Origin of the Reversible Thermochromic Properties of

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The Origin of the Reversible Thermochromic Properties of Polydiacetylenes is Revealed by Ultrafast Spectroscopy Junwoo Baek, Joonyoung Francis Joung, Songyi Lee, Hanju Rhee, Myung Hwa Kim, Sungnam Park, and Juyoung Yoon J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02671 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 5, 2016

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The Journal of Physical Chemistry Letters

The Origin of the Reversible Thermochromic Properties of Polydiacetylenes is Revealed by Ultrafast Spectroscopy Junwoo Baek,a‡ Joonyoung F. Joung,a‡ Songyi Lee,b‡ Hanju Rhee,c Myung Hwa Kim,b Sungnam Park,a* and Juyoung Yoonb* a

Department of Chemistry, Korea University, Seoul 136-701, Korea

b

Department of Chemistry and Nano Science, Ewha Womans University, Seoul, 120-750,

Korea c

Space-Time Resolved Molecular Imaging Research Team, Korea Basic Science Institute,

Seoul 136-713, Korea

ACS Paragon Plus Environment


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ABSTRACT: Polydiacetylenes (PDAs) with thermochromic properties undergo colorimetric transitions when the external temperature is varied. This capability has the potential to enable these materials to be used as temperature sensors. These thermochromic properties of PDAs stem from their temperature-dependent optical properties. In this work, we studied the temperature-dependent optical properties of Bis-PDA-Ph, which exhibits reversible thermochromic properties, and PCDA-PDA, which exhibits irreversible thermochromic properties, by UV-visible absorption and femtosecond transient absorption spectroscopy. Our results indicate that the electronic relaxation of PDAs occurs via an intermediate state in cases where the material exhibits reversible thermochromic properties, whereas the excited PDAs relax directly back to the ground state when irreversible thermochromic properties are observed. The existence of this intermediate state in the electronic relaxation of PDAs thus plays an important role in determining their thermochromic properties. These results are very important for both understanding and strategically modulating the thermochromic properties of PDAs.

TOC Graphic:

KEYWORDS. Thermochromic property, Colorimetric Sensor, Polydiacetylenes (PDAs), Femtosecond transient absorption spectroscopy, Temperature Sensor


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Polydiacetylenes (PDAs) have attracted much attention as potential materials for novel sensor systems because they undergo colorimetric transitions upon application of external stimuli.1-13 Most notably, the thermochromic properties of PDAs have been extensively used for the development of reversible temperature sensors because they exhibit a gradual color change from blue to red when they are heated from room temperature to a higher temperature (e.g., 120 °C).14-16 In some PDAs, the initial blue color is not recovered upon cooling from an elevated temperature to room temperature. Such partially reversible or irreversible thermochromic properties and the relatively narrow temperature range in which PDAs exhibit reversible thermochromic properties have been drawbacks in their application in sensor systems. Very recently, Yoon and coworkers successfully developed a new PDA derivative (abbreviated “Bis-PDA-Ph”) that consists of two PDAs linked via a p-phenylene group that exhibits exceptional thermochromic reversibility across a wide temperature range, from room temperature to 120 °C.16 They also demonstrated that the enhanced thermochromic properties result from the presence of hydrophobic interactions between the alkyl chains and the aryl moieties of Bis-PDA-Ph. Although the irreversible and reversible thermochromic properties of PDAs have been an important issue in the development of sensor systems, the physical reason for each is not yet clearly understood. For some PDAs, irreversible thermochromic properties have been demonstrated to be associated with irreversible structural changes, such as hydrogen-bond breaking.17-18



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Figure 1. Molecular structures of two representative PDAs. (a) Bis-PDA-Ph. Two PDA moieties are connected by a p-phenylene group. Hydrophobic interactions, as indicated by the dashed lines, within Bis-PDA-Ph are important for the stability of the overall structure. (b) PCDA-PDA. Hydrogen bonds, as indicated by the dashed lines, connect the alkyl chains in PCDA-PDA.

