π-Conjugated polymer-Eu3+ complexes: Versatile luminescent molecular probes for temperature sensing

DOI: 10.1039/c2ta00472k

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Journal of Materials Chemistry A p. 2256 - 2266 (2013)

Update date:2022-07-31

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Authors:

Balamurugan

Reddy Reddy

Jayakannan

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Article abstract of DOI:10.1039/c2ta00472k

We report π-conjugated polymer-Eu³⁺ ion complexes as new potential luminescent thermo-sensitive molecular probes. Carboxylic acid functionalized segmented π-conjugated polymers having oligophenylenevinylene (OPV) chromophores in the poly(ethyleneoxide) or polymethylene backbones were custom designed, synthesized and utilized as efficient photosensitizers for Eu³⁺ ions. These π-conjugated polymer-Eu³⁺ ion complexes were found to be thermo-sensitive and behaved as reversible 'turn-on' or 'turn-off' luminescent switches in solution and in solid state. Luminescent decay studies revealed that the red-emission from the Eu³⁺ ion excited state was highly sensitive to temperature which drove the functioning of optical switches. The decay rate constants followed a typical Arrhenius trend over a wide temperature range having similar activation energies. Both the nature as well as length of the segmented polymer chain that tied the OPV optical chromophores in the backbone determine the temperature range of the luminescent on-off process. The emission characteristics of the oligomer-Eu ³⁺ ion complex were found to be non-thermosensitive which emphasized the need for the segmented π-conjugated polymer ligand structure for the probes based on Eu³⁺ ion complexes. The present strategy opens up new concept and molecular design principles for π-conjugated polymer-lanthanide ion complexes as potential candidates for temperature sensitive luminescent molecular probes. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ta00472k

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Revised Manuscript Submitted to J. Mater. Chem.

(MS. ID. TA-ART-09-2012-000472)

π-Conjugated Polymer-Eu³⁺Complexes: A Versatile Luminescent

Molecular Probe for Temperature Sensing

A.Balamurugan^[a,b], M. L. P. Reddy^[b]and M. Jayakannan*^[a]1

^[a]Department of Chemistry, Indian Institute of Science Education and Research (IISER), Dr.

Homi Bhabha Road, Pune 411008, INDIA

^[b]Chemical Sciences & Technology Division, National Institute for Interdisciplinary Science

and Technology, Thiruvananthapuram-695019, Kerala, INDIA

Table of Contents

Polymer-Eu³⁺Thermosesntive Probe: New carboxylic functionalized π-conjugated

polymer-Eu³⁺ion complexes were designed for turn “On-Off’ thermosensitive luminescent

switches in solution and solid state

¹Corresponding author: jayakannan@iiserpune.ac.in ; Fax: +91-20-2590 8186.

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Abstract: We report π-conjugated polymer-Eu³⁺ion complexes as new potential

luminescent thermo-sensitive molecular probes. Carboxylic functionalized segmented π-

conjugated polymers having oligophenylenevinylene (OPV) chromophores in the

poly(ethyleneoxide) or polymethylene backbones were custom designed, synthesized and

utilized as efficient photosensitizer for Eu³⁺ion. These π-conjugated polymer-Eu³⁺ion

complexes were found to be thermo-sensitive and behaved as reversible ‘turn-On’ or ‘turn-

Off’ luminescent switches in solution and in solid state. Luminescent decay studies revealed

that the red-emission from the Eu³⁺ion excited state was highly sensitive to temperature

which drove the functioning of optical switches. The decay rate constants followed typical

Arrhenius trend over wide temperature range having similar activation energies. Both the

nature as well as length of the segmented polymer chain that tied the OPV optical

chromophores in the backbone determines the temperature range of the luminescent on-off

process. The emission characteristics of the oligomer-Eu³⁺ion complex was found to be non

thermosensitive which emphasized the need for the segmented π-conjugated polymer ligand

structure for the probes based on Eu³⁺ion complexes. The present strategy opens up new

concept and molecular design principles for π-conjugated polymer-lanthanide ion complexes

as potential candidates for temperature sensitive luminescent molecular probes.

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Introduction

Thermo-sensitive polymers are emerging as important classes of sensing materials for

accurate measurement of temperature in material science and in biological processes.¹These

classes of materials are typically made by blending or anchoring thermo-sensitive dye

molecules such as benzofuran, BODIPY, benzoxazole and pyridinyl on polyacrylic or

polystyrene backbone and the temperature dependent structural changes in the dyes were read

as sensing out-put signals.²Poly(N-isopropylacryamide) is another example for temperature-

sensitive polymer based on lower critical solution temperature (LCST) as a probe by the

selective precipitation of polymer chains in the heating/coolong cycles.³π-Conjugated

polymers (or oligomers) based on diacetylene linkages were also reported as thermo-

responsive probes due to their ability to change optical properties from blue to red depending

upon the temperature, pH and mechanical stress and so on.⁴Both the phase separations of dye

molecules from the polymer support and broad spectral width (absorbance or fluorescence,

larger half-width at maxima) of these dyes or π-conjugated polymer chains were found to be

