Unique fluorescence of boronic acid derived salicylidenehydrazone complexes with two perpendicular ICT: Solvent effect on PET process

Article abstract of DOI:10.1016/j.dyepig.2018.03.046

Solvent effects on the absorption and emission spectra of organic compounds have been widely observed. It is usually found that the positions, intensities and shapes of its bands are usually modified by these solvents. The solvent effects on intramolecula

Full text of DOI:10.1016/j.dyepig.2018.03.046

Accepted Manuscript
Unique fluorescence of boronic acid derived salicylidenehydrazone complexes with
two perpendicular ICT: Solvent effect on PET process
Boyu Zhang, Shichao Wang, Jingyun Tan, Xuanjun Zhang
PII:
S0143-7208(18)30327-9
10.1016/j.dyepig.2018.03.046
DYPI 6631
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 February 2018
Revised Date: 14 March 2018
Accepted Date: 23 March 2018
Please cite this article as: Zhang B, Wang S, Tan J, Zhang X, Unique fluorescence of boronic acid
derived salicylidenehydrazone complexes with two perpendicular ICT: Solvent effect on PET process,
Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.03.046.
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ACCEPTED MANUSCRIPT
Graphical Abstracts

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Unique fluorescence of boronic acid derived salicylidenehydrazone
complexes with two perpendicular ICT: solvent effect on PET
process
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Boyu Zhang, Shichao Wang, Jingyun Tan, Xuanjun Zhang*
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Faculty of Health Sciences, University of Macau, Macau SAR, China.
Email: xuanjunzhang@umac.mo
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Abstract: Solvent effects on the absorption and emission spectra of organic
compounds have been widely observed. It is usually found that the positions,
intensities and shapes of its bands are usually modified by these solvents. The solvent
effects on intramolecular charge transfer (ICT) process have been extensively
reported, whereas the solvent effect on photoinduced electron transfer (PET) process
is scarce. In this contribution, it was disclosed that boronic acid derived
salicylidenehydrazone complexes (BSs), BS-DPE and BS-DPBDE, with two
perpendicular ICT states developed a non-fluorescent PET compounds through
separating its two ICT states by boron node. Relative strong acidic proton in non-
hydrogen-bond accepting solvent were proved to actuate the vanishment of PET and
reinstatement of its fluorescence.
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Keywords: Solvent effects; Boronic acid; Salicylidenehydrazone; Photoinduced
electron transfer; T-shaped molecular geometry
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1. Introduction
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Solvent effects[1, 2] have been widely observed in different areas of chemistry such as
catalysis,[3] biochemistry[4] and photochemistry.[5] Of these observation, most concern its effects
on the position of chemical equilibria and/or the rates of chemical reactions. Furthermore, from a
photophysical point of view, the ground and excited state of solvated chromophore-containing
molecules in different solvents might undergo a physical perturbation as compared with its
isolated ones (gas state)[1]. Vibrational relaxation of electronic state with surrounding solvent are
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coincided with energy loss both from Franck-Condon excited (ground) state to equilibrium excited
(ground) state, leading to Stokes shift.[6] Many types of positive, negative and inverted
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fluorosolvatochromistic dyes have been synthesized and widely applied in bioimaging and
biosensing,[7] such as studying lipid domains,[8] apoptosis and endocytosis.[9] The theory of
solvent effects on the fluorescence assumes that the fluorophore is a point dipole residing in the
centre of a spherical cavity in a homogeneous and isotropic dielectric, which is so-called Lippert-
Mataga theory[10] but deviated from authentic condition. Therefore, it is of paramount
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importance to come up with and fully comprehend specific fluorophores solute/solvent interaction,
especially involving hydrogen bonding, electron-pair donor/electron-pair acceptor interactions or
others.[1, 11, 12]
Over the last few years were recognized a special solvent effect on ground-state reversible
isomerization of silica-rhodamines (SiR),[13] in which SiR-carboxyl exists predominantly
in its spirolactone form in dioxane-water mixtures of dielectric constant less than 30. This
seminal fluorogenic behaviour enabled multicolour imaging of target proteins in wash-free
conditions.[14] Also, based on a hypothesis which proposed that the break of fluorophore-
solvent hydrogen bond in the excited state account for fluorescence quenching,[15, 16]
flavones-based fluorophores with carbonyl as hydrogen bond acceptors were demonstrated
to be ideal wash-free bioprobe for mitochondria[17] and endoplasmic reticulum
imaging.[18] Our group also recently found that boronic acid derived
salicylidenehydrazone complexes (BSs) were ideal candidates for wash-free fluorescence
imaging of cellular organelles.[19] A remarkably hypsochromic shift of intramolecular
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charge-transfer (ICT) absorption band in water was observed than in apolar solvent, which
were attributed to lower the n-state energy of oxygen lone pairs of the carbonyl group
through hydrogen bonding with water.