In the present work, we explored the temperature-dependent optical properties of BisPDA-Ph and PCDA-PDA, using UV-visible absorption and frequency-resolved transient absorption (FRTA) spectroscopy, to elucidate the origins of their thermochromic properties in terms of electronic relaxation dynamics. Bis-PDA-Ph and PCDA-PDA were chosen as representative PDA model systems with distinct reversible (Bis-PDA-Ph) and irreversible (PCDA-PDA) thermochromic properties. The molecular structures of Bis-PDA-Ph and PCDA-PDA are shown in Fig. 1. Bis-PDA-Ph exhibits reversible thermochromic properties at temperatures up to 120 °C. PCDA-PDA is typical of the majority of PDAs, which exhibit either irreversible thermochromic properties or reversible properties only within a narrow


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temperature range. We carefully performed femtosecond FRTA experiments on Bis-PDA-Ph and PCDA-PDA at 20 and 80 °C, revealing important and direct evidence elucidating the origin of the reversible thermochromic properties of PDAs, as will be explained in detail. Temperature-dependent UV-visible absorption spectra of the two PDAs in aqueous solutions are displayed in Fig. 2. A strong optical absorption in PDAs results from a π→π* transition of the conjugated ene-yne backbone of PDAs.19-20 The absorption spectrum of BisPDA-Ph is gradually blue-shifted, and the color of its aqueous solution changes from blue to red as the temperature is increased from 20 to 80 °C. When the solution is returned to room temperature, the original absorption spectrum and color are completely recovered. Accordingly, Bis-PDA-Ph exhibits excellent reversible thermochromic properties, which are known to result from reversible structural changes in the PDA in the studied temperature range. In contrast, the absorption spectrum of PCDA-PDA (Fig. 2b) changes gradually between 20 and 60 °C, but undergoes a dramatic change at temperatures above 60 °C. Upon returning to room temperature, the absorption spectrum and color of PCDA-PDA are not recovered. The absorption spectra in Fig. 2b indicate reversible thermochromic properties for PCDA-PDA only up to 60 °C and clear irreversible properties above 60 °C. This behavior can be explained by the fact that PCDA-PDA undergoes a significant, irreversible structural change above 60 °C.17



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Figure 2. Temperature-dependent UV-visible absorption spectra of two PDAs. (a) Bis-PDAPh, (b) PCDA-PDA in aqueous solutions. The temperature was increased from 20 to 80 °C in increments of 10 °C. The thermochromic properties of Bis-PDA-Ph are reversible in the experimental temperature range. The thermochromic properties of PCDA-PDA are reversible up to 60 °C, but become irreversible at temperatures above 60 °C.

To study the electronic relaxation dynamics of Bis-PDA-Ph and PCDA-PDA in aqueous solutions, we carried out frequency-resolved transient absorption (FRTA) experiments. Two PDA samples at 20 and 80 °C were excited by a pump pulse at 630 and 520 nm, respectively, and their electronic relaxation dynamics were probed by a white-light continuum (a detailed experimental setup is presented in the Supporting Information).


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Figure 3. Frequency-resolved transient absorption signals measured with (a) Bis-PDA-Ph at 20 °C, (b) PCDA-PDA at 20 °C, (c) Bis-PDA-Ph at 80 °C, and (d) PCDA-PDA at 80 °C. In each graph, the spectral components and time-dependent populations of E1 (red), E2 (blue), and I (green), which were extracted from the global fitting analysis, are presented in the right and bottom panels, respectively.

In Fig. 3, the FRTA signals measured from Bis-PDA-Ph and PCDA-PDA at 20 and 80 °C are plotted with wavelength on the y-axis and time (log scale) on the x-axis. The FRTA signal shows how the spectral components of the electronically excited PDAs decay as a function of time. At short times, the FRTA signal is positive (red) at shorter wavelengths as a result of ground-state bleach (GSB) and stimulated emission (SE). In contrast, the signal is negative



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(blue) at longer wavelengths because of excited-state absorption (ESA). The FRTA signal decays to zero as the electronically excited PDA relaxes to its electronic ground state via different relaxation pathways depending on their respective electronic energy structures. Therefore, the FRTA signal contains dynamic information on electronic relaxation in terms of the energy structure and relaxation pathways of a molecular system. Close examination of the FRTA signals in Fig. 3 reveals some interesting and important features. First, the excited PDAs relax back to the ground state with both fast and slow decay components. In fact, the electronic relaxation of PCDA-PDA at 80 °C was observed to be reasonably well described by a biexponential function, as will be shown in the next section. Second, an intermediate state is observed in the FRTA signals of Bis-PDA-Ph at 20 and 80 °C and PCDA-PDA at 20 °C, as indicated by red-dotted circles in Fig. 3. No such intermediate state is observed in the FRTA signal of PCDA-PDA at 80 °C. In the case of Bis-PDA-Ph, the intermediate state is substantially populated at ~30 and ~80 ps at 20 and 80 °C, respectively. In contrast, the excited PCDA-PDA at 20 °C relaxes to its ground state via an intermediate state observed at ~50 ps, but it relaxes directly to its ground state at 80 °C. This observation is interesting and important in understanding the thermochromic behavior of these materials. As mentioned in the previous section, Bis-PDA-Ph has reversible thermochromic properties at temperatures up to 120 °C, whereas the thermochromic properties of PCDA-PDA become irreversible above 60 °C. Our experimental FRTA results indicate that the existence of the intermediate state, through which the electronic relaxations of the PDAs occur, is an important feature of PDAs with reversible thermochromic properties.