some of the inherent limitations in these materials for the sharp detection of temperature

changes.^4h,4iLanthanide metal ion complexes are important classes of inorganic luminescent

materials as they possessed unique optical properties such as strong and sharp emissions,

large stoke shifts, long lived excited states and so on.⁵Typically, small organic or

macromolecular ligands are coordinated with lanthanide ions and the photoemission are

occurred via excitation energy transfer from ligand to metal core.⁶The photophysical

characteristic and energy transfer pathways are completely different in lanthanide metal

complexes compared to other dye based systems like BODIPY (or poly(N-

isopropylacryamide)). Recent studies revealed that the excited state energy levels of Ln³⁺

ions were highly sensitive to the temperature and this property was exploited for temperature

sensing applications in Tb³⁺and Eu³⁺complexes.⁷However, the disassociation of metal-

ligand interaction of small molecular ligand- Ln³⁺complexes at the high temperature

restricted their thermal reversibility.⁸We have recently reported functionalized π-conjugated

polymers such as poly(phenylenevinylene) and poly(m-phenylene) as photosensitizers for

Tb³⁺and Eu³⁺ions to produce white light generation, selective red light emission and so on.⁹

The utilization of π-conjugated polymer backbone as photosensitizer ligand provided many

advantages including mechanical stability and film forming ability for polymer-lanthanide

metal ion complexes which are completely lacking in the small molecular ligand based

approaches.¹⁰Hybrid blends of non-luminescent polymer-lanthanide complexes were also

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reported and here the polymers typically employed to enhance the processability of the

lanthanides.¹¹Rybaltovskii et al. reported the doping of Eu(fod)₃molecules (fod =

6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadione) with polymers such as polypropylene

and oligo(urethane methacrylate) by soaking in a supercritical CO₂solution.¹¹ⁱThese blends

were tested for temperature sensor, however, the polymer chains did not participate in the

energy transfer mechanism and they were just used as mechanical support. Up to our

knowledge, there is no report known till date for utilizing the π-conjugated polymer-

lanthanide ion complexes as temperature-sensitive sources in the literature. This is partially

associated with the non-availability of suitable π-conjugated polymer with appropriate energy

levels for photoexcitation to lanthanide ions; as a result, both the materials development and

the thermo-sensitive mechanistic pathways were understood only at the premature level.

Hence, the development of new thermo-sensitive π-conjugated polymer-lanthanide ion

complexes are one of the very important research activity for both fundamental understanding

as well as developing new materials for innovative temperature sensing applications in the

polymer-inorganic chemistry interface.

Figure 1. Approach for thermo-sensitive

their turn-on and turn-off switching pathways

-conjugated polymer-Eu³⁺ion complexes and

Herein, we report a unique carboxylic acid functionalized π-conjugated polymer- Eu³⁺

ion complexes as thermo-sensitive luminescent molecular probes in both solution and in solid

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state (see figure 1). Blue light emitting oligophenylenevinylene (OPV) unit was chosen as

optical chromophore and new series of main chain segmented polymers containing

polymethylene or polyethylene oxides were prepared. These polymers were named as

“segmented polymers” because the polymer chains were segmented by OPV units at the

regular interval. The newly designed segmented OPV polymers were found to be efficient

photosensitizers for Eu³⁺ions to produce strong red emission from the metal center. The

polymer-Eu³⁺complexes were also found to show sharp changes in their emission

characteristics with respect to temperature changes in solution and in thin film. Depending on

the structure of the segmented polymer backbone, their temperature sensing ability ‘turn-on’

or ‘turn-off’^9cvaried from 20 to 100 °C (see figure 1). Photophysical experiments based on

absorbance, emission, and luminescent decay time measurements were carried out to trace the

mechanistic aspects of the luminescent pathways. The decay process was further investigated

using Arrhenius equation which suggested that the kinetic rate constants of the polymer

complexes followed typical linear relationship over a wide range of temperature in which the

probes were active. The nature as well as length of the segmented chain (hydrophilic or

hydrophobic) was found to be very crucial factor in determining their temperature sensing

ability. Thus, the present approach opens up new molecular design for π-conjugated polymer-

Eu³⁺ions complexes for temperature sensors application based on luminescent molecular

probes.

Results and Discussion

The carboxylic acid functionalized π-conjugated segmented polymers were

synthesized by A-A + B-B polycondensation approach via Wittig-Horner route as shown in

scheme-1. 4-Hydroxyphenylpropionc acid was converted into its methyl ester by acid

catalysed esterification to get methyl 3-(4-hydroxyphenyl)propanoate (1). The compound 1

was substituted with 2-ethylhexylbromide in the presence of K₂CO₃to give methyl 3-(4-((2-

ethylhexyl)oxy)phenyl)propanoate (2). It was treated with HBr/acetic acid in presence of p-

formaldehyde to yield the 3-(3,5-bis(bromomethyl)-4-(2-ethylhexyloxy)phenyl)propanoic

acid (3). The compound 3 was converted into its corresponding bis-phoponate ester (4) by

reacting with triethyl phosphite at 120° C. Oligo-ethyleneoxy substituted bis-benzaldehyde

derivatives were synthesized by two step processes: (i) triethyleneglycol, hexaethyleneglycol

and polyethylene glycol (PEG-400) were reacted with p-toluenesulfonylchloride in the

