Synthesis and characterization of a new chromophor with acid/base-sensitive fluorescence emission

Article abstract of DOI:10.1016/j.cclet.2011.03.013

A new two-photon absorption compound, 2-{4-[(dicyanomethylidene-5,5- dimethylcyclohexl)vinyl]phenyl}imidazo[4,5-f][1,10]phenanthroline (DDVPIP), was synthesized and characterized. The one-photon excited fluorescence (OPEF) and two-photon excited fluorescence (TPEF) of DDVPIP are sensitive to the acid/base of the solution, which are enhanced in basic solution but weakened in acidic solution. Charge-transfer (CT) states of DDVPIP were calculated through theory methods to explain its acid/base-sensitive fluorescent properties.

Full text of DOI:10.1016/j.cclet.2011.03.013

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 1051–1054
www.elsevier.com/locate/cclet
Synthesis and characterization of a new chromophor with
acid/base-sensitive fluorescence emission
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 13 December 2010
Available online 24 June 2011
Abstract
A new two-photon absorption compound, 2-{4-[(dicyanomethylidene-5,5-dimethylcyclohexl)vinyl]phenyl}imidazo[4,5-
f][1,10]phenanthroline (DDVPIP), was synthesized and characterized. The one-photon excited fluorescence (OPEF) and two-
photon excited fluorescence (TPEF) of DDVPIP are sensitive to the acid/base of the solution, which are enhanced in basic solution
but weakened in acidic solution. Charge-transfer (CT) states of DDVPIP were calculated through theory methods to explain its acid/
base-sensitive fluorescent properties.
# 2011 Hai Guang Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Imidazo[4,5-f][1,10]phenanthroline; Two-photon absorption; Two-photon fluorescence
Organic materials that can simultaneously absorb two photons have been extensively studied due to their increasing
applications in optical power limiting, two-photon upconverted lasing, confocal imaging, photodynamic therapy, and
three-dimensional optical data storage [1,2]. An important addition to those applications would be the development of
two-photon sensors for biological applications [3,4]. Imidazo[4,5-f][1,10]phenanthroline (abbreviated as IP) derivatives
and its metal complex has been demonstrated to be good emitters with interesting proton induced on-off emission
switching characteristics through a reversible acid/base interconversion of imidazole group [5–8]. On the other hand, the
isophorone-baseddopantisanexcellentred fluorescent materialfororganiclight-emittingdevice(OLED)[9,10]. Here, a
compound containing IP and isophorone was designed, which might be expected to be a fluorescent pH sensor.
1. Experimental
The compounds 1,10-phenanthroline-5,6-dione [11], 2-(4-formylphenyl)imidazo[4,5-f][1,10]phenanthroline
(fmp) [12], dicyanomethylidene-isophorone [9] were synthesized according to the literature methods. All other
reagents were obtained commercially and used as supplied (see Scheme 1).
* 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 # 2011 Hai Guang Zhang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.03.013

