Solid-emissive rhodamine: Hydrogen bonding-assisted efficient intermolecular fluorescence resonance energy transfer in the solid state

Article abstract of DOI:10.1080/10610278.2013.826807

Solid-emissive rhodamine complexes are obtained by mixing commercial rhodamine B (RhB) with the recently developed solid-emissive boron 2-(2′-pyridyl)imidazole (BOPIM) derivatives. The formation of intermolecular hydrogen bonds between RhB and BOPIM dyes plays a key role in the emission of RhB in the solid state. The disappearance of emissions from BOPIM dyes indicates the occurrence of efficient intermolecular fluorescence resonance energy transfer (FRET). The hydrogen bond also helps prevent the intermolecular interaction between the carboxyl moieties on RhB to alleviate concentration-induced fluorescence quenching because the emission of the complexes can be directly lightened by excitation at the RhB absorption (510 nm). Our results indicate that intermolecular FRET assisted by non-covalent interactions can be an efficient tool for constructing red or near-infrared solid emitters.

Full text of DOI:10.1080/10610278.2013.826807

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Solid-emissive rhodamine: hydrogen bonding-assisted
efficient intermolecular fluorescence resonance
energy transfer in the solid state
Qiong Caoa, Shuzhang Xiaoa, Ruohan Lia, Qifei Lina, Changying Yanga, Kun Zoua & Feijun
Dana
a Hubei Key Laboratory of Natural Products Research and Development, College of
Chemistry and Life Science, China Three Gorges University, Hubei Yichang, 443002, P.R.
China
Published online: 25 Aug 2013.
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To cite this article: Qiong Cao, Shuzhang Xiao, Ruohan Li, Qifei Lin, Changying Yang, Kun Zou & Feijun Dan (2014) Solid-
emissive rhodamine: hydrogen bonding-assisted efficient intermolecular fluorescence resonance energy transfer in the solid
state, Supramolecular Chemistry, 26:2, 105-110, DOI: 10.1080/10610278.2013.826807
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Supramolecular Chemistry, 2014
Vol. 26, No. 2, 105–110, http://dx.doi.org/10.1080/10610278.2013.826807
Solid-emissive rhodamine: hydrogen bonding-assisted efficient intermolecular fluorescence
resonance energy transfer in the solid state
Qiong Cao, Shuzhang Xiao*, Ruohan Li, Qifei Lin, Changying Yang, Kun Zou and Feijun Dan
Hubei Key Laboratory of Natural Products Research and Development, College of Chemistry and Life Science, China Three Gorges
University, Hubei Yichang 443002, P.R. China
(Received 22 May 2013; final version received 15 July 2013)
Solid-emissive rhodamine complexes are obtained by mixing commercial rhodamine B (RhB) with the recently developed
solid-emissive boron 2-(20-pyridyl)imidazole (BOPIM) derivatives. The formation of intermolecular hydrogen bonds
between RhB and BOPIM dyes plays a key role in the emission of RhB in the solid state. The disappearance of emissions
from BOPIM dyes indicates the occurrence of efficient intermolecular fluorescence resonance energy transfer (FRET).
The hydrogen bond also helps prevent the intermolecular interaction between the carboxyl moieties on RhB to alleviate
concentration-induced fluorescence quenching because the emission of the complexes can be directly lightened by
excitation at the RhB absorption (510 nm). Our results indicate that intermolecular FRET assisted by non-covalent
interactions can be an efficient tool for constructing red or near-infrared solid emitters.
Keywords: hydrogen bond; FRET; solid emission; rhodamine B; boron–fluorine
Introduction
the efficient intramolecular fluorescence resonance energy
transfer (FRET) from 2,20-biindene to the BODIPY core
(9). The same phenomenon has been observed for an
oligothiophene derivative containing a BODIPY chromo-
phore (10). Intense solid emission has been observed for
BODIPY because BODIPY absorbs the fluorescence of
oligothiophene in the solid state. Therefore, synthesised
dyads have unusually large Stoke shifts. However, the two
chromophores (donor and acceptor) are linked in a
conjugated manner, which increases synthesis difficulty.
Comparatively, intermolecular FRET is a simple and
efficient method of solving this problem. Intermolecular
energy transfer is based on the distance dependence
between a donor and an acceptor through dipole–dipole or
electron exchange interaction, resulting in decreased donor
and acceptor emission. A large number of commercially
available red-emissive dyes that emit in solutions exist, but
these dyes show no fluorescence in the solid state. Given
that solid-emissive dyes with a short wavelength are more
available, simply mixing two chromophores together may
enable efficient intermolecular FRET to produce long-
wavelength emission. However, some requisites must be
met for this strategy. First, the donor should emit in the solid
state with medium to high-quantum efficiency. Second, the
donor emission must match the acceptor absorption. Finally
and most importantly, the distance of the two chromophores
must be limited to a few nanometres to facilitate energy
transfer with high efficiency.
Organic fluorescent dyes are attracting considerable
attention because of their potential applications in
optoelectronic devices. The main concern is the improve-
ment of the emission efficiency of solid-state fluorescent
materials because highly emissive solids are key materials
in a range of technologies, including displays and
processing, chemical sensors and bio-imaging (1, 2).
Although conventional dyes tend to aggregate with
fluorescence quenching in the solid state, solid-emissive
dyes can be obtained by introducing bulky groups around
the chromophore to inhibit intermolecular p–p stacking
or by constructing non-covalent bonds to facilitate non-
parallel packing (3–6). However, dyes that can emit deep-
red or near-infrared (NIR) light with significant efficiency
in the condensed phase are less developed (7, 8). Recent
reports on red solid emitters focus on large p-conjugated
heterocycles. The synthesis for these dyes is normally
tedious with low emission efficiency. Intramolecular
energy transfer can be another efficient method of
developing red or NIR solid emitters because solid-
emissive dyes in the short wavelength region are usually
more available. For example, 2,20-biindene is structurally
simple but emits intense blue fluorescence in the solid
state. When the 4,4-difluoro-4-bora-3a,4a-diaza- s-inda-
cene (BODIPY) chromophore is conjugated with 2,20-
biindene, an intense long-wavelength emission of
BODIPY can be observed in the solid state because of
*Corresponding author. Email: shuzhangxiao@gmail.com
q 2013 Taylor & Francis

