Effects of Chromophore Conjugation Length and Concentration on the

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Effects of Chromophore Conjugation Length and Concentration on the Photostability of IndolineBased Nonlinear Optical Chromophore/Polymer Films Yasar Kutuvantavida, Grant V.M. Williams, Delower Bhuiyan, Sebastiampillai Raymond, and Andrew J. Kay J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509293m • Publication Date (Web): 22 Jan 2015 Downloaded from http://pubs.acs.org on February 1, 2015

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The Journal of Physical Chemistry

Effects of Chromophore Conjugation Length and Concentration on the Photostability of IndolineBased Nonlinear Optical Chromophore/Polymer Films Yasar Kutuvantavida1,2,†, Grant V. M. Williams2,3, M. Delower H. Bhuiyan1, Sebastiampillai G. Raymond*,1, and Andrew J. Kay1 1

Callaghan Innovation, PO Box 31310, Lower Hutt 5040, New Zealand.

2

School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600,

Wellington, New Zealand 3

The MacDiarmid Institute, Victoria University of Wellington, PO Box 600, Wellington, New

Zealand

ACS Paragon Plus Environment

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ABSTRACT Photostability measurements were made on host-guest thin films containing nonlinear optical chromophores with an indoline donor and amorphous polycarbonate. We find that the indolinebased chromophores are nearly 10 times more photostable than an analogous compound previously studied and which has a dihydopyridine donor. There is an enhancement of the photostability with increasing chromophore concentration and conjugation length. This can be attributed to a reduction in the average separation between the chromophores, and excited state singlet energy transfer to adjacent chromophores followed by eventual nonradiative decay. This leads to a reduction in the singlet oxygen generation rate as the chromophore concentration or conjugation length are increased.


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1. INTRODUCTION There is increasing interest in organic electro-optic (EO) polymer compounds because of their advantages over inorganic materials in a range of photonic devices such as components for broad band THz emitters and detectors,1 ultra-fast EO modulators2-7 and on-chip interferometers.2,7 The reported EO coefficients of a number of EO polymers greatly exceed those found in conventional single crystals (e.g. LiNbO3).2-5 A lower device drive voltage and ease of synthesis, including control over the geometrical shape, has also accelerated the research into EO polymers. The focus now is on improving the EO coefficient, the temporal stability of the macroscopic EO response, and the photostability of the chromophores.2-5,8-13 Using an efficient donor-acceptor group,10 changing the conjugation length,11 using site isolation groups,14,15 and tuning a given structure via modifying the polyene chain such as ringlocking10,11 are some of the methods being used to improve the efficiency of organic non-linear optical (NLO) chromophores at the molecular level. The molecular NLO effect in the chromophore is related to the degree of mixing of the ground state and the charge transfer excited state of the molecule as explained by a two-level model of molecular hyperpolarizability, β,16-19

β ∝ (µee − µ gg )

µ ge2

(1)

E ge2

where µgg is the dipole moment in the ground state, µee is the dipole moment in the excited state, µge is the transition dipole moment, Ege is the transition energy between two states and the subscripts g and e represent the ground state and the charge-transfer excited state. It has been found that µee - µgg, µge and Ege are correlated with the bond length alternation (BLA), which is


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the average difference between the double and single bond lengths of the conjugated structure.1619

This leads to a direct correlation between β and BLA.16-19 The efficiency of the chromophores for EO applications is generally expressed as the

product of β and the chromophore ground state dipole moment, µ, and it is called the figure of merit of the chromophore (µβ). Increasing the conjugation length can lead to a larger µβ value10,11,20 but the chromophores with large conjugation lengths can easily bend or twist,21 which may affect the resultant µβ of the chromophore. One possible method to overcome this problem is by ring-locking some of the double bonds in the conjugated pi-bridge that can increase the rigidity of the chromophore and the thermal stability.22,23 It has been suggested that chromophores with longer conjugation lengths and an increased number of double bonds are less photostable2,10,20 because double bonds are known to be susceptible to degradation by singlet oxygen24. However, it is important to realise that the chromophore photodegradation process occurs via optical stimulation into the chromophore charge-transfer band, energy transfer to a triplet oxygen that leaves it in a singlet oxygen state, followed by a chemical reaction with the singlet oxygen and a chromophore25,26 that reduces µβ to zero. Thus, the photostability is determined by the rate of singlet oxygen generation by photodegradation as well as the conjugation length. The rate of singlet oxygen generation in the case of energy transfer between the excited states of the chromophore and triplet oxygen is affected by how closely the energy difference of the relevant chromophore excited and ground states matches the energy difference between the oxygen triplet and singlet ground and excited states.27 Since the chromophore charge transfer gap energy is expected to reduce with increasing conjugation length10,11 and hence the chromophore ground state triplet to ground state singlet energy should also change, it is possible that the singlet oxygen generation rate can also change with chromophore conjugation length.


