Experimental and theoretical study of enol-keto prototropic tautomerism and photophysics of azomethine-BODIPY dyads

Article abstract of DOI:10.1039/c4cp02151g

In this study we report about two novel azomethine-BODIPY dyads 1 and 2. The two dyads have been, respectively, synthesized by covalent tethering of tautomeric ortho-hydroxy aromatic azomethine moieties including N-salicylideneaniline (SA) and N-naphthlideneaniline (NA) to a BODIPY fluorophore. Both of the two dyads 1 and 2 show enol-imine (OH) structures dominating in the crystalline state. Dyad 1 in the enol state is the most stable form at room temperature in most media, while enol-keto prototropic tautomerism of the NA moiety in solution is preserved in dyad 2, which can be reversibly converted between enol and keto forms in the environment's polarity. Visible illumination of dyad 2 in the enol state excites selectively the BODIPY fragment and then deactivates radiatively by emitting green light in the form of fluorescence, while the emission intensity of 2 in the keto state is quenched on the basis of the proton-coupled photoinduced electron transfer (PCPET) mechanism. This allows large fluorescence modulation between the two states of dyad 2 and generates a novel tautomerisable fluorescent switch. Theoretical calculations including calculated energies, potential energy surfaces (PESs) and intrinsic reaction coordinate (IRC) analysis further support that the single proton transfer reaction from an enol form to a transition state (TS) and from the TS to a keto form for 2 is easier to occur than that for 1, which accounts for the fluorescence quenching of 2 in methanol. The agreement of the experimental results and theoretical calculations clearly suggests that fluorescent and tautomeric components can be paired within the same molecular skeleton and the proton tautomerization of the latter can be designed to regulate the emission of the former. In addition, preliminary experiments revealed that 1 can be potentially used as a simple on/off fluorescent chemosensor which exhibited higher selectivity for Cu²⁺ over other common cations. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cp02151g

PCCP
View Article Online
View Journal | View Issue
PAPER
Experimental and theoretical study of enol–keto
prototropic tautomerism and photophysics of
azomethine–BODIPY dyads†
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 16290
Zhong-Hua Pan,a Jing-Wei Zhoub and Geng-Geng Luo*a
In this study we report about two novel azomethine–BODIPY dyads 1 and 2. The two dyads have been,
respectively, synthesized by covalent tethering of tautomeric ortho-hydroxy aromatic azomethine moieties
including N-salicylideneaniline (SA) and N-naphthlideneaniline (NA) to a BODIPY fluorophore. Both of the
two dyads 1 and 2 show enol-imine (OH) structures dominating in the crystalline state. Dyad 1 in the enol
state is the most stable form at room temperature in most media, while enol–keto prototropic tautomerism
of the NA moiety in solution is preserved in dyad 2, which can be reversibly converted between enol and
keto forms in the environment’s polarity. Visible illumination of dyad 2 in the enol state excites selectively
the BODIPY fragment and then deactivates radiatively by emitting green light in the form of fluorescence,
while the emission intensity of 2 in the keto state is quenched on the basis of the proton-coupled
photoinduced electron transfer (PCPET) mechanism. This allows large fluorescence modulation between the
two states of dyad 2 and generates a novel tautomerisable fluorescent switch. Theoretical calculations
including calculated energies, potential energy surfaces (PESs) and intrinsic reaction coordinate (IRC) analysis
further support that the single proton transfer reaction from an enol form to a transition state (TS) and from
the TS to a keto form for 2 is easier to occur than that for 1, which accounts for the fluorescence
quenching of 2 in methanol. The agreement of the experimental results and theoretical calculations clearly
suggests that fluorescent and tautomeric components can be paired within the same molecular skeleton
and the proton tautomerization of the latter can be designed to regulate the emission of the former. In
addition, preliminary experiments revealed that 1 can be potentially used as a simple on/off fluorescent
chemosensor which exhibited higher selectivity for Cu2+ over other common cations.
Received 17th May 2014,
Accepted 18th June 2014
DOI: 10.1039/c4cp02151g
www.rsc.org/pccp
hydrogen-bonding are an interesting class of compounds for
theoretical and experimental studies because they can undergo
1. Introduction
Tautomers are readily interconverted constitutional isomers, proton-tautomerism reaction leading to interesting properties
usually distinguished by a rearrangement of a labile hydrogen including thermo- and photochromism. As proton transfer in
atom and a double bond (e.g., enol–keto tautomerization).1 The these systems causes a change in optical properties, these mole-
equilibrium between tautomers is often rapid under normal cules can be utilized for the design of various optical switches and
conditions, being catalyzed by traces of acid or base present in storage devices.5 Due to significances of proton transfer and
most samples and solvents.2 Proton tautomerization is central tautomerism processes, there have been numerous studies of
to several fields of chemistry and biochemistry and plays a role in the ortho-hydroxy aromatic azomethines in the solid-state and in
pharmaceutical activity, enzyme activity, stabilization of base pairs solution. Generally, they display two possible tautomeric forms,
in duplex DNA, and self-assembly.3,4 ortho-Hydroxy aromatic the enol-imine (OH) form and the keto-enamine (NH) form.
azomethines (known as Schiff bases) possessing intramolecular Depending on the tautomers, two types of intramolecular
hydrogen bonds are observed in the ortho-hydroxy aromatic
a College of Materials Science and Engineering, Huaqiao University,
Schiff bases: O–HꢀꢀꢀN in enol-imine and N–HꢀꢀꢀO in keto-enamine
Xiamen 361021, P. R. China. E-mail: ggluo@hqu.edu.cn; Fax: +86-592-6162225
b School of Pharmaceutical Sciences, East Campus, Sun Yat-sen University,
Guangzhou 510006, P. R. China
tautomers. The enol-imine form is the most stable form at room
temperature but is in equilibrium with the keto-enamine form
relying on the substitution patterns of the aromatic rings and
the surrounding medium.6 Different analytical methods have
been used to show the presence of the enol-imine and keto-
enamine forms, among them are UV-vis, fluorescence, Raman,
† Electronic supplementary information (ESI) available: Additional calculations
and experimental measurements, figures and tables. CCDC 1000809 and
1000810. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c4cp02151g
16290 | Phys. Chem. Chem. Phys., 2014, 16, 16290--16301
This journal is © the Owner Societies 2014

View Article Online
Paper
PCCP
FT-IR, and NMR spectroscopy and X-ray crystallography techniques,
as well as combinations thereof.7 As a complement to these experi-
ments, a variety of computational chemistry methods has also been
applied.8 Both experimental and theoretical investigations have
shown that the reversible enol–keto tautomerism occurs via an
intramolecular H-transfer mechanism between ortho-hydroxy and
imine nitrogen forms.8 Enol–keto tautomerism has been used to
modulate magnetic dynamics of some lanthanide complexes.9
However, relatively less attention has been paid to attachment
of ortho-hydroxy azomethine fragments to organic chromo-
phores to study the effect of the enol 2 keto tautomerism
reaction on photophysical properties of chromophores.
Scheme 1 Design and synthesis of dyads 1 and 2.
characterized by standard analytical techniques (i.e., FT-IR spectro-
Seeking to look for suitable organic chromophores in favor scopy, NMR spectroscopy, mass spectrometry, elemental analysis
of grafting tautomeric components, our attention has turned to and cyclic voltammetry), which gave satisfactory data corresponding
the boron dipyrromethene (4,4-difluoro-4-bora-3a,4a-diaza-s- to their molecular structures. Solid-state IR spectra of 1 and 2 were
indacene, BDP or BODIPY) class of dyes. Recent efforts by other recorded in the 4000–400 cmꢁ1 region (Fig. S1 of ESI†). Generally,
groups and our laboratory have focused on developing BODIPY the non-hydrogen-bonded or a free hydroxyl group absorbs strongly
fluorescence sensors based on photoinduced electron transfer in the 3550–3700 cmꢁ1 region.13 Intramolecular hydrogen bonding
(PET) as a transduction mechanism.10 BODIPYs are well-known if present in the six-membered ring system would reduce the O–H
for their excellent photophysics such as strong absorption of stretching band to the 3550–3200 cmꢁ1 region.14 IR spectra of 1 and
visible light, high fluorescence quantum yields and good photo- 2 show a sharp band at 3441 and 3442 cmꢁ1 due to O–H stretching
stability.10,11 In addition, the optical properties of BODIPY are vibration. The characteristic region of 1700–1500 cmꢁ1 can be
tunable through structural modifications on the core of dye. used to identify the proton transfer of Schiff bases. Azomethine
These favorable features render BODIPY as widely used as a (CQN) bond stretching vibration was observed at 1623
fluorophore core for the construction of luminophores, molecular and 1626 cmꢁ1 for 1 and 2. The crystal phase purities of 1
probes or molecular logic gates, light-harvesting molecular arrays and 2 were confirmed from powder X-ray diffraction (PXRD)
and photodynamic therapy (PDT) agents.11 In this context, we patterns (Fig. S2 of ESI†). The thermal behavior of 1 and 2 as
decided to explore the possibility of integrating fluorescent and well as of SA and NA was studied by differential scanning
tautomeric components within the same molecular skeleton. calorimetry (DSC) (Fig. S3 of ESI†). The DSC thermogram
We report herein the synthesis, structures, photophysical properties of 1 and 2 exhibited only one exothermic peak at 288 1C
of two dyads (1 and 2) with high thermal stabilities, which consist (DH = 107.9 J gꢁ1) and 299 1C (DH = 86.21 J gꢁ1), respectively,
of ortho-hydroxy aromatic azomethines including N-salicylidene- ascribed to the melting point (Tm) of dyads. Obviously, the two
aniline (SA) and N-naphthlideneaniline (NA) coupled to a BODIPY dyads exhibit a higher melting point and are more stable than
fluorophore. Significant differences in the photophysical starting material 3 (Tm = 190 1C) or reference Schiff bases SA
behaviours of the two synthesized dyads related to proton- (Tm = 52 ꢂ 1 1C) and NA (Tm = 96 ꢂ 1 1C).
tautomerism are observed and discussed with the aid of density
functional theory (DFT) calculations.