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presence of sodium hydroxide to get their corresponding tosylates (5a-5c) and (ii) they were

further reacted with 4-hydroxybenzaldehyde in acetonitrile in the presence of K₂CO₃to yield

4,4'-(((ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl))bis(oxy))dibenzaldehyde (6a-6c). The

reaction of 1, 12-dibromododecane with 4-hydroxybenzaldehyde produced the alkylated bis-

benzaldehyde derivative (7). The segmented polymers were synthesised by reacting

equimolar amounts of dialdehyde (6 and 7) with bisphosphonate ester (4) in the presence of

potassium tertiarybutoxide as base in dry-THF. The polymerization reaction produced two

kinds of carboxylic functionalized oligophenyelenvinylene (OPV) containing segmented

polymers with either polyethyleneoxy chain or polymethylene chain segmented backbones.

These polymers were referred as Poly(OPV-X): Poly(OPV-TEG), Poly(OPV-HEG),

Poly(OPV-PEG400) and Poly(OPV-DD), where x represent the nature of the segments

TEG, HEG, PEG-400 and 1,12-dibromododecane, respectively. Similarly, structurally

identical model compound OPV-TEG was also synthesized by reacting bisphosphonate ester

4 with two equivalents benzaldehyde derivative (8) having triethyleneglycol monomethyl as

terminal units (see scheme SS1).

Scheme-1. Synthesis of Segmented

π-conjugated polymer having OPV units

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The structures of the monomers, oligomer and polymers were characterized by H-

NMR, ¹³C-NMR, FT-IR, and MALDI-TOF analysis (see supporting information). The

molecular weights of the polymers were determined by gel permeation chromatography

(GPC) in tetrahydrofuran using polystyrene as standard (GPC chromatograms are provided in

figure SF1). The molecular weights M_n, M_wand polydispersity are given in table ST-1 in the

supporting information. The molecular weights of the polymers were obtained in the range of

M_w= 18,000 – 37,000 with polydispersity of 1.9 – 3.0.¹³Thermogravimetric analysis showed

that the polymers were thermally stable up to 250 °C (see table ST-1 for values, TGA plots

are shown in figure-SF2). DSC analysis of the polymers did not show any melting or

crystallization peaks in the heating /cooling cycles indicating that they were highly

amorphous in nature (see figure SF3 and ST-1). With increase in the PEG chain length, the

glass transition temperature (T_g) of the segmented polymers decreased from 70 ºC to 8 ºC.

This confirmed that rigidity of the polymeric backbone decreased with increase in the PEG

chain length. The T_gof the rigid hydrophobic segmented polymer Poly(OPV-DD) was 84 ºC,

which was much higher compared to PEG-segmented polymers. The photophysical

properties of the polymers and oligomer were studied in solutions and drop cast films (see

figure SF4). In the solution, the absorption and emission spectra of the polymers were found

to be almost identical and their maxima were observed at 310 and 405 nm, respectively. The

absorption spectra of the polymers did not show any change in the solid state, however, their

emission maxima (at 410 nm) showed slight red shifted (10- 50 nm) with respect to the

difference in their polymer structures. The absorption and emission maxima of the OPV-TEG

were found identical to that of its polymer. The quantum yields of the polymers (also

oligomer) were determined using quinine sulfate (φ_r= 0.53, in 0.1N Conc. H₂SO₄) as a

standard (see table ST1).¹²The quantum yields of polymers (also oligomers) were obtained in

the range of 0.13-0.27 in chlorobenzene (table ST1) which is in accordance with earlier

reports. ^12c

Newly synthesized carboxylic acid functionalized polymers were complexed with

Eu³⁺ions using theonyl acetylacetone (TTA) as co-ligand in the polymer-Eu complex. FT-IR

spectra of these complexes showed peaks corresponding to the carbonyl stretching frequency

at 1615 cm^-1with respect to C=O…Eu³⁺complex and confirmed the formation of the

expected complex (see figure SF5).¹⁴In order to prove the binding of TTA and OPV

carboxylic acid functionality in the Eu³⁺ion complex, MALDI-TOF-MS analysis of the

OPV-Eu complex was carried out (see supporting information SF6). MALDI-TOF-MS

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spectra of the complex showed a mass peak at 1434.21amu with respect to (OPVTEG)(TTA)₂

Eu³⁺(H₂O)₂. The Eu³⁺ion complex was constituted by two TTA carboxylic units and one

OPV carboxylic unit for the charge neutralization at the metal center. Further, the eight

coordination of the Eu³⁺ion was satisfied by the coordination of two water molecules from

the solvent. Hence, both the carboxylic acid units from TTA and OPV units indeed

participated in the Eu³⁺ion complex formation. A similar MALDI-TOF-MS analysis for

polymer complex was found to be not successful due to the large polydispersity of the

samples. In order to confirm the formation of polymer-(TTA)-Eu³⁺ion complex (TTA was

used as co-ligand), the X-ray photoelectron spectroscopy (XPS) technique was employed.