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Thus, it is not clear if increasing the chromophore conjugation length for a given donor-acceptor pair makes the chromophore more or less photostable. It is also not known if ring-locking affects the photostability. In this paper we present the results from photostability measurements made on host-guest indoline-based chromophore/amorphous polycarbonate (APC) thin films. From previous studies we know that the hyperpolarizability, β, of these compounds increases with increasing conjugation length10,11 but it is not known if there is an adverse effect on the photostability. In the report we show that the photostability of the indoline-based systems increases with increasing conjugation length and chromophore concentration. Furthermore, the indoline-based chromophores are more photostable than the PYR-3 chromophore that we have previously studied.8,9,28 2. EXPERIMENTAL METHODS The methodology used to prepare the chromophores is the same as reported elsewhere.10,11,28 The structures of the chromophores used in the study are shown in Fig. 1, along with the structure of the PYR-3 compound that we have previously studied.8-10 Thin films were fabricated by spincoating solutions made by dissolving the required quantities of chromophore and amorphous polycarbonate (APEC 9389) in cyclohexanone. The solid to solvent mass ratio required to get the optimum viscosity to make ~2 µm films was 1:7.5. The films were spread onto a 25 mm x 25 mm glass substrate and spun at 1200 rpm for 10 seconds. Residual solvent was removed by drying the films at room temperature and then annealing at 80 °C overnight at ambient atmospheric pressure and then heating to 150 °C in a vacuum for 2 hours. This annealing temperature is well below the decomposition temperature (>245 °C) of the chromophores as well as the glass transition temperature (Tg) of the host polymer APC (218 °C).


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Figure 1. Structures of the chromophores used in this study. Also shown in the center is the structure of the PYR-3 chromophore.9,10 All the chromophores except PYR-3 have the same donor-acceptor pair. Previous studies have shown that ring-locking enhances the thermal stability of the chromophores by increasing the decomposition temperature from 249 °C to 278 °C.10 The trend reported in the literature29-33 and from our group11 shows that increasing the length of the conjugation between the donor and acceptor groups leads to an increase in the first static hyperpolarizability, β0, values. β0 is derived from β using the 2 level model2,34 and it is independent of the wavelength and hence it allows for a comparison between different chromophores. The film thicknesses were measured using a Metricon® 2010. The absorption spectra of the films was collected using a Perkin-Elmer® Lambda 1050™ UV-Vis-NIR spectrophotometer. Photostability experiments were made on the thin films in air and at atmospheric pressures at room temperature (15 °C to 21 °C) using laser wavelengths within the charge transfer absorption bands of the chromophores. The laser sources used in this study were diode lasers operating at


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635 nm or 655 nm. A uniform beam profile on the films was achieved by spatial filtering, expanding the incident laser beam, and using a pinhole on the film surface. A beam splitter was placed before the film to reflect part of the incident light to a reference photodiode. The remaining light was detected with another photodiode after traversing the film. 3. RESULTS AND ANALYSIS The absorption spectra of the films containing indoline-based chromophores with 2 (IND-3), 3 (IND-5), and 4 (IND-7) conjugated double bonds, and the ring-locked derivative of the IND-7 chromophore (IND-7R) are shown in Fig. 2a. The red-shift of the absorption spectra with increasing conjugation length shows that there is a decrease in the charge-transfer gap energy. The absorption bands of the chromophores also have additional blue-shifted shoulders that can be attributed to the excited vibronic states of the chromophores. Our previous studies have demonstrated that these blue shifted shoulders are not due to H-aggregation of the chromophores.10