The redox behaviour of the two dyads was investigated using
the cyclic voltammetry (CV) technique in dry DCM containing a
0.1 M TBAP background electrolyte (Table S1 of ESI†). For these
dyads, a single electrochemical peak is observed on reductive
scans that could be ascribed to formation of the BODIPY
p-radical anion (ꢁ1.69 V for 1 and ꢁ1.59 V for 2). On oxidative
scans, two peaks corresponding to the oxidation of the Schiff
2. Results and discussion
2.1. Synthesis and characterization
The starting BODIPY-linked aniline 3 was firstly synthesized base fragment and the formation of the BODIPY p-radical
in moderate yield from a reaction of 2,4-dimethylpyrrole and cation are observed for 1 and 2. For 2, there is a slight negative
4-nitro-benzaldehyde, followed by a conversion of the –NO2 group shift of the oxidation peak from the Schiff base fragment
to form an –NH2 substitute according to reported procedures.10e,12 (0.62 V) compared with that of 1 (0.80 V) due to the redundant
With the main aim of investigating the effect of prototropic benzene ring. The oxidation potentials of the BODIPY moiety in
tautomerism of ortho-hydroxy aromatic Schiff bases on the these dyads are similar (1.01 V for 1 and 1.04 V for 2). From
photophysics of BODIPY chromophores, azomethine–BODIPY these results of electrochemistry, it is inferred that the Schiff
dyads 1 and 2 functionalized with imines were prepared by a base unit and the BODIPY group are isolated and only minor, if
convenient one-step condensation reaction of 3 with 2-hydroxy- any, electronic communication takes place through the bridge
benzaldehyde or 2-hydroxynaphthaldehyde in a high yield in the ground state.
according to the synthetic procedures along with individual
Schiff bases (SA and NA) as well as a model TM-BODIPY, as
2.2. Crystal structural analyses
shown in Scheme 1. Dyads 1 and 2 were first documented Crystalline samples of azomethine–BODIPY dyads 1 and 2 were
by single-crystal X-ray crystallography (SCXRD) and further obtained by a slow vapour diffusion of hexane into a saturated
This journal is ©the Owner Societies 2014
Phys. Chem. Chem. Phys., 2014, 16, 16290--16301 | 16291

View Article Online
PCCP
Paper
SCXRD reveals the preference of the enol structure in the Schiff
base moiety in the solid state for 1 and 2, as indicated by
the bond parameters (C5–O1 = 1.359 Å, C7–O1 = 1.329 Å and
C7–N1 = 1.285 Å, C11–N1 = 1.291 Å). Furthermore, the aromati-
cities of enol forms of 1 and 2 can also be inferred from the
harmonic oscillator model aromaticity (HOMA) index.16 The
HOMA indices in the range of 0.900–0.990 and 0.500–0.800
correspond to aromatic and the non-aromatic rings, respectively.
In 1, the calculated HOMA indices for C1/C6 and C8/C13 are 0.968
and 0.992, respectively. These results show that C1/C6 and C8/C13
in 1 have purely aromatic character. In 2, the calculated HOMA
indices for C5/C10 and C12/C17 are 0.629 and 0.995, respectively,
which display that C12/C17 has purely aromatic character while
C5/C10 deviated from aromaticity. This difference in molecular
Fig. 1 Single crystal X-ray diffraction structures of 1 (a), 2 (b) with thermal
ellipsoids set at 50% probability, and the theoretical geometric structures
of 1 (c) and 2 (d).
CHCl3 solution of 1 or CH2Cl2 solution of 2. Single crystals configuration is responsible for different results of theoretical
collected were of suitable quality to undertake a structure calculations and optical properties as discussed below.
determination by SCXRD analysis, and crystallographic data
The existence of strong intramolecular O1–H1Dꢀ ꢀ ꢀN1 and
are given in Table S2 (ESI†). Both 1 and 2 crystallize in the O1–H1Bꢀ ꢀ ꢀN1 hydrogen bonds both producing S(6) ring motifs
%
triclinic space group P1 with one independent molecule within is observed in 1 and 2, respectively. These H-bonds can be
the asymmetric unit (Fig. 1a and b). In fact, each azomethine– characterized by O1ꢀ ꢀ ꢀN1 distances and are summarized in
BODIPY dyad constitutes a BODIPY unit as the fluorophore and Table 1. It is known that there is a strong correlation between
a Schiff base fragment as a tautomer. As previously observed in the strength of the H-bond and the delocalization of the system
some similar structures,10d,15 the BODIPY core B(N2F2) shows a of conjugated double bonds, and the effect is qualitatively
quasi-tetrahedron configuration with the average angle of F–B–F interpreted by the resonance-assisted hydrogen bond (RHAB)
of 108.81 and the average bond length of B–N and B–F to be model.17 The observed Oꢀ ꢀ ꢀN distances of 2.605(4) Å for 1 and
1.544 Å and 1.388 Å, respectively (Table S3 in the ESI†). In the 2.548(2) Å for 2 are apparently shorter than 2.656 Å which was
BODIPY moiety, the C9BN2 framework consisting of one central reported for O–Hꢀ ꢀ ꢀN in the class of the RAHB model. The
six-membered and two adjacent five-membered rings is essen- aggregations of 1 and 2 through a series of weak hydrogen
tially flat, with the maximum deviation from the least-squares bonds including C–HꢀꢀꢀF and edge-face C–Hꢀꢀꢀp interactions in the
mean plane for the 12 atoms of the indacene group being 0.083 Å crystal cells are shown in Fig. S4 in the ESI.† These F- and C-based
in 1 and 0.029 Å in 2, respectively. This geometry indicates the interactions can be further visualized by Hirshfeld surface calcula-
strongly delocalized p-system nature of the C9BN2 framework in tions and given in Fig. S5 in the ESI.†
1 and 2. However, this p-electron delocalization is interrupted
between the two B–N bonds (1.536–1.554 Å), which is in agree-
2.3. Computational studies
ment with the results reported for other BODIPYs.15 It was
The optimized parameters (bond lengths, bond angles, and
revealed that the introduction of two methyl groups at C-1 and
dihedral angles) of dyads 1 and 2 have been obtained using the
C-7 positions in the BODIPY core in 1 and 2 prevents the free
Gaussian 09 program with density functional theory (DFT), the
B3LYP method, and 6-31+G(d,p) as the basis set. The optimized
rotation of the meso-phenyl moiety, resulting in an almost ortho-
gonal configuration between the BODIPY core and the meso-
structures of 1 and 2 are depicted in Fig. 1c and d with
benzene moiety. The dihedral angle between the meso-phenyl ring
numbering of the atoms. The calculated structural parameters of 1
and the indacene plane (87.61) in 1 is analogous to that in 2 (86.51),
and 2 are presented in Table S3 (ESI†) along with the corre-
sponding values obtained from the experimental data. When the
X-ray structures of 1 and 2 are compared to their optimized counter-
parts, the bond lengths and bond angles computed by the B3LYP
method show a good correlation with the experimental values.
The remarkable conformational discrepancies are observed in the
orientation of the meso-phenyl ring in 1 and 2. The orientation
indicating the almost nonelectronic coupling nature between the
meso-phenyl ring and the C9BN2 unit in 1 and 2.
From the ORTEP view, the dihedral angle between the aromatic
ring systems in the Schiff base moiety of 1 is 26.11. This dihedral
angle decreases to 4.11 in 2. In general, ortho-hydroxy Schiff bases
undergo tautomerism involving proton transfer from the hydroxylic
oxygen to the imino nitrogen atom. The process of proton transfer
ends up with two tautomeric forms known as enol and keto
structures. The C7–N1 and C5–O1 bonds for 1 as well as C11–N1
and C7–O1 bonds for 2 are the most important indicators of the
tautomeric types. The C5–O1 and C7–O1 bonds are double bonds
for the keto-enamine tautomers, whereas these bonds display single
bond character in the enol-imine tautomers. The C7–N1 and
C11–N1 bonds are also double bonds in the enol-imine tautomers
and of single bond length in the keto-enamine tautomers.