The XPS measurements were carried out for Poly(TEG-OPV) (TTA) Eu³⁺complex and the

details are provided in the supporting information in figure SF7. The Eu3d and Eu4d core

level XPS spectra of the complex showed two doublets with binding energies at 1136.4 to

1166.2 eV and 139.3 to 144.6 eV, respectively. These energy values were exactly matched

15a

with earlier literature reports for Eu³⁺ion coordinated with oxygen atom.

The binding

energy of C1s photoelectrons were appeared as broad peak in the range of 283-292 eV which

15b

was further fitted with multiple-Guassian programme as reported by others.

The first

major peak 287.2 eV was assigned to C1s electron from C-C and C-H groups. The second

broad peak at 289.9 eV was assigned to the carbon atoms bonded to oxygen. The third high

energy peak at 291.1 eV was assigned to the carboxylic group carbon atom coordinated to

Eu³⁺ion. The actual binding energies of the present complex were almost similar to the

polyacrylic systems which was attributed to aliphatic COOH group in the conjugated OPV

chromophore.^15c,15dSimilarly O1s electron binding energies of the Eu³⁺complex matched

with the expected values (see supporting information).^15eThese results clearly confirmed that

the Eu³⁺metal ion was coordinated by the carboxylate unit of the OPV chromophore in the

polymeric chain.

Absorption, emission and excitation spectra of the polymer-Eu³⁺complexes were

recorded in solution (chlorobenzene) and in film (solid state). In figure 2a, the solution

excitation spectra of all Eu³⁺complexes showed broad absorption from 260 to 430 nm with

peak maxima at 350 nm corresponding to π-π^*transition of the aromatic OPV chromophore

(see also figure SF-8 and SF-9). The emission spectra of the polymer- Eu³⁺complexes in

solution (figure 2b) and film (figure 2c) showed characteristic Eu³⁺strong metal centered

sharp emission peaks at 580, 593, 614, 650 and 702 nm corresponding to the D₀→⁷F₀,

⁵D₀→⁷F₁, D₀→⁷F₂, D₀→⁷F₃(weak emission), and D₀→⁷F₄transitions, respectively.⁵In

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both solution as well as in the solid state, the emission spectra of the polymeric Eu³⁺

complexes did not show polymer self-emission (at 410 nm) from the π-conjugated backbone.

This confirmed that the excitation energy of the π-conjugated OPV chromophore in the

polymer backbone was completely transferred to the Eu³⁺ion during the photoexcitation

process. The photographs of polymer-Eu³⁺complexes showed homogeneous red emission in

solution and in film (see figure 2). Interestingly, the photosensitizing ability of the polymer-

Eu³⁺ion complexes was found to be almost same irrespective of the nature of the segmented

chains (hydrophilic PEG chain or hydrophobic dodecyl chain). Hence, it may be concluded

that the custom designed carboxylic functionalized OPV segmented polymers were efficient

photosensitizer for Eu³⁺ion in solution and in solid state. The absorption spectrum of OPV-

TEG-Eu³⁺and polymer- Eu³⁺complexes were almost same in solution (see figure SF8).

However, the solid state exciattion spectra of Polymer-Eu³⁺complexes were blue-shifted

comapred to their oligomer complex (see figure 2a). The reason for the blue-shift in the

polymer was attributed to the close packing of OPV chrmophores in the polymer chains

which was completely lacking in the oligomer species. The photoexcitation spectra of

structurally identical oligomer OPV-TEG-Eu³⁺complex are shown along with the polymers

in figure 2b and 2c. Both in solution (chlorobenzene) and in solid state, this oligomer

complex showed only weak emission from Eu³⁺ion (at 615 nm). Additionally, a strong self-

emission at 410 nm was clearly visible which suggested that the excitation energy from the

ligand to metal ions was not complete. The comparison of the emission spectra of the

Poly(TEG-OPV)-Eu³⁺and OPV-TEG-Eu³⁺complexes indicated that for a given identical

chromophore structure, the polymeric ligand was efficient photosensitizer for complete

excitation energy transfer from the ligand to the metal center compared to that of small

molecular systems.

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Figure 2. Excitation spectra (a) of the polymer and oligomer complex in chlorobenzene

(emission = 615 nm). Emission spectra of polymer and oligomer complex in chlorobenzene

solution (b) and in solid state (film)(c) (excitation = 360 nm). Luminescent decay profiles of

polymer and oligomer complex in chlorobenzene (d) (excitation source = 360 nm).

To further understand the the energy transfer mechanism and trace the role TTA

moiety in the complexes, singlet and triplet energy levels of polymeric and oligomeric

ligands were determined. The schematic energy transfer diagrams for polymers and oligomer

are shown in the supporting information (see figure SF10 and SF11). According to Latva’s

empirical rule, energy transfer from ligand to excited state of Eu³⁺metal ion was more

effective when ∆E (³ππ^*-⁵D₀) was equal to 2500 - 4000 cm^-1.^16aAs shown in figure SF11,

energy gap ∆E (³ππ^*-⁵D₀) between the triplet excited state of the polymer (or oligomer)

ligand to the excited state of Eu³⁺ion is obtained as 5755-5880 cm^-1. Therefore, according to

Latva’s rule, the direct energy transfer from the polymer ligand to Eu³⁺ion is not possible.