106
Q. Cao et al.
Rhodamine B (RhB) is a popular dye with red
the self-assembled complexes to verify the hydrogen bond
formation between the imidazole in BOPIM dyes and the
carboxylic acid in RhB during self-assemblies. During
these assemblies, the BOPIM dyes act as acceptors to
accept the proton, and RhB acts as the proton donor.
Organic carboxylic acid is weakly acidic but can lower the
electronic density of the BOPIM core once a hydrogen
bond is formed. Figure 1 shows an array of resonances that
correspond to the aromatic protons in the precursors and
the self-assembled complexes. The signal of the
carboxylic acid does not appear because of the fast proton
exchange with a minimal amount of water in the solution.
All samples show well-resolved resonances in CDCl3.
The protons on the pyridine rings (H1, H2 and H3) in the
assembled sample SA1 significantly shift downfield
compared with pure BOPIM1. This immediate shift
indicates a decrease in electronic density and leads to a
lower shielding effect on these protons. The proton on the
imidazole slightly shifts upfield, and no chemical shift
change is observed for the other protons. The different
changes in the protons may be due to the anisotropic field
effect of the carboxylic acid to the p system of the BOPIM
chromophore. The formation of the hydrogen bond clearly
affects the chemical environment of the BOPIM core more
severely than the terminal substituted groups (H or phenyl
rings) because almost no chemical shift change occurs for
the protons on phenyl rings. A similar trend is observed for
SA2 with BOPIM2 as the proton acceptor. The protons on
the pyridine rings shift downfield, as shown in Figure 1.
The protons on RhB are also affected by the
intermolecular interactions, especially the protons located
emission, and the derivatives of RhB are widely used for
chemical sensing and fluorescent labelling (11, 12). Solid-
emissive rhodamines can efficiently absorb visible light
and emit at around 600 nm, which can be applied in a light-
harvesting system (13, 14). However, few examples of
solid-emissive rhodamine derivatives exist because these
derivatives are compactly packed in the solid state (15,
16). Therefore, we propose to adopt the intermolecular
FRET technique to produce solid-emissive rhodamines.
Recently developed boron 2-(20-pyridyl)imidazole
(BOPIM) dyes are adopted as donors because these dyes
are solid emissive with a fluorescent quantum yield that
ranges from 0.10 to 0.37. The fluorescence of these dyes
also matches well with the absorption of the rhodamines
(17–19). Numerous techniques have been reported for
holding chromophores together to shorten the intermole-
cular distance between donor and acceptor such as the
construction of an intermolecular hydrogen bond (20, 21),
p–p interaction (22–29) and even van der Waals force
(30). BOPIM dyes have a naked N atom on the imidazole
ring that may form an intermolecular hydrogen bond with
the carboxyl group of RhB as indicated in Scheme 1. We
report a unique example of the highly solid-emissive
rhodamine complexes by constructing efficient FRET in
the solid state using an intermolecular hydrogen bond.
Results and discussion
The formation of an intermolecular hydrogen bond can
affect the electronic environment of donor and acceptor,
which further change the properties compared with
1
precursors. H NMR was obtained for the precursors and
Figure 1. 1H NMR of BOPIM dyes, RhB and the assemblies
Scheme 1. Self-assembly of the studied compounds.
(in CDCl3, solvent).
*