In this contribution, we report that a peculiar fluorescence turn-on property of two BSs
involved donor-acceptor diphenylpolyenes[20-22] as auxochrome. The versatile
fluorescent dye platform BSs were first reported last year, which were constructed from
modular assembly of boronic acids with schiff base ligand salicylidenehydrazone by
groups of Goris and Pischel.[23] Subsequently, they thoroughgoingly explored the
photophysical scope of the dyes through the variation of both the push-pull ICT character
at the ligand backbone and the electronic nature of the boronic acid derived moiety, the
latter of which have been demonstrated to be no influence on the Uv-vis absorption and
fluorescence properties.[24] The T-shaped molecular geometry of BSs sparked our
imagination on what photophysical properties will come into view if two perpendicular
ICT states were located in BSs (Scheme 1). To the extent of our knowledge, there has been
little research on fluorophore with this type molecular architecture.[25]
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2. Experimental
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2.1 General Information
All reagents were purchased from Aldrich or ShenZhen Dieckmann Technology Development co.,
LTD and were used without further purification, unless otherwise stated. HPLC grade solvent
were used in Uv-vis and fluorescence studies. Uv-vis spectra were recorded on a SHIMADZU Uv-
1800 spectrophotometer, with a quartz cuvette (path length 1 cm). The fluorescence spectra were
recorded with a HORIBA Fluorolog@-3 spectrofluorimeter. H and 13C NMR spectra were
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recorded on a BRUKER instrument (400 MHz and 100 MHz, respectively) and internally
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referenced to tetramethylsilane signal or residual protio solvent signals. Data for H NMR are
recorded as follows: chemical shift (δ, ppm), multiplicity (s, singlet; d, doublet; t, triplet; q, quartet;
m, multiplet), intergration, coupling constant (Hz). Data for 13C NMR are reported in terms of
chemical shift (δ, ppm). High resolution mass spectra for all the new compounds were done by an
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a Xevo G2-XS QTof spectrometer (Waters, USA). The fluorescence quantum yields (QYs) were
quantified using Fluorescein as the standard (Φf = 0.89, in 0.1 M NaOH). The QYs can be
calculated through adopting the following equation:
Φs = Φr(Arns2Fs)/(Asnr2Fr)
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where the subscripts s and r denote the sample and the standard (Fluorescein), respectively, Φ is
the quantum yield, F is the integrated emission intensity, A is the absorbance, and n is the
refractive index.
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Fluorescence lifetimes were detected by a DeltaTime™ TCSPC on Fluorolog®. The monitored
wavelength was 545 nm. Fluorescence decay histograms were recorded using the time-correlated
single photon counting technique in 4095 channels through a HORIBA Fluorolog@-3
spectrofluorimeter equipped with a HORIBA NanoLED source (N-455 nm). Histograms of the
instrument response functions and sample decays were obtained until it typically reached 1.0 × 104
counts. The fitting parameters (decay times and pre-exponential factors) were decided by
minimizing the reduced chisquare χ2.
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For a two-exponential decay the average lift time is given by following equation:
ꢁꢂꢃꢂꢄ + ꢁꢄꢃꢄꢄ
τꢀ =
ꢁꢂꢃꢂ + ꢁꢄꢃꢄ
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The time-dependent density functional theory (TD-DFT/B3LYP) calculations of BS-DPE, BS-
DPBDE were performed. All calculations were performed with the G03 software. The TD-DFT
calculation of the lowest 25 singlet-singlet excitation energies was calculated with a basis set
composed of 6-31G (d, p) for C, N, H, O, B atoms. The lowest 25-spin allowed singlet-singlet
transitions, up to the energy of about 5 eV, were taken into account for the calculation of the
absorption spectra.