The triplet state energy level of TTA was much lower than that of the triplet state of polymer

(or oligomer) and the same time the difference in the energy levels of TTA to the excited

state of Eu³⁺ion was only 2910 cm^-1. Hence, it may be postulated that during the

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photoexcitation process, the energy was initially transferred from triplet state of polymers (or

oligomer) to triplet state of the TTA and subsequently it was transferred to Eu³⁺ion metal

centre for the luminescence.^16bIn order to confirm this hypothesis, controlled experiments

were carried out by varying the amounts of TTA in the Poly(TEG-OPV)(TTA)-Eu³⁺ion

complex. The Poly(TEG-OPV) polymer was complexed with Eu³⁺ion using various

equivalents of TTA (zero, one, two and three mole equivalents) and further these complexes

were subjected for excitation and emission studies (see supporting information for detailed

characterization, figure SF12 and SF13). In the absence of the TTA, the emission spectrum of

the Eu³⁺complexes showed a large amount of self-emission from the OPV polymer backbone

indicating the incomplete photoexcitation energy transfer to the metal centre. With further

increase in the TTA in the complex, the self-emission of OPV was vanished and complete

photoexcitation energy transfer from the polymer chromophores occurred. Further to confirm

the enhanced photosensitizing ability of the polymer, luminescent decay lifetime

measurements were carried out and their decay profiles are given in figure 2d. The lifetime

decay profiles of the Eu³⁺complexes were fitted with first order decay (see figure SF14 for

other polymer complexes, see table ST2). The D₀excited state lifetime of the polymer-Eu³⁺

complexes was obtained as 270 µs whereas the lifetime of OPV-TEG/Eu³⁺complex was

found much lower (180 µs) due to partial self-emission. Hence, the photosensitizing ability of

the macromolecular ligand was much more robust and superior compared to small organic

ligands.

The temperature dependent luminescent studies were carried out for the polymer

(alone) and its Eu³⁺ion complexes in chlorobenzene and the photographs are shown in figure

3. The photographs of the vials showed a strong blue emission from the OPV chromophores

in the polymer backbone and their luminescent intensity was less influenced by the

temperature changes from 20-100 °C (see emission spectra in the figure SF15 and SF16).

Interestingly, the polymer- Eu³⁺ion complex showed a large variation in their red color

luminescence depending upon their temperature of the solution. For instance, the strong and

sharp red-emission at 20 °C was gradually reduced while heating up to 100 °C and their

luminescent behavior was found to be completely reversible in the subsequent cooling cycles

from 100 to 20 °C. The emission spectra of Poly (OPV-HEG)-Eu³⁺complex in both heating

and cooling cycles are shown in figure 4 [see figure SF15 and figure SF16 for other

complexes]. In figure 4a, upon heating the solution from 20 to 100° C, the characteristic

emission peak intensities of the Eu³⁺ion (major peak at 615 nm) gradually decreased and the

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complexes became completely non-luminescent above 70 °C. In the subsequent cooling

cycles, from 100 °C to 20 °C, the luminescent peak at 615 nm re-emerged below 70 °C and

stayed up to the room temperature. The reversibility of the temperature sensing behaviours is

checked for atleast 10 cooling and heating cycles and they are perfectly reproducible. This

clearly supported the fact that the photosensitizing ability of the polymer-Eu³⁺ion complexes

was highly sensitive to the temperature of the medium.

Figure 3. Photographs of polymer (a) and polymer-Eu³⁺complex (b) in chlorobenzene at

different temperatures. The sample vials were excited with hand held UV-lamp (excitation

= 360 nm).

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Figure 4. Temperature dependent emission spectra of Poly(OPV-HEG) in chlorobenzene (a)

and thin film (b) in heating and cooling cycles (excitation excitation = 360 nm). Plots of PL

intensity versus temperature (c) and their break points (d) of polymer complexes.

The temperature dependent emission characteristics of the polymer-Eu ion complexes

was further checked in other protic and hydrocarbon solvents such as: N,N-

dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), 1,4-xylene

and 1,2-dichlorobenzene (ODCB) . The variable temperature emission spectra of the

polymer-Eu³⁺complex in these solvents are given in the supporting information (figure

SF17). Along with the temperature dependent luminescent behaviors, partial self-emission

from the OPV chromophores were also observed in THF, DMSO and DMF solvents. On the

other hand, ODCB, chlorobenzene and xylene did not show any chromophore self-emission

and only the temperature dependent emission behaviors were observed from the metal center.

Hence, the protic solvents like DMF and DMSO were not good solvents for thermo-sensing

studies. Further, the effect of concentrations of the polymer chromophores on the thermo-

sensing ability was also investigated for 10^-6M to 10^-5M OPVs in chlorobenzene (see

supporting information, figure SF18). It was found that the thermo-response behaviors of the

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polymer-Eu³⁺complex were retained even at very low concentration (10^-6M) and suggesting

that they polymer-lanthanide ion system is very potential for temperature sensing

applications.