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Figure 4. Kinetic model analysis. (a) Kinetic model with two excited electronic states (E1 and E2) and an intermediate state (I), (b) Kinetic model with two excited electronic states (E1 and E2). PDAs are excited by a pump pulse (hνpump) to two nearly degenerate electronic states (E1 and E2), and electronic relaxation occurs directly to the electronic ground state (G) or via an intermediate state (I). The corresponding rate constants are indicated by k1, k2, and k3, (c) Fitted FRTA signal and the time-dependent populations of E1, E2, and I states.



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To understand the electronic relaxation dynamics of PDAs with reversible or irreversible thermochromic properties, the FRTA signals were analyzed by using the kinetic models presented in Fig. 4. In reality, PDAs are complex molecular systems with electronic excited states that are expected to be highly congested. However, to make the kinetic analysis simple and include only the key features, the PDAs are assumed to be excited to nearly degenerate electronic states (E1 and E2) with an equal probability by the pump pulse (hνpump) and to subsequently relax either directly back to the ground state (G) or via an intermediate state (I) depending on their thermochromic behavior (Fig. 4). For PDAs with reversible thermochromic properties (Fig. 4a), the E2 state relaxes directly to the ground state (G) with a rate constant of k2, whereas the E1 state relaxes to the ground state via an intermediate state (k1 and k3). For PDAs with irreversible thermochromic properties (Fig. 4b), both the E1 and E2 states relax directly back to the ground state with rate constants k1 and k2, respectively (a detailed kinetic model analysis is presented in the Supporting Information). The FRTA signal was decomposed into three time-dependent components (E1, E2, and I states) using the global fitting method (Fig. 4c) allowing extraction of the kinetic rate constants (see the Supporting Information for details).21-23 The rate constants derived from the kinetic model are summarized in Table 1. The time-dependent populations and their corresponding spectra extracted from the global fitting analysis are shown in the bottom and right panel, respectively, of each FRTA signal in Fig. 3. The population relaxations of the E1 and E2 states are displayed in red and blue in the bottom panel of each FRTA signal, respectively, and the time-dependent population of the intermediate state is shown in green. The spectral contributions of the E1, E2, and I states, which are represented with the same color scheme, are shown in the right panel in each FRTA signal in Fig. 3.


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Table 1. Kinetic parameters obtained from the global fit analysis

Temp. (°C)

1/k1 (ps)

1/k2 (ps)

1/k3 (ps)

20

0.77

5.7

270

80

1.3

16

4100

20

0.90

11

550

80

1.3

40

-

Bis-PDA-Ph

PCDA-PDA

Some important features uncovered in the results of our kinetic model analysis in Table 1 and Fig. 4c are worthy of discussion. First, the population relaxation (k2) of the E2 state, which is a common relaxation pathway in PDAs, occurs on the tens-of-picoseconds time scale and is observed to be faster at 20 °C than at 80 °C. Second, the E1 state decays to the intermediate state when it is available, but otherwise decays directly to the ground state. The time scale of the population relaxation (k1) of the E1 state is not significantly dependent on whether the intermediate state is available or not. In the case of PDAs with reversible thermochromic properties, the intermediate state is populated as the E1 state decays. Third, the intermediate state decays on the hundreds-of-picoseconds time scale (k3) and prolongs the overall electronic relaxation of the PDAs, resulting in reversible thermochromic behavior. Before closing this section, we note that we used the kinetic model in Fig. 4 to explain the fundamental features of the electronic relaxation dynamics of PDAs with irreversible or reversible thermochromic properties, with the minimum possible number of rate constants used to describe the overall kinetics. Different kinetic models with additional rate constants



