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Tuning the fluorescence behavior of liquid crystal molecules containing Schiff-base: Effect of solvent polarity
Accepted Manuscript Tuning the fluorescence behavior of liquid crystal molecules containing Schiff-base: Effect of solvent polarity Ashok K. Satapathy, Santosh K. Behera, Ankit Yadav, Laxmi Narayan Mahour, C.V. Yelamaggad, K.L. Sandhya, Balaram Sahoo PII:
S0022-2313(18)32395-0
DOI:
https://doi.org/10.1016/j.jlumin.2019.02.056
Reference:
LUMIN 16322
To appear in:
Journal of Luminescence
Received Date: 22 December 2018 Revised Date:
17 February 2019
Accepted Date: 26 February 2019
Please cite this article as: A.K. Satapathy, S.K. Behera, A. Yadav, L.N. Mahour, C.V. Yelamaggad, K.L. Sandhya, B. Sahoo, Tuning the fluorescence behavior of liquid crystal molecules containing Schiff-base: Effect of solvent polarity, Journal of Luminescence (2019), doi: https://doi.org/10.1016/ j.jlumin.2019.02.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Tuning the Fluorescence Behavior of Liquid Crystal Molecules Containing Schiff-Base: Effect of Solvent Polarity
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Ashok K. Satapathya,b, Santosh K. Beherac, Ankit Yadavd, Laxmi Narayan Mahourd, C. V. Yelamaggade, K. L. Sandhyab, †, Balaram Sahood, † a
Department of Physics, Acharya Institute of Technology, Bangalore 560107, India
b
Department of Physics,Ramaiah Institute of Technology, Bangalore 560054, India
Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India d
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c
Materials Research Centre, Indian Institute of Science, Bangalore 560012, India Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
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e
†
Corresponding author(s):
Email: [email protected] (B. Sahoo) Tel: +91 80 2293 2943 Email: [email protected] (K. L. Sandhya) Tel: +91 96321 48972
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Abstract: We report the influence of intermolecular and intramolecular hydrogen bonding on the excited state proton transfer (ESPT) emission behavior of two bent core liquid crystal (BLC)
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molecules, C54H63NO9 (BLC3) and C60H75NO9 (BLC4), having a Schiff-base and two long alkyl chains at its two ends. Fluorescence spectra of these BLC molecules dispersed in different solvents show dual emission (at ~ 365 nm and ~ 425 nm) due to formation of keto and enol
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tautomers. We observed that the population of these keto and enol tautomers and the corresponding intensities of fluorescence emission are strongly influenced by the solvent
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polarity. In protic solvents, formation of intermolecular hydrogen bond with the Schiff-base of the BLC molecules is highly favoured than the intramolecular hydrogen bonding in the BLCs. This intermolecular hydrogen bonding drastically reduces the population of the keto tautomers in the excited state, resulting in enhanced enol fluoroscence band along with a weak keto emission
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band. The observed intensity of the enol fluorescence band is the highest for the most polar solvent (methanol). On the other hand, in aprotic solvents, the intramolecular hydrogen bonding is highly favored, which leads to the formation of keto tautomers in the excited state. Hence, an
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intense keto emission band is observed for the aprotic solvents along with a weak enol emission band. From the time resolved fluorescence spectroscopy studies we observed a longer life time
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for the keto band than that for the enol band. This is also related to the delayed emission associated with the vibrational bands resulting from the bulky alkyl chains attached to the ends of the BLC molecules.
Keywords: Bent-core liquid crystals; Intramolecular hydrogen bond; Intermolecular hydrogen bond; ESPT; Effect of solvents; Photoluminescence.
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1. Introduction In recent time photophysical studies of dual emitters remain a very active field of research. These dyes found emerging applications in biological and chemical systems due to red shifted
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emission which is highly Stokes shift [1-12]. In this category, dyes undergoing an excited-state intramolecular proton transfer (ESIPT) are chiefly attractive. ESIPT is a very fast phototautomerisation process taking place along an intramolecular hydrogen bond between
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proton donor and proton acceptor and that are significantly tuned upon electronic excitation [7, 13-17]. The pre–requisite for ESIPT is the presence of intramolecular hydrogen bond between
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proton donor (–OH and –NH2) and proton acceptor (=N– and –C=O) groups near each other in a molecule. ESIPT usually results in dual fluorescence with emission from both excited enol and excited keto tautomer. The notable properties of the ESIPT dyes are dual emission, large Stokes shift and ultrafast process. ESIPT dyes have found applications in many fields such as in
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chemosensors, electroluminescent materials, laser dyes, and UV-photostabilisers, white organic light-emitting diodes (OLED) [17-22]. The spectral properties of the ESIPT dyes depend on hydrogen bonding, rotamerisation process and pH of the surrounding medium and substituents
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present on donor and acceptor units [23-27]. The fluorescence properties of the ESIPT fluorophores are tunable by changing these parameters. Several series of compounds undergo
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ESIPT and they have been investigated with both experimental and theoretical approaches [14, 28-32]. However, the effect of hydrogen bonding on proton transfer process can be used to tune the excited state emission properties of various dyes, which attracts a lot of attention [15]. Zhou et al. reported the effects of intermolecular hydrogen bonding on the dual fluorescence of methyl salicylate [33]. They investigated that the intermolecular hydrogen bond plays an important role for the dual fluorescence of methyl salicylate in alcoholic solvents. Subsequently, the effect of
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solvent polarity on different class of ESIPT dyes including salicylideneaniline, hydroxyacetophenones, hydroxyflavones, hydroxyphenyl-oxazole(HPO), hydroxyphenyl-benzoxazoles (HPBO), hydroxyphenyl-benzothiazole (HPBT) and hydroxyphenyl-benzimidazoles (HPBI)
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were extensively investigated [14]. Park et al. reported a novel class of ESIPT-active molecule forming columnar liquid crystals with enhanced fluorescence [34]. In this work, we studied the effect of sovent polarity on the favorability of hydrogen bond formation and the subsequent
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effect on ESIPT behavior of two bent core liquid crystal (BLC) molecules containing salicylideneaniline core (Schiff-base) attached with two alkyl chains at their ends, as shown in
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Fig. 1.
Fig. 1. Chemical structures of (a) BLC3 and (b) BLC4.
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2. Materials and Methods The chemical structures of the two bent core liquid crystals (BLCs) used for this study,
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C54H63NO9 (BLC3) and C60H75NO9 (BLC4), are provided in Fig. 1. The details of synthetic procedure and the liquid crystal behavior of these BLCs are reported elsewhere [35]. Both liquid crystals are similar in structure except the alkyl groups attached at their ends. BLC3 constitutes of alkyl groups –C10H21 and –C10H21, whereas BLC4 contains –C10H21 and –C16H33 at their two
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ends. The UV-Vis absorption spectra (Fig. S1 and Fig. S2, electronic supplementary information (ESI) file) of the BLCs were recorded using “Perkin Elmer Lambda 750 UV-Visible”
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spectrophotometer. All the measurements were performed with 1.5 µM concentration of the solute in different solvents to avoid aggregation and self-quenching. The HPLC grade cyclohexane, n-hexane, 2-propanol, ethanol, methanol are used as solvents for this study, where the polarity of the solvents are 0.006, 0.009, 0.546, 0.654 and 0.762 (with respect to the unity
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(1.0) polarity of water). Fluorescence spectra were recorded using HORIBA Jobin Yvon Fluoromax-4 spectrofluorimeter. The concentration of the solute and the solvents used for fluorescence study were same as that of absorption study. The fluorescence lifetimes were
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measured from time-resolved intensity decay by time correlated single-photon counting (TCSPC) method on an Edinburgh instrument (Life-Spec II) using 303 nm LED. The
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fluorescence decay was analyzed by deconvolution method using the DAS6 software. The obtained values of χ2 for all the fittings were close to 1 and weighted residual values were between + 4 and - 4.