Table 1 Experimental and theoretical hydrogen-bond geometries (Å, deg)
for 1 and 2
D–Hꢀ ꢀ ꢀA
D–H
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
D–Hꢀ ꢀ ꢀA
X-ray diffraction for 1
B3LYP/6-31+G(d,p) for 1
X-ray diffraction for 2
B3LYP/6-31+G(d,p) for 2
1.02(4)
1.00
1.02(2)
1.01
1.65(5)
1.74
1.61(3)
1.65
2.605(4)
2.634
2.548(2)
2.563
153.8(3)
147.2
150.9(2)
148.0
16292 | Phys. Chem. Chem. Phys., 2014, 16, 16290--16301
This journal is © the Owner Societies 2014

View Article Online
Paper
PCCP
of the meso-phenyl ring is defined by the torsion angles
C7–N1–C8–C13 [153.1(2)1 for 1] and C11–N1–C12–C17 [177.2(1)1
for 2]. These torsion angles have been calculated to be 145.061 and
146.111 for B3LYP, respectively. Such conformational discrepancies
can be explained by the fact that the calculations assume an
isolated molecule where the intermolecular Coulombic interaction
with the neighboring molecules is absent, whereas the experi-
mental results correspond to interacting molecules in the
crystal lattice, as described in the aforementioned crystal
structure analysis. When the geometries of hydrogen bonds
in the optimized structures of 1 and 2 are examined, it is seen
Fig. 2 Relative energy profiles during the proton transfer process of 1 (a)
and 2 (b).
that O–Hꢀ ꢀ ꢀN intramolecular hydrogen bonds exist between stable enol form. Fig. 2a shows two minima representing the
the phenol O atom for 1 (naphthol O atom for 2) and the imine stable forms of 1. The keto form corresponds to a local minimum
N atom. For 1, O–H, Hꢀ ꢀ ꢀN, and O–Hꢀ ꢀ ꢀN values are 1.02(4) Å, while the global minimum represents the stable enol form. The
1.65(5) Å, and 153.8(3)1 for X-ray diffraction, and 1.00 Å, 1.74 Å, potential energy barrier needed for the enol to keto form conver-
and 147.21 for B3LYP. For 2, O–H, Hꢀ ꢀ ꢀN, and O–Hꢀ ꢀ ꢀN values sion process in 1 was calculated to be 6.08 kcal molꢁ1
.
are 1.02(2) Å, 1.61(3) Å, and 150.9(2)1 for experimental data, and
We have also considered 2 for further study to know whether
1.01 Å, 1.65 Å, and 148.01 for theoretical values. There is good the nature of proton transfer of 1 and 2 is the same. As seen
matching between the calculated hydrogen-bond geometries from Table 2, the calculated total energy of the enol form of 2
and those obtained from the X-ray diffraction structures. The is only slightly lower than the keto form in the gas phase
presence of the hydrogen bond appears to be an important (0.81 kcal molꢁ1). Similar to dyad 1, PES scan of 2 shows two
property of the molecule, stabilizing its conformation in the distinct minima corresponding to enol and keto tautomers.
crystal, as shown in the molecular modelling part, this is also Here the enol form is slightly stable than the keto form. The energy
visible in the model obtained for the molecules discussed.
barrier in going from the enol to keto form is 3.34 kcal molꢁ1, which
The enol-imine (NH) and keto-enamine (OH) tautomerisms is obviously smaller than that of 1. Such a small energy barrier is
of dyads 1 and 2 are given in Scheme S1 (ESI†). To investigate easily overcome by a light-weight particle, such as a proton at room
these tautomeric stabilities, quantum chemical calculations temperature. Therefore, at the ground state there will always be a
were carried out for the enol and keto forms of 1 and 2. Some mixture of keto and enol forms with a larger percentage of the enol
important physicochemical properties such as total, HOMO, and form in the gas phase and the keto form in solution.
LUMO energies and the dipole moment (m) were also calculated
To further examine the whole process of enol–keto tautomeriza-
with the same level of theory, and the results are given in Table 2. tion via single proton transfer in dyads 1 and 2 in more detail, we
First comparing the total energies of the two tautomers for 1, the performed an intrinsic reaction coordinate (IRC) analysis for each
enol tautomer is substantially more stable (4.91 kcal molꢁ1) than case (see ESI†). The nature of the stationary points was confirmed by
the keto form. This is an expected result since the enol form of 1 means of a vibrational analysis. The whole prototropic tautomerism
has two purely aromatic rings as supported by the above process of 1 and 2 is actually subdivided into two parts: (i) transfer
structural analysis and ortho-hydroxy azomethine generally pre- from the enol form (OH) to the transition state (TS) and (ii) from the
fers the enol structure. The intramolecular proton transfer for 1 TS to the proton transfer keto form (NH). Fig. 3 shows the calculated
was investigated in the gas phase by performing a potential free energy profiles and the structures of the enol form, TS, and
energy surface (PES) scan at the B3LYP/6-31+G(d,p) level in order keto form for 1 and 2. For enol–keto tautomerization of 1, the
to determine its effects on the molecular geometry. The process free energy of the keto form is very close to that of the TS
was started from optimized enol-imine geometry by selecting the (3.4 kcal molꢁ1 vs. 3.5 kcal molꢁ1) while larger than that of the
O–H bond as a redundant internal coordinate. The graph of the enol form (3.4 kcal molꢁ1 vs. 0 kcal molꢁ1). While for 2, both
relative energy versus the O–H bond distances is given in Fig. 2. the barrier energy and product free energy are on the verge
The energy values were calculated relative to the energy of the of the reactant free energy (1.4 kcal molꢁ1 vs. 0.8 kcal molꢁ1 vs.
0 kcal molꢁ1). These calculated results indicate that compared
to 1, the single proton transfer reaction of 2 is easier to occur,
and 2 is more inclined to obtain an enol–keto mixture while 1 is
in vacuum and in methanol for 1 and 2 and their tautomers
Table 2 Calculated energies, dipole moments, and frontier orbital energies
not. With a closer analysis of structures, we can see that both
the structures of reactants and transition states of 1 and 2 exist
Tautomer ETOTAL (hartree) EHOMO (eV) ELUMO (eV) m (D)
Gas phase 1-enol
ꢁ1469.54123405 ꢁ5.6793
ꢁ1469.53340411 ꢁ5.7579
ꢁ1623.18996163 ꢁ5.6602
ꢁ1623.18870347 ꢁ5.7209
ꢁ2.7007
ꢁ2.7884
ꢁ2.6814
ꢁ2.7483
4.8157 differently while the product states are similar. For the reactant
(e = 1)
1-keto
2-enol
2-keto
3.8781
state, the distance of the N–H hydrogen-bond of the reaction
center in 1 is larger than that in 2, indicating that the proton
5.4163
4.4635
transfers from the O atom to the N atom are more relaxed in 2.
For the TS, the distances of N–H and O–H are equivalent in 2
while in 1 the distance of O–H is larger than that of N–H,
CH3OH
1-enol
ꢁ1469.55686104 ꢁ5.8172
ꢁ1469.55389163 ꢁ5.8317
ꢁ1623.20631485 ꢁ5.8143
ꢁ1623.20871348 ꢁ5.8153
ꢁ2.8177
ꢁ2.8398
ꢁ2.8145
ꢁ2.8313
6.7180
5.5145
7.2598
(e = 32.7) 1-keto
2-enol
2-keto
5.9880 indicating that the structure of the TS is similar to the structure
This journal is ©the Owner Societies 2014
Phys. Chem. Chem. Phys., 2014, 16, 16290--16301 | 16293

View Article Online
PCCP
Paper
Table 3 Spectral parameters of 1 and 2 in different solvents
ET(30)a
labs
(max/nm)
Log
(emax
lem
(max/nm)
b
Solvent
(kcal molꢁ1
)
)
Ffl
1
MeOH
CH3CN
THF
CH2Cl2
Toluene
55.4
45.6
37.4
40.7
33.9
499
498
501
502
505
4.92
4.93
4.95
4.95
4.92
512
511
515
516
518
0.38
0.48
0.53
0.55
0.57
2
MeOH
CH3CN
THF
CH2Cl2
Toluene
55.4
45.6
37.4
40.7
33.9
499
498
501
502
505
4.93
4.94
4.95
4.95
4.96
513
512
516
516
519
0.05
0.03
0.30
0.27
0.40
a
b
Solvent polarity index. 1,3,5,7-Tetramethyl-8-phenyl–BODIPY was
used as a standard (Ffl = 0.72 in tetrahydrofuran).
Fig. 4 UV-Vis absorption spectra of 1 and SA (a) as well as of 2 and NA
(b) in methanol at room temperature, path length 1.0 cm, concentration
1 ꢃ 10ꢁ5 M for each sample.
and TM-BODIPY. The absorption maximum (499 nm) of 1
with the molar absorption coefficient as high as 8.03 ꢃ
104 Mꢁ1 cmꢁ1 is as expected for the TM-BODIPY dye, which
is due to the 0–0 vibrational band of the S0 - S1 (p–p*)
transition localized on the BODIPY unit.19
Fig. 3 Transition state from enol to keto form of 1 (a) and 2 (b) at the
B3LYP/6-31+G(d,p) level (kcal molꢁ1).