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2.2 Synthesis
(Z)-2-(((E)-4-(diethylamino)-2-hydroxybenzylidene)hydrazono)-2-phenylacetic acid (Ligand)
(Scheme 2-1). Adopting a literature procedure with some reasonable modification,[23]
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hydrazonoe (2.5 mmol, 490 mg) was completely dissolved in 5 mL water firstly and 20 mL
methanol was added. After shaking moderately, this solution was eluted through a columned
packed with Amberlyst@ 15 (5 g wet resin exchanged with 10 mL methanol for three times) and
dropped into a methanolic solution of SA (2.5 mmol, 483 mg in 5 mL methanol). The ultimate
solution was stirred for 2 h at room temperature and then incubation at 4
in refrigerator
overnight. The slurry was filtered and washed with cold methanol (5 mL) three times, and
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afterwards collected as orange powder: yield 65%; H NMR (400 MHz, CDCl3) δ 8.16 (s, 1H),
8.09 – 8.01 (m, 2H), 7.71 (d, J = 2.3 Hz, 1H), 7.43 (td, J = 7.0, 5.8, 3.0 Hz, 3H), 7.19 (d, J = 9.2
Hz, 1H), 6.36 (dd, J = 9.2, 2.3 Hz, 1H), 3.59 (q, J = 7.1 Hz, 4H), 1.31 (t, J = 7.1 Hz, 6H). 13C
NMR (101 MHz, DMSO-d6) δ 167.3, 164.9, 161.9, 159.5, 152.4, 134.7, 131.9, 131.7, 129.6,
127.3, 106.5, 104.9, 97.3, 44.5, 13.0. HRMS-ESI calcd for C19H22N3O3 ([M+H]+), 340.1661;
found, 340.1703.
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(4-boronobenzyl)triphenylphosphonium
bromide.
A
solution
containing
(4-
(bromomethyl)phenyl)boronic acid (5.2 mmol, 1.12 g) and triphenylphosphane (5.72 mmol, 1.5 g)
in 160 mL MeCN was refluxed for 12 h. After the completion of reaction, the solvent was
removed in vacuo and the residue was triturated with diethyl ether (3 × 10 mL) and collected as
white precipitate: yield 95%; 1H NMR (400 MHz, DMSO-d6) δ 7.88 – 7.78 (m, 3H), 7.71 – 7.51
(m, 17H), 6.90 (dd, J = 8.1, 2.5 Hz, 2H), 4.98 (d, J = 15.4 Hz, 2H). 13C NMR (101 MHz, DMSO-
d6) δ 135.6, 135.6, 134.9, 134.8, 134.6, 134.5, 130.6, 130.5, 130.4, 130.4, 118.7, 117.9, 29.0.
HRMS-ESI calcd for C25H23BO2P ([M-Br]+), 397.1523; found, 397.1613.
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8-(diethylamino)-2,5-diphenyl-3H,5H-5l4,12l4-benzo[5,6][1,3,2]oxazaborinino[2,3-
b][1,3,4,2]oxadiazaborinin-3-one (BS-Ph) (Scheme 2-4). The derivate of phenylboronic acid
(0.3 mmol) was dissolved in CH3CN (3 mL), toward which the ligand (0.33 mmol, 112 mg) was
added. The resulting solution was refluxed until the termination of reaction. And then, the solution
was concentrated and the residue was triturated with cold methanol for washing away minor
impurities. [19]Orange solid; Yield 95%; 1H NMR (400 MHz, CDCl3) δ 8.21 (s, 1H), 7.96 – 7.88
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(m, 2H), 7.41 – 7.27 (m, 5H), 7.18 – 7.08 (m, 4H), 6.29 (dd, J = 9.1, 2.4 Hz, 1H), 6.13 (d, J = 2.3
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Hz, 1H), 3.37 (ddt, J = 16.6, 14.8, 7.4 Hz, 4H), 1.16 (t, J = 7.2 Hz, 6H). 13C NMR (101 MHz,
CDCl3) δ 161.6, 157.3, 155.8, 154.0, 153.6, 134.6, 132.7, 130.8, 130.8, 129.5, 128.2, 127.8,
127.6, 107.7, 106.4, 98.7, 45.4, 12.8. HRMS-ESI calcd for C25H25BN3O3 ([M+H]+), 426.1989;
found, 426.1873.