The fluorescence quantum yields of BLC3 and BLC4 samples were determined relative to that of quinine sulphate (Φr = 0.546) and calculated on the basis of the following equation [36].
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Φ s I s Ar ns2 = Φ r I r As nr2
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where Φs is the fluorescence quantum yield of the samples, Is and Ir are the integrated fluorescence area, and As and Ar are the absorbance values for the sample and reference solutions, respectively. ns and nr are the refractive indices for the sample and reference solutions,
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respectively.
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3. Results and Discussion
The absorption spectra of BLC3 and BLC4 exhibit multi absorption bands in n-hexane, i.e., absorption at ~ 225 nm, ~265 nm and ~315 nm. These band positions remain unaffected
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with solvent polarity (Fig. S1). The origin of the bands is π-π* and/or n-π* transitions.
Fig. 2. The structure of the emitting species enol and keto in the excited state.
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The presence of the Schiff-base (C=N) in the BLC molecules allows the existence of two tautomeric forms in the excited state of the compounds, enol and keto, as shown in Fig. 2. In general, molecules containing proton donor and proton acceptor nearly upon photoexcitation
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under goes a cyclic process. These two forms can be foreseen to cause the dual emissive behavior of the compounds as schematically shown in Fig. 3. The measured fluorescence spectra of both the BLCs are shown in Fig. 4. Clearly, upon photoexcitation at 310 nm, BLC3 and BLC4
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emit dual emission fluorescence at ~ 365 nm and ~ 425 nm. We observed a weak-intensity shorter wavelength band (at ~ 365 nm) and a strong-intensity longer wavelength band (at ~ 425
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nm) for aprotic solvents (n-hexane and cyclohexane). Note that, n-hexane and cyclohexane have a very small difference in their polarity (n-hexane: 0.009, cyclohexane: 0.006, with respect to the reference polarity of water:1.000). However, for less polar cyclohexane we observed an intense longer wavelength emission band, in comparison to that for n-hexane. On the other hand, in
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protic solvents (such as methanol, ethanol and 2-propanol) the intensities of these bands are reversed. As the polarity of the protic solvents increase, the intensity of the shorter wavelength emission band increases. In addition, a marginal blue shift is observed for longer wavelength
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band for the protic solvents. This suggests that the longer wavelength emission was due to ESIPT form keto tautomer and shorter wavelength band is due to normal emission of excited enol [26].
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In excited state proton transfer the excited state keto is less polar than ground state enol
tautomer, as a result, with an increase in polarity of the solvents the energy gap between these states increases leading to a blue shift. Similar blue shift is also observed for other reported ESIPT exhibiting molecules. The shorter wavelength band is due to emission from excited enol tautomer and the longer wavelength is due to keto tautomer formed from excited enol in excited state. The solvent polarity greatly influences the easy formation of keto in excited state.
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Fig. 3. Mechanistic pathways for the fluroscence emission for the keto (produced from cis-enol)
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and enol (produced from trans-enol) tautomers.
Fig. 4. Fluorescence spectra of (a) BLC3 and (b) BLC4 in different aprotic and protic solvents, λexc=310 nm. 8
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The solvent polarity modulation of fluorescence is quite interesting. It is well studied that keto tautomer are efficiently produced in hydrocarbon solvents [25-30, 36]. In alcoholic (protic) solvent, there exists competition between intramolecular bonding of the nearest hydrogen with
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the Schiff-base and intermolecular bonding of hydrogen of the solvents with the Schiff-base.
Fig. 5. The scheme of formation of intramolecular and intermolecular hydrogen bonding in
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aprotic and protic solvents, respectively.
Therefore, these compounds show different photophysical behavior in solutions of
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different polarities. In aprotic n-hexane, the proton donor -OH group and proton acceptor =N site (Fig. 5 (a)) forms strong intramolecular hydrogen bonding, which facilitates the proton transfer
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and upon photo excitation it emits strong keto emission. However, in protic solvents, the -OH group of the solvents form intermolecular hydrogen bond with =N site. Hence, in protic solvents, there is favorable formation of intermolecular hydrogen bonding in these BLCs (Fig. 5 (b)). This prevents the ease of (intramolecular) proton transfer and reduces the keto emission in photo excitation [25]. Their results summarize the weak normal emission (at ~365 nm) and strong tautomer emission (at ~425 nm) in n-hexane and cyclohexane. Likewise, a strong normal
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emission (at ~365 nm ) and weak tautomer emission (at ~425 nm) in n-hexane and cyclohexane, were observed. Note that due to overlapping of the FTIR line positions (at about 3330 cm-1) of N-H and O-H stretching vibrations, the effect of intramolecular or intermolecular hydrogen
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bonding in the FTIR absorption spectra could not be observed (see ESI, Fig. S3 and Fig. S4). The time resolved fluorescence spectra of our BLC3 and BLC4 samples are given in Fig. 6. For understanding the behavior of the BLCs in protic and aprotic solvents, we have chosen
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methanol as protic and n-hexane as aprotic solvent. The decay intensities I(t), shown in (Fig. 6) clearly shows two decay modes associated with two life times, τ1 and τ2. Here, the mean life (τ) ଵ
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is the time required for the intensity of fluorescence to reach a value of ( )th of the initial
intensity. The decay curves (Fig. 6) are fit with two separate profiles, as given by the following equation.
−t
−t
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I (t ) = A + B1e τ1 + B2eτ 2
The fraction of intensities (in percentage) contributed by the two modes are calculated by:
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Fi (%) =
Biτ i
×100
2
∑Bτ j =1
j
j
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The lifetimes obtained from fitting of the decay curves are given in Table 1. It is clear
from the table that the lifetime τ1 is always lower than that of τ2, for a particular solvent at a particular emission wavelength. For both the decay modes (τ1 and τ2) in BLC3 and BLC4, the longer wavelength emission band λem= 450 nm has longer lifetime and the shorter wavelength emission band λem= 365 nm has shorter lifetime. We compare the decay behavior of both the BLCs in protic (methanol) and aprotic (n-hexane) solvents. For both BLCs at λem= 450 nm, τ2 is
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lower for protic solvent than aprotic solvent, but at λem= 365 nm, τ2 is longer for protic solvents than aprotic solvents. Again for both the BLCs, at λem= 365 nm, the value for τ1 for protic and aprotic solvents are not very different, but at λem= 450 nm, the protic solvents have lower τ1 in
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comparison to aprotic solvents. Furthermore, considering the high PL intensity observed at λem = 365 nm for protic solvents and at λem= 450 nm for the aprotic solvents (Fig. 4), we may assign the shorter lifetime τ1 to the decay of enol form of the molecule while that of τ2 to the keto form
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of the molecule. The comparison of the lifetimes for BLC3 and BLC4 suggests that the length of
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the alkoxy chains at the end of the molecules have insignificant role in the fluorescence behavior of the molecules. Moreover, the longer lifetime τ2 obtained for both the BLCs suggests that the origin of the long tail in the emission spectra are due to vibrational bands resulting from the bulky alkoxy chains attached to the ends of the BLC molecules.