Dyad 2 also shows an intense absorption band centered
at 499 nm with the absorption coefficient of around 7.97 ꢃ
104 Mꢁ1 cmꢁ1. In addition, 2 displays a shoulder at the short
wavelength (high-energy) side, centered at about 469 nm, and is
attributed to the 0–1 vibrational band of the same transition.
The moderate higher-energy transitions of the NA unit in 2 were
observed at 315 and 375 nm, attributed to p–p* transition of the
enol form of NA. Interestingly, an absorption band at 438 nm
was observed in the spectrum of 2 in CH3OH, which may belong
to the keto form of ortho-hydroxy Schiff bases.20 In contrast, any
of the product state in 1 and to some extent means that the
proton-transfer-distance from the reactant to the TS in 1 is longer
than that in 2. Therefore, the single proton transfer reaction of 2 is
easier to occur. The reason for this result may be the redundant
benzene ring of 2 which could increase the conjugative effect and
the resonant structure compared to 1 (Fig. S6 in the ESI†).
2.4. Photophysical properties
The optical properties of 1 and 2 were first characterized by band belonging to the keto-enamine form in 1 was not observed
using UV-vis absorption spectroscopy in solvents of varying with a value greater than 400 nm. Referring to previous studies it
polarity (Table 3 and Fig. S7 of ESI†). Fig. 4 shows the absorp- was concluded that 2 exists in both enol and keto forms in
tion spectra of 1 and 2 as well as of references SA and NA in CH3OH while 1 is in favor of the enol state and not in keto in
CH3OH. The absorption spectra of the studied dyads exhibit most organic solvents at room temperature.8a,21 The keto form is
features that can be easily assigned to specific subunits. SA important in the solution and stabilized by the polar solvents
exhibits two moderate absorption peaks centered at 269 nm through solute–solvent interactions. Here hydrogen bonding
and 348 nm, respectively, assignable to p - p* and n - p* between the hydroxyl group of methanol and the nitrogen atom
transitions.18 The model TM-BODIPY presents a sharp and of Schiff base has played a crucial role in the proton transfer
intense absorption peak at 500 nm (Fig. S8 in the ESI†),19
a
process. It can be noted that in these dyads there is no any
typical feature of the BODIPY dyes.15 The absorption spectrum significant absorption band which cannot be assigned to specific
of dyad 1 is nearly the superposition of the spectra of SA individual subunits: this suggests that the interaction among the
16294 | Phys. Chem. Chem. Phys., 2014, 16, 16290--16301
This journal is © the Owner Societies 2014

View Article Online
Paper
PCCP
various components is weak and attachment of the Schiff base
fragment to the BODIPY entity causes minimal perturbation
of its optical properties, so that these dyads can be regarded as
multicomponent systems.
For a better understanding of the experimental data for the
optical properties, the optimization of both keto and enol
forms of 1 and 2 was carried out at the B3LYP/6-31+G(d,p) level
using a polarizable continuum model (PCM) in the presence of
methanol (e = 32.7) (Table 2). Our theoretical results allow the
conclusion that the total energy of the enol form of 1 obtained
by the PCM method is still more stable than the keto form,
though both HOMO and LUMO energy values become more
and more negative in going from the gaseous phase to the
solution phase with the increase of the dielectric constant
value. However, calculated energies of 2 in methanol show that
the enol-imine form is less stable than the keto-enamine form by
6.30 kJ molꢁ1, which indicates that it is energetically favorable to
have carbon–carbon double bonds rather than carbon–nitrogen
Fig. 5 (a) Absorption spectra and (b) fluorescence spectra (lex = 470 nm)
of
2 in toluene–methanol mixtures with volume ratios of toluene;
(c) photographs of the solutions of 2 in a mixture of the methanol–toluene
solvent system with different toluene fractions under UV illumination.
double bonds. In other words, the process of intramolecular been studied in a mixture of methanol–toluene, in which the
proton transfer evoked by the influence of solvent results in toluene content ( ftoluene) was varied in the range of 0–100 vol%.
destabilization of the OH form (a decrease in the prevalence of The UV-Vis spectra of 2 in the methanol–toluene solvent system
the OH form and an increase in that of the NH form) and showed several isosbestic points at 410, 364, and 324 nm, which
destabilization of the aromatic form. Thus, the keto-enamine definitely point out the existence of two molecular absorbing
form of 2 is much more stable than enol-imine, which accounts species in equilibrium (Fig. 5a). The luminescent properties of 2
for available UV-Vis experimental results.
were also investigated in the methanol–toluene system and the
Steady-state fluorescence spectra of 1 and 2 were also recorded results are given in Fig. 5b. The emission intensity of 2 increased
in various solvents with increasing polarity from toluene to dramatically as the fraction of toluene ftoluene increased, and the
methanol, and the details are gathered in Table 2 and Fig. S9 logarithm of the intensity and the fraction have a good linear
(ESI†). From the viewpoint of fluorescence, the structures of relationship with R2 4 0.99 as shown in the inset of Fig 5b. The
BODIPY-linked Schiff bases can be composed of two fragments, visual emission color of 2 in the methanol–toluene system when
the BODIPY moiety as a fluorophore and the Schiff base moiety excited using a hand-held 365 nm UV lamp is shown in Fig 5c,
as a fluorescence switch, which modulates the fluorescence which is consistent with the above analysis. Overall, these findings
quantum yield (Ffl) of the fluorophore, since they are ortho- of fluorescence contrast with the above observations of UV-Vis
gonal to each other. The starting material 3 was reported spectra. Therefore, 1 retains a moderate fluorescence quantum
to display quenched emission in methanol with Ffl o 1%.10e yield while the lower value of Ffl for 2 indicates a highly efficient
The quenching phenomenon has all the hallmarks of photo- fluorescence quenching by a thermodynamically allowed proton-
induced electron transfer (PET) from the phenylamino unit to coupled photoinduced electron transfer (PCPET) from the keto
the boradiazaindacene fluorophore. Upon condensation with unit of NA to the singlet excited state of the BODIPY moiety.
salicylaldehyde to form 1, the reduction potential of the non-
To shed light on the photoinduced activation of an electron
bonding electron pair on the nitrogen atom decreased due to transfer process within an azomethine–BODIPY dyad, computa-
the formation of an imine functional group. In other words, tions on 1 and 2 before and after prototropic tautomerism are also
electron transfer from the electron-deficient imine moiety performed and displayed in Fig. S10 (ESI†) and Fig. 6. As shown in
to the electron-rich BODIPY fluorophore became less feasible. Fig. 6, before prototropic tautomerism of 2, the HOMO energy level
The frontier orbital diagram also indicates that the highest of the enol form of the NA moiety (ꢁ6.26 eV) is obviously lower
occupied molecular orbital (HOMO) energy of the phenylamino than that of the BODIPY unit (ꢁ5.78 eV), therefore, the PET
unit (ꢁ5.38 eV) was higher than the HOMO energy of imine process can be suppressed and the emission of the BODIPY
(ꢁ6.26 eV), thereby preventing PET quenching of the emission, fluorophore is on. The energy level of HOMO (ꢁ5.56 eV) of
exhibiting that the photoluminescence quantum efficiencies the keto form of the NA moiety should increase above that of
vary between 0.57 and 0.38 (Table 3).
the HOMO (ꢁ5.78 eV) of the fluorophore BODIPY only after
In contrast, for 2, the value of Ffl is up to 0.40 in toluene, prototropic tautomerism. Under these conditions, the electron
and declines to only 0.05 in methanol. We speculate that the transfer process becomes exergonic and favorable. As a conse-
marked difference in fluorescence quantum yields between 1 and quence, when the BODIPY moiety of 2 is photoexcited, one
2 in pure methanol is consistent in the prototropic tautomerism, electron can be transferred from the keto form of NA to the
and derives from an increase in the keto form for 2 compared BODIPY moiety with a concomitant fluorescence quenching.
with that of its analog 1. In order to experimentally strengthen
The calculated frontier molecular orbital (FMO) energies
the above speculation, the photo-physical properties of 2 had and surfaces of 2 are shown in Fig. 7. As can be found, both
This journal is ©the Owner Societies 2014
Phys. Chem. Chem. Phys., 2014, 16, 16290--16301 | 16295

View Article Online
PCCP
Paper
Fig. 6 Proposed proton-coupled photoinduced electron transfer
(PCPET) mechanism between the BODIPY moiety and the keto unit of 2.
the contours of the electronic distribution in HOMO and LUMO
states of 2 in the enol form are located almost completely on
the BODIPY moiety, while the HOMO ꢁ 1 and LUMO + 1 are
located mainly on the NA fragment. It is postulated that the
HOMO–LUMO transition corresponds to the emissive p - p*
excited state as observed in the reference dye TM-BODIPY and
other BODIPY derivatives.10 As a result, the electronic transition
between HOMO and LUMO for 2 in the enol form is limited
only to the BODIPY moiety, leading to an intensive intrinsic
fluorescence from the BODIPY moiety of 2 in the enol form.