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can be used as well. However, the key and fundamental features of the electronic relaxation dynamics of the two PDAs extracted from such kinetic models would not be substantially different. Lastly, we used QM/MM methods to calculate the optimized molecular structures of model PDAs at 20 and 80 °C and their electronic energies (See the Supporting Information for details).24 The carboxylic acids in the model PCDA-PDA are all hydrogen-bonded at 20 °C; however, their hydrogen bonds are all broken at 80 °C (See Fig. S6 in the Supporting Information). Such a structural change in PCDA-PDA leads to a significant rearrangement of its electronic energies at 80 °C (See Fig. S7). In contrast, the electronic energies of the model Bis-PDA-Ph are observed to be less dependent on temperature because of its structural robustness. In conclusion, we studied the comparative electronic relaxation dynamics of Bis-PDAPh and PCDA-PDA to understand their observed reversible or irreversible thermochromic properties. Bis-PDA-Ph exhibits excellent reversible thermochromic properties up to 120 °C, whereas PCDA-PDA exhibits reversible thermochromic behavior in only a narrow temperature range and irreversible behavior at temperatures above 60 °C. Our experimental FRTA results indicate that the electronic relaxation of PDAs with reversible thermochromic properties occurs via an intermediate state, whereas the excited PDAs with irreversible thermochromic properties relax directly to the ground state. This observation is interesting and is important in understanding and strategically modulating the thermochromic properties of PDAs. In the case of Bis-PDA-Ph, the excellent observed reversible thermochromic properties result from the existence of unique hydrophobic interactions between alkyl chains, as well as π-π interactions between aryl groups.16 As the temperature is increased, PDAs undergo a gradual colorimetric transition as a result of gradual changes in their backbone


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structures because of thermal energy. PDAs display a common intermediate state as an electronic structural marker for the recovery of their initial structures when the temperature is decreased. However, PDAs appear to lose their reversible thermochromic properties at higher temperatures when significant structural changes are thermally induced. Their electronic energy structure is altered by these structural changes; consequently, the intermediate state no longer exists. The existence of the intermediate state in the electronic relaxation of PDAs is direct evidence for reversible thermochromic properties of PDAs. The intermediate state responsible for the reversible thermochromic properties may result from a common molecular structure and is shown to play an important role in recovering the original molecular structure of the molecule. However, the identification of the intermediate state in the electronic structure associated directly with the molecular structure is not experimentally straightforward. Nonetheless, detailed electronic structure calculations can help identify the intermediate state responsible for the reversible thermochromic properties of PDAs.

EXPERIMENTAL METHODS Materials. Unless otherwise noted, materials were obtained from commercial suppliers and were used without further purification. Flash chromatography was carried out on silica gel (230-400 mesh). 1H NMR and

13

C NMR spectra were recorded using a 300 MHz NMR

spectrometer. Chemical shifts were expressed in ppm and coupling constants (J) in Hz. Synthesis of Bis-PCDA-Ph. Synthesis of Bis-PCDA-Ph was achieved by modifying previous work.16 To a solution containing 0.750 g (2.01 mmol) of 10, 12-pentacosadiynoic acid in 20 mL of methylene chloride was added 0.05 mL (7.44 mmol) oxalyl chloride. After 1 h of stirring, one drop of dimethylformamide (DMF) was added to the solution and the resulting mixture was stirred for 4 h. To the residue obtained by concentration of the solution was



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added a solution containing 0.10 g (1.00 mmol) of hydroquinone and 0.6 mL (6 mmol) of triethylamine in 20 mL of THF. The resulting solution was stirred 24 h at room temperature under N2. The residue obtained by concentration was subjected to silica gel column chromatography (CH2Cl2 100%) to give Bis-PCDA-Ph (0.72 g, 84%) as a white solid. 1H NMR (300 MHz, CDCl3): δ (ppm): 0.83-0.87 (t, J = 6.9 Hz, 6H), 1.23-1.53 (m, 52H), 1.681.73 (m, 9H), 1.69-1.76 (m, 4H), 2.20-2.24 (m, 8H), 2.50-2.55 (t, J = 7.5 Hz, 4H), 7.06 (s, 4H).

13

C NMR (300 MHz, CDCl3) δ (ppm) 172.15, 148.05, 122.38, 77.44, 77.01, 76.59,

65.31, 34.31, 31.92, 29.65, 29.63, 29.61, 29.48, 29.35, 29.10, 29.08, 29.03, 28.90, 28.86, 28.75, 28.35, 28.29, 24.86, 22.63, 19.21, 19.19. FAB HRMS m/z = 823.6606 [M]+, calc. for C56H87O4 = 824.3044.