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(E)-8-(diethylamino)-5-(4-(4-(dimethylamino)styryl)phenyl)-2-phenyl-3H,5H-5l4,12l4-
benzo[5,6][1,3,2]oxazaborinino[2,3-b][1,3,4,2]oxadiazaborinin-3-one (BS-DPE) (Scheme 2
-
5). (E)-(4-(4-(dimethylamino)styryl)phenyl)boronic acid in Scheme 2-2 was synthesized on the
previous reports (Spectroscopic sugar sensing by a stilbene derivative with push (Me2N-)-pull
((HO)2B-)-type substituents). The derivate of phenylboronic acid (0.3 mmol) was dissolved in
CH3CN (3 mL), toward which the ligand (0.33 mmol, 112 mg) was added. The resulting solution
was refluxed until the termination of reaction. And then, the solution was concentrated and the
residue was dissolved in tiny amount toluene and then purified by column chromatography
(Hexane : EtOAc=10:1). Pale orange solid: yield 65%. 1H NMR (400 MHz, Chloroform-d) δ 8.31
(s, 1H), 8.07 – 7.99 (m, 2H), 7.51 – 7.30 (m, 9H), 7.24 (d, J = 9.1 Hz, 1H), 6.98 (d, J = 16.3 Hz,
1H), 6.85 (d, J = 16.2 Hz, 1H), 6.78 – 6.66 (m, 2H), 6.39 (dd, J = 9.1, 2.4 Hz, 1H), 6.24 (d, J = 2.3
Hz, 1H), 3.46 (ddt, J = 23.8, 14.5, 7.0 Hz, 4H), 2.98 (s, 6H), 1.26 (t, J = 7.2 Hz, 6H). 13C NMR
(101 MHz, CDCl3) δ 161.6, 157.3, 155.8, 154.1, 153.5, 150.0, 137.6, 134.5, 132.7, 131.1, 130.8,
129.5, 128.2, 128.1, 127.4, 126.1, 125.3, 124.8, 112.5, 107.7, 106.4, 98.7, 45.4, 40.5, 12.7.
HRMS-ESI calcd for C35H36BN4O3 ([M+H]+), 571.2880; found, 571.2463.
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8-(diethylamino)-5-(4-((1E,3E)-4-(4-(dimethylamino)phenyl)buta-1,3-dien-1-yl)phenyl)-2-
phenyl-3H,5H-5l4,12l4-benzo[5,6][1,3,2]oxazaborinino[2,3-b][1,3,4,2]oxadiazaborinin-3-one
(BS-DPBDE) (Scheme 2-6). The derivate of phenylboronic acid (0.3 mmol) was dissolved in
CH3CN (3 mL), toward which the ligand (0.33 mmol, 112 mg) was added. The resulting solution
was refluxed until the termination of reaction. And then, the solution was concentrated and the
residue was dissolved in tiny amount toluene and then purified by column chromatography
(Hexane : EtOAc=10:1). Pale orange solid: yield 54%. 1H NMR (400 MHz, Chloroform-d) δ 8.31
(s, 1H), 8.06 – 7.98 (m, 2H), 7.51 – 7.20 (m, 10H), 6.88 (dd, J = 15.3, 10.5 Hz, 1H), 6.81 – 6.66
(m, 3H), 6.54 (t, J = 16.1 Hz, 2H), 6.39 (dd, J = 9.2, 2.4 Hz, 1H), 6.24 (d, J = 2.4 Hz, 1H), 3.47
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(qt, J = 14.4, 7.3 Hz, 4H), 2.98 (s, 6H), 1.27 (t, J = 7.0 Hz, 6H). 13C NMR (101 MHz, CDCl3) δ
161.6, 157.3, 155.8, 154.1, 153.5, 137.3, 134.5, 132.7, 132.6, 131.1, 130.8, 129.5, 129.4, 128.2,
127.4, 125.6, 125.4, 112.5, 107.6, 106.4, 98.7, 45.4, 40.5, 29.7, 12.7. HRMS-ESI calcd for
C37H38BN4O3 ([M+H]+), 597.3037; found, 597.2697.