Supramolecular Chemistry
107
close to the carboxyl unit. These protons (Ha, Hb and Hc)
shift upfield, indicating that the electronic density is
enriched because of proton loss during the formation of the
intermolecular hydrogen bond. The other protons far from
the carboxyl moiety are barely affected; thus, no chemical
shift change is observed. However, the effect of the
hydrogen bond is usually weak, and the largest chemical
shift change is no more than 0.09 ppm. This result proves
the formation of the intermolecular hydrogen bond and
excludes the hypothesis of protonation. Deprotonation of
RhB by NaOD induced a 0.347 ppm upfield shift for Ha.
In order to prove that the hydrogen bond is formed
between carboxyl and imidazole, 19F NMR was recorded
for BOPIM1 and SA1. As expected, they show almost the
same resonance patterns (single quarter: 2156.19 ppm for
BOPIM1 and 2156.021 ppm for SA1), which excludes the
possible interactions of carboxyl with F atoms.
Therefore, FRET can be incurred if the distance is limited
to several nanometres as verified by the concentration-
dependent fluorescent measurement of SA1 (Figure 3(B)).
In a dilute solution (1026 M), only the emission ascribed to
BOPIM1 is observed, and no emissive peak exists at
around 580 nm because the fluorescent intensity of
BOPIM1 is much higher than that of RhB under 365 nm
excitation (the molar absorption coefficient of BOPIM1 at
365 nm is larger). This result proves that no FRET occurs
in such a dilute solution. With increased concentration of
1025 M, a shoulder emission ascribed to RhB appears at
572 nm. The increase in concentration facilitates the
shortening of the distance between BOPIM1 and RhB by
hydrogen bond formation. Therefore, the fluorescence
intensity ratio IRhB/IBOPIM1 continues to increase with
increasing concentration (IRhB/IBOPIM1 ¼ 0.12, 0.36 and
36.54 at 1025, 1024 and 1023 M, respectively). In fact, the
emission peak of BOPIM1 in 1023 M solution becomes very
small, and the red emission centred at 598 nm becomes the
main emissive peak with .97% FRET efficiency. Further
increasing concentration can reach quantitative because the
emission of BOPIM1 completely disappears. A sample
containing the same RhB concentration as the blank was
compared to elicit the fluorescent enhancement ascribed to
FRET. The fluorescence intensity of RhB (1022 M) is only
25% that of SA1 (1022 M). This result indicates that most of
the red fluorescence originates from the emission of
BOPIM1. Similar results are obtained for SA2, as shown in
Figure 3(C). The quantum yield of BOPIM2 is notably
higher than that of BOPIM1 (17), and the fluorescence of
BOPIM2 also better matches with that of RhB (Figure 1(A)).
Consequently, much stronger red fluorescence is observed
for SA2. Taking the fluorescence intensity of blank RhB as
the standard (concentration ¼ 1022 M), the fluorescence
ratio ISA1/IRhB is 4.07, but ISA2/IRhB is raised to 6.53.
FT-IR is another useful tool for characterising
intermolecular hydrogen bonds. RhB shows a character-
istic absorption band at 1693 cm21 because of the
stretching vibration of the carboxylic acid dimers (the
monomer should show an absorption band at longer wave
number), as shown in Figure 2. This band diminishes and
new bands centred at 1710 and 1708 cm21 arose in the
self-assembled SA1 and SA2, respectively, due to
the complex formation between the imidazole unit of the
BOPIM dyes and the carboxyl unit of RhB. Another
evidence for the self-assembled complexes are the
presence of broad bands at 1920 and 2444 cm21 for
SA1, and at 2025 and 2428 cm21 for SA2, which indicate a
neutral OZH . . . N hydrogen bond interaction between the
carboxylic acid on RhB and the N atom on imidazole
moieties (31, 32).
An efficient FRET process requires spectral overlap-
ping between the emission band of the donor and the
absorption band of the acceptor. The fluorescence of
BOPIM dyes (donor) ranging from 470 to 600 nm matches
the absorption of RhB (acceptor), as shown in Figure 3(A).
The association constant is also important for their
fluorescent properties. By fluorescent titration, their
association constants are measured to be 3.71 £ 104 M21
Figure 2. FT-IR of BOPIM dyes, RhB and the assemblies.

