Off-on-off luminescent switching of a dye containing imidazo[4, 5-f][1, 10]phenanthroline

Article abstract of DOI:10.1016/j.cclet.2010.12.005

A new acid-base fluorescent switch containing both imidazo[4,5-f][1,10]- phenanthroline and triphenylamine groups has been synthesized. Its fluorescence emissions and absorptions can be reversibly changed through protonation/ deprotonation of imidazole and amine moiety by controlling the intramolecular charge transfer (ICT) process, leading to off-on-off fluorescent molecular switching.

Full text of DOI:10.1016/j.cclet.2010.12.005

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 647–650
www.elsevier.com/locate/cclet
Off–on–off luminescent switching of a dye containing
imidazo[4, 5-f][1, 10]phenanthroline
b
c
b
Hai Guang Zhang a,b, , Xu Tang Tao , Kao Shan Chen , Chun Xue Yuan ,
*
Shi Na Yan a, Min Hua Jiang b
a College of Chemistry Science, Qufu Normal University, Qufu 273165, China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
c School of Life Science, Shandong University, Jinan 250100, China
Received 10 September 2010
Abstract
A new acid–base fluorescent switch containing both imidazo[4,5-f][1,10]-phenanthroline and triphenylamine groups has been
synthesized. Its fluorescence emissions and absorptions can be reversibly changed through protonation/deprotonation of imidazole
and amine moiety by controlling the intramolecular charge transfer (ICT) process, leading to off–on–off fluorescent molecular
switching.
# 2010 Hai Guang Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Fluorescence switch; Imidazo[4,5-f][1,10]phenanthroline; Triphenylamine; Intramolecular charge transfer (ICT)
The design and synthesis of fluorescent molecules exhibiting a switch function is an active area that has been
intensively studied, which promises tremendous potential applications in the field of sensor fabrication [1–3].
Imidazo[4,5-f][1,10]phenanthroline (abbreviated as IP) derivatives have shown interesting proton induced on–off
emission switching characteristics [4–7]. On the other hand, aromatic amines are typical examples of pH fluorescent
switches [8–10]. In this work, we report the synthesis and study of a new proton induced fluorescent molecular switch
2-(40-di(4-methylphenyl)-aminophenyl)imidazo[4,5-f][1,10]-phenanthroline (1).
1. Experimental
The compound 1,10-phenanthroline-5,6-dione [11] was synthesized according to a method in the literature. The
other reagents were obtained commercially and used as supplied. The compound 1 was prepared according to the
procedures in the literature [12,13] (Scheme 1).
1H NMR and 13C NMR spectra were recorded on Bruker A vance 300 MHz instrument. The electrospray mass
spectrum (ES-MS) was determined on an ABI 4000 mass spectrograph. UV–vis absorption spectra were recorded on a
* Corresponding author at: College of Chemistry Science, Qufu Normal University, Qufu 273165, China.
E-mail address: haiguangzhang2006@126.com (H.G. Zhang).
1001-8417/$ – see front matter # 2010 Hai Guang Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.12.005

6[()TD$FIG] 48
H.G. Zhang et al. / Chinese Chemical Letters 22 (2011) 647–650
Scheme 1. Regents and conditions: (a) H2SO4/HNO3/KBr, reflux, 2 h, 95%; (b) NH4Ac/CH3COOH, reflux, 3 h, 62%.
TU-1800 SPC spectrophotometer. Fluorescence emission spectra were measured using a Hitachi F-4500 fluorescence
spectrophotometer.
A mixture of 1,10-phenanthroline-5,6-dione (5 mmol), ammonium acetate (100 mmol), aldehyde (6 mmol) and
glacial acetic acid (60 mL) was refluxed for about 2 h, then cooled to room temperature. The yellow precipitate was
collected and washed with hot water after the reaction solution being neutralized with concentrated aqueous ammonia.
The crude product was purifies by recrystallization from ethanol to obtain compound 1 as yellow needle crystals
(yield: 62%). 1H NMR (300 MHz, DMSO-d6): d 13.57 (s, 1H), 9.02 (d, 2H), 8.91 (d, 2H), 8.12 (d, 2H), 7.82 (m, 2H),
7.18 (d, 4H), 7.04 (m, 6H), 2.30 (s, 6H). 13C NMR (300 MHz, DMSO-d6): d 151.27, 149.37, 148.15, 144.62, 143.75,
136.22, 133.65, 130.72, 130.05, 127.84, 126.57, 125.57, 123.81, 122.81, 119.73, 20.91. ES-MS, C33H25N5 (M++H) m/
z (%) Calcd.: 492.6, Found: 492.6 (100%).
2. Results and discussion
The variations in UV–vis spectra during the titration of NaOH aqua to the neutral DMSO solutions are shown in
Fig. 1. Specifically, during NaOH titration, the absorptions at 292 nm that can be attributed to local p–p* electron
transition in imidazo[4,5-f][1,10]phenanthroline group progressively decreased until the absorption almost
completely disappeared, simultaneously, a new absorption band around 311 nm gradually formed and developed.
Particularly, the presence of an isosbestic points at 299 nm indicates that only two species coexisted at the
equilibrium. Meanwhile, the absorption band at 366 nm assigned to ICT was strengthened and blue-shifted 5 nm.
Particularly, upon the addition of NaOH, the colors of the solutions of the compounds gradually changed from
colorless to greenish blue. However, the color as well as the absorption spectra can be recovered after adding
appropriate amount of HCl, indicating the occurrence of reversible reactions. On the other hand, during the titration
of the neutral solutions with the HCl solution, no appreciable changes in absorption spectra were observed until 20
[()TD$FIG]
Fig. 1. UV–vis spectra of title compound during the titration of NaOH aqua (c = 5 ꢀ 10ꢁ5 mol/L).