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3. Results and discussion
3.1 Synthesis
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Two derivatives of diphenylethene (DPE) or diphenylbutadiene (DPBDE), possessing a
dimethylamino and a boronic acid group as electron-donor and electron-withdrawing
groups, were settled on assembly with Schiff base ligand to create targeted molecules, BS-
DPE and BS-DPBDE. The B-N dative bond in BSs could furnish an extra molecular
stability,[26] which might also modify the electronic properties of auxiliary ICT state.
DPE-BA (boronic acid) and DPBDE-BA were synthesized by the Wittig reaction (Scheme
2
) between the corresponding benzaldehydes and the para-boronic acid derivative of the
benzyltriphenylphosphonium (TPP) bromide,[27] which were then converted into BS-
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DPE and BS-DPBDE by simple condensation with salicylidenehydrazone ligand.[23, 24]
3.2 Distinctive optical properties
Unexpectedly, light tangelo BS-DPE and BS-DPBDE are essentially nonfluorescent in the
solid state (Fig. 1b) and show very weak fluorescence in chloroform (Fig. 1c), which is in
marked contrast to BS-Ph with high fluorescence quantum yield (QY ca. 0.5-0.7) and
brightness in chloroform as previous reports.[19, 23] Serendipitously, it was found BS-
DPE and BS-DPBDE are highly luminescent in deuterated chloroform (CDCl3) (Fig. 1d)
during NMR test, in striking contrast to in chloroform. It spurred us into a detailed test on
its absorption and emission spectra in different solvent.Fig. 1e shows that BS-DPE has a
dual absorption band trait in nine selected solvents, where the low-energy band from 400
nm to 525 nm were attributed to ICT absorption of the ligand backbone, and the high-
energy band within the range of 300 nm – 400 nm originated from ICT state of DPE.[20]
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The spectra in CDCl3 and HEPES buffer were unpredicted, the former of which shows a
large hypsochromic shift of high-energy band from 357 nm to 320 nm in comparison with
that in CHCl3, the latter exhibits that both of bands broadened and stretched out to 410 nm
and 550nm, indicating water molecules have obvious influence on both of ICT ground
states. Our earlier report revealed that BS-Ph had just one local chromophore absorption
band located at 400 nm, which is an outcome of interaction of backbone ICT state with
water.[19] Based on the above facts in HEPES buffer, it could be concluded that the
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horizontal and vertical ICT states have independent features, but also interact with each
other and offer an ensemble solvatochromism. Fluorescence spectra follow the same trend
as the absorption spectra (Fig. 1f). High QY up to 0.43 was attained in CDCl3, whereas no
fluorescent signal was detected in HEPES. A red shift was observed in high polar solvent
in comparison with apolar solvents (average 520 nm), up to 15 nm, 32 nm and 17 nm for
PEG400, DMSO and CH3OH, respectively. For BS-DPBDE, the spectral features are
comparable to that of BS-DPE, but with three distinctions: (1) High energy-band is located
at 344 nm for CDCl3 and near 385 nm for others; (2) The intensities of high energy-band
are higher than that of low energy-band in all solvents except CDCl3; (3) The QY of BS-
DPBD (0.52) is higher than BS-DPE in CDCl3 (Table 1). Due to much higher brightness (ε
×Φs) of BS-DPBDE, it was selected for further discussion.
3.3 Turn-on phenomena and characterization.
The extraordinary brightness in CDCl3 made us think about the concealed factor in
fluorescence turn-on. Hydrochloric acid as the main decomposed product of CHCl3 was
tested first.[28] As shown in Fig. 2a-b and Fig. S1-S2, the absorption and emission spectra
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of BS-DPBDE in different solvents were compared with that of corresponding solvent
added HCl up to 15 uM. Distinctly, the spectral feature of BS-DPBDE in CHCl3 with HCl
was identical with that in CDCl3, which indicates that it is easy for CDCl3 to release DCl as
acid for actuating fluorescence of BS-DPBDE. For other solvents with adding HCl,
unexpectedly, there are no significant enhancement of fluorescence intensities even in
apolar solvent toluene, and also no occurrence of high-energy absorption band blue-shift.