By contrast, the calculation results reveal that both the LUMO
and HOMO ꢁ 1 orbitals for 2 in the keto form are mostly
contributed from the BODIPY moiety, while the HOMO and
LUMO + 1 orbitals are mainly located at the NA fragment. The
significant distribution difference between HOMO and LUMO
of 2 in the keto form shows that there is electron transfer from
the NA fragment (keto form) to the BODIPY core. And this is
likely related to the PET process between the NA fragment (keto
form) and the BODIPY core, which quenches the fluorescence
of the BODIPY component.
Fig. 7 Molecular orbital surfaces and energy levels given in parentheses
for LUMO + 1, LUMO, HOMO, and HOMO ꢁ 1 of enol and keto forms of 2
computed at the B3LYP/6-31+G(d,p) level in methanol solvent.
Fe3+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+, and Zn2+ were studied in a
MeOH/CHCl3 (20 : 1, v/v) solution (Fig. 8a and Fig. S11, ESI†).
Changes in the fluorescence properties of 1 caused by various metal
ions were measured once the emission intensity was constant.
When 1 equiv. of Cu2+ was added to the solution of 1, the
fluorescence intensity will be reduced by 80%, which suggested
that 1 showed a particular response to Cu2+ ions compared to those
of other metal ions of similar electronic structure. It is well-known
that the paramagnetic Cu2+ center has a pronounced quenching
effect on fluorophores through a photoinduced electron or energy
transfer mechanism.23 Additionally, among the relevant para-
magnetic metal ions, Cu2+ has a particularly high thermo-
dynamic affinity for ligands with N or O as the chelating element,
and with a fast metal-to-ligand binding kinetic process.24
2.5. Dyad 1 as an ‘on-off’ fluorescent chemosensor for Cu2+
Dyad 1 was preliminarily chosen to detect metal ions based on
the following considerations: (1) as described in the foregoing
discussion, 1 in the enol-imine state is the most stable form
at room temperature in solution and shows moderate fluores-
cence quantum yields in most medium compared to dyad 2.
(2) Schiff bases are well known to be good ligands for metal
ions, and Schiff bases incorporating a fluorescent moiety are
appealing tools for optical sensing of metal ions.22
Fig. 8 (a) Emission spectra of 1 (1.0 ꢃ 10ꢁ5 M) in the solution of MeOH/
CHCl3 (20 : 1, v/v) upon addition of 3 equiv. of different metal nitrate salts;
(b) fluorescent spectra of 1 (1.0 ꢃ 10ꢁ5 M, excited at 470 nm) upon addition
Initially, the absorption and fluorescence response of 1
toward the nitrate salts of Cu2+, Ag+, Al3+, Ca2+, Co2+, Cr3+,
of 0–1.0 equiv. of Cu2+
.
16296 | Phys. Chem. Chem. Phys., 2014, 16, 16290--16301
This journal is © the Owner Societies 2014

View Article Online
Paper
PCCP
Selectivity is a matter of necessity for a chemosensor. In addition, in view of ortho-hydroxy aromatic Schiff bases
To investigate the selectivity of 1 for Cu2+ over other relevant exhibiting the interesting phenomenon of photochromism,25 the
cations, the fluorescence intensity changes upon addition of detailed investigation of the relationship between fluorescence
various competitive cations were examined, 3 equiv. of most modulation and photochromic behavior of these azomethine–
other metal ions showed no interference to the sensing of Cu2+ BODIPY dyads as well as other BODIPY-linked Schiff bases both
by 1. Additionally, to explore the effects of anionic counterions in solution and in the solid state when exposed to light of different
on the sensing behavior of 1 to metal ions, fluorescence wavelengths is under progress in our laboratory.
responses of 1 to chloride, nitrate, perchlorate and sulfate salts
with Cu2+ were examined and it can be seen that anions had no
influence on the fluorescence of 1 (Fig. S12, ESI†). The fluores-
cence intensity of 1 exhibits a gradual reduction upon the
4. Experimental
4.1. Materials
addition of 0–1.0 equiv. of Cu2+ and saturation upon the addition
of 1.0–3.0 equiv. of Cu2+, revealing that the stoichiometry of the
complex formed between 1 and Cu2+ ions is 1 : 1 (Fig. 8b).
Overall, current preliminary experiments revealed that 1 can be
potentially used as a simple fluorescent chemosensor which
exhibited high selectivity for Cu2+ over other common cations.
To further confirm the quenching mechanism, the coordination
environment of the 1–Cu2+ complex and the corresponding
association constant (K), more experiments including Job’s
method, ESI-MS, X-ray crystallography and NMR titration as well
as DFT calculations are currently in progress.
All the materials for the syntheses were purchased from commer-
cial suppliers and used without further purification. Air- and
moisture-sensitive reactions were carried out under a nitrogen
atmosphere using oven-dried glassware. Solvents were dried by
standard literature methods26 before being distilled and stored
under nitrogen over 3 Å molecular sieves prior to use. Water was
deionized. Thin-layer chromatography (TLC) was performed using
silica gel plates and flash column chromatography was conducted
over silica gel (200–300 meshes) with the eluent reported in
parentheses, both of which were obtained from the Qingdao
Ocean Chemicals.
3. Conclusions
4.2. Instrumentation
Two new dyads made of azomethine and BODIPY subunits, 1
and 2, have been prepared, together with their parent species 1H and 13C NMR spectra were recorded at room temperature
SA, NA and TM-BODIPY, and the photophysical properties of all using a Bruker PLUS 400 spectrometer with tetramethylsilane
the compounds have been investigated in fluid solution. Weak (TMS, 0.00 ppm) as an internal standard and CDCl3 as a
electronic interactions between Schiff base units and BODIPY solvent. Chemical shift multiplicities are reported as s = singlet,
fragments take place in the ground states of the two dyads, d = doublet, and br = broad singlet. Coupling constant ( J)
whose absorption spectra resemble those of their constituting values are given in Hz. Mass spectrometry (MS) experiment was
groups. The marked difference in luminescence between 1 carried out in the positive ion mode on a Bruker Esquire HCT
and 2 in methanol is observed and obviously consistent with ion trap mass spectrometer (Billerica, MA) coupled with a
the prototropic tautomerism of 2. Irradiation of dyad 2 in homemade electrospray ionization (ESI) device. Parameters of
the enol state excites the BODIPY component, which then the ESI source were optimized to enhance the signal intensity.
returns to its ground state by emitting light in the form of The pressure of nebulizing nitrogen, the flow rate of desolvation
fluorescence. However, the keto state of 2 can donate an gas, and the temperature of desolvation gas were set to 8 psi, 1 L
electron to the excited fluorophore. As a result of the proton- minꢁ1, and 250 1C, respectively. Electrochemical experiments
coupled photoinduced electron transfer (PCPET) process, the were performed using a three-electrode system. The working
excited state of the BODIPY component is quenched and the electrode was 2 mm Pt with a Pt wire as an auxiliary electrode
associated fluorescence is suppressed. Theoretical calculations and a 0.01 M Ag/AgNO3 solution as the reference electrode.
of total energies, potential energy surfaces (PESs) and the The sample solution contained an azomethine–BODIPY dyad
intrinsic reaction coordinate (IRC) analysis also support that (1.0 ꢃ 10ꢁ3 M) and 0.1 M tetrabutylammonium hexafluoro-
the single proton transfer reaction from an enol form to a phosphate (TBAP) as a supporting electrolyte in dry acetonitrile.
transition state (TS) and from the TS to the keto form for 2 is easier FT-IR spectra were recorded from KBr pellets in the 4000–400 cmꢁ1
to occur than that for 1, which accounts for the fluorescence range on a Nicolet AVATAT FT-IR360 spectrometer. C, H, and N
quenching phenomenon of 2. Additionally, 1 was preliminarily microanalyses were carried out using an EA 1110 analyzer from
chosen as an on/off type fluorescent chemosensor for Cu2+ over CE Instruments. The experimental powder X-ray diffraction
other common cations.
patterns were recorded on a Panalytical X-Pert Pro diffractometer
This contribution not only attempts to integrate the fluorescent with Cu Ka radiation equipped with an X’Celerator detector.
and tautomeric components within the same molecular Differential scanning calorimetry (DSC) data were collected on a
skeleton, but also presents that the proton tautomerization Netzsch-DSC-200F3 instrument at a heating rate of 10 1C minꢁ1
can be designed to regulate the emission of an organic chromo- from 25 to 310 1C. Samples were heated in open aluminum pans
phore between a nonemissive state and an emissive one. under a nitrogen gas flow of 20 mL minꢁ1
.