Table 1. The Fluorescence decay life time (τ) obtained from least squares fitting of the decay
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plots shown in Fig. 6. The errorbar in both cases is estimated to be about ± 0.03 ns. Wavelength (λ λem nm)
τ1 (± ± 0.03) (ns)
τ2 (± ± 0.03) (ns)
χ2
BLC3
365
1.44(28.12%)
6.35 (71.88%)
1.02
450
5.78(52.88%)
13.6(47. 12%)
1.01
365
1.36 (29.05%)
9.60 (70.95%)
1.5
(Methanol)
450
2.01 (15.69%)
9.76 (84.31%)
1.2
BLC4
365
1.01 (24.78 %)
6.05 (75.22 %)
1.1
(Hexane)
450
5.74 (51.55 %)
14.3 (48.45%)
1.0
BLC4
365
1.11 (26.19%)
8.97 (73.81%)
1.4
(Methanol)
450
1.53 (14.42 %)
10.6 (85.58 %)
1.2
(Hexane)
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BLC3
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Sample (Solvent)
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Fig. 6. Fluorescence decay plot for BLC3 (upper panel) and BLC4 (lower panel) for emission wavelength of (a) λem= 365 nm and (b) λem= 425 nm, measured using n-hexane and methanol as
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solvent. The instrumental response function (IRF) is also shown in each plot. The estimated total quatntum yields of the BLC3 and BLC4 samples (Table 2) show that
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both the molecules have very low quantum yields in protic solvents and quite good quantum yields in aprotic solvents. This behavior is similar to some other reported ESIPT molecules [23,
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27], indicating that the molecules are potential candidates for organic light emitting diode, optoelectronic and sensing applications. To demonstarte the usefulness of the samples for sensor applications, we exposed all the samples (conc. ~ 1.5 µM) in visible (white) light and UV light of wavelength ~ 365 nm and ~ 254 nm. The resulting optical images shown in Fig. 7 suggests that both the BLC samples in non-polar solvents have stronger green emission when exposed to 254 nm UV-light, correspondingly, stronger blue emission when exposed to 365 nm UV-light and stronger yellowish emission when exposed to visible white light. 12
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Table 2.The total quantum yield (Φs) of BLC3 and BLC4 in different solvents ΦS (BLC4) 0.046 0.099 0.215 0.397 0.533
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ΦS (BLC3) 0.035 0.085 0.126 0.173 0.725
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Solvent Methanol Ethanol 2-Propanol Cyclohexane n-Hexane
Fig. 7. Optical images of the samples in solvents of different polarity, under exposure of white visible light and UV light of two fifferent wavelengths (365 nm and 254 nm): (a) BLC-3 sample and (b) BLC-4 sample. 13
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4.Conclusion We have explored the photophysical characteristics of two bent core liquid crystals C54H63NO9 (BLC3) and C60H75NO9 (BLC4). We confirmed that the nature (proticity) of the
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chosen solvent plays a very important role in deciding the intramolecular and intermolecular hydrogen bond formation behavior of the BLCs. This inter-molecular/intra-molecular hydrogen bond formation controls the overall excited state intramolecular proton transfer (ESIPT)
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emission behavior of the molecule. For protic solvents the BLCs show intense enol emission band through the intermolecular bonding of the proton of the solvent with the Schiff-base,
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whereas for aprotic solvent the BLCs show intense keto emission through the intramolecular hydrogen bonding. With increase in polarity of the protic solvents the enol emission band intensifies. Our results are useful for understanding and tuning the fluorescence behavior of bent
Conflicts of interest
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core liquid crystals containing Schiff-base.
There are no conflicts of interest to declare.
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Acknowledgements
The author S K Behera is grateful for the Dr. DS Kothari UGC fellowship. New Delhi, India.
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The authors thank Mr. Rajeev Kumar (MRC, IISc Bangalore) for his help during initial test experiments.
Appendix A. Supplementary data Supplimentary data associated with this article. File name: Electronic Supporting Information.
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References [1] E. A. Margulies, J. L. Logsdon, C. E. Miller, L. Ma, E. Simonoff, R. M. Young, G. C. Schatz, and M. R. Wasielewski: Direct observation of a charge-transfer state preceding high-yield
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singlet fission in terrylenediimide thin films, J. Am. Chem. Soc. 2017, 139, 663−671.
[2] M. Mamada, K. Inada, T. Komino, W. J. Potscavage, Jr. H Nakanotani, and C Adachi, Highly efficient thermally activated delayed fluorescence from an excited-state
SC
intramolecular proton transfer system, ACS Cent. Sci. 2017, 3, 769−777.
[3] B. Li, L. Zhou, H. Cheng, Q. Huang, J. Lan, L. Zhou and J. You, Dual-emissive 2-(2′-
M AN U
hydroxyphenyl) oxazoles for high performance organic electroluminescent devices: discovery of a new equilibrium of excited state intramolecular proton transfer with a reverse intersystem crossing process, Chem Sci. 2018, 9, 1213–1220.
[4] S. L. Bondareva, S. A. Tikhomirova, T. F. Raichenoka, O. V. Buganova, R. G. Fedunovb, S. S. Khokhlovab, A. I. Ivanovb, V. K. Ol'khovikc, N. A. Galinovskiic, Fluorescence quenching
TE D
of 2,7-diaminoxanthone in alcohols by hydrogen bonding: An experimental and theoretical research, J. Luminescence, 2018, 198, 226–235.
[5] C. Azarias, S. Budzák,A. D. Laurent, G. Ulrich and D. Jacquemin, Tuning ESIPT fluorophores
EP
into dual emitters, Chem. Sci., 2016, 7, 3763-3774.
AC C
[6] V. S. Padalkar, A. Tathe, V. D. Gupta, V. S. Patil, K. Phatangare and N. Sekar, Synthesis and photo-physical characteristics of ESIPT inspired 2-substituted benzimidazole, benzoxazole and benzothiazole fluorescent derivatives, J. Fluorescence., 2012, 22, 311–322.
[7] J. Seo, S. Kim, and S. Y. Park, Strong solvatochromic fluorescence from the intramolecular charge-transfer state created by excited-state intramolecular proton transfer, J. Am. Chem. Soc. 2004, 126, 11154-11155.
15
ACCEPTED MANUSCRIPT
[8] J. Liang, B. Z Tang and Bin Liu, Specific light-up bioprobes based on AIEgen conjugates, Chem Soc Rev. 2015, 44, 2798-811.
Transfer Reaction, J. Phys. Chem. A 2013, 117, 1400-1405.
RI PT
[9] J. Lee, C. H. Kim, and T. Joo, Active Role of Proton in Excited State Intramolecular Proton
[10] H. Konoshima, S. Nagao, I. Kiyota, K. Amimoto,N. Yamamoto, M. Sekine, M. Nakata, K. Furukawaa and H. Sekiya, Excited-state intramolecular proton transfer and charge transfer in
SC
2-(2′-hydroxyphenyl)benzimidazole crystals studied by polymorphs-selected electronic
M AN U
spectroscopy, Phys. Chem. Chem. Phys., 2012, 14, 16448–1645.
[11] T. Mutai, H. Tomoda, T. Ohkawa, Y. Yabe and K. Araki, Switching of PolymorphDependent ESIPT Luminescence of an Imidazo[1,2-a]pyridine Derivative, Angew. Chem. Int. Ed. 2008, 47, 9522 –9524.
[12] P. Singh, H. Singh, R. Sharma, G. Bhargava and S. Kumar, Diphenylpyrimidinone–
TE D
salicylideneamine – new ESIPT based AIEgens with applications in latentfingerprinting, J. Mater. Chem. C, 2016, 4, 11180-11189.
[13] J. Pina, D. Sarmento, Marco Accoto, P.L Gentili, L. Vaccaro, A. Galvão and J. S. S de
EP
Melo, Excited-state proton transfer in indigo, J. Phys. Chem. B, 2017, 121, 2308−2318.
AC C
[14] V. S. Padalkar, and Shu Seki, Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters, Chem. Soc. Rev., 2016, 45, 169-202.