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Figure 2. (a) Absorption spectra of indoline-based chromophores shown in Fig.1 in APC with chromophore concentrations of 7.5×1019 cm-3. (b) Absorption spectra of IND-7 for IND-7 concentrations, N, of 0.78×1019 cm-3, 2.4×1019 cm-3, 3.8×1019 cm-3, and 6.1×1019 cm-3. The arrow indicates increasing IND-7 concentration. Inset: Plot of the integrated absorption coefficient, αI, against the IND-7 concentration. The line is a linear fit to the data. Ring-locking leads to a higher relative intensity of the secondary absorption at shorter wavelengths when compared to that of IND-7. This may be due to a change in the energy level hybridisation due to ring-locking, which leads to a reduced energy gap.10 It is generally expected that there is a trade-off between the stability and NLO response of the chromophore,20 however, no significant change in NLO response was observed as reported in recent studies on this compound.11 A small red-shift is also observed as a result of this reduced energy gap. The optical absorption coefficients of films containing different IND-7 chromophore concentrations in APC are shown in Fig. 2b. The integrated absorption coefficient is plotted in the inset. It is linear with the concentration and it indicates that the absorption cross section does not change with concentration. This confirms that aggregation is not occurring within the films. There is a small red shift and a change in the spectral shape with increasing concentration, which are likely to be due solvatochromic effects. It is known that continual optical stimulation into the charge-transfer band leads to oxygen-mediated

photodegradation

and

this

is

evidenced

by

an

increase

in

the

transmittance.8,9,22,26,35 Two main processes are believed to cause photodegradation. They are S1+3O2→T1+1O2 and T1+3O2→S0+1O2 where 3O2 is triplet oxygen, 1O2 is singlet oxygen, and S0, S1, and T1 are the chromophore singlet ground state, singlet excited state, and triplet excited


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state, respectively. They result in the generation of singlet oxygen that can react with the chromophore and lead to a photodegraded product with a negligible hyperpolarizability. The simplest photodegradation model assumes that the photodegradation quantum efficiency, B-1, is polarization independent and the photodegraded products do not absorb at the excitation wavelength. In our case photodegradation leads to a complete loss of the chargetransfer band above ~500 nm and hence there is no absorption from the photodegraded products during our photodegradation measurements. Thus, the rate equations for the photon flux and chromophore concentration while photodegrading the chromophores can be written as,35 ∂ n (t , z ) = −σN (t , z ) n(t , z ) ∂z

(2)

∂N (t , z ) = −σN (t , z ) B −1 n(t , z ) ∂t

(3)

where t is the time, z is the depth into the film, n(t,z) is the photon flux and N(t,z) is the chromophore concentration at a depth z in the film, and σ is the absorption cross-section at the irradiating wavelength. It can then be shown from Eqs. 2 and 3 that the transmittance, T(t), can be written as,35

T (t ) =

T (∞) 1 + [T (∞) / T (0) − 1] exp(−t / τ )

(4)

where T(∞) is the bleached transmittance, T(0) is the initial transmittance, τ=B/σn0, and n0 is the initial photon flux. It is possible to rearrange Eq. 4 to obtain F(t)=[T(∞)/T(t)-1]/[T(∞)/T(0)-1] and hence,8


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F (ξ ) = exp(− B −1ξ )

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(5)

where ξ = σn0t. Thus, B-1 can be obtained from the value of ξ when F(ξ)=1/e. The results from photodegradation measurements on chromophore/APC films are shown in Fig. 3 where the data are plotted as F(ξ) vs. ξ. It is apparent that F(ξ) is not a single exponential, which is likely to be due to a polarization dependent B-1.9 However, it is still possible to define a ξ1/e to characterize the photodegradation, which is the value of ξ when F(ξ)=1/e.8

Figure 3. (a) Plot of F(ξ) vs. ξ for a IND-7/APC film at different light intensities during photodegradation using a 655 nm diode laser. The IND-7 concentration was 3.8×1019 cm-3. The light intensities were 0.77 mW/mm2, 1.6 mW/mm2, 7.4 mW/mm2, 15 mW/mm2, and 32


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mW/mm2. The arrow indicates increasing intensity. The film was 2.1 µm thick. Inset: plot of ξ1/e against the optical intensities for IND-7 (circles) and ~2µm PYR-3/APC film with a PYR-3 concentration of 8.4×1019 cm-3 (plus signs).9 (b) Plot of F(ξ) vs. ξ during photodegradation of a IND-7/APC film with increasing concentrations of 2.4×1019 cm-3, 3.8×1019 cm-3, and 6.1×1019 cm-3. The inset is a plot of ξ1/e against the chromophore concentration. It can be seen in Fig. 3a that F(ξ) extends to larger ξ with increasing optical intensity for a IND-7/APC film with an IND-7 concentration of 3.8×1019 cm-3 that was exposed to 655 nm light. This has previously been observed in PYR-3/APC films and it was attributed to the high optical intensities leading to oxygen depletion due to chromophore photodegradation.8,9 The resultant ξ1/e is plotted in the inset to Fig. 3a where it can be seen that ξ1/e increases with increasing optical intensity. Also shown are data from a PYR-3/APC film9 where it is apparent that ξ1/e is nearly 10 times greater for the IND-7/APC film and shows that the IND-7/APC film is more photostable. Fig. 3b shows F(ξ) for IND-7/APC films with different IND-7 concentrations. It can be seen that F(ξ) extends to larger ξ with increasing IND-7 concentrations, which indicates that films with higher IND-7 concentrations are more photostable. The resultant ξ1/e is plotted in the inset to Fig. 3b. The increased photostability with increasing chromophore concentration may be due to a reduction in the rate of singlet oxygen generation caused by chromophore excited state singlet energy transfer to adjacent chromophores and eventually to non-radiative recombination sites. This process is shown schematically in Fig. 4 where optical absorption leads to occupation of the chromophore S1 state and either emission and decay to the chromophore S0 ground state or intersystem crossing to the chromophore T1 state that results in the generation of singlet oxygen via the S1+3O2→T1+1O2 and T1+3O2→S0+1O2 processes and chromophore degradation.