This journal is ©the Owner Societies 2014
Phys. Chem. Chem. Phys., 2014, 16, 16290--16301 | 16297

View Article Online
PCCP
Paper
4.3. Spectroscopic measurements and determination of
fluorescent quantum yields
Synthesis of 8-(1-azastyryl(2-hydroxynaphthaldehyde))-4,4-
difluoro-1,3,5,7-tetramethyl 4-bora-3a,4a-diaza-s-indacene (2).
3 (339 mg, 1 mmol) and 2-hydroxy-1-naphthaldehyde (172 mg,
1 mmol) were refluxed for 6 h under a nitrogen atmosphere at
78 1C in 50 mL of absolute ethanol. When TLC analysis showed
that the reaction was complete, the solution was cooled to room
temperature. The product was collected by filtration as a red
solid. Recrystallization from CH2Cl2–EtOH (356 mg, 75% yield);
M.p. = 299 1C. vmax(KBr pellet) cmꢁ1: 3442 (nOH), 1626 (nCQN),
1600, 1576, 1548, 1511, 1466, 1408, 1359, 1309, 1257, 1201, 1155,
1117, 1091, 1073, 1038, 983, 893, 829, 803, 767, 744, 707, 506,
Azomethine–BODIPYs were dissolved in various solvents to
acquire optical measurements. Acetonitrile, methanol, tetra-
hydrofuran, dichloromethane and toluene were all individually
used in preparing a BODIPY solution. UV-vis absorption and
steady-state fluorescence spectroscopic studies are performed
on a UV-2100 (Shimadzu) spectrophotometer and a F-7000 (Hitachi)
spectrophotometer at room temperature. The slit width was
5 nm for both excitation and emission. Samples for absorption
and emission measurements were contained in 1 cm ꢃ 1 cm
quartz cuvettes. Measurements were made using optically dilute
1
477. H NMR (400 MHz, CDCl3): 1.50 (s, 6H), 2.60 (s, 6H), 6.03
(s, 2H), 7.26 (d, J = 9.2 Hz, 1H), 7.41 (t, J = 7.7 Hz, 3H), 7.58
(dd, J = 17.0, 7.9 Hz, 3H), 7.78 (d, J = 7.9 Hz, 1H), 7.89
(d, J = 9.2 Hz, 1H), 8.19 (d, J = 8.4 Hz, 1H), 9.48 (s, 1H).
solutions after deoxygenation by purging with dried N2.
sample
fl
The relative fluorescence quantum yields (F
) of the
samples were obtained by comparing the area under the
corrected emission spectrum of the test sample with that of
13C NMR (100 MHz, CDCl3): d 14.62, 14.70, 74.34, 76.68,
77.00, 77.21, 77.32, 112.28, 116.48, 121.56, 121.87, 126.17,
127.82, 128.49, 129.17, 129.46, 130.92, 132.96, 135.60, 140.29,
148.04, 156.01. ESI-MS m/z (C30H26BF2N3O) calculated: 493.4,
found: 516.3 (M + Na), 532.3 (M + K).
the standard. Only dilute solutions with an absorbance below 0.1 at
sample
fl
the excitation wavelength lex were used. The F
values were
calculated using synthesized 8-phenyl-4,4-difluoro-1,3,5,7-tetra-
standard
methyl 4-bora-3a,4a-diaza-s-indacene (compound 3, F
=
fl
0.72 in tetrahydrofuran)27 as a fluorescence standard using the
Synthesis of 8-(1-azastyryl(2-hydroxybenzaldehyde))-4,4-
difluoro-1,3,5,7-tetramethyl 4-bora-3a,4a-diaza-s-indacene (1).
3 (339 mg, 1 mmol) and 2-hydroxy-1-benzaldehyde (125 mg,
1 mmol) were refluxed for 6 h under a nitrogen atmosphere at
78 1C in 50 mL of absolute ethanol. When TLC analysis showed
that the reaction was complete, the solution was cooled to room
temperature. The product was collected by filtration as an orange
solid. Recrystallization from CHCl3–EtOH (300 mg, 80% yield);
M.p. = 288 1C. vmax(KBr pellet) cmꢁ1: 3441 (nOH), 1623 (nCQN),
1597, 1573, 1545, 1508, 1467, 1409, 1365, 1306, 1279, 1195, 1154,
1120, 1086, 1052, 978, 910, 842, 810, 761, 708, 665, 581, 479.
1H NMR (400 MHz, CDCl3): d 1.48 (s, 6H), 2.59 (s, 6H), 6.03
(s, 2H), 7.00 (t, J = 7.5 Hz, 1H), 7.10 (d, J = 8.2 Hz, 1H), 7.39
(d, J = 8.3 Hz, 2H), 7.41–7.49 (m, 4H), 8.74 (s, 1H), 13.08 (s, 1H).
13C NMR (100 MHz, CDCl3): d 14.75, 14.64, 76.71, 77.02, 77.23,
77.34, 117.40, 119.03, 119.29, 121.35, 122.00, 129.09, 129.32,
131.46, 132.51, 133.58, 133.66, 140.95, 143.00, 149.10, 155.71,
161.26, 163.45. ESI-MS m/z (C26H24BF2N3O) calculated: 443.3,
following equation:
sample
standard
F
= F
ꢃ (Isample/Istandard) ꢃ (Astandard/Asample
)
fl
fl
ꢃ (nsample/nstandard 2
)
sample
standard
where F
and F
are the emission quantum yields of
fl
fl
the sample and the reference, respectively, Astandard and Asample
are the measured absorbances of the reference and the sample
at the excitation wavelength, respectively, Istandard and Isample
are the area under the emission spectra of the reference and the
sample, respectively, and nstandard and nsample are the refractive
indices of the solvents of the reference and the sample,
sample
fl
respectively. The F
values reported in this work are the
average values of multiple (generally three), fully independent
measurements.
4.4. Synthetic procedures
The syntheses of the starting BODIPY compounds NH2–BODIPY found: 466.3 (M + Na), 482.3 (M + K).
(3) and NO2–BODIPY (4) were achieved using literature methods
8-Phenyl-4,4-difluoro-1,3,5,7-tetramethyl 4-bora-3a,4a-diaza-
and characterized by 1H NMR, 13C NMR and elemental analysis s-indacene (5). 5 was synthesized according to the literature
to determine their structures.10e,12
reports.15 1H NMR (400 MHz, CDCl3) d 1.39 (s, 6H, CH3), 2.58
8-(4-Nitro-phenyl)-4,4-difluoro-1,3,5,7-tetramethyl 4-bora-3a,4a- (s, 6H, CH3), 6.00 (s, 2H), 7.28–7.32 (m, 2H, phenyl), 7.49–7.50
1
diaza-s-indacene (4). H NMR (400 MHz, CDCl3) d 1.37 (s, 6H), (m, 3H, phenyl); 13C NMR (100 MHz, CDCl3) d 14.3, 14.6, 121.2,
2.57 (s, 6H), 6.03 (s, 2H), 7.54 (d, 2H, J = 8.8 Hz), 8.39 (d, 2H, 127.9, 128.9, 129.1, 131.4, 135.0, 141.7, 143.1, 155.4. Anal. cacld
J = 8.8 Hz); 13C NMR (100 MHz, CDCl3) d 14.5, 121.7, 124.4, for C19H19BF2N2: C, 70.40; H, 5.91; N, 8.64. Found: C, 70.44;
129.6, 130.6, 138.3, 141.9, 142.5, 148.4, 156.3. Anal. cacld for H, 5.88; N, 8.67. Since 5 is an internal reference dye for the
C
19H18BF2O2N3: C, 61.81; H, 4.91; N, 11.38. Found: C, 61.78; determination of quantum yields it is regularly checked for
H, 4.87; N, 11.35.
stability and purity.
8-(4-Amino-phenyl)-4,4-difluoro-1,3,5,7-tetramethyl 4-bora-3a,4a-
4,4-Difluoro-1,3,5,7-tetramethyl 4-bora-3a,4a-diaza-s-indacene
1
diaza-s-indacene (3). H NMR (400 MHz, CDCl3) d 1.51 (s, 6H), (the model TM-BODIPY). TM-BODIPY was prepared using a
2.57 (s, 6H), 4.44 (br, 2H), 5.99 (s, 2H), 6.86 (d, 2H, J = 4.0 Hz), literature procedure.19 13C NMR (125 MHz, CDCl3) d 11.3, 14.6,
7.05 (d, 2H, J = 6.8 Hz); 13C NMR (150 MHz, CDCl3) d 14.5, 14.6, 119.1, 120.2, 133.3, 141.2, 156.7. ESI-MS m/z (C13H15BF2N2)
115.5, 119.7, 120.9, 124.7, 128.8, 128.9, 132.0, 142.6, 143.2, 146.9, calculated: 248.1, found: 271.1 (M + Na).
154.9. Anal. cacld for C19H20BF2N3: C, 67.28; H, 5.94; N, 12.39.
Found: C, 67.33; H, 5.91; N, 12.31.