[15] H. Lin, X. Chang, D. Yan, W.H
Fang and Ganglong Cui, Tuning excited-state-
intramolecular-proton transfer (ESIPT) process and emission by co-crystal formation, a combined experimental and theoretical study, Chem. Sci., 2017, 82086-82090.
16
ACCEPTED MANUSCRIPT
[16] C.Y Peng, J.Y Shen, Y. T Chen, P.J Wu, W. Y Hung, W.P Hu, and Pi-Tai Chou, Optically triggered stepwise double-proton transfer in an intramolecular proton relay: A Case Study of 1,8-Dihydroxy-2- naphthaldehyde, J. Am. Chem. Soc. 2015, 137, 14349−14357.
RI PT
[17] K. Sakai, T. Tsuzuki, Y. Itoh, M. Ichikawa, and Y. Taniguchi, Using proton-transfer laser dyes for organic laser diodes, Appl. Phys. Lett., 2005, 86, 081103.
[18] M. J. Paterson, M. A. Robb, L. Blancafort and A. D. DeBellis, Mechanism of an
SC
Exceptional Class of Photostabilizers: A Seam of Conical Intersection Parallel to Excited State Intramolecular Proton Transfer (ESIPT) in o-Hydroxyphenyl-(1,3,5)-triazine, J. Phys.
M AN U
Chem. A, 2005, 109, 7527–7537.
[19] J. Zhao, S. Ji, Y. Chen, H. Guo and P. Yang, Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials, Phys. Chem. Chem.
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Phys., 2012, 14, 8803–8817.
[20] J. Wu, W. Liu, J. Ge, H. Zhang and P. Wang, New sensing mechanisms for design of fluorescent chemosensors emerging in recent years, Chem. Soc. Rev., 2011, 40, 3483–3495.
EP
[21] K. C. Tang, M. J. Chang, T.Y. Lin, H.A. Pan, T. C. Fang, K.Y. Chen, W.Y. Hung, Y.H. Hsu and P.T. Chou,Fine Tuning the Energetics of Excited-State Intramolecular Proton
AC C
Transfer (ESIPT): White Light Generation in A Single ESIPT System, J. Am. Chem. Soc., 2011, 133, 17738–17745.
[22] V. Luxami and S. Kumar, Molecular half-subtractor based on 3,3′-bis(1H-benzimidazolyl-2yl)[1,1′]binaphthalenyl-2,2′-diol, New J. Chem., 2008, 32, 2074–2079.
17
ACCEPTED MANUSCRIPT
[23] F. A. S. Chipem, S. K. Behera and G. Krishnamoorthy,Ratiometric fluorescence sensing ability of 2-(2′-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues towards metal ions, Sens. Actuators, B, 2014, 191, 727–733.
RI PT
[24] S. K. Behera,G. Sadhuragiri, P. Elumalai, M. Sathiyendiran, and G. Krishnamoorthy, Exclusive Tautomer Emission from a 2-(2′-Hydroxyphenyl) benzimidazole Derivative, RSC Adv., 2016, 6, 59708-59717.
SC
[25] S. K. Behera, Ananda Karak, and G. Krishnamoorthy, Photophysics of 2-(4′-Amino-2′hydroxyphenyl)-1Himidazo-[4,5c] pyridine and Its Analogues: Intramolecular proton transfer
M AN U
versus intramolecular charge transfer, J. Phys. Chem. B, 2015, 119,2330–2344.
[26] S. K. Behera, A. Murkherjee, G. Sadhuragiri, P. Elumalai, M. Sathiyendiran, M. Kumar, B. B. Mandal, and G. Krishnamoorthy, Aggregation induced enhanced and exclusive highly stoke shifted emission from an excited state intramolecular proton transfer exhibiting
TE D
molecule, Faraday Discuss., 2017, 196, 71–90.
[27] P. Majumdar and J. Zhao, 2-(2-Hydroxyphenyl)-benzothiazole (HBT)-Rhodamine Dyad: Acid-Switchable Absorption and Fluorescence of Excited-State Intramolecular Proton
EP
Transfer (ESIPT), J. Phys. Chem. B, 2015, 119, 2384–2394.
[28] X. Zhang, J, Liu, Solvent dependent photophysical properties and near-infrared solid-state state
intramolecular
proton
transfer
AC C
excited
(ESIPT)
fluorescence
of
2,4,6-
trisbenzothiazolylphenol, Dyes and Pigments, 2016, 125, 80-88.
[29] Zhe Tanga, Meiheng Lu, Kangjing Liu, Yanliang Zhao, Yutai Qi, Yi Wang Peng Zhang, Panwang Zhou, Solvation effect on the ESIPT mechanism of
2-(4′-amino-2′-
hydroxyphenyl)- 1H-imidazo-[4,5-c]pyridine, J. Photochem. Photobiol. A: Chem., 2018, 367, 261–269.
18
ACCEPTED MANUSCRIPT
[30] Qin Wang,Yahui Niu,Rong Wang, Haoran Wu,and Yanrong Zhang, Acid-Induced Shift of Intramolecular Hydrogen Bonding Responsible
for Excited-State Intramolecular Proton
Transfer, Chem. Asian J., 2018, 13, 1735 – 1743.
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[31] Pi-Tai Chou, Wei-Shan Yu, Yi-Ming Cheng, Shih-Chieh Pu, Yueh-Chi Yu, Yu-Chung Lin, Chien-Huang Huang, and Chao-Tsen Chen,Solvent-Polarity Tuning Excited-State Charge Coupled Proton-Transfer Reaction in p-N,N-Ditolylaminosalicylaldehydes, J. Phys. Chem. A,
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2004, 108, 6487-6498.
[32] A.K. Satapathy, S.K. Behera, R. Kumar, K.L. Sandhya, C.V. Yelamaggad, B. Sahoo,
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Excited state intramolecular proton transfer emission in bent core liquid crystals, J. Photochem. Photobiol. A: Chem., 2018, 358, 186–191.
[33] P. Zhou, M.R. Hoffmann, K.L. Han, G. He, New insights into the dual fluorescence of methyl salicylate: effects of intermolecular hydrogen bonding and salvation, J. Phys. Chem.
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B, 2015, 119, 2125–213.
[34] J. Seo, S. Kim, S.H. Gihm, C.R. Park, S.Y. Park, Highly fluorescent columnar liquid crystals with elliptical molecular shape: oblique molecular stacking and excited-state
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intramolecular proton-transfer fluorescence, J. Mater. Chem., 2007, 17, 5052.
[35] C.V. Yelamaggad, I.S. Shashikala, G.G. Nair, D.S.S. Rao, S.K. Prasad, Nonsymmetrical
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five-ring achiral banana-shaped liquid crystals comprisingsalicylaldimine mesogenic segment, J. Chem. Res., 2006, 9, 612–616.
[36] A. Crosby, J.N. Demas, Measurement of Photoluminescence Quantum Yields. A Review, J. Phys. Chem., 1971, 75, 991−1024.
[37] Y.T Chen, P.J Wu, C.Y Peng, J. Y Shen, Cheng-Cheng Tsai, W-P Hu and Pi-Tai Chou, A study of the competitive multiple hydrogen bonding effect and its associated excited-state proton transfer tautomerism, Phys. Chem. Chem. Phys., 2017, 19, 28641—28646. 19
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Table of Content
The intensity of fluoroscence emission can be tuned through solvent polarity via formation of intramolecular or intermolecular hydrogen bonds.
Highlights:
Excited State Proton Transfer emission of the LCs can be tuned through the solvent polarity.
•
Intensity of keto emission increases with increase in polarity of the protic solvents.