108
Q. Cao et al.
Figure 3. (A) Spectral overlap of the emissions of BOPIM dyes and absorption of RhB in chloroform; (B) concentration-dependent
fluorescent spectra of SA1 in chloroform; (C) concentration-dependent fluorescent spectra of SA2 in chloroform. (Ex: 365 nm) and (D)
excitation spectra of SA1, SA2 and mixtures of BOPIM dyes with RhBE.
for SA1 and 5.19 £ 104 M21 for SA2. Higher association
constant for SA2 may make it show higher fluorescent
intensity than SA1 at the same concentration. The reason
may lie in that the phenyl ring in SA1 rotates fast in
solutions (17), which makes it more difficult to form the
hydrogen bond.
in the prepared solid film. This emission peak is ascribed to
the aggregate form because of the small steric hindrance of
the form (17). The aggregate form is about 15% through the
integration of the fluorescent band.
RhB is a highly fluorescent dye in the dilute solution.
The fluorescence of RhB is completely quenched in the
solid state with an absorption that broadens because of the
aggregate formation through the intermolecular inter-
actions between carboxyl groups (Figure 4(A)). However,
the formation of the intermolecular hydrogen bond
between BOPIM dyes and RhB can significantly decrease
the aggregation degree of RhB. Figure 4(A)–(C) shows
that the absorption bands of RhB in SA1 and SA2 become
much narrower than before. The half-peak width is 78 nm
in SA1 and 77 nm in SA2, which are much smaller than
that of pure RhB (110 nm) and indicates that the formation
of the intermolecular hydrogen bond can significantly
alleviate aggregate formation.
Given the above results, FRET may efficiently occur in
the solid state because this state is the most concentrated
form. BOPIM derivatives show intense emissions in
solutions and in the solid state because of the non-parallel
packing mode in the solid state induced by the formation of
multiple intermolecular non-covalent bonds (such as
CZH . . . N, CZH . . . B and CZH . . . F) (17–19). These
derivatives also have a large Stokes shift given the efficient
intramolecular charge transfer from the electron-rich groups
to the electron-accepting BOPIM skeleton. BOPIM2 shows
almost the same emissive band in the solid state as that
in the dilute solution because the terminal two phenyl rings
act as insulators to further inhibit aggregate formation.
A shoulder peak at around 470 nm is observed for BOPIM1
The self-assemblies show efficient FRET in the
concentrated solution and in the solid state. Figure 4(B)

Supramolecular Chemistry
109
Figure 4. (A) Spectral overlap of the emissions of BOPIM dyes and absorption of RhB in solid film; (B) UV–vis absorption of SA1,
fluorescence of SA1 and RhB in solid film (Ex: 365 nm); (C) UV–vis absorption of SA2, fluorescence of SA2 and RhB in solid film (Ex:
365 nm) and (D) fluorescent spectra in solid film (Ex: 510 nm for SA1, SA2 and RhB; 365 nm for RhBE þ BOPIM1 and
RhBE þ BOPIM2).
shows that a sharp emissive peak ascribed to RhB appears
and that the emission from BOPIM1 significantly
decreases as shown in the solution. However, one small
peak at around 470 nm exists, which occupies 15% of the
entire emission in accordance with the fluorescence of
BOPIM1 in the solid state. The aggregate form of the
BOPIM1 cannot convert its fluorescence to RhB. The
aggregate is probably formed through the N...N inter-
action, which inhibits the formation of an intermolecular
hydrogen bond between BOPIM1 and RhB.
hydrogen bond. No intermolecular hydrogen bond is
formed because no free carboxyl unit exists. No red
emission is observed in the mixtures, proving that the
intermolecular hydrogen bond between BOPIMs and RhB
plays a key role in this approach (Figure 4(D)). And the
green emissions observed are ascribed to BOPIM dyes. The
hydrogen bond can also alleviate aggregate formation
because SA1 and SA2 also show red solid-emission under a
510 nm excitation, where no FRET occurs (Figure 4(D)).
Therefore, the excitation spectra of SA1 and SA2 show two
excitation bands around 410 and 510 nm (Figure 3(D)).
We investigated SA2 with BOPIM2 as the donor to
prove this hypothesis. BOPIM2 has two phenyl rings at the
terminal ends and can inhibit aggregate formation because
of the steric effect. Only one emissive peak accordingly
appears at 524 nm in the solid state. The fluorescence
spectrum of SA2 expectedly shows only one sharp, long-
wavelength emission peak, which is ascribed to RhB. The
emission peak from BOPIM2 completely disappears.
BOPIM dyes are mixed with rhodamine B ethyl ester
(RhBE) to investigate the function of the intermolecular
Conclusions
Unique solid-emissive rhodamine complexes are devel-
oped by simply mixing RhB with BOPIM dyes. The
intermolecular hydrogen bond can significantly alleviate
aggregate formation. Most importantly, the bond facili-
tates intermolecular energy transfer from BOPIM dyes
to RhB, which increases the emission intensity of RhB.

110
Q. Cao et al.
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BOPIM dyes and commerically available RhB were mixed
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