Figure 5. Temperature dependent luminescent decay profiles of Poly(OPV-HEG) in

chlorobenzene in the heating (a) and cooling (b) cycles (excitation λ = 360 nm). Plots of τ₁

versus temperature (c) and decay rate constants verses 1/T (d) of the polymer complexes.

In order to check the thermo-sensitive luminescent characteristics in the solid state,

thin films were cast on quartz plate and subjected to photoluminescence studies. In figure 4b,

both the emission spectra and the photographs clearly support the thermo-reversibility of

luminescence properties of these π-conjugated polymer- Eu³⁺complexes in the solid state.

This is for the first time, thermo-sensitive lanthanide complexes were found to show

complete thermo-reversible luminescent behavior in solution as well as in solid state. It is

very important to mentioned that the solid state samples in fact showed small amount of

emission at 100 °C which was completely absent in the solution (figure SF19). This

observation was attributed to the closer packing of the polymer chains in film whereas they

remained apart in solution at high temperature. All other polymer complexes were also found

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to be thermo-reversible in their luminescent behaviors (the details are provided in the figure

SF15 and SF16). In order to compare the thermo-reversibility across the various segmented

polymers, the optical density (O.D.) of the OPV chromophores were maintained as O.D. =

0.1 in chlorobenzene and their emission spectra were recorded at various temperatures (see

figure SF15 and SF16). The PL-intensity of these spectra at 615 nm (with respect to Eu³⁺ion)

were plotted against the temperature and shown in figure 4c. From the plots, it is clearly

evident that all the polymer complexes showed perfect temperature dependent luminescent

properties. However, the temperature at which the turn-on/turn-off and vice versa occurred

was varied with polymer structures. The temperature break points are nothing but the region

in which the luminescent probe became active in the temperature window of 20 to 100 °C.

These break points were determined by linear fits and the values are plotted for different

polymers and shown in figure 4d. These values suggested that with the increase in the PEG

chain length in the segmented polymer, the break points were increased from 55° to 72 °C

(for TEG, HEG and PEG-450 segmented units). The sensitivity of the thermo-probe was

determined as reported by Brites et al. based on the emission intensity and the values are

summarized in table ST4). ^7a,7jThese molecular probes were found to have sensitivity of 1.5

to 1.6 K^-1which matched with the literature reports. ^[7a]This suggested that the temperature

range at which the turn-on or turn-off probes worked as highly dependent on the structure of

the polymer backbone. Based on the appropriate chain backbone, one could selectively

choose a particular polymer (see scheme-1) for sensing sharp temperature change within 10°

difference. Thus, the segmented polymer structural design (like OPV chromophores) in the

backbone is a very crucial parameter for making successful thermo-reversible π-conjugated

polymer-Eu³⁺complex systems.

Further, temperature dependent luminescence decay profiles of the complexes were

recorded to trace their radiative pathways. The luminescent decay profiles for Poly(OPV-

HEG)-Eu³⁺are given in figure 5a [see figure SF20 for other complexes]. The luminescent

decays became predominant at higher temperatures (from 20 to 100° C) and the decay

profiles were found to be completely reversibility in the subsequent cooling cycles (see figure

5b). The same trend was observed for other polymer-Eu³⁺complexes in the heating and

cooling cycles (see figure SF20). The decay profiles were fitted with exponential decay fits

and their lifetime values are given in table ST3 in the supporting information. Up to 70 °C,

the decay profiles followed single exponential fit whereas at higher temperatures (above 70

°C) they were best fitted with bi-exponential fits. The plots of fast decay lifetime (τ₁) versus

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temperature are showed in figure 5c. The thermo-sensing trends of the plots in the lifetime

were almost identical to that of observed based on PL-intensity of the polymer complexes

(see figure 4c). Therefore, the reproducibility of the thermo-sensitive characteristics of the

polymer-Eu³⁺ion complexes were confiremd by two independent photophysical techniques:

(i) vataion in the PL-intesity (see figure 4c) and (ii) PL excited state lite times (figure 5c).

Hence, the turn-On and turn-off luminescent behaviors are molecular properties of the π-

conjugated polymer- Eu³⁺ion complexes and mainly controlled by the segmented polymer

chain structure in solution and in solid state.

Arrhenius equation directly correlates the radiative decay rate constant of the

luminescent properties with respect to the change in the temperatures as:¹⁷

k = Ae ^(-Ea/RT)

(1)

where E_ais energy gap between the emitting levels (equivalent to activation energy), A is

Arhenius pre-exponential factor, R the gas constant and T is absolute temperature. For decay

rate constants at various temperatures (∆T), the above expression is modified as:^7a

ln (k-k₀) = lnA - (E_a/RT)

(2)

where k is decay rate constant at temperature at T and k₀is the decay rate constant in which

the molecule is less influenced by thermal disturbance (k at 20º C). The rate constant (k) is

determined from the equation k = 1/τ₁, where τ₁is the fast decay life time of the excited

species.^7f,7bFrom the plots ln(k-k₀) vs 1/T (see figure 5d), the activation energy (E_a) and the

frequency factor ‘A’ is directly obtained from the slope and intercept, respectively (see table

ST4 for values). The activation energy (E_a) of all the polymer complexes was found in the

range of 3.14 - 3.62×10²KJ mol^-1which are in accordance with earlier reports.^[7f,8]This

confirmed that the nature of the decay of the excited species from ⁵D₀emitting level of Eu³⁺

ion followed typical Arrhenius linear trend with temperature irrespective of the variation in

segmented polymer structure backbone. Therefore, the variation in the temperature break

points (see figure 4d) with respect to the segmented polymer structures were not due to the

difference in the ⁵D₀emitting level of Eu³⁺ion in their complexes.