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Intra-molecular (inter-molecular) hydrogen bonds prevail in aprotic (protic) solvents.
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The intensity of keto- (enol-) emission is stronger in aprotic (protic) solvents.
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S0022-2313(18)32395-0
DOI:
https://doi.org/10.1016/j.jlumin.2019.02.056
Reference:
LUMIN 16322
To appear in:
Journal of Luminescence
Received Date: 22 December 2018 Revised Date:
17 February 2019
Accepted Date: 26 February 2019
Please cite this article as: A.K. Satapathy, S.K. Behera, A. Yadav, L.N. Mahour, C.V. Yelamaggad, K.L. Sandhya, B. Sahoo, Tuning the fluorescence behavior of liquid crystal molecules containing Schiff-base: Effect of solvent polarity, Journal of Luminescence (2019), doi: https://doi.org/10.1016/ j.jlumin.2019.02.056. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Tuning the Fluorescence Behavior of Liquid Crystal Molecules Containing Schiff-Base: Effect of Solvent Polarity
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Ashok K. Satapathya,b, Santosh K. Beherac, Ankit Yadavd, Laxmi Narayan Mahourd, C. V. Yelamaggade, K. L. Sandhyab, †, Balaram Sahood, † a
Department of Physics, Acharya Institute of Technology, Bangalore 560107, India
b
Department of Physics,Ramaiah Institute of Technology, Bangalore 560054, India
Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India d
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c
Materials Research Centre, Indian Institute of Science, Bangalore 560012, India Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India
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e
†
Corresponding author(s):
Email: [email protected] (B. Sahoo) Tel: +91 80 2293 2943 Email: [email protected] (K. L. Sandhya) Tel: +91 96321 48972
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Abstract: We report the influence of intermolecular and intramolecular hydrogen bonding on the excited state proton transfer (ESPT) emission behavior of two bent core liquid crystal (BLC)
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molecules, C54H63NO9 (BLC3) and C60H75NO9 (BLC4), having a Schiff-base and two long alkyl chains at its two ends. Fluorescence spectra of these BLC molecules dispersed in different solvents show dual emission (at ~ 365 nm and ~ 425 nm) due to formation of keto and enol
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tautomers. We observed that the population of these keto and enol tautomers and the corresponding intensities of fluorescence emission are strongly influenced by the solvent
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polarity. In protic solvents, formation of intermolecular hydrogen bond with the Schiff-base of the BLC molecules is highly favoured than the intramolecular hydrogen bonding in the BLCs. This intermolecular hydrogen bonding drastically reduces the population of the keto tautomers in the excited state, resulting in enhanced enol fluoroscence band along with a weak keto emission
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band. The observed intensity of the enol fluorescence band is the highest for the most polar solvent (methanol). On the other hand, in aprotic solvents, the intramolecular hydrogen bonding is highly favored, which leads to the formation of keto tautomers in the excited state. Hence, an
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intense keto emission band is observed for the aprotic solvents along with a weak enol emission band. From the time resolved fluorescence spectroscopy studies we observed a longer life time
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for the keto band than that for the enol band. This is also related to the delayed emission associated with the vibrational bands resulting from the bulky alkyl chains attached to the ends of the BLC molecules.
Keywords: Bent-core liquid crystals; Intramolecular hydrogen bond; Intermolecular hydrogen bond; ESPT; Effect of solvents; Photoluminescence.
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1. Introduction In recent time photophysical studies of dual emitters remain a very active field of research. These dyes found emerging applications in biological and chemical systems due to red shifted
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emission which is highly Stokes shift [1-12]. In this category, dyes undergoing an excited-state intramolecular proton transfer (ESIPT) are chiefly attractive. ESIPT is a very fast phototautomerisation process taking place along an intramolecular hydrogen bond between
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proton donor and proton acceptor and that are significantly tuned upon electronic excitation [7, 13-17]. The pre–requisite for ESIPT is the presence of intramolecular hydrogen bond between
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proton donor (–OH and –NH2) and proton acceptor (=N– and –C=O) groups near each other in a molecule. ESIPT usually results in dual fluorescence with emission from both excited enol and excited keto tautomer. The notable properties of the ESIPT dyes are dual emission, large Stokes shift and ultrafast process. ESIPT dyes have found applications in many fields such as in
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chemosensors, electroluminescent materials, laser dyes, and UV-photostabilisers, white organic light-emitting diodes (OLED) [17-22]. The spectral properties of the ESIPT dyes depend on hydrogen bonding, rotamerisation process and pH of the surrounding medium and substituents
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present on donor and acceptor units [23-27]. The fluorescence properties of the ESIPT fluorophores are tunable by changing these parameters. Several series of compounds undergo
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ESIPT and they have been investigated with both experimental and theoretical approaches [14, 28-32]. However, the effect of hydrogen bonding on proton transfer process can be used to tune the excited state emission properties of various dyes, which attracts a lot of attention [15]. Zhou et al. reported the effects of intermolecular hydrogen bonding on the dual fluorescence of methyl salicylate [33]. They investigated that the intermolecular hydrogen bond plays an important role for the dual fluorescence of methyl salicylate in alcoholic solvents. Subsequently, the effect of
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solvent polarity on different class of ESIPT dyes including salicylideneaniline, hydroxyacetophenones, hydroxyflavones, hydroxyphenyl-oxazole(HPO), hydroxyphenyl-benzoxazoles (HPBO), hydroxyphenyl-benzothiazole (HPBT) and hydroxyphenyl-benzimidazoles (HPBI)
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were extensively investigated [14]. Park et al. reported a novel class of ESIPT-active molecule forming columnar liquid crystals with enhanced fluorescence [34]. In this work, we studied the effect of sovent polarity on the favorability of hydrogen bond formation and the subsequent
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effect on ESIPT behavior of two bent core liquid crystal (BLC) molecules containing salicylideneaniline core (Schiff-base) attached with two alkyl chains at their ends, as shown in
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Fig. 1.
Fig. 1. Chemical structures of (a) BLC3 and (b) BLC4.
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2. Materials and Methods The chemical structures of the two bent core liquid crystals (BLCs) used for this study,
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C54H63NO9 (BLC3) and C60H75NO9 (BLC4), are provided in Fig. 1. The details of synthetic procedure and the liquid crystal behavior of these BLCs are reported elsewhere [35]. Both liquid crystals are similar in structure except the alkyl groups attached at their ends. BLC3 constitutes of alkyl groups –C10H21 and –C10H21, whereas BLC4 contains –C10H21 and –C16H33 at their two
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ends. The UV-Vis absorption spectra (Fig. S1 and Fig. S2, electronic supplementary information (ESI) file) of the BLCs were recorded using “Perkin Elmer Lambda 750 UV-Visible”
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spectrophotometer. All the measurements were performed with 1.5 µM concentration of the solute in different solvents to avoid aggregation and self-quenching. The HPLC grade cyclohexane, n-hexane, 2-propanol, ethanol, methanol are used as solvents for this study, where the polarity of the solvents are 0.006, 0.009, 0.546, 0.654 and 0.762 (with respect to the unity
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(1.0) polarity of water). Fluorescence spectra were recorded using HORIBA Jobin Yvon Fluoromax-4 spectrofluorimeter. The concentration of the solute and the solvents used for fluorescence study were same as that of absorption study. The fluorescence lifetimes were
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measured from time-resolved intensity decay by time correlated single-photon counting (TCSPC) method on an Edinburgh instrument (Life-Spec II) using 303 nm LED. The
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fluorescence decay was analyzed by deconvolution method using the DAS6 software. The obtained values of χ2 for all the fittings were close to 1 and weighted residual values were between + 4 and - 4.