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In toluene and THF, addition of HCl even caused the decrease in fluorescence a little bit.
This fact points out that the fluorescent turn-on is not only an acid-actuation process, but
also dependence on solvent. Next, the chloride solvents C2H4Cl2 and CH2Cl2 were used to
further investigate this solvent effect on fluorescence turn-on. From Fig. 2c, it can be seen
that the low-energy band were heightened, and also that the low energy band lost its
vibrational fine-structure when HCl was added into CHCl3 and C2H4Cl2, which are
consonant with intensified fluorescence in CHCl3 and C2H4Cl2 with HCl (Fig. 2d).
Afterwards, UV-vis and emission titration experiments of BS-DPBDE (10 µM) in CHCl3
were recorded with increasing concentrations of HCl (Fig. 2e-f). The continuous addition of
HCl from 1 uM to 20 uM induced a dramatic change in the spectra that accompanied two
isosbestic points at 350 and 445 nm. The presence of an isosbestic point signified the
presence of an equilibrium process between turn-on and turn-off states. No further spectral
change was registered when HCl was added beyond the mole ratio of 1.25 equiv. The
fluorescence intensity increase caused by addition of HCl to BS-DPBDE is linearly
proportional to HCl concentration in the 1-7.5 uM range (Fig. 2f). Ensuingly, acetic acid,
Trifluoroacetate (TFA) and sulfuric acid were selected for testing whether or not all acids
could actuate the fluorescence of BS-DPBDE. Fig. 2g presents the fluorescence intensity
change (I/I0) upon adding acid into CHCl3 solution, which manifest that acids with pKa
lower than 3.0 enable the CHCl3 solution of BS-DPBDE to be emitted. The spectral
properties of BS-DPE were comparable with BS-DPBDE and the data were shown in Fig.
S5.
Fluorescence decay parameters of the BS-DPBDE in different solvents without or with
acids are listed in Table 2. Fluorescence decay profiles were satisfactorily fitted with a
two-exponential model and lifetimes show apparent effect of solvent, much shorter
lifetimes for polar solvent THF (1.23 ns) and DMSO (0.61 ns).[29] The faster component
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of the lifetime can be attributed to the ICT species, and the slower component is due to the
presence of the LE molecules[30]. Remarkably, roughly twice prolongation of τ1 were
registered in CHCl3 upon adding HCl and H2SO4 over others, especially in contrast to that
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of TFA and acetic acid (Fig. 3a). It indicates that there might have some fresh species with
longer lifetime, associated with high fluorescence. Schiff-base backbone and two
dialkylamino-group confused us as to whether BS-DPBDE was degraded and which
dialkylamino base group was vulnerable to attack by acid. Further, proton NMR titrations
were employed to disclose the underlying mechanism. DMSO-d6 with 1.2 M acids was
added to CDCl3 solution of BS-DPBDE (10 mM), ultimately with a volume ratio of 1: 100,
circumventing the solvent effects on chemical shifts. The signals of proton 7, 8 and 9 are of
non-movement under all tests, regarded as reference peak together with residual signal of
CDCl3 (Fig. 3c), signifying that Schiff-base backbone is stable in CDCl3 with a small
amount of acid although it might be unstable in acidic aqueous solution (Fig. S8). With
gradual addition of HCl, the larger downfield shift of the dimethyl group proton 1 and
aromatic protons 4 were observed, whereas the smaller downfield shift of that as replacing
HCl with TFA. It is well-known that chemical shift is a measure of electron density near
the proton being measured. And thus, it can be concluded that stronger acidic proton as
electron acceptor makes protons 1 and 4 in BS-DPBDE deshielded heavily.[31] In contrast,
proton 2 and 3 of the diethyl-amino group are less affected even though its high pKa value
(N-diethylaniline, 6.56 vs N-dimethylaniline, 5.06[32]). The peaks appeared under
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addition of HCl to 0.8 equiv. (asterisk peaks) were contributed to slow-proton-exchange of
protonated diethyl-amino group on the NMR time scale.[33]
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3.4 Molecular mechanism and calculations.
To understand the fluorosolvatochromic behaviour, the fluorescence intensity of BS-
DPBDE in different solvent with 1.5 equiv. HCl were associated with three solvatochromic
parameters: polarity parameter (ET(30)), hydrogen-bond-donating parameter (
hydrogen-bond-donating parameter ( ) (Fig. 3b). It is apparent that no correlation is
observed for ET(30) and , whereas indicates that the fluorescence of BS-DPBDE is
lighted up only in strong acid containing non-hydrogen-bond accepting solvents,
choroform, whose value approximate zero. Density functional theory (DFT) calculations
α), and
β
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α
β
revealed that the HOMOs, for both of BS-DPE and BS-DPBDE, are located on the donor-
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acceptor diphenylpolyenes, whereas the LUMOs on the Schiff-base backbone (Fig. 4 and
Table 3). It is in sharp contrast with HOMO and LUMO of BS-Ph, both of which located
on the Schiff-base backbone.[23] The non-superimposed HOMO and LUMO distribution
support the photoinduced electron transfer (PET) process in these two molecules, which
weakened and even quenched the fluorescence of the Schiff base backbone fluorophore.
All of clues demonstrate that hydrogen-bond accepting solvents might enfeeble the
electron withdrawing ability of acidic proton, and hence impeded the vanishment of PET
process. We believe that this unique fluorescence phenomena might broaden our notion of
fluorosolvatochromism and be applied in diverse fields.[34]
274
4. Conclusions
275
276
277
278
279
280
281
282
283
In closing, BSs derivatives, BS-DPE and BS-DPBDE, with two perpendicular ICT states
have been synthesized and its spectroscopic and photophysical properties are described.
We disclosed that boron in BSs as a node separated its two ICT states (Schiff base
backbone ICT and diphenylpolyenes ICT states) and developed non-fluorescent PET
compounds. Peculiarly, our work provides indisputable prerequisite for its fluorescent turn-
on, that is, stronger acidic proton in non-hydrogen-bond accepting solvents. We believe
that the unique fluorescence of BS-DPE and BS-DPBDE would not only spur new
application in diverse field, but also provide the foundation for modification of
conventional theories.
284
Acknowledgements
285
286
287
This work was supported by the Macao Science and Technology Development Fund under
Grant No.: 052/2015/A2, 082/2016/A2, the Research Grant of University of Macau under
grant No.: MYRG2016-00058-FHS, and MYRG2017-00066-FHS.
288
Appendix A. Supplementary data
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Supplementary data related to this article can be found at
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Scheme 1 T-shape molecular geometry of BSs with two perpendicular ICT states (The DFT optimized
structure) and the structures of BS-Ph, BS-DMA, BS-DPE and BS-DPBDE
369
Scheme 2 Synthetic routine of BS-Ph, BS-DPE and BS-DPBDE
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372
Table 1 Spectral Properties and Fluorescence Quantum Yield (Φs) of the BS-DPE and BS-
DPBDE in different solvents.
b
c
λabs
ε
λem
∆
Compd
solvent
Φs
（nm）
321
(M-1cm-1)
(nm)
518
517
522
508
517
535
552
537
(nm)
40
BS-DPE
CDCl3
CHCl3
CH2Cl2
Toluene
THF
478 53000
0.43
357
478 52000
477 52000
472 54000
471 55000
478 43000
478 42000
473 43000
39
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
359
45
355
36
355
46
PEG400
DMSO
CH3OH
361
57
363
74
351
64
20 mM
HEPES
356
471 38000
__a
BS-DPBDE
CDCl3
CHCl3
CH2Cl2
Toluene
THF
344
387
390
387
384
387
386
379
469 58000
478 56000
477 55000
472 59000
472 55000
477 48000
479 47000
473 46000
518
517
522
509
520
533
544
553
49
39
45
37
48
56
65
80
0.52
0.02
0.02
0.02
<0.01
<0.01
<0.01
<0.01
PEG400
DMSO
CH3OH
20 mM
HEPES
389
465 40000
__a
373
374
a Not determined due to the weak fluorescence. b λ ex = at 470 nm. c Stokes shift.
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Fig. 1 Image of compounds BS-Ph, BS-DPE and BS-DPBDE (a) under vis light; (b) in solid states, (c) dissolved in
CHCl3 or (d) in CDCl3 under 365 nm UV light. Absorption (e, g) and emission spectra (f, h) of BS-DPE (e, f) and
BS-DPBDE (10 uM) (g, h) in CDCl3, CHCl3, CH2Cl2, toluene, THF, PEG400, DMSO, CH3OH and HEPES buffer
(20 mM). (excitation at 450 nm, slit: 1 nm/1 nm)
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Fig. 2 Absorption (a, c) and emission (b, d) spectra of 10 uM BS-DPBDE in different solvents with or without 15
uM HCl. Notes: for (b), CHCl3 (black), toluene (red), THF (green), EtOAc (blue), CH3OH (cyan), DMSO
(magenta), PEG400 (yellow) and 20 mM HEPES buffer (dark yellow); short dot line, with 15 uM HCl. Absorption
(e) and emission (f) titration spectra in the presence of varying concentrations of HCl in CHCl3. (g) the emission
ratio (I/I0) upon adding acid into CHCl3 solution. (excitation at 450 nm, slit: 1 nm/1 nm)
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Fig. 3 (a) The distribution of fluorescence decay times of BS-DPBDE in CHCl3 with 15 uM acids
(BS-DPBDE, 10 uM), data from Table 2 and Fig S3-4 (b) Relationship between the fluorescent
intensity of BS-DPBDE in 15 uM HCl solution and the solvatochromic parameters ET(30),
hydrogen-bond-donating parameter (α), and hydrogen-bond-accepting parameter (β) of solvents.
1
Partial Data are repeated from Fig.2; (c) H NMR titration of BS-DPBDE with HCl and TFA in
CDCl3 (BS-DPBDE, 10 mM).
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Table 2 Fluorescence Decay Parameters of the BS-DPBDE investigated in different solvent with
or without acid
2
τ1 (ns)
τ2 (ns)
α1
α2
χR
τꢀ (ns)
1
2
CHCl3
CHCl3 with 15 uM HCl
CH2Cl2
0.60
1.27
0.74
0.64
0.90
0.87
0.05
0.09
0.34
0.51
0.57
1.27
0.59
2.45
2.24
2.41
2.22
2.23
2.10
1.25
1.06
0.80
2.24
2.44
2.11
2.50
0.18
0.18
0.32
0.38
0.20
0.39
0.26
0.22
0.62
0.97
0.26
0.42
0.17
0.82
0.82
0.68
0.62
0.80
0.61
0.74
0.78
0.38
0.03
0.74
0.58
0.83
2.36
2.13
2.20
1.98
2.11
1.84
1.23
1.04
0.61
0.72
2.30
1.86
2.41
1.7
1.9
1.9
1.7
1.6
1.6
3.8
6.0
2.0
2.6
1.5
1.6
1.8
3
4
CH2Cl2 with 15 uM HCl
Toluene
5
6
Toluene with 15 uM HCl
THF
7
8
THF with 15 uM HCl
DMSO
9
10
11
12
13
DMSO with 15 uM HCl
CHCl3 with 15 uM TFA
CHCl3 +15 uM H2SO4
CHCl3 +15 uM Acetic acid
382
383
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Fig. 4 Molecular orbitals (LUMO and HOMO) and HOMO/LUMO energy gaps of BS-DPE and
BS-DPBDE.
385
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Table 3 Calculated single-photon related spectral properties of BS-DPE, BS-DPBDE in the gas
phase.
λmax(nm) a
Comp.
∆E1(eV) b
f
OI c
452.10
462.85
2.74
2.68
0.1196
0.0296
149 → 152 (H-2 → L)
157 → 159 (H-1 → L)
2
389
390
a. Peak position of the longest absorption band; b. The energy gap of the one-photon
absorption band; c. TD-DFT method with the orbitals Involved (OI).
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Highlights
ò Two boronic acid derived salicylidenehydrazone complexes (BSs) with two
perpendicular ICT states, BS-DPE and BS-DPBDE, were synthesized and
characterized.
ò The non-fluorescent PET characteristic of BS-DPE and BS-DPBDE were
disclosed.
ò Relative strong acidic proton in non-hydrogen-bond accepting solvent were
proved to actuate the vanishment of PET and reinstatement of its fluorescence.