[()TD$FIG]
H.G. Zhang et al. / Chinese Chemical Letters 22 (2011) 647–650
649
Fig. 2. The emission fluorescent spectra of compound 1 during the titration of NaOH (a) or HCl aqua (b) (excitation at 366 nm) (c = 5 ꢀ 10ꢁ5 mol/L).
times mole amount of HCl relative to the titrand has been added, indicating a negligible effect of protonation on the
ground states of 1.
The variations in fluorescent spectra of compound 1 responding to the addition of the NaOH or HCl solutions are
shown in Fig. 2. Specifically, at room temperature, the compound 1 at concentration c = 5 ꢀ 10ꢁ5 mol/L exhibited
strong fluorescence emissions at 500 nm (excitation at 366 nm). However, upon the addition of NaOH, its emissions
decreased significantly while the maximum emission wavelength was red-shifted about 36 nm. The emission spectra
of 1 no longer changed remarkably after more than 3 times mole amount NaOH were added, indicating almost
complete deprotonation of imidazole. The fluorescence quantum yields FF was determined by the method in Ref. [14]
with Cormarin 307 in methanol (FF = 0.57) was adopted as a reference. The FF of 1 was calculated as 0.63, while the
FF of deprotonated 1 was calculated as 0.14, which was only 22% of that of 1, it can be attributed to the quenching of
fluorescence emissions caused by the photo-induced electron transfer (PET) from IP to phenylamine group that was
induced by enhanced electron density in IP group as a result of the deprotonation of imidazolyl group. On the other
hand, as Fig. 2(b) shown, the fluorescent emissions of 1 decreased accompanying the addition of HCl into the neutral
solutions until their emissions have been completely quenched, the shift of the maximum emission wavelength cannot
be observed. When the HCl or NaOH solution was added into the above basic or acid solutions, the fluorescent
[()TD$FIG]
Fig. 3. Electron distributions as well as HOMO and LUMO energy levels of 1, 1-OH (deprotonated) and 1-H (protonated).

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H.G. Zhang et al. / Chinese Chemical Letters 22 (2011) 647–650
emission gradually increased until the initial intensity was completely recovered. In short, the compound 1 behaved as
an excellent off–on–off emission switch via deprotonation/protonation by adding base/acid.
To be able to gain a deeper insight into the ICT process of 1 and its (de)protonated state, the theoretical calculations
have been conducted based on Gauss03 program in which the compounds were perfectly optimized at the B3LYP/6-
31G(d) level. The electron distribution in HOMO and LUMO energy states of 1, 1-OH (deprotonated) and 1-H
(protonated) are shown in Fig. 3. As shown in Fig. 3, the electron distribution in the frontier MOs of 1 revealed a
HOMO–LUMO excitation in which the electron distribution extended from TPA moiety to the IP moiety, exhibiting
strong ICT migration characteristics. Meanwhile, the electron distribution in the frontier MOs of deprotonated 1
revealed a HOMO–LUMO excitation in which the electron migrated from IP to TPA moiety, indicating the occurrence
of an efficient PET process. It was well consistent with the PET quenching observed in the base solutions in the
experiments. For protonated 1, we calculated the molecule that the H+ was connected with the N atom of phenylamine
group but not imidazolyl group because of the more basic of phenylamine group than imidazolyl group. As shown in
Fig. 3, an electron migration from the IP moiety to the phenyl ring of TPA moiety has been observed in protonated 1 by
the electron distribution in HOMO–LUMO excitation. However, the ICT migration from TPA to IP cannot be found
here, which is consistent with the observed fluorescent quenching in the acid solutions during the experiments.
3. Conclusion
To conclude, we designed and synthesized a new dye containing both imidazo[4,5-f][1,10]phenanthroline and
triphenylamine groups. Particularly, an acid/base induced absorption and fluorescent molecular switch has been
demonstrated by modulating the ICT process from phenylamine group to the IP group via protonation/deprotonation.
Meanwhile, the electron distributions of HOMO–LUMO excitation in compound 1 have been obtained via theoretical
calculations, which were well consistent with the results of the fluorescence emission changes observed during the
experiments.
Acknowledgments
The authors greatly acknowledge the financial support of the State National Natural Science Foundation of China
(No. 50325311) and National 863 Project Foundation of China (No. 2007AA10Z334).
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