# Photoinduced Change of UV-Absorption and Redox Potential of Alkyne-Bridged Diferrocenyl Azobenzene Derivative

Authors: Limin Han; Jianchen Bai; Quanling Suo; Meihua Luo; Lifeng Zhang
Journal: Research Letters in Inorganic Chemistry
 (2008)

---

## Abstract

An alkynyl-coupled diferrocenylpropane and azobenzene derivative has been synthesized. Photoinduced change of UV-absorbance and shifts of redox potential of synthesized compound were elucidated by time-dependent density function theory (TD-DFT) calculations, indicating that isomerization of trans to cis configuration occurs by UV irradiation.


---

## Body

## 1. Introduction

Since the
structures and properties of intelligent azo molecules are
facilely and reversibly changeable by the application of external physical and
chemical stimuli, they have attracted much attention relative to the development
of new molecule-based devices [1]. One category of such molecules
comprises transition metal complexes with conjugated spacers to azo unit, in
which the combination of flexibility of metal d- and azo π-orbitals
can yield unique optical, magnetic, and electronic properties [2–4]. For example, when the ferrocene is covalently bound to an azobenzene, the isomerization of azo functional group may lead to change
of intrinsic redox potential of ferrocene [5]. Inversely, isomerization behavior of azo-structure can also be
influenced by the connected metal complex [6].Though attention has been focused on photoisomerization
of metal complexes-azobenzene systems, there are no reports on the alkynyl-bridged diferrocene and azo system, especially, for the interpretation of absorbency change of UV-spectrum
and electrochemical redox
potential shift in this system. In this paper, we synthesized an alkynyl-bridged diferrocenyl azobenzene derivatives by using documental methods [7–10], and investigated the
photoinduced azo isomerization and redox potential
shift of ferrocene.


## 2. Experimental

### 2.1. Material Preparation

Diferrocenylpropane and diferrocenypropanyl acetylene
(DFA) were prepared by
previous methods [11], the Diferrocenylpropanyl
acetylene azophenol (DFAA) was synthesized by the Sonogashira coupling reaction
of DFA with iodic azophenol (see Scheme 1). Under N2 atmosphere DFA (165 mg, 0.51 mmol) and iodic azophenol
(162 mg, 0.5 mmol) were stirred in diisopropylamine (20 mL) with Pd(PPh3)2Cl2 (6.4 mg, 0.009 mmol) and CuI (5.3 mg, 0.0275 mmol) for 20 hours under reflux.
The solvent of the resulting orange mixture was removed in vacuum. The residue
was dissolved in a minimal amount of CH2Cl2. The filtrate
was concentrated and subjected to chromatographic separation on a neutral
alumina column. Elution with hexane-dichloromethane
(4:1, v/v) afforded a yellow band and
red band, the red band product was obtained by recrystallizing from hexane-dichloromethane at −20C°. Yield, 55.1%. Anal. Calc: C, 70.28; H, 5.10; N,
4.14. Found: C, 69.79; H, 5.42; N, 4.43%. IR(KBr): 3088 [Cp vC–H]; 2197[C≡C, vC≡C]; 1593, 1495, [C6H4, vC=C]; 1424[–N=N–, vN=N]; 1265[–OH δO–H]; 1101, 1029[Cp γC–H]; 820[Cp δC–H]cm−1. H1 NMR 10.3 (m, 1H, –OH);
6.9–7.8 (m, 8H, –C6H4–N=N–C6H4–);
4.0–4.5 (m, 9H, –Cp)⋅1.5‐1.6 (m, 6H, –C(CH3)2) ppm. 13C NMR(CDCl3,δ): 161.2, 150.9, 145.3, 132.0, 124.9, 122.5,
116.0 (–C6H4–N=N–C6H4–); 91.4,
85.3(C≡C); 71.2, 69.8, 69.2, 63.9 (Cp) ppm. MS(EI) 632(M+).Scheme 1
Synthesis procedure of DFAA.


### 2.2. Characterization

IR
spectra were recorded on a Nicolet FTIR spectrometer as KBr discs. Elemental
analysis was carried out on an Elementar var III-type analyzer.H1
and C13 NMR spectra in CDCl3 were recorded on an Inova 500 FT-MHz spectrometer. The mass spectra were
determined using a Polaris QMS and a Micromass Autospec Ultima-TOF instrument.
The electrochemical properties were determined with a CHI760C type analyzer. UV spectra were recorded on a Shimadzu
UV-3150 spectrometer.The quantum calculations were performed using
Gaussian03 program [12]. The
geometries of DFAA and the frequencies were evaluated using
the DFT level of the three-parameter compound functional of Becke (B3LYP) [13]. The 6-31G(d) basis set was used for all atoms. The geometry
structures of neutral molecules were optimized under no constraint.