Preparation and polymerization of polymers. Bis-PCDA-Ph monomer (8.3 mg, 0.01 mmol) was dissolved in THF (1 mL) and injected into 9 mL of deionized water while shaking the mixed solution to yield a total monomer concentration of 1 mM. The sample was then sonicated at 80 °C for 40 min. The resulting solution was filtered through a 0.8 µm filter, giving a filtrate that was cooled down to 4 °C for 12 h. Polymerization was carried out at room temperature by irradiating the solution with a 254 nm UV light (1 mW/cm2) for 40 s.

UV-visible absorption spectroscopy. UV-visible absorption spectra of the sample solutions were measured in a 10×10 mm quartz cuvette by using a Cary 100 spectrometer (Varian) in the wavelength range of 190-900 nm. The baselines were corrected by using UV-visible spectra of the reference solution. For the temperature-dependent UV-visible absorption experiments, the temperature of the sample cell was controlled by using a temperature controller (Cary Dual cell peltier accessory, Varian) by which the temperature of the cell can


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be increased by the minimum increment of 0.1 °C. For a given temperature, the temperature of the cell was kept constant at least for 10 minutes to ensure that the thermal equilibrium was established before the absorption spectrum was taken. After the temperature-dependent experiments was finished, the sample solution was quickly cooled to 20 °C and the UV-vis absorption spectrum was taken again. And two UV-visible absorption spectra were compared before and after the temperature-dependent experiments.

Frequency-resolved transient absorption (FRTA) experiments. In FRTA experiments, PDAs were electronically excited by visible light pulses and the subsequent relaxation was monitored by using white light probe pulses.25-27 The TA experimental setup is shown in Fig. S1. A femtosecond Ti: Sapphire oscillator and amplifier system (SpectraPhysics) generated 800 nm pulses with 1 mJ per pulse and 35 fs pulse duration operating at 1 kHz. 800 nm pulse was used to pump an optical parametric amplifier (OPA, SpectraPhysics) which then generated the signal (1.2-1.6 µm) and idler (1.6-2.4 µm) in the near-IR range. The pump pulses at 630 nm or 520 nm were generated by using second harmonic generation of the signal or fourth harmonic generation of the idler from the OPA, respectively. White light was generated by focusing 800 nm pulses onto a 2 mm thick sapphire disk. The instrumental response function of the TA experimental setup was ca. 200 fs. The probe beam was focused onto the sample by using a parabolic mirror (focal length = 10 cm) and its diameter was approximately 200 µm at the sample position. The pump beam was focused by using a lens with a 20 cm focal length and the diameter of the pump beam was almost 3 times larger than that of the probe beam at the sample position. The power of the pump beams was ca. 200 µW. After passing through the sample, the probe beam was collimated with a parabolic mirror (focal length = 10 cm), dispersed through a monochromator, and detected by using a Si



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photodiode. TA signals were measured with a lock-in amplifier by chopping the pump beam at 500 Hz. The TA signal was obtained as a fractional change in the transmitted probe beam,

∆T ( t , λ ) / T ( λ ) , by28-29 ∆T ( t , λ ) T ( t , λ ) pump− on − T ( t , λ )pump −off = T (λ ) [T ( λ )pump −on + T ( λ )pump −off ] / 2

(1)

where T ( t , λ )pump −on and T ( t , λ ) pump −off were the intensities of the transmitted probe beam with the pump beam on and off, respectively. The sample solutions were housed in a 1 mm thick glass cell for TA experiments at room temperature (22 °C). For temperature-controlled TA experiments, a home-made temperature controlled cell with a Teflon spacer was used and its temperature was controlled by using a temperature controller and measured with a K-type thermocouple. The temperature of the cell was maintained within 0.2 °C at desired temperatures during the TA experiments.

ACKNOWLEDGMENTS This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2012R1A3A2048814 for J.Y.; No. 2013R1A1A2009991 for S.P.). The FRTA data were measured by using the femtosecond multi-dimensional spectroscopic system at KBSI. Mass spectral data were obtained from the Korea Basic Science Institute (Daegu) on a Jeol JMS 700 high resolution mass spectrometer.

ASSOCIATED CONTENT Supporting Information.


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Transient absorption experimental setup, kinetic models, global fitting method, QM/MM calculations of model Bis-PDA-Ph and PCDA-PDA, 1H NMR and 13C NMR spectra of BisPCDA-Ph. This information can be found free of charge on the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (J. Yoon) *Email: [email protected] (S. Park) Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest.



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Absorption

Kinetics

Associated

with

Higher


Exciton

States

in

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Origin of the Reversible Thermochromic Properties of

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