The fluorescence quantum yields of BLC3 and BLC4 samples were determined relative to that of quinine sulphate (Φr = 0.546) and calculated on the basis of the following equation [36].
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Φ s I s Ar ns2 = Φ r I r As nr2
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where Φs is the fluorescence quantum yield of the samples, Is and Ir are the integrated fluorescence area, and As and Ar are the absorbance values for the sample and reference solutions, respectively. ns and nr are the refractive indices for the sample and reference solutions,
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respectively.
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3. Results and Discussion
The absorption spectra of BLC3 and BLC4 exhibit multi absorption bands in n-hexane, i.e., absorption at ~ 225 nm, ~265 nm and ~315 nm. These band positions remain unaffected
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with solvent polarity (Fig. S1). The origin of the bands is π-π* and/or n-π* transitions.
Fig. 2. The structure of the emitting species enol and keto in the excited state.
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The presence of the Schiff-base (C=N) in the BLC molecules allows the existence of two tautomeric forms in the excited state of the compounds, enol and keto, as shown in Fig. 2. In general, molecules containing proton donor and proton acceptor nearly upon photoexcitation
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under goes a cyclic process. These two forms can be foreseen to cause the dual emissive behavior of the compounds as schematically shown in Fig. 3. The measured fluorescence spectra of both the BLCs are shown in Fig. 4. Clearly, upon photoexcitation at 310 nm, BLC3 and BLC4
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emit dual emission fluorescence at ~ 365 nm and ~ 425 nm. We observed a weak-intensity shorter wavelength band (at ~ 365 nm) and a strong-intensity longer wavelength band (at ~ 425
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nm) for aprotic solvents (n-hexane and cyclohexane). Note that, n-hexane and cyclohexane have a very small difference in their polarity (n-hexane: 0.009, cyclohexane: 0.006, with respect to the reference polarity of water:1.000). However, for less polar cyclohexane we observed an intense longer wavelength emission band, in comparison to that for n-hexane. On the other hand, in
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protic solvents (such as methanol, ethanol and 2-propanol) the intensities of these bands are reversed. As the polarity of the protic solvents increase, the intensity of the shorter wavelength emission band increases. In addition, a marginal blue shift is observed for longer wavelength
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band for the protic solvents. This suggests that the longer wavelength emission was due to ESIPT form keto tautomer and shorter wavelength band is due to normal emission of excited enol [26].
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In excited state proton transfer the excited state keto is less polar than ground state enol
tautomer, as a result, with an increase in polarity of the solvents the energy gap between these states increases leading to a blue shift. Similar blue shift is also observed for other reported ESIPT exhibiting molecules. The shorter wavelength band is due to emission from excited enol tautomer and the longer wavelength is due to keto tautomer formed from excited enol in excited state. The solvent polarity greatly influences the easy formation of keto in excited state.
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Fig. 3. Mechanistic pathways for the fluroscence emission for the keto (produced from cis-enol)
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and enol (produced from trans-enol) tautomers.
Fig. 4. Fluorescence spectra of (a) BLC3 and (b) BLC4 in different aprotic and protic solvents, λexc=310 nm. 8
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The solvent polarity modulation of fluorescence is quite interesting. It is well studied that keto tautomer are efficiently produced in hydrocarbon solvents [25-30, 36]. In alcoholic (protic) solvent, there exists competition between intramolecular bonding of the nearest hydrogen with
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the Schiff-base and intermolecular bonding of hydrogen of the solvents with the Schiff-base.
Fig. 5. The scheme of formation of intramolecular and intermolecular hydrogen bonding in
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aprotic and protic solvents, respectively.
Therefore, these compounds show different photophysical behavior in solutions of
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different polarities. In aprotic n-hexane, the proton donor -OH group and proton acceptor =N site (Fig. 5 (a)) forms strong intramolecular hydrogen bonding, which facilitates the proton transfer
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and upon photo excitation it emits strong keto emission. However, in protic solvents, the -OH group of the solvents form intermolecular hydrogen bond with =N site. Hence, in protic solvents, there is favorable formation of intermolecular hydrogen bonding in these BLCs (Fig. 5 (b)). This prevents the ease of (intramolecular) proton transfer and reduces the keto emission in photo excitation [25]. Their results summarize the weak normal emission (at ~365 nm) and strong tautomer emission (at ~425 nm) in n-hexane and cyclohexane. Likewise, a strong normal
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emission (at ~365 nm ) and weak tautomer emission (at ~425 nm) in n-hexane and cyclohexane, were observed. Note that due to overlapping of the FTIR line positions (at about 3330 cm-1) of N-H and O-H stretching vibrations, the effect of intramolecular or intermolecular hydrogen
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bonding in the FTIR absorption spectra could not be observed (see ESI, Fig. S3 and Fig. S4). The time resolved fluorescence spectra of our BLC3 and BLC4 samples are given in Fig. 6. For understanding the behavior of the BLCs in protic and aprotic solvents, we have chosen
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methanol as protic and n-hexane as aprotic solvent. The decay intensities I(t), shown in (Fig. 6) clearly shows two decay modes associated with two life times, τ1 and τ2. Here, the mean life (τ) ଵ
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is the time required for the intensity of fluorescence to reach a value of ( )th of the initial
intensity. The decay curves (Fig. 6) are fit with two separate profiles, as given by the following equation.
−t
−t
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I (t ) = A + B1e τ1 + B2eτ 2
The fraction of intensities (in percentage) contributed by the two modes are calculated by:
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Fi (%) =
Biτ i
×100
2
∑Bτ j =1
j
j
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The lifetimes obtained from fitting of the decay curves are given in Table 1. It is clear
from the table that the lifetime τ1 is always lower than that of τ2, for a particular solvent at a particular emission wavelength. For both the decay modes (τ1 and τ2) in BLC3 and BLC4, the longer wavelength emission band λem= 450 nm has longer lifetime and the shorter wavelength emission band λem= 365 nm has shorter lifetime. We compare the decay behavior of both the BLCs in protic (methanol) and aprotic (n-hexane) solvents. For both BLCs at λem= 450 nm, τ2 is
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lower for protic solvent than aprotic solvent, but at λem= 365 nm, τ2 is longer for protic solvents than aprotic solvents. Again for both the BLCs, at λem= 365 nm, the value for τ1 for protic and aprotic solvents are not very different, but at λem= 450 nm, the protic solvents have lower τ1 in
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comparison to aprotic solvents. Furthermore, considering the high PL intensity observed at λem = 365 nm for protic solvents and at λem= 450 nm for the aprotic solvents (Fig. 4), we may assign the shorter lifetime τ1 to the decay of enol form of the molecule while that of τ2 to the keto form
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of the molecule. The comparison of the lifetimes for BLC3 and BLC4 suggests that the length of
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the alkoxy chains at the end of the molecules have insignificant role in the fluorescence behavior of the molecules. Moreover, the longer lifetime τ2 obtained for both the BLCs suggests that the origin of the long tail in the emission spectra are due to vibrational bands resulting from the bulky alkoxy chains attached to the ends of the BLC molecules.
Table 1. The Fluorescence decay life time (τ) obtained from least squares fitting of the decay
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plots shown in Fig. 6. The errorbar in both cases is estimated to be about ± 0.03 ns. Wavelength (λ λem nm)
τ1 (± ± 0.03) (ns)
τ2 (± ± 0.03) (ns)
χ2
BLC3
365
1.44(28.12%)
6.35 (71.88%)
1.02
450
5.78(52.88%)
13.6(47. 12%)
1.01
365
1.36 (29.05%)
9.60 (70.95%)
1.5
(Methanol)
450
2.01 (15.69%)
9.76 (84.31%)
1.2
BLC4
365
1.01 (24.78 %)
6.05 (75.22 %)
1.1
(Hexane)
450
5.74 (51.55 %)
14.3 (48.45%)
1.0
BLC4
365
1.11 (26.19%)
8.97 (73.81%)
1.4
(Methanol)
450
1.53 (14.42 %)
10.6 (85.58 %)
1.2
(Hexane)
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BLC3
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Sample (Solvent)
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Fig. 6. Fluorescence decay plot for BLC3 (upper panel) and BLC4 (lower panel) for emission wavelength of (a) λem= 365 nm and (b) λem= 425 nm, measured using n-hexane and methanol as
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solvent. The instrumental response function (IRF) is also shown in each plot. The estimated total quatntum yields of the BLC3 and BLC4 samples (Table 2) show that
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both the molecules have very low quantum yields in protic solvents and quite good quantum yields in aprotic solvents. This behavior is similar to some other reported ESIPT molecules [23,
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27], indicating that the molecules are potential candidates for organic light emitting diode, optoelectronic and sensing applications. To demonstarte the usefulness of the samples for sensor applications, we exposed all the samples (conc. ~ 1.5 µM) in visible (white) light and UV light of wavelength ~ 365 nm and ~ 254 nm. The resulting optical images shown in Fig. 7 suggests that both the BLC samples in non-polar solvents have stronger green emission when exposed to 254 nm UV-light, correspondingly, stronger blue emission when exposed to 365 nm UV-light and stronger yellowish emission when exposed to visible white light. 12
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Table 2.The total quantum yield (Φs) of BLC3 and BLC4 in different solvents ΦS (BLC4) 0.046 0.099 0.215 0.397 0.533
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ΦS (BLC3) 0.035 0.085 0.126 0.173 0.725
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Solvent Methanol Ethanol 2-Propanol Cyclohexane n-Hexane
Fig. 7. Optical images of the samples in solvents of different polarity, under exposure of white visible light and UV light of two fifferent wavelengths (365 nm and 254 nm): (a) BLC-3 sample and (b) BLC-4 sample. 13
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4.Conclusion We have explored the photophysical characteristics of two bent core liquid crystals C54H63NO9 (BLC3) and C60H75NO9 (BLC4). We confirmed that the nature (proticity) of the
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chosen solvent plays a very important role in deciding the intramolecular and intermolecular hydrogen bond formation behavior of the BLCs. This inter-molecular/intra-molecular hydrogen bond formation controls the overall excited state intramolecular proton transfer (ESIPT)
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emission behavior of the molecule. For protic solvents the BLCs show intense enol emission band through the intermolecular bonding of the proton of the solvent with the Schiff-base,
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whereas for aprotic solvent the BLCs show intense keto emission through the intramolecular hydrogen bonding. With increase in polarity of the protic solvents the enol emission band intensifies. Our results are useful for understanding and tuning the fluorescence behavior of bent
Conflicts of interest
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core liquid crystals containing Schiff-base.
There are no conflicts of interest to declare.
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Acknowledgements
The author S K Behera is grateful for the Dr. DS Kothari UGC fellowship. New Delhi, India.
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The authors thank Mr. Rajeev Kumar (MRC, IISc Bangalore) for his help during initial test experiments.
Appendix A. Supplementary data Supplimentary data associated with this article. File name: Electronic Supporting Information.
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References [1] E. A. Margulies, J. L. Logsdon, C. E. Miller, L. Ma, E. Simonoff, R. M. Young, G. C. Schatz, and M. R. Wasielewski: Direct observation of a charge-transfer state preceding high-yield
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singlet fission in terrylenediimide thin films, J. Am. Chem. Soc. 2017, 139, 663−671.
[2] M. Mamada, K. Inada, T. Komino, W. J. Potscavage, Jr. H Nakanotani, and C Adachi, Highly efficient thermally activated delayed fluorescence from an excited-state
SC
intramolecular proton transfer system, ACS Cent. Sci. 2017, 3, 769−777.
[3] B. Li, L. Zhou, H. Cheng, Q. Huang, J. Lan, L. Zhou and J. You, Dual-emissive 2-(2′-
M AN U
hydroxyphenyl) oxazoles for high performance organic electroluminescent devices: discovery of a new equilibrium of excited state intramolecular proton transfer with a reverse intersystem crossing process, Chem Sci. 2018, 9, 1213–1220.
[4] S. L. Bondareva, S. A. Tikhomirova, T. F. Raichenoka, O. V. Buganova, R. G. Fedunovb, S. S. Khokhlovab, A. I. Ivanovb, V. K. Ol'khovikc, N. A. Galinovskiic, Fluorescence quenching
TE D
of 2,7-diaminoxanthone in alcohols by hydrogen bonding: An experimental and theoretical research, J. Luminescence, 2018, 198, 226–235.
[5] C. Azarias, S. Budzák,A. D. Laurent, G. Ulrich and D. Jacquemin, Tuning ESIPT fluorophores
EP
into dual emitters, Chem. Sci., 2016, 7, 3763-3774.
AC C
[6] V. S. Padalkar, A. Tathe, V. D. Gupta, V. S. Patil, K. Phatangare and N. Sekar, Synthesis and photo-physical characteristics of ESIPT inspired 2-substituted benzimidazole, benzoxazole and benzothiazole fluorescent derivatives, J. Fluorescence., 2012, 22, 311–322.
[7] J. Seo, S. Kim, and S. Y. Park, Strong solvatochromic fluorescence from the intramolecular charge-transfer state created by excited-state intramolecular proton transfer, J. Am. Chem. Soc. 2004, 126, 11154-11155.
15
ACCEPTED MANUSCRIPT
[8] J. Liang, B. Z Tang and Bin Liu, Specific light-up bioprobes based on AIEgen conjugates, Chem Soc Rev. 2015, 44, 2798-811.
Transfer Reaction, J. Phys. Chem. A 2013, 117, 1400-1405.
RI PT
[9] J. Lee, C. H. Kim, and T. Joo, Active Role of Proton in Excited State Intramolecular Proton
[10] H. Konoshima, S. Nagao, I. Kiyota, K. Amimoto,N. Yamamoto, M. Sekine, M. Nakata, K. Furukawaa and H. Sekiya, Excited-state intramolecular proton transfer and charge transfer in
SC
2-(2′-hydroxyphenyl)benzimidazole crystals studied by polymorphs-selected electronic
M AN U
spectroscopy, Phys. Chem. Chem. Phys., 2012, 14, 16448–1645.
[11] T. Mutai, H. Tomoda, T. Ohkawa, Y. Yabe and K. Araki, Switching of PolymorphDependent ESIPT Luminescence of an Imidazo[1,2-a]pyridine Derivative, Angew. Chem. Int. Ed. 2008, 47, 9522 –9524.
[12] P. Singh, H. Singh, R. Sharma, G. Bhargava and S. Kumar, Diphenylpyrimidinone–
TE D
salicylideneamine – new ESIPT based AIEgens with applications in latentfingerprinting, J. Mater. Chem. C, 2016, 4, 11180-11189.
[13] J. Pina, D. Sarmento, Marco Accoto, P.L Gentili, L. Vaccaro, A. Galvão and J. S. S de
EP
Melo, Excited-state proton transfer in indigo, J. Phys. Chem. B, 2017, 121, 2308−2318.
AC C
[14] V. S. Padalkar, and Shu Seki, Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters, Chem. Soc. Rev., 2016, 45, 169-202.
[15] H. Lin, X. Chang, D. Yan, W.H
Fang and Ganglong Cui, Tuning excited-state-
intramolecular-proton transfer (ESIPT) process and emission by co-crystal formation, a combined experimental and theoretical study, Chem. Sci., 2017, 82086-82090.