## 2.1. Material Preparation

Diferrocenylpropane and diferrocenypropanyl acetylene
(DFA) were prepared by
previous methods [11], the Diferrocenylpropanyl
acetylene azophenol (DFAA) was synthesized by the Sonogashira coupling reaction
of DFA with iodic azophenol (see Scheme 1). Under N2 atmosphere DFA (165 mg, 0.51 mmol) and iodic azophenol
(162 mg, 0.5 mmol) were stirred in diisopropylamine (20 mL) with Pd(PPh3)2Cl2 (6.4 mg, 0.009 mmol) and CuI (5.3 mg, 0.0275 mmol) for 20 hours under reflux.
The solvent of the resulting orange mixture was removed in vacuum. The residue
was dissolved in a minimal amount of CH2Cl2. The filtrate
was concentrated and subjected to chromatographic separation on a neutral
alumina column. Elution with hexane-dichloromethane
(4:1, v/v) afforded a yellow band and
red band, the red band product was obtained by recrystallizing from hexane-dichloromethane at −20C°. Yield, 55.1%. Anal. Calc: C, 70.28; H, 5.10; N,
4.14. Found: C, 69.79; H, 5.42; N, 4.43%. IR(KBr): 3088 [Cp vC–H]; 2197[C≡C, vC≡C]; 1593, 1495, [C6H4, vC=C]; 1424[–N=N–, vN=N]; 1265[–OH δO–H]; 1101, 1029[Cp γC–H]; 820[Cp δC–H]cm−1. H1 NMR 10.3 (m, 1H, –OH);
6.9–7.8 (m, 8H, –C6H4–N=N–C6H4–);
4.0–4.5 (m, 9H, –Cp)⋅1.5‐1.6 (m, 6H, –C(CH3)2) ppm. 13C NMR(CDCl3,δ): 161.2, 150.9, 145.3, 132.0, 124.9, 122.5,
116.0 (–C6H4–N=N–C6H4–); 91.4,
85.3(C≡C); 71.2, 69.8, 69.2, 63.9 (Cp) ppm. MS(EI) 632(M+).Scheme 1
Synthesis procedure of DFAA.


## 2.2. Characterization

IR
spectra were recorded on a Nicolet FTIR spectrometer as KBr discs. Elemental
analysis was carried out on an Elementar var III-type analyzer.H1
and C13 NMR spectra in CDCl3 were recorded on an Inova 500 FT-MHz spectrometer. The mass spectra were
determined using a Polaris QMS and a Micromass Autospec Ultima-TOF instrument.
The electrochemical properties were determined with a CHI760C type analyzer. UV spectra were recorded on a Shimadzu
UV-3150 spectrometer.The quantum calculations were performed using
Gaussian03 program [12]. The
geometries of DFAA and the frequencies were evaluated using
the DFT level of the three-parameter compound functional of Becke (B3LYP) [13]. The 6-31G(d) basis set was used for all atoms. The geometry
structures of neutral molecules were optimized under no constraint.


## 3. Results and Discussions

The synthesized DFAA was characterized using FT-IR,
NMR, MS, and elemental analysis data. However, because there was an azo group
in the molecule, the product may be a mixture of trans and cis configuration,
it was very difficult to obtain a single crystal of DFAA.


### 3.1. UV Absorption Change

The UV-vis spectra
of DFAA in tetrahydrofuran (THF) showed an intense azoπ‐π* band at λmax=364nm and a weak visible band at 450 nm, the latter was
primarily ascribable to a transition from the  e1g of ferrocene to the π* of azo group [14]. The irradiation of DFAA solution with 365 nm UV light
led to a decrease in the absorbance of azo π‐π* band, which
indicated the trans-to-cis isomerization of DFAA (Figure 1), and the
photostationary state (PSS) of DFAA at a nonradiation condition should be
transconfiguration, or transform was a main configuration in mixture of trans
and cis product.Figure 1
UV-vis absorption spectral change of DFAA (2 mM) in THF solution upon irradiation with UV light at 365 nm for (a) 30 minutes, (b) 50 minutes, (c) 70 minutes, (d) 90 minutes, (e) 100 minutes.


### 3.2. Redox Potential Shift

The cyclic voltammogram of DFAA in THF solution containing a supporting electrolyte ofBu4NClO4 in a sealed thin quartz cell shows that there are two reversible 1e− oxidation waves at E = 0.24 and 0.51 V versus Ag/Ag+ reference electrode (Figure 2(a)). E = 0.24 V was ascribed to the oxidation of ferrocene unit at position B, and 0.51 V was the oxidation of ferrocene unit at position A because the
withdrawing electron azo group and alkynyl were connected to it [15] (see Scheme 1). The potential difference between
ferrocene unit A and B is 270 mV, which is similar to that of diferrocenyl
propane derivatives [16]. After the cell was irradiated with UV
at 365 nm for 1.5 hours, we repeated the cyclic voltammetric measurement,
oxidation potential at 0.51V of ferrocene unit A negatively shifted to 0.48 V
(Figure 2(b)), and there was no clear potential shift of ferrocenyl unit B in
curve a and b.Figure 2
Cyclic voltammogram
of DFAA in solution ofCH2Cl2–Bu4NClO4 (0.1 M)
(a) before irradiation and (b) after irradiation.


### 3.3. Calculations

In order to understand the change of absorbency and
oxidation potential, the geometry of trans-DFAA was optimized by three
parameterized Becke-Lee-Yang-Parr (B3LYP) hybrid
exchange-correlation function [17]. In transoptimized structure (see
Figure 3(b)), the azo and phenyl groups are
located nearly on a
plane in optimized molecular structure, the dihedral angle of Ph–N=N–Ph was
near to 0°, the N=N bond length was 1.282Å, and others structural data were consistent with documented reported [18]. Comparing to trans structural datas,
cis-DFAA was also optimized by B3LYP/DFT (see Figure 3(a)), the
resulted N=N bond length was 1.265Å, shorter than that of trans, and
Ph–N=N–Ph dihedral angle of cis-DFAA was 12°, similar to
other cis-azobenezene
derivatives [19, 20]. Because two phenyls lose their coplanarity in the cis configuration, the π electrons prefer to localize in N=N bond compared to the large conjugated system of trans configuration.
The bond length was shortened and electron withdrawing function of N=N was
reduced. Therefore, in the optimized structure of trans DFAA, the Mulliken charge numbers of Fe(II) in ferrocene units A and B (see Scheme
1) were 0.6210 and 0.6005, respectively, and those of cis were 0.6110 and
0.6003. For the ferrocene unit in position A, charge difference was 0.0100
between trans and cis configuration, with no apparent difference exhibited
between trans and cis configuration in position B. This phenomenon may give us
a possible reason for the oxidation potential of ferrocene A being
slightly shifted
negatively after irradiated by 365 nm light.Optimized molecular structure of cis and trans DFAA.
(a)(b)Calculated
excitation energies in the trans and cis forms were in reasonable agreement
with the experimental values. For the trans DFAA, molecular orbitals contributing to the main UV
absorption at 364 nm was the excitation of electron from orbital 160 to 165, not
the excitation from the HOMO (164) to LUMO (165), and the oscillator strengths(ftrans) contributed to
this excitation was 0.8396. For the cis DFAA, molecular orbitals contributing
to the main UV absorption at 364 nm was the excitation of electron from orbitals
of 160, 159, 158 to 165, not from HOMO (164) to LUMO (165). The oscillator
strengths (fcis) contributed to excitation was 0.4072. Comparing the calculated results, the
excitation energy of trans and cis form was 3.4193 and 3.4173 eV, no apparent difference
existed. However, the difference of oscillator strengths (f) was obvious, Δf value
between the trans and cis was 0.4324, this helped us to understand why the
absorbance of trans-DFAA decreased with the irradiation of UV light at 365 nm
and to identify the isomerization of trans to cis DFAA. MLCT (from
Fe d orbital to azo π orbital) band of DFAA in experimental and calculated
results was not very obvious in the calculations of trans configuration;
excitation at 462 nm with oscillator strengths (f) 0.2698 can be attributed to MLCT, but we did obtained the
similar data in the calculations of cis form. The reason might be derived from the
optimized structure of cis form, in which
the ferrocene unit lost
the conjugated plane, and the avenue of electron transfer from Fe d orbital to azo π orbital is interdicted partially.


## 3.1. UV Absorption Change

The UV-vis spectra
of DFAA in tetrahydrofuran (THF) showed an intense azoπ‐π* band at λmax=364nm and a weak visible band at 450 nm, the latter was
primarily ascribable to a transition from the  e1g of ferrocene to the π* of azo group [14]. The irradiation of DFAA solution with 365 nm UV light
led to a decrease in the absorbance of azo π‐π* band, which
indicated the trans-to-cis isomerization of DFAA (Figure 1), and the
photostationary state (PSS) of DFAA at a nonradiation condition should be
transconfiguration, or transform was a main configuration in mixture of trans
and cis product.Figure 1
UV-vis absorption spectral change of DFAA (2 mM) in THF solution upon irradiation with UV light at 365 nm for (a) 30 minutes, (b) 50 minutes, (c) 70 minutes, (d) 90 minutes, (e) 100 minutes.