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However, if a nearby chromophore is close enough, it is possible that there can be energy transfer to the adjacent chromophore S1 state and eventually to a non-radiative decay site.9,36 Chromophore S1 excited state energy transfer to a S1 state in an adjacent chromophore is a dipole-dipole process and the probability varies as 1/r6 where r is the distance between the chromophores37 and hence the energy transfer rate will increase as the chromophore concentration increases. Thus, there will be an increase in the chromophore S1 energy transfer non-radiative recombination rate as the chromophore concentration increases and this will lead to a reduction in the single oxygen generation rate and result in an increase in ξ1/e.

Figure 4. Energy level diagram for the chromophores showing a non-radiative decay process. The diagram on the left shows optical excitation to the S1 excited state. It can then decay to the S0 state or to the T1 state by intersystem crossing leading to singlet oxygen generation. In an isolated chromophore the T1 state can decay to the ground S0 state by intersystem crossing (ISC) in the presence of triplet oxygen and resulting in singlet oxygen. If there are nearby chromophores (middle diagram), it is possible to get excited state energy transfer (ET) via a dipole-dipole mechanism to the adjacent chromophore. This process can continue until a nonradiative decay site is found (right diagram).


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F(ξ) vs ξ from films containing IND-3, IND-5, IND-7, and IND-7R are plotted in Fig. 5 for the same 655 nm laser intensity (2 mW/mm2) and thicknesses (~1.7 µm). It is apparent that ξ1/e is largest for IND-7 that has the longest conjugation length. This can be seen more clearly in Fig. 6 where ξ1/e is plotted (left axis) against the conjugation length, nc. It is also clear that ξ1/e for IND-7R is nearly the same as that for IND-7 and hence ring-locking has not significantly affected the photostability. Interestingly, the static hyperpolarizability, |β0|, also increases with increasing conjugation length.11 |β0| was obtained from the dynamic hyperpolarizability, β, using the 2-level model16-19 and can be written as,

β0 =

3e 3 µ 0 ∆µ

(6)

hω 0 2

where β = β 0 G (ω / ω 0 ) , e is the electron charge, µ0 is the permittivity of free space, ∆µ is the difference between the ground state and first excited state dipole moments, ℏ is Planck's constant divided by 2π, ω0 is the resonant frequency, ω is the operating frequency and G(ω/ω0) is a function that depends on the nonlinear optics process2 (e.g. linear electro-optic, etc.). The increase in |β0| can be seen Fig. 6 (right axis) where |β0| is plotted against the conjugation length. Thus, indoline-based chromophores with longer conjugation lengths have a larger |β0| and they are more photostable, which is particularly useful for device applications.


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Figure 5. Plot of F(ξ) vs. ξ for IND-3, IND-5, IND-7, and IND-7R chromophores in APC when photodegrading using a 635 nm laser with a light intensity of 2 mW/mm2. The concentrations were 7.5×1019 cm-3. The films were all ~1.7 µm thick. The reason for ξ1/e and |β0| increasing with nc is likely to be different in both cases. In the case of photodegradation, the average size of the chromophores will increase with increasing nc. Thus, if the chromophore concentrations are the same for indoline-based chromophores in APC with different nc, then the average separation between chromophores will be smaller for larger nc. Since, as we have mentioned above, chromophore S1 energy transfer to an adjacent chromophore S1 level and eventually to non-radiative recombination sites is strongly dependent on the average separation37 then the energy transfer and hence the chromophore S1 non-radiative decay probability will increase with increasing nc. This will result in a reduction in the singlet oxygen generation rate and hence lead to an increase in ξ1/e. The increase in |β0| with increasing conjugation length may be due to the decrease in the charge-transfer gap energy that is seen in Fig. 2 where from Eq. 6 in can be seen that a decrease in resonant frequency leads to an increase in β.


