# The Effect of Tb and Sm Ions on the Photochromic Behavior of Two Spiropyrans of Benzoxazine Series in Solution

Authors: Esam Bakeir; G. M. Attia; Maria Lukyanova; Boris Lukyanov; M. S. A. Abdel-Mottaleb
Journal: Research Letters in Physical Chemistry
 (2008)

---

## Abstract

The photochromism of [7′-hydroxy-8′-formyl-3-methyl-4-oxospiro[1,3-benzoxazin-2,2′-[2H-1]benzopyran],SP(I),[7′-hydroxy-8′-formyl-3-benzyl-4-oxospiro[1,3-benzoxazin-2,2′-[2H-1]benzopyran] SP(II) and their coordination with Tb3+and Sm3+ ions have been studied in DMF. UV/vis induced-color development due to heterolytic bond cleavage of SP(I) and SP(II) is greatly influenced by complexation with
the lanthanide ions. The irradiation-induced color enhancement due to ring opening and thermal decoloration of the open forms of SP(I), SP(II) follows first-order kinetics. Physical characteristics of the studied systems such as colorability and
relaxation time of thermal bleaching parameters were determined. Moreover, light-energy transfer-induced luminescence of lanthanide ions via coordination with the two
spirobenzoxazines was monitored.


---

## Body

## 1. Introduction

Recently, many  profitable applications
of  photochromic  dyes,
particularly spirooxazines, either as passive or active
devices, have been proposed [1–13].In some cases, it has been reported that
thermal equilibrium between the closed (colorless form) and opened form
(colored merocyanine quinonoid form) is affected by the change in solvent
polarity  [14, 15], since polar solvents promote the formation of the
colored form at room temperature in the absence of light. The equilibrium
between both forms is strongly displaced upon irradiation to the side of
open-chain colored photomerocyanine and spontaneously converts to the colorless
spiro form to reach thermal equilibrium immediately after removing the light [15].
The metal-ion
coordination ability of photochromic spirocyclic compounds adequately
substituted is of a great interest and is being the topic of several recent
studies [12, 13]. Search for molecules possessing better performances is valuable
and it is important to continue to explore this subject. Here, we report
on the possibility to stabilize the colored open forms of the recently
synthesized spirobenzoxazines SP(I) and SP(II) toward thermal bleaching by
coordination with Tb3+ and Sm3+ ions in polar DMF solvent.
Moreover, the expected light-energy transfer-induced characteristic
luminescence of lanthanide ions via complexation with the merocyanine quinonoid
open forms of the two spirobenzoxazines will be explored.


## 2. Experimental

### 2.1. Materials

The metal chlorides  (Sigma-Aldrich, 99.99%)
were used  as received. The synthesis of [7′-hydroxy-8′-formyl-3-methyl-4-oxospiro [1,3-benzoxazin-2,2′-[2H-1]benzopyran], SP(I), [7′-hydroxy-8′-formyl-3-benzyl-4-oxospiro
[1,3-benzoxazin-2,2′-[2H-1] benzopyran] SP(II) were described previously
[16, 17].
Spectroscopic pure grade solvents were used.


### 2.2. Instruments

UV-visible
absorption spectra were recorded on a range (200–650 nm) using λ-Helios SP Pye-Unicam spectrophotometer
and/or Ocean Optics US4000 fiber optics
spectrophotometer. Continuous irradiation experiments were performed using a
150 W xenon arc lamp (PTI-LPS-220
Photon Technology International, USA)  operated at 70 W. Photochemical reactions
were carried out in the spectrophotometric quartz cell with a homogeneously
spread light on the cell window to avoid stirring. Measurements were made on
aerated solutions.Fluorescence
spectra were measured in the range (290–750 nm) using Shimadzu RF5301 (PC)
spectrofluorophotometer.


### 2.3. Kinetics Measurements

The
ring-closure reaction after photocoloration was monitored directly after
removal of light at room temperature. First-order rate constants were obtained
from the linear lnA versus time descending curves. By extrapolation of
the obtained ln A/t plots to zero time, the absorbance Ao of the open form or its complexes at t = 0 was related to their
“colorabilities”  [18–21] using the expression (Ao/cSPb), where cSP is the initial concentration of SP
and b is the optical path length.


## 2.1. Materials

The metal chlorides  (Sigma-Aldrich, 99.99%)
were used  as received. The synthesis of [7′-hydroxy-8′-formyl-3-methyl-4-oxospiro [1,3-benzoxazin-2,2′-[2H-1]benzopyran], SP(I), [7′-hydroxy-8′-formyl-3-benzyl-4-oxospiro
[1,3-benzoxazin-2,2′-[2H-1] benzopyran] SP(II) were described previously
[16, 17].
Spectroscopic pure grade solvents were used.