## 3.2. Redox Potential Shift

The cyclic voltammogram of DFAA in THF solution containing a supporting electrolyte ofBu4NClO4 in a sealed thin quartz cell shows that there are two reversible 1e− oxidation waves at E = 0.24 and 0.51 V versus Ag/Ag+ reference electrode (Figure 2(a)). E = 0.24 V was ascribed to the oxidation of ferrocene unit at position B, and 0.51 V was the oxidation of ferrocene unit at position A because the
withdrawing electron azo group and alkynyl were connected to it [15] (see Scheme 1). The potential difference between
ferrocene unit A and B is 270 mV, which is similar to that of diferrocenyl
propane derivatives [16]. After the cell was irradiated with UV
at 365 nm for 1.5 hours, we repeated the cyclic voltammetric measurement,
oxidation potential at 0.51V of ferrocene unit A negatively shifted to 0.48 V
(Figure 2(b)), and there was no clear potential shift of ferrocenyl unit B in
curve a and b.Figure 2
Cyclic voltammogram
of DFAA in solution ofCH2Cl2–Bu4NClO4 (0.1 M)
(a) before irradiation and (b) after irradiation.


## 3.3. Calculations

In order to understand the change of absorbency and
oxidation potential, the geometry of trans-DFAA was optimized by three
parameterized Becke-Lee-Yang-Parr (B3LYP) hybrid
exchange-correlation function [17]. In transoptimized structure (see
Figure 3(b)), the azo and phenyl groups are
located nearly on a
plane in optimized molecular structure, the dihedral angle of Ph–N=N–Ph was
near to 0°, the N=N bond length was 1.282Å, and others structural data were consistent with documented reported [18]. Comparing to trans structural datas,
cis-DFAA was also optimized by B3LYP/DFT (see Figure 3(a)), the
resulted N=N bond length was 1.265Å, shorter than that of trans, and
Ph–N=N–Ph dihedral angle of cis-DFAA was 12°, similar to
other cis-azobenezene
derivatives [19, 20]. Because two phenyls lose their coplanarity in the cis configuration, the π electrons prefer to localize in N=N bond compared to the large conjugated system of trans configuration.
The bond length was shortened and electron withdrawing function of N=N was
reduced. Therefore, in the optimized structure of trans DFAA, the Mulliken charge numbers of Fe(II) in ferrocene units A and B (see Scheme
1) were 0.6210 and 0.6005, respectively, and those of cis were 0.6110 and
0.6003. For the ferrocene unit in position A, charge difference was 0.0100
between trans and cis configuration, with no apparent difference exhibited
between trans and cis configuration in position B. This phenomenon may give us
a possible reason for the oxidation potential of ferrocene A being
slightly shifted
negatively after irradiated by 365 nm light.Optimized molecular structure of cis and trans DFAA.
(a)(b)Calculated
excitation energies in the trans and cis forms were in reasonable agreement
with the experimental values. For the trans DFAA, molecular orbitals contributing to the main UV
absorption at 364 nm was the excitation of electron from orbital 160 to 165, not
the excitation from the HOMO (164) to LUMO (165), and the oscillator strengths(ftrans) contributed to
this excitation was 0.8396. For the cis DFAA, molecular orbitals contributing
to the main UV absorption at 364 nm was the excitation of electron from orbitals
of 160, 159, 158 to 165, not from HOMO (164) to LUMO (165). The oscillator
strengths (fcis) contributed to excitation was 0.4072. Comparing the calculated results, the
excitation energy of trans and cis form was 3.4193 and 3.4173 eV, no apparent difference
existed. However, the difference of oscillator strengths (f) was obvious, Δf value
between the trans and cis was 0.4324, this helped us to understand why the
absorbance of trans-DFAA decreased with the irradiation of UV light at 365 nm
and to identify the isomerization of trans to cis DFAA. MLCT (from
Fe d orbital to azo π orbital) band of DFAA in experimental and calculated
results was not very obvious in the calculations of trans configuration;
excitation at 462 nm with oscillator strengths (f) 0.2698 can be attributed to MLCT, but we did obtained the
similar data in the calculations of cis form. The reason might be derived from the
optimized structure of cis form, in which
the ferrocene unit lost
the conjugated plane, and the avenue of electron transfer from Fe d orbital to azo π orbital is interdicted partially.


## 4. Conclusions

In summary, the DFAA was synthesized by Sonogashira coupling
reaction, and the phemomena of absorbance change and redox potential shift of
DFAA were interpreted in terms of the optimization of structures and the
singlet excitation calculations of DFAA, which indicated that the synthsized DFAA
was mainly in trans configuration, but it could isomerize to a cis form by the irradiation of UV light with wavelength of
365 nm.



---

*Source: 906851-2008-10-09.xml*