ether gives 110 mg of DCM-Fc in 15% yield. H NMR (400
MHz, CDCl3, ppm) δ: 2.40 (s, 3H), 4.19 (s, 5H), 4.50 (s, 2H),
4.54 (s, 2H), 6.30 (d, J=15.8 Hz, 1H), 6.52 (s, 1H), 6.57 (s,
1H), 7.32 (d, J=15.8 Hz, 1H). 13C NMR (100 MHz, CDCl3,
ppm) δ: 20.08, 57.88, 68.50, 69.91, 71.52, 79.43, 105.27,
106.32, 114.99, 115.47, 139.78, 156.60, 159.52, 161.85.
HRMS (TOF-ESI+) m/z: Calcd. for C21H16FeN2O: 369.0612;
Found: 368.0613. FT-IR (KBr, cm−1): 3419.03, 2209.57
(C≡N), 2195.37 (C≡N), 1456.45, 1045.66, 957.83, 833.72.
2.2.2 Synthesis of compound DCM-Fc
The reaction was performed with DCM-Fc (160.0 mg, 0.43
mmol) and dimethylaminobenzaldehyde (60.0 mg, 0.40
mmol) in 20 mL anhydrous toluene under argon. And then,
both piperidine (0.2 mL) and acetic acid (0.4 mL) were
added. The contents were refluxed, and monitored by TLC
(dichloromethane:petroleum ether). After reaction (10 h),
the reaction mixture was cooled and the solvent was removed
under reduced pressure. Subsequently, a reddish brown
solid was obtained. Crystallization from chloroform and
petroleum ether gives 94.0 mg of DCM-N-Fc in 46% yield.
1H NMR (400 MHz, CDCl3, ppm) δ: 3.09 (s, 6H), 4.24 (s,
5H), 4.53 (s, 2H), 4.60 (s, 2H), 6.38 (d, J=15.8 Hz, 1H),
6.55 (d, J=15.8 Hz, 1H), 6.57 (s, 1H), 6.62 (s, 1H), 6.75 (d,
J=8.8 Hz, 2H), 7.39 (d, J=15.8 Hz, 1H), 7.46 (d, J=15.8 Hz,
1H), 7.51 (d, J=8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3,
ppm) δ: 40.13, 56.74, 68.42, 69.8, 71.34, 79.62, 105.07,
By means of CV in tetrahydrofuran, the electrochemical

4
Yang et al. Sci China Chem
Figure 2 Electrochemical oxidative potential of DCM-N-Fc, DCM-N and
DCM-Fc in THF vs. SCE at room temperature (color online).
Figure 1 (a) Electronic absorption spectra of DCM-N-Fc, DCM-N and
DCM-Fc in THF, 1.0×10−5 mol L−1; (b) emission spectrum of DCM-N,
excitation at 461 nm in THF, 1.0×10−5 mol L−1 (color online).
rocene, the fluorescence can be switched on upon ferrocene
oxidation in DCM-N-Fc. As a result of the electrochem-
ical behaviour of DCM-N-Fc, a redox peak can be identi-
fied in the CV of DCM-N-Fc, which attributes to the readi-
ness of the conversion from Fe(II) to Fe(III) and the stability
of Fe(III). Corresponding to the Fe(II)-Fe(III) inter valence
electron transfer (IVET), the ICT band intensity rises while a
emission band in the 610 nm range appears.