16
ACCEPTED MANUSCRIPT
[16] C.Y Peng, J.Y Shen, Y. T Chen, P.J Wu, W. Y Hung, W.P Hu, and Pi-Tai Chou, Optically triggered stepwise double-proton transfer in an intramolecular proton relay: A Case Study of 1,8-Dihydroxy-2- naphthaldehyde, J. Am. Chem. Soc. 2015, 137, 14349−14357.
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[17] K. Sakai, T. Tsuzuki, Y. Itoh, M. Ichikawa, and Y. Taniguchi, Using proton-transfer laser dyes for organic laser diodes, Appl. Phys. Lett., 2005, 86, 081103.
[18] M. J. Paterson, M. A. Robb, L. Blancafort and A. D. DeBellis, Mechanism of an
SC
Exceptional Class of Photostabilizers: A Seam of Conical Intersection Parallel to Excited State Intramolecular Proton Transfer (ESIPT) in o-Hydroxyphenyl-(1,3,5)-triazine, J. Phys.
M AN U
Chem. A, 2005, 109, 7527–7537.
[19] J. Zhao, S. Ji, Y. Chen, H. Guo and P. Yang, Excited state intramolecular proton transfer (ESIPT): from principal photophysics to the development of new chromophores and applications in fluorescent molecular probes and luminescent materials, Phys. Chem. Chem.
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Phys., 2012, 14, 8803–8817.
[20] J. Wu, W. Liu, J. Ge, H. Zhang and P. Wang, New sensing mechanisms for design of fluorescent chemosensors emerging in recent years, Chem. Soc. Rev., 2011, 40, 3483–3495.
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[21] K. C. Tang, M. J. Chang, T.Y. Lin, H.A. Pan, T. C. Fang, K.Y. Chen, W.Y. Hung, Y.H. Hsu and P.T. Chou,Fine Tuning the Energetics of Excited-State Intramolecular Proton
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Transfer (ESIPT): White Light Generation in A Single ESIPT System, J. Am. Chem. Soc., 2011, 133, 17738–17745.
[22] V. Luxami and S. Kumar, Molecular half-subtractor based on 3,3′-bis(1H-benzimidazolyl-2yl)[1,1′]binaphthalenyl-2,2′-diol, New J. Chem., 2008, 32, 2074–2079.
17
ACCEPTED MANUSCRIPT
[23] F. A. S. Chipem, S. K. Behera and G. Krishnamoorthy,Ratiometric fluorescence sensing ability of 2-(2′-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues towards metal ions, Sens. Actuators, B, 2014, 191, 727–733.
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[24] S. K. Behera,G. Sadhuragiri, P. Elumalai, M. Sathiyendiran, and G. Krishnamoorthy, Exclusive Tautomer Emission from a 2-(2′-Hydroxyphenyl) benzimidazole Derivative, RSC Adv., 2016, 6, 59708-59717.
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[25] S. K. Behera, Ananda Karak, and G. Krishnamoorthy, Photophysics of 2-(4′-Amino-2′hydroxyphenyl)-1Himidazo-[4,5c] pyridine and Its Analogues: Intramolecular proton transfer
M AN U
versus intramolecular charge transfer, J. Phys. Chem. B, 2015, 119,2330–2344.
[26] S. K. Behera, A. Murkherjee, G. Sadhuragiri, P. Elumalai, M. Sathiyendiran, M. Kumar, B. B. Mandal, and G. Krishnamoorthy, Aggregation induced enhanced and exclusive highly stoke shifted emission from an excited state intramolecular proton transfer exhibiting
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molecule, Faraday Discuss., 2017, 196, 71–90.
[27] P. Majumdar and J. Zhao, 2-(2-Hydroxyphenyl)-benzothiazole (HBT)-Rhodamine Dyad: Acid-Switchable Absorption and Fluorescence of Excited-State Intramolecular Proton
EP
Transfer (ESIPT), J. Phys. Chem. B, 2015, 119, 2384–2394.
[28] X. Zhang, J, Liu, Solvent dependent photophysical properties and near-infrared solid-state state
intramolecular
proton
transfer
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excited
(ESIPT)
fluorescence
of
2,4,6-
trisbenzothiazolylphenol, Dyes and Pigments, 2016, 125, 80-88.
[29] Zhe Tanga, Meiheng Lu, Kangjing Liu, Yanliang Zhao, Yutai Qi, Yi Wang Peng Zhang, Panwang Zhou, Solvation effect on the ESIPT mechanism of
2-(4′-amino-2′-
hydroxyphenyl)- 1H-imidazo-[4,5-c]pyridine, J. Photochem. Photobiol. A: Chem., 2018, 367, 261–269.
18
ACCEPTED MANUSCRIPT
[30] Qin Wang,Yahui Niu,Rong Wang, Haoran Wu,and Yanrong Zhang, Acid-Induced Shift of Intramolecular Hydrogen Bonding Responsible
for Excited-State Intramolecular Proton
Transfer, Chem. Asian J., 2018, 13, 1735 – 1743.
RI PT
[31] Pi-Tai Chou, Wei-Shan Yu, Yi-Ming Cheng, Shih-Chieh Pu, Yueh-Chi Yu, Yu-Chung Lin, Chien-Huang Huang, and Chao-Tsen Chen,Solvent-Polarity Tuning Excited-State Charge Coupled Proton-Transfer Reaction in p-N,N-Ditolylaminosalicylaldehydes, J. Phys. Chem. A,
SC
2004, 108, 6487-6498.
[32] A.K. Satapathy, S.K. Behera, R. Kumar, K.L. Sandhya, C.V. Yelamaggad, B. Sahoo,
M AN U
Excited state intramolecular proton transfer emission in bent core liquid crystals, J. Photochem. Photobiol. A: Chem., 2018, 358, 186–191.
[33] P. Zhou, M.R. Hoffmann, K.L. Han, G. He, New insights into the dual fluorescence of methyl salicylate: effects of intermolecular hydrogen bonding and salvation, J. Phys. Chem.
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B, 2015, 119, 2125–213.
[34] J. Seo, S. Kim, S.H. Gihm, C.R. Park, S.Y. Park, Highly fluorescent columnar liquid crystals with elliptical molecular shape: oblique molecular stacking and excited-state
EP
intramolecular proton-transfer fluorescence, J. Mater. Chem., 2007, 17, 5052.
[35] C.V. Yelamaggad, I.S. Shashikala, G.G. Nair, D.S.S. Rao, S.K. Prasad, Nonsymmetrical
AC C
five-ring achiral banana-shaped liquid crystals comprisingsalicylaldimine mesogenic segment, J. Chem. Res., 2006, 9, 612–616.
[36] A. Crosby, J.N. Demas, Measurement of Photoluminescence Quantum Yields. A Review, J. Phys. Chem., 1971, 75, 991−1024.
[37] Y.T Chen, P.J Wu, C.Y Peng, J. Y Shen, Cheng-Cheng Tsai, W-P Hu and Pi-Tai Chou, A study of the competitive multiple hydrogen bonding effect and its associated excited-state proton transfer tautomerism, Phys. Chem. Chem. Phys., 2017, 19, 28641—28646. 19
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Table of Content
The intensity of fluoroscence emission can be tuned through solvent polarity via formation of intramolecular or intermolecular hydrogen bonds.
Highlights:
Excited State Proton Transfer emission of the LCs can be tuned through the solvent polarity.
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Intensity of keto emission increases with increase in polarity of the protic solvents.
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Intra-molecular (inter-molecular) hydrogen bonds prevail in aprotic (protic) solvents.
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The intensity of keto- (enol-) emission is stronger in aprotic (protic) solvents.
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