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Figure 6. Plot of the ξ1/e (left axis) and |β0| (right axis11) as a function of the number of conjugated double bonds (nc) for IND chromophores. ξ1/e was measured using a 635 nm laser with a light intensity of 2 mW/mm2. The concentrations were 7.5×10-19 cm-3 and the thicknesses of the films were all ~1.7 µm. |β0| was obtained from β measured at 1300 nm and using the 2 level model.11 4. CONCLUSIONS In conclusion, we find that the indoline-based chromophores are nearly 10 times more photostable than the PYR-3 chromophore that has a dihydopyridine donor. We also find that ξ1/e increases with increasing optical intensity, which is due to a reduction in the oxygen concentration in the films and hence a decrease in the oxygen-mediated photodegradation. ξ1/e increases with increasing chromophore concentration and this is most likely due to excited state singlet energy transfer between chomophores to non-radiative decay sites. Ring-locking has no significant effect on the photostability. Since it was previously reported that |β0| is also not significantly altered by ring-locking, then ring-locking is a potentially viable method to increase the thermal stability and hence the temporal stability of the 2nd order NLO coefficient after poling. We find that ξ1/e and |β0| both increase with increasing conjugation length. This is advantageous for device applications as it shows that an increase in the molecular NLO response


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is also accompanied by an increase in photostability. The enhancement of ξ1/e is likely to be due to an average decrease in the separation between chromophores due to their increased size. This can lead to increased energy transfer from the singlet excited state to adjacent chromophores and eventually to non-radiative decay sites. The net result will be an enhancement of ξ1/e. The increase in |β0| with increasing conjugation length may be due to the systematic decrease in the charge-transfer transition energy.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +64-4-9313078 Present Address †

Institute of Photonics & Quantum Electronics, and Institute of Microstructure Technology,

Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany. Author Contributions YK and GVMW developed the research plan, analyzed the results, and wrote the paper; YK made the thin films and performed the measurements along with SGR; MDHB and AJK synthesized the organic NLO chromophores and assisted in thin film fabrication. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge funding from MBIE (C08X0704, C08X01206) and the MacDiarmid Institute for Advanced Materials and Nanotechnology. ABBREVIATIONS NLO nonlinear optical; EO electro-optic; APC amorphous polycarbonate; BLA bond-lengthalternation; ET energy transfer; ISC intersystem crossing.


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REFERENCES (1) McLaughlin, C. V.; Hayden, L. M.; Polishak, B.; Huang, S.; Luo, J.; Kim, T.-D.; Jen, A. K. Y. Wideband 15 THz Response Using Organic Electro-Optic Polymer Emitter-Sensor Pairs at Telecommunication Wavelengths. Appl. Phys. Lett. 2008, 92, 151107. (2) Dalton, L. Nonlinear Optical Polymeric Materials: From Chromophore Design to Commercial Applications, in Polymers for Photonic Applications I, Advances in Polymer Science, Ed. K. S. Lee, Springer-Verlag: New York, 2002, pp 1-86. (3) Bosshard, Ch.; Sutter, K.; Prêtre, Ph.; Hulliger, J.; Flörsheimer, M.; Kaatz, P.; Günter, P. Organic Nonlinear Optical Materials, Gordon and Breach: Switzerland, 2001. (4) Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Electric Field Poled Organic Electro-optic Materials: State of the Art and Future Prospects. Chem. Rev. 2010, 110, 25-55. (5) Sullivan, P. A.; Dalton, L. R. Theory-Inspired Development of Organic Electro-optic Materials. Acc. Chem. Res. 2010, 43, 10-18. (6) Chen, D.; Fetterman, H. R.; Chen, A.; Steier, W. H.; Dalton, L. R.; Wang, W.; Shi, Y. Demonstration of 110 GHz Electro-Optic Polymer Modulators. Appl. Phys. Lett. 1997, 70, 3335-3337. (7) Dalton, L.; Harper, A.; Ren, A.; Wang, F.; Todorova, G.; Chen, J.; Zhang, C.; Lee, M. Polymeric Electro-Optic Modulators: From Chromophore Design to Integration with Semiconductor Very Large Scale Integration Electronics and Silica Fiber Optics. Ind. Eng. Chem. Res. 1999, 38, 8-33.


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TABLE OF CONTENTS GRAPHIC


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Effects of Chromophore Conjugation Length and Concentration on the

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