Figure 3 shows the changes in both absorption and fluores-
cence of DCM-N-Fc upon oxidation by electrochemisty. Be-
fore oxidation, the ICT is inhibited, there is no fluorescence
during which the Fc unit acts as the donor and DCM moi-
ety as the acceptor. After oxidation, the recording of emis-
sion spectra shows an emerging band with a maximum near
610 nm that disappears when the potential is stepped back
to 0 V (Figure 3(b)). At the same time, the character is also
reversible in absorption spectra (Figure 3(a)). However, the
reference DCM-N exhibited nearly no alteration in either ab-
sorption or fluorescence spectra (Figure S1, Supporting In-
characterization of DCM-N-Fc and reference compounds
DCM-Fc and DCM-N is performed (Figure 2). CV analysis
suggested that each compound undergoes one reversible
oxidation. The oxidation is reversible (ipc/ipa=1.19–2.50)
and diffusion-controlled (ipc/v1/2) constant in the scan rate
(v) range (0.05–6.0 V s−1) for one-electron transfer (∆Ep) is
0.09–0.39 V for the ferrocene/ferroceniun (FcH/FcH+) cou-
ple. The oxidation potential of DCM-N-Fc showed almost
no difference even with extended conjugaton. Reduction
potential of DCM-N-Fc is observed at −1.15 V and this may
arise from the functionalized pyran moiety rather from the
ferrocene entity. It is reasonable to assume that the added
electronic radical anions remain locating on the pyran core.
As expected, the redox potential values for the ferrocene
termini are confined to DCM-Fc. Table 1 and Table 2 sum-
marized their corresponding redox potential data.
Due to the excellent intrinsic electrochemical signals of fer-
Table 1 Electrochemical data of DCM-N-Fc and reference compounds ferrocene, DCM-Fc and DCM-N a)
Compounds
Fc-H
Epa (V)
0.68
−
Epc (V)
0.29
−
ΔEp (V)
0.39
−
ipa/ipc
1.37
−
Epa* (V)
−
Epc* (V)
−
ΔEp* (V)
ipa*/ipc*
−
−
−
−
−
−
DCM-N
DCM-Fc
DCM-N-Fc
0.66
0.67
0.57
0.53
0.09
0.14
2.50
1.19
−1.21
−1.20
−1.43
−1.30
0.22
0.10
0.29
0.24
a) * represents quasi-reversible. And Epa* is the potential of quasi-reversible oxidation peak; Epc* is the potential of quasi-reversible reduction peak; ΔEp*
is the potential difference of quasi reversible redox; ipa*/ipc* is the ratio of quasi reversible redox current.
Table 2 Electrochemical data and ΔGET for DCM-N-Fc, DCM-N and DCM-Fc a)
Compounds
Fc-H
E0ox (eV)
0.49
−
E*0red (eV)
E(S0→S1) (eV)
E (eV)
−
E* (eV)
−
ΔGET (eV)
−
−
−
−
−
−
DCM-N
0.92
−
−1.39
−
DCM-Fc
0.62
0.60
−1.32
−1.25
2.28
2.55
−0.34
−0.60
DCM-N-Fc
0.86
−1.06
a) e2/εr=−0.1 eV, E0ox=(Epa+Epc)/2, E*red=(E*pa+E*pc)/2.

Yang et al. Sci China Chem
5
Figure 3 (a) Time dependent UV-Vis spectra of DCM-N-Fc oxidation by electrochemistry; (b) time dependent fluorescent spectra of DCM-N-Fc oxidation
by electrochemistry (THF, 1.0×10−5 mol L−1, λex=486 nm) (color online).
formation online). Also, there is no fluorescence recovery
in the compound DCM-Fc. Thus it is demonstrated that the
DCM-N-Fc exhibits an electrofluorochromic behavior by the
switch between the PET and ICT. Reversibility of the flu-
orescence shows evidence that in this kind of compounds
(D1-π-A-π-D2), the fluorescence can be switched on electro-
chemically.
