Two-photon Absorption Activity Research Articles

Advancing Two-Photon Photodynamic Therapy Over NIR-II Excitable Conjugated Microporous Polymer with NIR-I Emission.

The availability of second near-infrared (NIR-II) excitable two-photon photosensitizers with NIR-I emission for efficient photodynamic therapy (PDT) is limited by challenges in molecular design. In this study, a NIR-II light-excitable two-photon conjugated microporous polymer (Tph-Dbd) with emission in the NIR-I region is developed. The large conjugated system and delocalized electronic structures endow Tph-Dbd with a large two-photon absorption cross-section under NIR-II light excitation. Moreover, the efficient electron acceptor and donor units within the π-conjugated backbones result in NIR-I emission for high signal-to-background ratio imaging, as well as separated highest occupied molecular orbitaland lowest unoccupied molecular orbital distributions for excellent singlet oxygen generation ability. The excellent NIR-II excitable two-photon absorption activity, NIR-I emission, good biocompatibility, and high photostability allow Tph-Dbd to be used for efficient in vitro fluorescence imaging guided PDT. Moreover, the impressive photothermal effect of Tph-Dbd can overcome the limitations of PDT in the treatment of hypoxic tumors. This study highlights a strategy for designing NIR-II excitable two-photon photosensitizers for advanced PDT.

Chromophore Planarity, -BH Bridge Effect, and Two-Photon Activity: Bi- and Ter-Phenyl Derivatives as a Case Study.

In this work, we have employed electronic structure theories to explore the effect of the planarity of the chromophore on the two-photon absorption properties of bi- and ter-phenyl systems. To that end, we have considered 11 bi- and 7 ter-phenyl-based chromophores presenting a donor-π-acceptor architecture. In some cases, the planarity has been enforced by bridging the rings at ortho-positions by -CH2 and/or -BH, -O, -S, and -NH moieties. The results presented herein demonstrate that in bi- and ter-phenyl systems, the planarity achieved via a -CH2 bridge increases the 2PA activity. However, the introduction of a bridge with the -BH moiety perturbs the electronic structure to a large extent, thus diminishing the two-photon transition strength to the lowest electronic excited state. As far as two-photon absorption activity is concerned, this work hints toward avoiding -BH bridge(s) to enforce planarity in bi- and ter-phenyl systems; however, one may use -CH2 bridge(s) to achieve the enhancement of the property in question. All of these conclusions have been supported by in-depth analyses based on generalized few-state models.

Impact of the Protein Environment on Two-Photon Absorption Cross-Sections of the GFP Chromophore Anion Resolved at the XMCQDPT2 Level of Theory.

The search for fluorescent proteins with large two-photon absorption (TPA) cross-sections and improved brightness is required for their efficient use in bioimaging. Here, we explored the impact of a single-point mutation close to the anionic form of the GFP chromophore on its TPA activity. We considered the lowest-energy transition of EGFP and its modification EGFP T203I. We focused on a methodology for obtaining reliable TPA cross-sections for mutated proteins, based on conformational sampling using molecular dynamics simulations and a high-level XMCQDPT2-based QM/MM approach. We also studied the numerical convergence of the sum-over-states formalism and provide direct evidence for the applicability of the two-level model for calculating TPA cross-sections in EGFP. The calculated values were found to be very sensitive to changes in the permanent dipole moments between the ground and excited states and highly tunable by internal electric field of the protein environment. In the case of the GFP chromophore anion, even a single hydrogen bond was shown to be capable of drastically increasing the TPA cross-section. Such high tunability of the nonlinear photophysical properties of the chromophore anions can be used for the rational design of brighter fluorescent proteins for bioimaging using two-photon laser scanning microscopy.

Two-Photon Absorption Activity of BOPHY Derivatives: Insights from Theory

We present a theoretical study of a two-photon absorption (2PA) process in dipolar and quadrupolar systems containing two BF2 units. For this purpose, we considered 13 systems studied by Ponce-Vargas et al. [J. Phys. Chem. B2017, 121, 10850−1085829136383] and performed linear and quadratic response theory calculations based on the RI-CC2 method to obtain the 2PA parameters. Furthermore, using the recently developed generalized few-state model, we provided an in-depth view of the changes in 2PA properties in the molecules considered. Our results clearly indicate that suitable electron-donating group substitution to the core BF2 units results in a large red-shift of the two-photon absorption wavelength, thereby entering into the desired biological window. Furthermore, the corresponding 2PA strength also increases significantly (up to 30-fold). This makes the substituted systems a potential candidate for biological imaging.