[(Scheme_1)TD$FIG]
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H.G. Zhang et al. / Chinese Chemical Letters 22 (2011) 1051–1054
Scheme 1. Regents and conditions: (a) H2SO4/HNO3/KBr, reflux, 2 h, 95%; (b) NH4Ac/CH3COOH, reflux, 3 h, 70%; (c) CH3COOH/C5H10N,
DMF/benzene (90% for 2, 32% for DDVPIP).
1H NMR and 13C NMR spectra were recorded on Bruker A Vance 400 MHz instrument. The electrospray mass
spectrum (ES-MS) was determined on an ABI 4000 mass spectrograph. The liner absorption spectra were recorded on
a TU-1800 SPC spectrophotometer. OPEF spectra were obtained on an Edinburgh FLS920 spectrofluorometer. The
fluorescence quantum yields FF was determined by the method in the reference with Cormarin 307 in methanol
(FF = 0.57) was adopted as a reference [13]. TPEF spectra were noted on an OOIBASE32 spectrophotometer. The
pump laser beam came from a mode-locked Ti:sapphire laser system, pulse duration 200 fs, repetition rate of 76 MHz
(Coherent Mira900-D). TPA cross-section (s) was measured using two-photon-induced fluorescence measurement
technique [14] and fluorescein in water with pH 11 was selected as the standard (s = 36 GM) [15].
2-(4-Formylphenyl)imidazo[4,5-f][1,10]phenanthroline (fmp) 3.24 g (0.01 mol) and dicyanomethylidene-iso-
phorone 1.86 g (0.01 mol) were dissolved in 40 mL DMF, and 1 mL hexahydropyridine (C5H10N), 1 mL glacial acetic
acid (CH3COOH) were added to the solution respectively. Stirring was continued for 1 h at room temperature during
which the reaction mixture become to red slowly. Then 100 mL dry benzene was added and the reaction mixture was
heated under reflux for 8 h. After benzene was distilled off, the residual reaction mixture was cooled to room
temperature and 200 mL water was added, many precipitate was formed. The precipitate was collected through
filtration and recrystallized from ethanol to give red crystals of DDVPIP (1.57 g, 0.0032 mol, 32%). Mp: 154 8C. 1H
NMR (400 MHz, TFA): d 9.50 (d, 2H), 9.30 (d, 2H), 8.34 (q, 2H), 8.18 (d, 2H), 7.87 (d, 2H), 7.27 (q, 2H), 7.01 (s, 1H),
2.67 (s, 2H), 2.56 (s, 2H), 1.09 (s, 6H). 13C NMR (100 MHz, TFA): d 25.49, 31.23, 38.05, 42.39, 75.83, 119.21, 124.33,
124.88, 126.61, 127.79, 128.39, 132.68, 134.44, 135.62, 136.34, 142.50, 146.77, 150.55, 155.86, 173.94. ES-MS
(M++H) m/z (%): Calcd. for C32H24N6 493.7, Found 493.7 (100%).
2. Results and discussion
Variations of liner absorption, OPEF and TPEF spectra of DDVPIP due to acid/base changes were investigated in
mixed neutral solvent: THF/H2O, acidic solvent: THF/0.1 mol/L HCl aqueous solution and basic solvent: THF/
0.1 mol/L NaOH aqueous solution. The mixed ratio in three solvents was 9:1 (v/v). The liner absorption, OPEF and
TPEF spectra of DDVPIP in neutral, acidic and basic solvents were shown in Fig. 1.
As shown in Fig. 1(a), in the neutral THF/H2O, there was one strong absorption band with maxima peak located at
433 nm. But in basic solution, the whole absorption spectra became featureless and much broader with the longer
wavelength absorption was increased while the shorter wavelength absorption was reduced. In the acidic solution, the
maxima peak located at 413 nm and the spectral profile was not changed compared with in neutral solution. As shown
in Fig. 1(b), in the neutral THF/H2O, excited by 360 nm wavelength, DDVPIP emission fluorescence with maxima
peak located at 571 nm. The F is 0.27. Compared with neutral solution, the maxima peak of DDVPIP in basic solution
underwent a blue shift and the strength was enhanced, F increased to 0.53. While in acidic solution, the OPEF spectra
were vanished significantly, F decreased to 0.056. As shown in Fig. 1(c), a similar change of TPEF of DDVPIP took
place as that of OPEF in these solvents. In the neutral THF/H2O, excited by 800 nm wavelength, DDVPIP emission
two-photon fluorescence with the maxima peak at 615 nm, which TPA cross-section was obtained as 46 GM.
Compared with neutral solution, the TPEF of DDVPIP in basic solution was intensified and its maxima peak located at