In general, the emission of Eu³⁺ion complexes were influenced at higher

temperatures in two ways: (i), the deactivation of Eu³⁺ion ⁵D₀excited state and non-radiative

decay of the excitation energy and (ii) less excitation energy transfer from the ligand to the

europium excited states (⁵D₀and ⁵D₁). These two processes are schematically shown in figure

6a. Upon photoexcitation (excitation λ = 360 nm), OPV chromophores in the polymer chains

excited to their higher energy states. The excited polymeric ligand species could lose its

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excitation energy in two possible ways: (i) self-emission of conjugated chromophore in the

blue region (emission at λ = 360 nm) (route-1) or (ii) transfer the excitation energy from π-

conjugated chromophore to metal ions and subsequently produced sharp emission in the

range of 570-720 nm from the Eu³⁺metal center (route-2). The photographs of the vials in

figure 3b and their respective emission spectra in figure 4 confirmed that the excitation

energy was completely transferred from the polymer to the metal center (through route-2) at

20 °C for red-emission (the polymer self-emission was completely absent in both solution

and solid state, see figure 4b and 4c). Therefore, the vanishing of red-emission at 100 °C is

occured either by the deactivation of the excited states Eu³⁺ion (⁵D₀excited state) or may be

due to the structural changes in the polymer chains at high temperatures.

S₂

ISC

S₁

⁵D₁

⁵D₀

λ = 420 nm

(a)

λ = 615 nm

(Route-1)

(Route-2)

⁷F_j

S₀

Eu³⁺

π-Conjugate

Eu³⁺

Shorter

Heat

Segment

Chain Length

Eu³⁺

(b)

Eu³⁺

Cool

Eu³⁺

Longer

Eu³⁺

Heat

Cool

Heat

Cool

(c)

Eu³⁺

Figure 6. Mechanism of energy transfer processes in the polymer-Eu³⁺ion complexes (a).

The intercation of OPV chromophores in short chain (b) or long chain segmented polymers

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Recently, Paris et al. and Shiraki et al. independently reported the change in the

polymer topology as one of the main reasons for the temperature sensing of the BODIPY

attached PMMA and curdlan complexed with polythiophene.^3e,18In the case of BODIPY, the

emission intensity increased reversibly while assembling the chrmophores towards the lower

critical solution temperature (LCST). In curdlan, the exposure to the methanol vapour was

used as stimuli to partially disassociate its complexation with polythiophene to attain

vaporchromism. On the other hand, in the present investigation, the polymer backbone itself

acting as chromophores compared to these examples. Therefore, the change in the topology

of the polymer chains was investigated by variable temperature absorption and emission

studies. For this study, two PEG-based segmented polymers [Poly(OPV-TEG) and

Poly(OPV-HEG)] and polymethylene segmented Poly(OPV-DD) were chosen for variable

tempetaure studies in chlorobenzene (see figure SF21 and SF22). The variable temperature

absorbance spectra of the polymers (alone) did not show any aggregated peaks in the heating

and cooling cycles (see figure SF21). Hence, the occurrence of large change in the topology

of the polymer chains as the cause for the temperature dependent luminescent behavior of

metal ions was rule out. The straight line in the K-values (in figure 5d) for the complexes

over the temperature confirmed that all the polymer complexes followed almost similar decay

pathway for D₀→ F_jtransitions. Therefore, the energy deactivation process was attributed

to the thermo-sensitivity and their turn-On and turn-Off’ processes. The deactivation

mechanism was further confirmed by carefully analyzing the emission spectra of the

polymer- Eu³⁺ion complexes in the region which corresponded to the polymer self-emission

(see figure SF23). With increase in the temperature, the self-emission of polymer did not

show any major changes in the complex (very faint blue self-emission and not to the extent of

the strong blue-emission of polymer alone, compare figure 3a and 3b at 100 °C). Therefore,

neither the route-1 process (see figure 6a) or change in the polymer topology significantly

influenced the decay of Eu³⁺ion emission at high temperatures. Hence, the deactivation of

Eu³⁺ion excited state (⁵D₀) primarily accounted for the thermal decay of red-luminescence at

high temperatures.

Nevertheless, it is not possible to ignore the role of the polymer structure completely.