## 2.2. Instruments

UV-visible
absorption spectra were recorded on a range (200–650 nm) using λ-Helios SP Pye-Unicam spectrophotometer
and/or Ocean Optics US4000 fiber optics
spectrophotometer. Continuous irradiation experiments were performed using a
150 W xenon arc lamp (PTI-LPS-220
Photon Technology International, USA)  operated at 70 W. Photochemical reactions
were carried out in the spectrophotometric quartz cell with a homogeneously
spread light on the cell window to avoid stirring. Measurements were made on
aerated solutions.Fluorescence
spectra were measured in the range (290–750 nm) using Shimadzu RF5301 (PC)
spectrofluorophotometer.


## 2.3. Kinetics Measurements

The
ring-closure reaction after photocoloration was monitored directly after
removal of light at room temperature. First-order rate constants were obtained
from the linear lnA versus time descending curves. By extrapolation of
the obtained ln A/t plots to zero time, the absorbance Ao of the open form or its complexes at t = 0 was related to their
“colorabilities”  [18–21] using the expression (Ao/cSPb), where cSP is the initial concentration of SP
and b is the optical path length.


## 3. Results and Discussion

### 3.1. Absorption Spectra

The absorption spectrum of SP(I) and
SP(II) displayed two bands at 276 and 370 nm,
Table1, which are solvent independent
(methanol, ethanol, and DMF solvents were tried). UV light-induced color
development of both compounds was only remarkably observed in DMF solution.Table 1
The experimental values of absorption
maxima of SP and its open form (λSP, λopen), SP ring
opening rate constant
(k1), thermal open form ring closure rate constant (k−1),
equilibrium constant of the reversible reaction (Ke), relaxation
time of open form (τopen-SP), and colorability in DMF at 295 ± 1K. Error limits of the kinetics
parameters are in the order of about 5%.
CompoundλSP(nm)λopen(nm)k−1×103
(min−1)k1×103 (min−1)τopen-SP (min)KeColorability M−1cm−1×     10−2SP(I)2764093.16192316607.36370SP(II)2764047.96529125661.30370Upon
irradiation of 60μM SP(I) solutions, the absorption spectra at 276 nm and 370 nm
decreased and a new band at 409 nm appeared and its intensity increased by
increasing the irradiation time. Addition of lanthanide chloride solution in
DMF showed no change in the absorption band of SP(I) in the dark. However,
irradiation of the (1 : 1 molar ratio) solution of SP(I) in presence of Tb3+ and Sm3+, respectively, led to a new band at 412 nm and 414 nm,
Figure 1. Similar behavior observed for SP(II) in presence of lanthanide ions
(Tb3+, Sm3+) in DMF,
Table 1. Thermal bleaching was
monitored spectrophotometrically. First-order kinetics rate constants of the
reversible close  ⇌  open reactions (see Scheme 2) were determined
graphically (Figure 2, as an example) and data are collected in
Tables 1 and 2.Table 2
The experimental values of
absorption, M(III)-SP formation rate constant(k2), thermal ring
closure rate constant (k−2), equilibrium (Ke) constant of
the reversible reaction, relaxation time τM(III)-open-SP, and colorability in
DMF at 295 ± 1 K. Error
limits of the kinetics parameters are in the order of about 5%.
Complexesλopen-M(III) (nm)k−2×103 (min−1)k2×103 (min−1)τM(III)-open-SP (min)KeColorability M−1cm−110−2Tb(III)-SP(I)4124.046612471637.02Sm(III)-SP(I)4144.55258219567.85Tb(III)-SP(II)4128.16327122407.73Sm(III)-SP(II)4144.77225209477.72Scheme 1Figure 1
UV/vis absorption spectra of SP(I) and its complexes with Tb3+ and Sm3+ ions in DMF before and after light irradiation ([SP(I)] =
[Ln3+] =  60 μM)
(at 295 ± 1K).Figure 2
Effect of time of irradiation on the UV/vis absorption spectra of DMF solution
of a mixture of 60μM Sm3+ and 60 μM SP(II) (inset back ring closure
reaction and the first-order plots of thermal decoloration of SP in dark) after
UV irradiation at  295 ±1K.Scheme 2
Illustration of the reversible structural
transformation of SP to the MC form in response to light in the absence and
presence of Ln3+ metal ion.Color development rate constant(k1) of the
photomerocyanine form and the thermal bleaching rate constant (k−1) are used to estimate the equilibrium constant (Ke=k1/k−1)  of the forward photochemical ring opening reaction and
the backward thermal bleaching one. The results of Ke are
summarized in Table 1 for the SP(I) and SP(II). Data for Ln3+-SP (I
and II) appear in Table 2. The relaxation time of the open form (τopen-SO)
given in Table 1 was obtained from the first-order rate constant using the
expression τ=1/k−1  [18–21]. The
obtained relaxation time of SP and its complexes with lanthanide ions are
relatively high [22], reflecting highly stabilized color form. The colored open
MC form of the methyl-substituted SP(I) is more stable than that of the bulky benzyl derivative reflecting the
influence of the constituent’s nature and size. Moreover, the presence of the
lanthanide ions generally enhances the colorability of both SP derivatives
reflecting the result of coordination with the MC forms.Benzyl group substituent in SP(II) relative  to the
smaller methyl group substituent in SP(I) accelerates both rates of
photocoloration  and
thermal  bleaching as reflected in the higher values of the
rate constants  for SP(II) shown in Table1. A significant decrease in
relaxation time and colorability of the open form (MC quinonoid form) of the
SP(II) was also induced due to the effect of more bulky benzyl group. Generally
speaking, the presence of Ln3+ metal ions induced more bathochromic
shift in the absorption band of the MC form, enhance colorability and
relaxation time. Exception is the case of Tb3+-SP(I) complexes,
where colorability and relaxation time slightly decrease.