H.G. Zhang et al. / Chinese Chemical Letters 22 (2011) 1051–1054
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[(Fig._1)TD$FIG]
Fig. 1. Liner absorption (a) (c = 1 Â 10À5 mol/L), OPEF (b) (excited by 360 nm wavelength, c = 1 Â 10À5 mol/L) and TPEF (c) (excited by 800 nm
wavelength, c = 1 Â 10À4 mol/L) spectra of DDVPIP in THF (with and without acid or base).
615 nm which was blue-shifted about 70 nm, and TPA cross-section increased to 75 GM. While the TPEF in acid
solution was weakened and TPA cross-section decreased to 6 GM.
From above, we can conclude that liner absorption, OPEF and TPEF of DDVPIP were sensitive to the acid/base
solutions: deprotonation of DDVPIP in basic solutions could induce OPEF and TPEF enhancement while protonation
of DDVPIP in acidic solutions could result in weakened OPEF and TPEF. This might be explained by an increase or
decrease of p-donor property of the N atom in deprotonated or protonated imidazole. After deprotonation of the
imidazole induced by NaOH, one negative charge was formed on the N atom which could easily delocalize to other
moiety in the molecular. This indicated that the p-donor property of the N atom in the imidazole increased, and the p
electrons on N atom of imidazole could easily transit into the molecule orbitals, which was necessary for strong OPEF
and TPEF. But in the protonated DDVPIP, the p electrons on N atom connected with one H+ will be difficult to transit
into the molecule orbitals, so p-donor property of the N atom in the imidazole decreased and the OPEF and TPEF are
both weakened. On the other hand, we proposed that the electron withdrawal property of dicyano moiety in the
molecular is crucial for the OPEF and TPEF enhancement and blue-shift of deprotonated DDVPIP, and an
intramolecular charge transfer (ICT) from deprotonated IP to dicyano moiety along with phenyl and three vinyl groups
contributes the OPEF and TPEF of deprotonated DDVPIP.
Charge-transfer (CT) states play a key role for both one and two-photon absorption and emission behaviors. To gain
an insight into the CT process of DDVPIP, the theoretical calculations were conducted based on Gauss03 program in
which the structures of the compounds were perfectly optimized at the B3LYP/6-31G(d) level. But in acidic solutions,
the H+ atom could connect with the N atom of imidazole, dicyano or phenanthroline moieties. Thus, we could not
identify which form of the molecule of protonated DDVPIP could be calculated. So we calculated only DDVPIP and
deprotonated DDVPIP.
[(Fig._2)TD$FIG]
Fig. 2. Electron density distribution of frontier molecular orbitals of DDVPIP (marked 1) and deprotonated DDVPIP (marked 2).

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H.G. Zhang et al. / Chinese Chemical Letters 22 (2011) 1051–1054
Fig. 2 illustrates their electron density distribution of the frontier molecular orbital. In the HOMO of DDVPIP, the
electrons were mainly concentrated on the imidazo, phenanthroline and phenyl moieties, while in the LUMO of
DDVPIP, the electrons spread over the molecule along the molecular axis to the three vinyl and dicyano moieties. In
the deprotonated DDVPIP, the imidazole changed from one weak donor to one stronger donor group, the delocalizable
electrons of imidazole were increased. Upon excitation, the electrons were mainly transferred from imidazole,
phenanthroline and benzyl to the three vinyl and dicyano moieties. The calculated electron levels of DDVPIP were
À0.209 eV (HOMO), À0.111 eV (LUMO), and deprotonated DDVPIP were À0.085 eV (HOMO), À0.024 eV
(LUMO). Obviously, the electron transition from the HOMO to the LUMO was accompanied by CT along the
molecules.
3. Conclusion
In conclusion, a new TPA compound, DDVPIP, was designed and synthesized. The liner absorption, OPEF and
TPEF of DDVPIP in the neutral, acidic and basic THF/H2O were investigated and they are all sensitive to the acid/base
of the solution. Meanwhile, theoretical calculations revealed CT along the molecules upon excitation to the excited
state in DDVPIP and deprotonated DDVPIP.
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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