References N-salicylideneaniline (SA) and N-naphthlideneaniline
(NA) are synthesized by the appropriate condensation of the
16298 | Phys. Chem. Chem. Phys., 2014, 16, 16290--16301
This journal is © the Owner Societies 2014

View Article Online
Paper
PCCP
corresponding aldehyde and aniline according to the standard 4.6. Computational details
procedures as previously reported.8c,28 Pure SA and NA were
Theoretical calculations have been performed using the Gaus-
sian 09 software package.35 For the purpose of accordance with
experimental results, the molecular structures of 1 and 2 are
taken from the crystal structures before optimization. The
geometry optimization of enol and keto forms as well as the
transition states (TS) of 1 and 2 in the gas phase is performed by
carrying out density functional theory (DFT) calculations using
Becke’s three-parameter exchange and Lee–Yang–Parr correla-
tion functionals (B3LYP) with a combination of the 6-31+G(d,p)
basis set. The harmonic vibrational frequencies of the studied
structures are calculated at the same level to characterize their
existence on the potential energy surface (PES). The minimum
energy structures are ensured by the absence of any imaginary
frequency whereas any transition state is characterized by the
presence of only one imaginary frequency. In the solution
phase, the geometry optimization of the studied structures is
performed at the same level using a polarizable continuum
model (PCM). All molecular orbitals were visualized using the
software GaussView 5.0.
obtained after recrystallization twice from absolute ethanol.
The melting points agreed with those in the literature and the
elemental analyses were in accord. Other analysis data are as
1
follows. SA: M.p. 51–53 1C. H NMR(400 MHz, CDCl3): d 6.98
(q, 1H, J = 7.3 Hz), 7.09 (d, 1H, J = 8.3 Hz), 7.33 (m, 3H), 7.45
(m, 4H), 8.67 (s, 1H). FT-IR transmission (KBr pellet): 3433 cmꢁ1
(nOH), 1615 cmꢁ1 (nCQN), 1590, 1572, 1507, 1496, 1485, 1453,
1402, 1361, 1277, 1186, 1169, 1150, 1116, 1074, 1032, 982, 917,
896, 843, 780, 757, 691, 547, 523 cmꢁ1. UV-Vis, lmax/nm,
(e/L molꢁ1 cmꢁ1), (CH3OH): 269 nm (11000), 348 nm (10570).
Anal. calcd for C13H11NO (197.23): C, 79.17; H, 5.62; N, 7.10.
Found: C, 79.12; H, 5.57; N, 7.02. NA: M.p. 95–97 1C.
1H NMR(400 MHz, CDCl3): d 7.14 (d, 1H, J = 9.12 Hz), 7.37
(d, 2H, J = 8.52 Hz), 7.44 (m, 2H, J = 13.48 Hz), 7.71 (d, 1H,
J = 7.93 Hz), 7.80 (d, 1H, J = 9.11 Hz), 8.07 (d, 1H, J = 8.59 Hz).
FT-IR transmission (KBr pellet): 3437 cmꢁ1 (nOH), 1626 cmꢁ1
(nCQN), 1570, 1542, 1489, 1476, 1420, 1329, 1248, 1181, 1163,
1076, 1023, 967, 911, 873, 824, 750, 694 cmꢁ1. UV-Vis, lmax/nm,
(e/L molꢁ1 cmꢁ1), (CH3OH): 315 nm (17 283), 357 (12 228),
435 nm (22 000) and 455 nm (20 400). Anal. cacld for
C
17H13NO (247.3): C, 82.57; H, 5.30; N, 5.66. Found: C, 82.61;
Acknowledgements
H, 5.35; N, 5.55.
This work was financially supported by the National Natural
Science Foundation of China (no. 21201066), the Natural
Science Foundation of Fujian Province (no. 2011J01047), the
outstanding Youth Scientific Research Cultivation Plan of
Colleges and Universities of Fujian Province (JA13008), and
the Promotion Program for Young and Middle-aged Teacher
in Science and Technology Research of Huaqiao University
(ZQN-PY104). Dr Hai-Feng Su (Xiamen University) is greatly
acknowledged for assistance with the ESI-MS experiments and
we thank Dr Hong-Xin Mei (Xiamen University) for help with
the single-crystal X-ray diffraction data collection. Zhong-Hua
Pan also thanks the National Innovative Foundation Project for
undergraduates (no. 201310385004).
4.5. Crystallization experiments and X-ray crystallography
The quality of single crystals depends on the purity of the used
material. Hence, the synthesized target dyads were highly
purified (recrystallization at least twice before performing
crystallization experiments). Better quality crystals of 1 and 2
were obtained by slow vapour diffusion of n-hexane into a
saturated CHCl3 solution of 1 or CH2Cl2 solution of 2 at room
temperature.
Intensity data for 1 and 2 were collected on a Rigaku R-AXIS
RAPID Image Plate single-crystal diffractometer using a graphite-
monochromated Mo Ka radiation source (l = 0.71073 Å). Single
crystals of 1 and 2 with appropriate dimensions were chosen
under an optical microscope and mounted on a glass fiber for
data collection at low temperature (173 ꢂ 2 K). Absorption
correction was applied by correction of symmetry-equivalent
reflections using the ABSCOR program.29 All structures were
solved by direct methods using SHELXS-9730 and refined by full-
matrix least-squares on F2 using SHELXL-9731 via the program
interface X-Seed.32 Non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms attached to oxygen in 1 and 2 were
located using difference Fourier maps and other hydrogen atoms
were refined isotropically using a riding model with Uiso values
1.2–1.5 times those of their parent atoms. All structures were
examined using the Addsym subroutine PLATON33 to ensure
that no additional symmetry could be applied to the models.
Crystal structure views were obtained using Diamond v3.1.34
Details of the data collection conditions and the parameters of
the refinement process are given in Table S2 (ESI†). Selected
bond lengths and angles are listed in Table S3 (ESI†). The
hydrogen bond geometries for 1 and 2 are shown in Table 1.
Notes and references
1 J. Toullec, Adv. Phys. Org. Chem., 1982, 18, 1–261.
2 (a) S. K. Pollack and W. J. Hehre, J. Am. Chem. Soc., 1977, 99,
4845–4846; (b) H. E. Zimmermann, Acc. Chem. Res., 1987,
20, 263–268; (c) J. Toullec and J. E. Dubois, J. Am. Chem. Soc.,
1974, 96, 3524–3532.
3 (a) R. Glaser and M. Lewis, Org. Lett., 1999, 1, 273–276;
(b) R. S. Phillips, B. Sundararaju and N. G. Faleev, J. Am.
Chem. Soc., 2000, 122, 1008–1014; (c) P. T. Chou, Y. C. Chen,
C. Y. Wei and W. S. Chen, J. Am. Chem. Soc., 2000, 122,
9322–9323; (d) O. K. Abou-Zied, R. Jimenez and
F. E. Romesberg, J. Am. Chem. Soc., 2001, 123, 4613–4614;
(e) P. Naumov, A. Sekine, H. Uekusa and Y. Ohashi, J. Am.
Chem. Soc., 2002, 124, 8540–8541; ( f ) M. J. Thompson,
D. Bashford, L. Noodleman and E. D. Getzoff, J. Am. Chem.
Soc., 2003, 125, 8186–8194.
This journal is ©the Owner Societies 2014
Phys. Chem. Chem. Phys., 2014, 16, 16290--16301 | 16299

View Article Online
PCCP
Paper
4 (a) D. S. Lawrence, T. Jiang and M. Levett, Chem. Rev., 1995,
95, 2229–2260; (b) R. P. Sijbesma and E. W. Meijer, Curr.
Opin. Colloid Interface Sci., 1999, 4, 24–32; (c) L. Forlani,
L. V. Kiew, L. Y. Chung and K. Burgess, Chem. Soc. Rev.,
2013, 42, 77–88; (c) A. C. Benniston and G. Copley, Phys.
Chem. Chem. Phys., 2009, 11, 4124–4131.
E. Mezzina, C. Boga and M. Forconi, Eur. J. Org. Chem., 2001, 12 A. Cui, X. Peng, J. Fan, X. Chen, Y. Wu and B. Guo,
2779–2785; (d) G. M. Whitesides and B. Grzybowski, Science,
2002, 295, 2418–2421.
5 (a) B. L. Feringa, W. F. Jager and B. de Lange, Tetrahedron,
J. Photochem. Photobiol., A, 2007, 186, 85–92.
13 A. Teimouri, A. N. Chermahini, K. Taban and
H. A. Dabbagh, Spectrochim. Acta, Part A, 2009, 72, 369–377.
1993, 49, 8267–8470; (b) K. Nakatani and J. A. Delaire, Chem. 14 H. A. Dabbagh, A. Teimouri, A. N. Chermahini and
Mater., 1997, 9, 2682–2684; (c) J. Harada, H. Uekusa and M. Shahraki, Spectrochim. Acta, Part A, 2008, 69, 449–459.