If one assumes that the variation in the polymer structures do not have a role to play on the

luminescent properties, then all the polymer complexes could be expected to show identical

temperature break points. However, the temperature region at which the turn-On/Off probe

works (temperature break points) was mainly decided by the structure of the segmental

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polymers (see figure 4c, 4d and 5c). To explain this point, two polymer structures Poly(OPV-

HEG) and Poly(OPV-TEG) were chosen and their possible chain orientations with respect to

temperature changes in solution are schematically shown in figure 6b and 6c. These two

polymer strcutures were varied only by the number of –CH₂CH₂O- units which tied the OPV

chromophores in the backbone by either triethylene or hexaethylene glycol segments.

Though both polymers have identical OPV chromophores for photosensitization, the flexible

HEG-segmented polymer complex showed break points at 72 °C which was almost 18 °C

much higher than that of the TEG-polymer complex (at 57 °C). As shown in the figure-6, the

conjugated chromophore-Eu³⁺ion species in the Poly(OPV-TEG)-Eu³⁺complex were

expected to be kept closely whereas they were apart in the more flexible Poly(OPV-HEG)-

Eu³⁺complex. While increasing the temperature, the closely placed species were more

susceptible to thermal changes; as a result the excited state deactivation process became

active at much lower temperatures. As a result, the thermo-sensitive turn-On/Off process

occured at much lower temperature in Poly(OPV-TEG)-Eu³⁺complex (at 57 °C). When the

OPV chromophores weer kept apart in the chains, they tend to undergo only partial

separation from metal-ion center at low temperature and need more thermal energy to for

complete disassociation. As a result, the turn On/Off process became active only at higher

temperature (at 72 °C). Hence, in the present investigation, the luminescence probe active

temperature region as directly dictated by the length of the segmented polymer chain.

Interestingly, all the polymer-Eu³⁺complexes were very stable and showed complete

reversibility in the temperature dependent luminescence process in solution as well as in solid

state. The molecular thermo-probes demonstrated here could be very useful for tracing the

temperature changes in the biological systems or studying the molecular assemblies in

material science below 100 °C. We are currently exploring these possibilities which will be

addressed in the future work. Thus, appropriate design of the optical chromophores and their

arrangements in the polymer main chains are two crucial parameters for photosensitizing as

well as thermo-sensing ability of π-conjugated polymers-lanthanide metal ions complexes.

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Conclusions

In summary, new class of segmented π-conjugated oligo-phenylenevinylene polymers

with carboxylic acid functionality were synthesized through tailor made approaches and

successfully utilized as photosensitizer for Eu³⁺ion. Upon photoexcitation, the excitation

energy transfer from the blue luminescent polymeric ligand to metal center produced strong

and sharp metal centerd red-emission. These polymer complexes were found to show

complete reversible temperature-sensitive luminescence properties in solution as well as in

the solid state (in thin films). Detailed photophysical studies and the luminescent decay

dynamics revealed that the thermal deactivation of the Eu³⁺ion accounted for the decay of

the red luminescence at the high temperature. The temperature break points at which the

‘Turn-On and Turn-Off’ luminescence switch (red to colorless) occurred was found to be

highly dependent on the structure of the segmented chain backbone. Based on the detailed

photophysical analysis, the change in the topology of the polymer at the high temperature

was ruled out. Segmented polymer chains with shorter spacer length showed the break point

at much lower temperatures compared to that of their long flexible counterparts. Hence, it

could be concluded that the arrangements of optical chromophores in the segmented polymer

chain backbone directly control the temperature break points. Further, structurally similar

oligomer-complex was found to be less efficient as photosensitizer and also did not show any

thermo-sensitive behavior. Thus, the design of the segmented π-conjugated polymer is a very

crucial factor for making reversible thermo-sensitive luminescent molecular probes. In the

present investigation, this concept was demonstrated by designing appropriate π-conjugated

segmented polymers with photoactive OPV chromophores and their complexes with Eu³⁺ion.

Though the approach demonstrated here pertains to π-conjugated polymers having OPV

chromophores and Eu³⁺ion, however, it is not restricted to any particular type of π-

conjugated polymeric systems or lanthanide ions. This finding opens up new concept in

macromolecular ligand design for lanthanide ions and their potential for many applications in

temperature sensitive molecular probes.

Experimental Section

The experimental section containing syntheses and characterization of compounds,

GPC, Thermal analysis (TGA and DSC), Photophysical studies of polymers and their

Eu³⁺complexes, Temperature dependent absorption and emission spectra of polymers,

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polymer-Eu³⁺complexes in solution and solid state and luminescent decay profiles Eu³⁺

complexes are provided. ¹H NMR, ¹³C-NMR, MALDI spectra, XPS spectra are also provided

in the supporting information.

Acknowledgment

Research grant from Department of Science and Technology (DST), New Delhi, INDIA,

under nanomission initiative project SR/NM/NS-42/2009 and SR/S1/OC-54/2009 are

acknowledged. The research work described in the manuscript is entirely carried out at

IISER-Pune and A. Balamurugan thanks the Director, IISER-Pune for extending the research

facilities for his Ph.D. thesis work. A. Balamurugan also thanks CSIR, India for SRF

fellowship. Authors thank Dr. C. S. Gopinath, NCL, Pune for XPS measurements.

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Figure and Scheme Captions: