Rational Molecular Design and Concise Synthesis of High-Performance Near-Infrared Chromophores for Optical Modulation and Phototheranostics
高效近紅外生色團分子的合理設計、簡易合成及應用於光調製和光診療
Student thesis: Doctoral Thesis
Author(s)
Related Research Unit(s)
Detail(s)
| Awarding Institution | |
|---|---|
| Supervisors/Advisors |
|
| Award date | 2 Aug 2023 |
Link(s)
| Permanent Link | https://scholars.cityu.edu.hk/en/theses/theses(4e010900-6f03-4c7c-bec9-7200a1163ca8).html |
|---|---|
| Other link(s) | Links |
Abstract
High performance organic and polymeric electro-optic (EO) materials and devices are of intense interest to meet the challenge by encoding electrical signals into the optical domain for ultrafast signal processing in future 5G/6G technologies. Compared with inorganic materials, organic electro-optic (EO) materials display significant advantages such as large EO coefficients, ultrafast response time, and excellent processibility in integrated photonic platforms. The past two decades have witnessed significant research progress in the fundamental understanding of organic EO materials’ structure-property relationship and the demonstration of device prototypes. One major challenge in the recent research of organic EO materials is whether donor-π-bridge-acceptor (D-π-A) chromophores exhibiting large molecular hyperpolarizabilities (β) can be rationally designed, efficiently synthesized, and reliably processed to construct poled waveguide films with large EO coefficients, good optical transparency, and excellent stability. It determines the central theme of this research on the concise synthesis, material processing, and systematic characterization of high-performance organic EO materials for photonic applications.
Chapter 2 focuses on the selection of effective donor units to enhance the β value of tricyanofuran-based (TCF) dipolar heptamethines, including indoline (F1), benzo[e]indoline (F2), benz[cd]indoline (F3), and Michler’s base derivatives (M1 and M2). The linear and nonlinear optical (NLO) properties of these chromophores have been thoroughly investigated, and the relationship between molecular and bulk response has been analyzed. These push-pull heptamethines with synthetic efficacy exhibit high near-infrared absorption, excellent chemical stability, and large hyperpolarizabilities (β) varied from 1023×10-30 esu for F1, 3047×10-30 esu for M1, and 3547×10-30 esu for F3 at 1304 nm in poled films, respectively. Where, chromophores taking benz[cd]indoline, and Michler’s base derivative as donor units lead to the best EO potential for EO application.
In Chapter 3, a 3D-shaped 4,5-diphenyl-oxazol-2-ylsulfanyl group is introduced to the central position of the π-conjugated bridge of TCF-based push-pull heptamethines. Crystallographic studies show a very uncommon crystal packing mode of one-dimensional chain structure with vanished C-H···N (cyano) contacts and negligible π-π stacking. Such reduced intermolecular interactions and fine-tuned bond length alternation lead to NLO performance breakthrough of these chromophores in poled polymeric films, including much better EO coefficients up to 465% in improvement, higher polar order, and larger molecular hyperpolarizabilities (up to 4135×10-30 esu at 1304 nm), over the results from the simple chloro-substituted heptamethines.
In chapter 4, bis(4-fluorophenyl)-substituted TCF (FP-TCF) is introduced to the second-order nonlinear optics. The FP-TCF acceptor is synthesized through two consecutive multiple steps, one-pot reactions to achieve a good overall yield of 50%, which is higher than that of making CF3-TCF. The reduced π-π stacking and fine-tuned bond-length alternation boost the nonlinear optical activities of FP-TCF-based heptamethines in plasmon-coupled EO polymer waveguides, leading to larger EO coefficients (up to 205%) over those of the TCF-based chromophores. More importantly, we demonstrate an ultra-large molecular first hyperpolarizability of 8437×10-30 esu at 1304 nm for M1a-FP-ON, which exceeds the results of two best-performing but synthetically demanding push-pull tetraene chromophores.
After achieving high first hyperpolarizability, chapter 5 focuses on enhancing the loading density of chromophores and optimizing film processing. The tert-butyldiphenylsilyl (TBDPS) group is introduced to one arm of Michler’s base as a good rigid isolation group. After TBDPS is introduced to the donor unit, a higher loading density can be achieved. For M3-FP-PhON, the EO coefficient (r33) is 135.6 pm/V at the number density of 1.6×1020/cm3 in poly(styrene-co-methyl methacrylate) (P(S-co-MMA)). The SnO2 colloid dispersion is spin-coated on ITO glass with an annealing temperature of no more than 150 ºC. Due to its low-lying valence band, the SnO2 layer is a very effective barrier to reduce the hole injection and suppresses the space charge accumulation during the poling process, leading to decreased current density and improved EO coefficient. With the help of the SnO2 barrier layer, the r33 increases to 149.1 pm/V.
Chapter 6 discusses about improving the optical transparency of organic EO materials at the telecomm wavelengths by controlling the absorption bands of chromophores in the near infrared. When the 4,5-diphenyl-oxazol-2-ylsulfanyl is replaced by the aromatic substituents, the optical loss of chromophores in P(S-co-MMA) with a loading density of 0.6×1020/cm3 at 1304 nm decreased from 37.1 dB/cm for M1a-FP-ON to 1.5 dB/cm for M1a-Ph-FP. The decreased optical loss is caused by the blue shift of the maximum absorption wavelength (λmax) and decreased Urbach energy of the chromophore. M1a-Ph-FP displays a similar r33 value with M1a-FP-ON, 121.0 pm/V at 1304 nm, and improved thermal stability with Td increasing from 233 ºC for M1a-FP-ON to 252 ºC for M1a-Ph-FP.
Through collaboration, chapter 7 explores the new application of nonlinear optical chromophores in phototheranostics for their strong optical absorption in the near-infrared region. A diradicaloid molecule (DRM) connecting a strong donor (N,N-bis(4-butoxyphenyl)thiophen-2-amine) and a robust acceptor (croconic acid) has demonstrated one of the highest mass extinction coefficient (ε) of ∼220 L·g-1·cm-1 at 854 nm. The assembled nanoparticles of DRM showed good water dispersibility, excellent photostability, and high photothermal conversion efficiency (PCE) of 68%. We also synthesize organic small molecules through a photochemical reaction under ambient temperature by using a portable UV lamp with high yields (71-80%). The nonplanar small molecules (NSMs) containing Michler’s base donors and the tricyanoquinodimethane (TCQ) acceptor exhibit broad absorption edge over 1000 nm, high NIR extinction coefficients, and remarkable intramolecular rotation upon photoexcitation. The assembled nanoparticles (NSMN) possess good biocompatibility and a high PCE of 75%.
Chapter 2 focuses on the selection of effective donor units to enhance the β value of tricyanofuran-based (TCF) dipolar heptamethines, including indoline (F1), benzo[e]indoline (F2), benz[cd]indoline (F3), and Michler’s base derivatives (M1 and M2). The linear and nonlinear optical (NLO) properties of these chromophores have been thoroughly investigated, and the relationship between molecular and bulk response has been analyzed. These push-pull heptamethines with synthetic efficacy exhibit high near-infrared absorption, excellent chemical stability, and large hyperpolarizabilities (β) varied from 1023×10-30 esu for F1, 3047×10-30 esu for M1, and 3547×10-30 esu for F3 at 1304 nm in poled films, respectively. Where, chromophores taking benz[cd]indoline, and Michler’s base derivative as donor units lead to the best EO potential for EO application.
In Chapter 3, a 3D-shaped 4,5-diphenyl-oxazol-2-ylsulfanyl group is introduced to the central position of the π-conjugated bridge of TCF-based push-pull heptamethines. Crystallographic studies show a very uncommon crystal packing mode of one-dimensional chain structure with vanished C-H···N (cyano) contacts and negligible π-π stacking. Such reduced intermolecular interactions and fine-tuned bond length alternation lead to NLO performance breakthrough of these chromophores in poled polymeric films, including much better EO coefficients up to 465% in improvement, higher polar order, and larger molecular hyperpolarizabilities (up to 4135×10-30 esu at 1304 nm), over the results from the simple chloro-substituted heptamethines.
In chapter 4, bis(4-fluorophenyl)-substituted TCF (FP-TCF) is introduced to the second-order nonlinear optics. The FP-TCF acceptor is synthesized through two consecutive multiple steps, one-pot reactions to achieve a good overall yield of 50%, which is higher than that of making CF3-TCF. The reduced π-π stacking and fine-tuned bond-length alternation boost the nonlinear optical activities of FP-TCF-based heptamethines in plasmon-coupled EO polymer waveguides, leading to larger EO coefficients (up to 205%) over those of the TCF-based chromophores. More importantly, we demonstrate an ultra-large molecular first hyperpolarizability of 8437×10-30 esu at 1304 nm for M1a-FP-ON, which exceeds the results of two best-performing but synthetically demanding push-pull tetraene chromophores.
After achieving high first hyperpolarizability, chapter 5 focuses on enhancing the loading density of chromophores and optimizing film processing. The tert-butyldiphenylsilyl (TBDPS) group is introduced to one arm of Michler’s base as a good rigid isolation group. After TBDPS is introduced to the donor unit, a higher loading density can be achieved. For M3-FP-PhON, the EO coefficient (r33) is 135.6 pm/V at the number density of 1.6×1020/cm3 in poly(styrene-co-methyl methacrylate) (P(S-co-MMA)). The SnO2 colloid dispersion is spin-coated on ITO glass with an annealing temperature of no more than 150 ºC. Due to its low-lying valence band, the SnO2 layer is a very effective barrier to reduce the hole injection and suppresses the space charge accumulation during the poling process, leading to decreased current density and improved EO coefficient. With the help of the SnO2 barrier layer, the r33 increases to 149.1 pm/V.
Chapter 6 discusses about improving the optical transparency of organic EO materials at the telecomm wavelengths by controlling the absorption bands of chromophores in the near infrared. When the 4,5-diphenyl-oxazol-2-ylsulfanyl is replaced by the aromatic substituents, the optical loss of chromophores in P(S-co-MMA) with a loading density of 0.6×1020/cm3 at 1304 nm decreased from 37.1 dB/cm for M1a-FP-ON to 1.5 dB/cm for M1a-Ph-FP. The decreased optical loss is caused by the blue shift of the maximum absorption wavelength (λmax) and decreased Urbach energy of the chromophore. M1a-Ph-FP displays a similar r33 value with M1a-FP-ON, 121.0 pm/V at 1304 nm, and improved thermal stability with Td increasing from 233 ºC for M1a-FP-ON to 252 ºC for M1a-Ph-FP.
Through collaboration, chapter 7 explores the new application of nonlinear optical chromophores in phototheranostics for their strong optical absorption in the near-infrared region. A diradicaloid molecule (DRM) connecting a strong donor (N,N-bis(4-butoxyphenyl)thiophen-2-amine) and a robust acceptor (croconic acid) has demonstrated one of the highest mass extinction coefficient (ε) of ∼220 L·g-1·cm-1 at 854 nm. The assembled nanoparticles of DRM showed good water dispersibility, excellent photostability, and high photothermal conversion efficiency (PCE) of 68%. We also synthesize organic small molecules through a photochemical reaction under ambient temperature by using a portable UV lamp with high yields (71-80%). The nonplanar small molecules (NSMs) containing Michler’s base donors and the tricyanoquinodimethane (TCQ) acceptor exhibit broad absorption edge over 1000 nm, high NIR extinction coefficients, and remarkable intramolecular rotation upon photoexcitation. The assembled nanoparticles (NSMN) possess good biocompatibility and a high PCE of 75%.