In order to apply the redox switch of luminescence, we
checked this possibility by chemical oxidation via Fe3+ as
the oxidizing agent. Initially, upon successive titration with
Fe3+, it is observed that the intensity of absorption spectrum
at 486 nm decreased linearly and remained constant after ad-
dition of 2.0 equiv. Fe3+ (Figure 4(a)). Correspondingly, a
new emission band at 610 nm starts to appear when excited
at 486 nm. It is notable that the fluorescence intensity at λmax
610 nm increases linearly upon successively quantitative ad-
ditions of Fe3+ (Figure 4(b)). The results above showed the
same trend with electrochemical method. Subsequently, it
is demonstrated that the prime ratio between the DCM-N-Fc
and Fe3+ is 1:1, and the fluorescence intensity about 610 nm
of DCM-N-Fc remains constant. As expected, the reference
DCM-N shows unchangeable both UV-Vis spectra and flu-
orescence upon addition of Fe3+ as well as the compound
DCM-Fc. Figure 5 shows one-electron oxidation mechanism
of DCM-N-Fc. Confirmedly, the fluorescence of DCM-N-Fc
is strongly quenched by the ferrocenyl unit via PET.
The fluorescence lifetime τ of selected compounds is
presented in Table 3. The fluorescence decay of three com-
pounds is consistent with double exponential fitting curve
(Figure S2). It should be emphasized to reference compound
DCM-N that excitation leads first to a locally excited state
(LE) which is almost no emissive, then to the “relaxed
intramolecular charge-transfer (RICT) state which has a
good quantum yield. The fast decay component (τ1, 4.04
ns, 60.42%) corresponds to the LE state, and the slow one
(τ2, 15.65 ns, 39.58%) is attributed to the RICT state. For
DCM-N-Fc, it is a totally different argument. First, exci-
tation leads the case of metal locally excited state (MLLE)
attributing to the ferrocenyl unit, and it is slow and little
(τ2, 4.62 ns, 9.28%). Further, the particular case of metal to
ligand charge transfer (MLCT) is formed, and it is fast and
dominant (τ1 1.67 ns, 90.72%). These values clearly show
the absence of fluorescent signal in DCM-N-Fc, because PET
is usually faster than the luminescence emission process if
Figure 4 Absorption (a) and fluorescence spectra (b) of DCM-N-Fc (THF, 1.0×10−5 mol L−1) in the presence of Fe(ClO4)3 at concentrations of 0.0, 0.2, 0.4,
0.6, 0.8, 1.0, 1.2, 1.6 and 2.0 equiv. Inset of (a): absorption ratio upon titration of Fe(ClO4)3 monitored at 486 nm after 5 min. Inset of (b): fluorescence ratio
upon titration of Fe(ClO4)3 monitored at 610 nm after 5 min. λex=486 nm (color online).

6
Yang et al. Sci China Chem
Figure 5 One-electron oxidation mechanism of DCM-N-Fc (color online).
Table 3 Fluorescence decay parameters of selected compounds in THF sol-
vent
τ1 (ns)/
A1 (%)
τ2 (ns)/
A2 (%) a) b)
kET
(109 s−1) c)
d)
Compounds
ΦET
a) b)
DCM-N
DCM-N-Fc
4.04/60.42
4.62/9.28
2.49/79.86
15.65/39.58
1.67/90.72
10.25/20.14
0.184
0.351
0.154
0.74
0.59
0.38
Figure 6 (a) Frontier molecular orbitals of DCM-N-Fc; (b) spin density
difference spin(α)-spin(β) for DCM-N-Fc (color online).
DCM-N-Fc(III)
a) Excitation wavelength λex=486 nm; b) the relative amplitudes A were
k /
k /
1
calculated from fitting the data by the equations Y = A1e
+ A2e 2 and
A1+A2=100%; c) kET=1/τ−1/τref(DCM); d) ΦET=kETτ, τ with the largest amplitude
occupied molecular orbital (HOMO), since the electron den-
sity is mainly centered at the DCM core, although with a
small contribution from the ferrocenyl termini. The lowest
unoccupied molecular orbital (LUMO), as expected, spreads
over the whole molecule backbone and does not confined only
on the ferrocenyl moieties. Regarding the quenching mech-
anism, both electron transfer and energy transfer are thermo-
dynamically feasible. The DCM chromophore provides an
orbital that has energy between those HOMO and LUMO of
the fluorophore, and an intramolecular PET can occur from a
filled orbital to the HOMO of the DCM fluorophore depend-
ing on the nature of the “nearby” orbital incorporated. Deac-
tivation processes without radiation can therefore take place,
and fluorescence quenching is observed. Accordingly, the re-
moval of the “nearby” orbital between HOMO and LUMO
of the fluorophore results in fluorescence restore (Figure 3(b)
and Figure 4(b)). As demonstrated, the dimethylamino group
in the DCM-N-Fc, which is also another effective electron
donor, leads molecular stability to block the PET from the
ferrocene at an excited state.
use in the equations.
favorable PET thermodynamics is arranged. In contrast, the
fluorescence decays of DCM-N-Fc(III) are attributed to the
mechanism involving both the LE state and RICT state dur-
ing which followed by complex MLLE, so the slow one (τ2
10.25 ns, 20.14%) should correspond to an RICT state. In all
the above considerations, the fluorescence of the DCM flu-
orophore should be recovered when the PET path from the
ferrocene termini to the DCM moiety is inhibited by the oxi-
dation of the ferrocene unit (Figure 5).
Fluorescence quenching rate constant is correlated with the
free energy change, ΔG, involved in the actual electron trans-
fer process. The energies of the pyran first excited singlet
states E (S0→S1) of DCM-N-Fc and DCM-Fc were obtained
from the average values of the absorption and emission
band (Table 1). The free energy can be calculated according
to the equation: ΔGET=E0ox−E*0red−E(S0→S1)−e2/εr. Four
compounds in which fluorescence quenching is likely to
occur by the ΔG-values, as shown in Table 2. This series
of compounds display a kET/ΔGET correlation in the marcus
region (Table 2 and Table 3). Here the more negative ΔGET is
highly beneficial for an increase of kET from 0.184 (DCM-N)
to 0.351 (DCM-N-Fc). In DCM-N-Fc, the free energy
ΔG(DCM-N-Fc) (−0.60 eV) implies a PET channal, which can be
taken as a measure of fluorescence quenching. As expected,
only the fluorescent intensity of the DCM-N-Fc being of a
typical D1-π-A-π-D2 structure, in which located at 610 nm,
is triggered upon oxidation from ferrocene to ferrocenium
(either chemical or electrochemical method). Whereas, the
fluorescence recover is not observed in both DCM-N and
DCM-Fc.
4 Conclusions
We have successfully synthesized a novel DCM derivative
DCM-N-Fc, which allows the straightforward incorpora-
tion of ICT and PET donors on the unimolecular platform,
achieving “off-on” switching of luminescence upon oxida-
tion from ferrocene to ferrocenium either electrochemically
or chemically. In DCM-N-Fc, the free energy ΔG(DCM-N-Fc)
,
fluorescence lifetime, and DFT calculation verify the ther-
modynamic feasibility in the PET path, which is relative
to fluorescence quenching. The competition between the
PET and ICT is well-tuned by changing the redox state of
ferrocene unit, and manipulating the fluorescence intensity
of DCM choromophore at around 610 nm with an electro-flu-
orochromic behavior.
To obtain in-depth mechanistic insight, the geometry of
DFT calculation was investigated. The orbit modelling of the
“trans-” configuration of DCM-Fc is shown in Figure 6. The
conjugation is clearly predicted by the calculations of highest

Yang et al. Sci China Chem
7
Acknowledgments This work was supported by the National Key
3371–3389
Research and Development Program (2016YFA0200300), the National
Natural Science Foundation of China for Distinguished Young Scholars
(21325625) and Key Project (21636002), Natural Science Foundation of
China (1607044), Oriental Scholarship, Fundamental Research Funds for
the Central Universities (WJ1416005, 2016 MS108), Science and Technol-
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