Y. Ohashi, J. Am. Chem. Soc., 1999, 121, 5809–5810; 15 (a) Z. Chen, H. Rohr, K. Rurack, H. Uno, M. Spieles,
(d) H. Pizzala, M. Carles, W. E. E. Stone and A. Thevand,
J. Chem. Soc., Perkin Trans. 2, 2000, 935–939; (e) K. Ogawa,
J. Harada, T. Fujiwara and S. Yoshida, J. Phys. Chem. A, 2001,
105, 3425–3427; ( f ) V. Vargas and L. Amigo, J. Chem. Soc.,
Perkin Trans. 2, 2001, 1124–1134; (g) K. Amimoto,
B. Schulz and N. Ono, Chem. – Eur. J., 2004, 10,
4853–4871; (b) M. Broring, R. Kruger, S. Link, C. Kleeberg,
S. Kohler, X. Xie, B. Ventura and L. Flamigni, Chem. – Eur. J.,
2008, 14, 2976–2983; (c) X. D. Yin, Y. J. Li, Y. L. Zhu, X. Jing,
Y. L. Li and D. B. Zhu, Dalton Trans., 2010, 39, 9929–9935.
H. Kanatomi, A. Nagakari, H. Fukuda, H. Koyama and 16 HOMA index was defined by the formulas: HOMA ¼
n
ꢀ
ꢁ
P
1
T. Kawato, Chem. Commun., 2003, 870–871; (h) J. H. Chong,
M. Sauer, B. O. Patrick and M. J. MacLachlan, Org. Lett., 2003,
5, 3823–3826.
6 (a) T. Suzuki and T. Arai, Chem. Lett., 2001, 124–127;
(b) J. J. Charette and E. De Hoffmann, J. Org. Chem., 1979,
44, 2256–2262; (c) P. E. Hansen, F. Duus and P. Schmitt, Org.
Magn. Reson., 1982, 18, 58–61.
1 ꢁ
ai Ropt ꢁ Ri 2, where n represents the total number
n i¼1
of bonds in the ring, Ri is an individual bond length, ai is an
empirical constant equal to 257.7 and Ropt is equal to 1.388
Å for CC bonds. For purely aromatic compounds, HOMA
index is equal to 1 while it is equal to 0 for nonaromatic
compounds. See: (a) T. M. Krygowski, J. Chem. Inf. Comput.
Sci., 1993, 33, 70–78; (b) T. M. Krygowski and
¨
7 (a) H. Unver, Spectrosc. Lett., 2001, 34, 783–791; (b) S. R. Salman,
J. C. Lindon and R. D. Farrant, Magn. Reson. Chem., 1993, 31,
991–994; (c) S. H. Alarcon, A. C. Olivieri and A. Nordon, Tetra-
hedron, 1995, 51, 4619–4626; (d) J. W. Ledbetter, J. Phys. Chem.,
1982, 86, 2449–2451; (e) V. Vargas C, J. Phys. Chem. A, 2004, 108,
´
M. K. Cyranski, Chem. Rev., 2001, 101, 1385–1419.
17 G. Gilli and P. Gilli, J. Mol. Struct., 2000, 552, 1–15.
18 J. W. Ledbetter, J. Phys. Chem., 1966, 70, 2245–2249.
19 L. X. Wu and K. Burgess, Chem. Commun., 2008, 4933–4935.
¨
´
281–288; ( f ) M. Sliwa, S. Letard, I. Malfant, M. Nierlich,
20 (a) H. Unver, M. Kabak, M. D. Zengin and T. N. Durlu,
¨
P. G. Lacroix, T. Asahi, H. Masuhara, P. Yu and K. Nakatani,
Chem. Mater., 2005, 17, 4727–4735; (g) H. Tanak, J. Phys. Chem.
A, 2011, 115, 13865–13876.
J. Chem. Crystallogr., 2001, 31, 203–209; (b) H. Unver,
¨
M. Yildiz, M. D. Zengin, S. Ozbey and E. Kendi, J. Chem.
Crystallogr., 2001, 31, 211–216.
8 (a) L. Antonov, W. M. F. Fabian, D. Nedeltcheva and
F. S. Kamounah, J. Chem. Soc., Perkin Trans. 2, 2001,
´
21 R. Gawinecki, A. Kuczek, E. Kolehmainen, B. Osmiałowski,
T. M. Krygowski and R. Kauppinen, J. Org. Chem., 2007, 72,
5598–5607.
´
1173–1179; (b) S. H. Alarcon, A. C. Olivieri and
´
M. Gonzalez-Sierra, J. Chem. Soc., Perkin Trans. 2, 1994,
22 (a) Y. Dong, J. Li, X. Jiang, F. Song, Y. Cheng and C. Zhu, Org.
Lett., 2011, 13, 2252–2255; (b) S. Kim, J. Y. Noh, K. Y. Kim,
J. H. Kim, H. K. Kang, S. W. Nam, S. H. Kim, S. Park, C. Kim
and J. Kim, Inorg. Chem., 2012, 51, 3597–3602.
23 (a) Q. Y. Wu and E. V. Anslyn, J. Am. Chem. Soc., 2004, 126,
14682–14683; (b) M. H. Lim, B. A. Baik and S. J. Lippard,
J. Am. Chem. Soc., 2006, 128, 14364–14373.
1067–1070; (c) W. M. F. Fabian, L. Antonov, D. Nedeltcheva,
F. S. Kamounah and P. J. Taylor, J. Phys. Chem. A, 2004, 108,
7603–7612; (d) A. Filarowski and I. Majerz, J. Phys. Chem. A,
2008, 112, 3119–3126; (e) P. I. Nagy and W. M. F. Fabian,
J. Phys. Chem. B, 2006, 110, 25026–25032.
9 Y. N. Guo, X. H. Chen, S. F. Xue and J. K. Tang, Inorg. Chem.,
2011, 50, 9705–9713.
¨
24 (a) R. Kramer, Angew. Chem., Int. Ed., 1998, 37, 772–773;
10 (a) G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int.
Ed., 2008, 47, 1184–1201; (b) N. Boens, V. Leen and
W. Dehaen, Chem. Soc. Rev., 2012, 41, 1130–1172;
(c) T. Ueno, Y. Urano, H. Kojima and T. Nagano, J. Am.
Chem. Soc., 2006, 128, 10640–10641; (d) G. G. Luo, J. X. Xia,
K. Fang, Q. H. Zhao, J. H. Wu and J. C. Dai, Dalton Trans.,
2013, 42, 16268–16271; (e) Z. H. Pan, G. G. Luo, J. W. Zhou,
J. X. Xia, K. Fang and R. B. Wu, Dalton Trans., 2014, 43,
8499–8507; ( f ) H. Lu, S. S. Zhang, H. Z. Liu, Y. W. Wang,
Z. Shen, C. G. Liu and X. Z. You, J. Phys. Chem. A, 2009, 113,
14081–14086.
(b) J. Hu, F. T. Lu¨, L. P. Ding, S. J. Zhang and Y. Fang,
J. Photochem. Photobiol., A, 2007, 188, 351–357.
25 (a) E. Hadjoudis and I. M. Mavridis, Chem. Soc. Rev., 2004,
33, 579–588; (b) K. M. Hutchins, S. Dutta, B. P. Loren and
L. R. MacGillivray, Chem. Mater., 2014, 26, 3042–3044.
26 J. A. Riddick, W. B. Bunger and T. K. Sakano, Organic
Solvents, Wiley-Interscience, New York, 4th edn, 1986.
27 Y. C. Wang, D. K. Zhang, H. Zhou, J. L. Ding, Q. Chen,
Y. Xiao and S. X. Qian, J. Appl. Phys., 2010, 108, 455–462.
28 H. E. Smith, S. L. Cook and M. E. Warren, J. Org. Chem.,
1964, 29, 2265–2267.
11 (a) A. Loudet and K. Burgess, Chem. Rev., 2007, 107,
4891–4932; (b) A. Kamkaew, S. H. Lim, H. B. Lee,
29 T. Higashi, ABSCOR, Empirical Absorption Correction based on
Fourier series Approximation, Rigaku Corporation, Tokyo, 1995.
16300 | Phys. Chem. Chem. Phys., 2014, 16, 16290--16301
This journal is © the Owner Societies 2014

View Article Online
Paper
PCCP
30 G. M. Sheldrick, SHELXS-97, Program for X-ray Crystal
Structure Determination, University of Gottingen, Germany,
1997.
31 G. M. Sheldrick, SHELXL-97, Program for X-ray Crystal Struc-
ture Refinement, University of Gottingen, Germany, 1997.
32 L. J. Barbour, X-Seed, A software tool for Supramolecular
Crystallography, Supramol. Chem., 2001, 1, 189–191.
33 A. L. Spek, Implemented as the PLATON Procedure, a Multi-
purpose Crystallographic Tool, Utrecht University, Utrecht,
The Netherlands, 1998.
34 K. Brandenburg, DIAMOND, Version 3.1f, Crystal Impact
GbR, Bonn, Germany, 2008.
35 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta,
F. Ogaliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian, Inc., Wallingford CT, 2009.
This journal is ©the Owner Societies 2014
Phys. Chem. Chem. Phys., 2014, 16, 16290--16301 | 16301