### 3.2. Luminescence Spectra of Lanthanide Complexes

The ligand-centered luminescence was not observed in Tb3+ and Sm3+
complexes, whereas the typical characteristic narrow
emission bands of the Tb3+ (5D4→7F6, 7F5, 7F4, and 7F3)
and Sm3+
(4G5/2→6H5/2, 6H7/2, 6H9/2,
and 6H11/2)
(at λex =  360 nm) ions can be detected in polar protic
solvents upon excitation of the SP absorption band (see Figure 3).This
indicates efficient energy transfer from the excited open MC form to the Ln3+ ions having lower energy levels [23].Figure 3
The sensitized luminescence spectra of 60μM Tb3+
in the presence of 60 μM SP(I,II) in DMF at room temperature.


### 3.3. Conclusion

The  substituent nature and the presence of
lanthanide metal ions in solution of recently synthesized photochromic spirobenzopyrans of benzoxazine series induced
significant changes in its photochromic parameters. It could be generally
concluded that the presence of lanthanide ions significantly enhances the rate
constants of both color development of the light-induced formation of the open
merocyanine-like quinonoid species and the rate constant of its thermal
bleaching. In most cases, colorability is enhanced. Moreover, strong
characteristic sensitized luminescence of the lanthanide ions was observed due
to efficient population of its emissive  states via energy transfer from the
open forms of the spirobenzoxazines.


## 3.1. Absorption Spectra

The absorption spectrum of SP(I) and
SP(II) displayed two bands at 276 and 370 nm,
Table1, which are solvent independent
(methanol, ethanol, and DMF solvents were tried). UV light-induced color
development of both compounds was only remarkably observed in DMF solution.Table 1
The experimental values of absorption
maxima of SP and its open form (λSP, λopen), SP ring
opening rate constant
(k1), thermal open form ring closure rate constant (k−1),
equilibrium constant of the reversible reaction (Ke), relaxation
time of open form (τopen-SP), and colorability in DMF at 295 ± 1K. Error limits of the kinetics
parameters are in the order of about 5%.
CompoundλSP(nm)λopen(nm)k−1×103
(min−1)k1×103 (min−1)τopen-SP (min)KeColorability M−1cm−1×     10−2SP(I)2764093.16192316607.36370SP(II)2764047.96529125661.30370Upon
irradiation of 60μM SP(I) solutions, the absorption spectra at 276 nm and 370 nm
decreased and a new band at 409 nm appeared and its intensity increased by
increasing the irradiation time. Addition of lanthanide chloride solution in
DMF showed no change in the absorption band of SP(I) in the dark. However,
irradiation of the (1 : 1 molar ratio) solution of SP(I) in presence of Tb3+ and Sm3+, respectively, led to a new band at 412 nm and 414 nm,
Figure 1. Similar behavior observed for SP(II) in presence of lanthanide ions
(Tb3+, Sm3+) in DMF,
Table 1. Thermal bleaching was
monitored spectrophotometrically. First-order kinetics rate constants of the
reversible close  ⇌  open reactions (see Scheme 2) were determined
graphically (Figure 2, as an example) and data are collected in
Tables 1 and 2.Table 2
The experimental values of
absorption, M(III)-SP formation rate constant(k2), thermal ring
closure rate constant (k−2), equilibrium (Ke) constant of
the reversible reaction, relaxation time τM(III)-open-SP, and colorability in
DMF at 295 ± 1 K. Error
limits of the kinetics parameters are in the order of about 5%.
Complexesλopen-M(III) (nm)k−2×103 (min−1)k2×103 (min−1)τM(III)-open-SP (min)KeColorability M−1cm−110−2Tb(III)-SP(I)4124.046612471637.02Sm(III)-SP(I)4144.55258219567.85Tb(III)-SP(II)4128.16327122407.73Sm(III)-SP(II)4144.77225209477.72Scheme 1Figure 1
UV/vis absorption spectra of SP(I) and its complexes with Tb3+ and Sm3+ ions in DMF before and after light irradiation ([SP(I)] =
[Ln3+] =  60 μM)
(at 295 ± 1K).Figure 2
Effect of time of irradiation on the UV/vis absorption spectra of DMF solution
of a mixture of 60μM Sm3+ and 60 μM SP(II) (inset back ring closure
reaction and the first-order plots of thermal decoloration of SP in dark) after
UV irradiation at  295 ±1K.Scheme 2
Illustration of the reversible structural
transformation of SP to the MC form in response to light in the absence and
presence of Ln3+ metal ion.Color development rate constant(k1) of the
photomerocyanine form and the thermal bleaching rate constant (k−1) are used to estimate the equilibrium constant (Ke=k1/k−1)  of the forward photochemical ring opening reaction and
the backward thermal bleaching one. The results of Ke are
summarized in Table 1 for the SP(I) and SP(II). Data for Ln3+-SP (I
and II) appear in Table 2. The relaxation time of the open form (τopen-SO)
given in Table 1 was obtained from the first-order rate constant using the
expression τ=1/k−1  [18–21]. The
obtained relaxation time of SP and its complexes with lanthanide ions are
relatively high [22], reflecting highly stabilized color form. The colored open
MC form of the methyl-substituted SP(I) is more stable than that of the bulky benzyl derivative reflecting the
influence of the constituent’s nature and size. Moreover, the presence of the
lanthanide ions generally enhances the colorability of both SP derivatives
reflecting the result of coordination with the MC forms.Benzyl group substituent in SP(II) relative  to the
smaller methyl group substituent in SP(I) accelerates both rates of
photocoloration  and
thermal  bleaching as reflected in the higher values of the
rate constants  for SP(II) shown in Table1. A significant decrease in
relaxation time and colorability of the open form (MC quinonoid form) of the
SP(II) was also induced due to the effect of more bulky benzyl group. Generally
speaking, the presence of Ln3+ metal ions induced more bathochromic
shift in the absorption band of the MC form, enhance colorability and
relaxation time. Exception is the case of Tb3+-SP(I) complexes,
where colorability and relaxation time slightly decrease.


## 3.2. Luminescence Spectra of Lanthanide Complexes

The ligand-centered luminescence was not observed in Tb3+ and Sm3+
complexes, whereas the typical characteristic narrow
emission bands of the Tb3+ (5D4→7F6, 7F5, 7F4, and 7F3)
and Sm3+
(4G5/2→6H5/2, 6H7/2, 6H9/2,
and 6H11/2)
(at λex =  360 nm) ions can be detected in polar protic
solvents upon excitation of the SP absorption band (see Figure 3).This
indicates efficient energy transfer from the excited open MC form to the Ln3+ ions having lower energy levels [23].Figure 3
The sensitized luminescence spectra of 60μM Tb3+
in the presence of 60 μM SP(I,II) in DMF at room temperature.


## 3.3. Conclusion

The  substituent nature and the presence of
lanthanide metal ions in solution of recently synthesized photochromic spirobenzopyrans of benzoxazine series induced
significant changes in its photochromic parameters. It could be generally
concluded that the presence of lanthanide ions significantly enhances the rate
constants of both color development of the light-induced formation of the open
merocyanine-like quinonoid species and the rate constant of its thermal
bleaching. In most cases, colorability is enhanced. Moreover, strong
characteristic sensitized luminescence of the lanthanide ions was observed due
to efficient population of its emissive  states via energy transfer from the
open forms of the spirobenzoxazines.



---

*Source: 314898-2008-03-27.xml*