Enhanced Two-Photon Absorption in Two Triphenylamine-Based All-Organic Compounds.

Two-photon absorption (TPA) enables the excitation of molecules by comparatively lower energy photons with longer penetration depth and higher spatial precision control, which advances the uses of organic molecules in various applications. In this work, we report two simple all-organic molecules C42H33N (compound 3) and C138H168N4 (compound 14) with strong TPA and fluorescent emission activity. Density functional theory calculations show that the enhanced oscillator strengths could be responsible for improved TPA and emission activity for compound 14 compared to those for 3. The degradation of C138H168N4 under focused laser illumination without circulation is almost negligible within 5 h, making it a candidate for laser dyes. Solid-state measurements confirm the presence of a direct band gap for electron transition that determines the channel to release the absorbed energy and functionality of the studied molecules. This work emphasizes that a high TPA cross-section and selectable energy relaxation (fluorescent emission or heat dissipation) are equally important to the design of advanced functional TPA molecules.

Synthesis and two-photon properties of phenothiazine-based dendrimers with tetraphenylmethane cores

We have synthesized two new two-photon active dendritic molecules with tetraphenylmethane core with four phenothiazine-based arms. They show good solubility in organic solvents, including dichloromethane, chloroform, toluene and tetrahydrofuran. It is found that the two dendrimers display strong two-photon absorption and two-photon excited up-conversion fluorescence at 800 nm. • Two new phenothiazine-based dendrimers with tetraphenylmethane cores have been synthesized. • The dendritic molecules display two-photon absorption activity. • The two-photon absorption cross section increased with the increasing of the conjugation degree.

Nonlinear optical properties and markedly higher two photon absorption of ordered c-shaped plasmon-active metal nanostructures

Development of materials with highly nonlinear optical activity represents an intensively studied discipline due to potentially unique applications of such materials in photonics and information technologies. In this work the creation of ordered nanostructured arrays of c-shaped metals (Ag, Au, Al, and Pd) and characterization of their nonlinear optical properties are described. Large scale ordered arrays were created by the excimer laser pattering of a polymer surface and subsequent coating with Ag, Au, Al, and Pd. The successful creation of required structures was confirmed by the conductive AFM and FIB-SEM techniques. Linear optical response of the structures was examined using the UV–Vis technique and strong excitation of surface plasmon polariton resulting in appearance of strong absorption band was confirmed. The Z-scan technique with femtosecond laser pulses was used to determine the nonlinear optical response of the prepared metal arrays. It was found that the strong two-photon absorption appears under the illumination at the wavelength corresponding to the surface plasmon excitation. Extensive comparison with available literature data shows that the present structures exhibit markedly higher two-photon absorption activity.

Activated Type I and Type II Process for Two-Photon Promoted ROS Generation: The Coordinated Zn Matters.

The construction of novel classes of photosensitizers for efficient reactive oxygen species (ROS) generation is of great interest, yet it is a challenge. In this work, a bis(terpyridine)zinc(II) complex (namely, ZnL1) with two-photon absorption activity as an efficient ROS photogenerator was synthesized. Benefiting from the coordinated Zn, the decreased singlet-triplet energy gap favors the intersystem crossing process facilitating the singlet oxygen (1O2) generation via energy transfer. In addition, it makes the superoxide radical (O2·-) generation easier. This is an extremely rare study on two-photon excited ROS generation by activating type I and type II processes based on a cheaper and bioaccessible Zn complex.

Hybrids of gold nanoparticles and oligo(p-phenyleneethynylene)s end-functionalized with alkynylruthenium groups: Outstanding two-photon absorption in the second biological window

Oligo(p-phenyleneethynylene)s (OPEs) end-capped with (alkynyl)bis(diphosphine)ruthenium and thiol/thiolate groups stabilize ca. 2 nm diameter gold nanoparticles (AuNPs). The morphology, elemental composition and stability of the resultant organometallic OPE/AuNP hybrid materials have been defined using a combination of molecular- and nano-material chacterization techniques. The hybrids display long-term stability in solution (more than a month), good solubility in organic solvents, reversible ruthenium-centered oxidation, and transparency beyond 800 nm, and possess very strong nonlinear absorption activity at the first biological window, and unprecedented two-photon absorption activity in the second biological window (σ2 up to 38,000 GM at 1,050 nm).

Arylamine-coumarin based donor-acceptor dyads: Unveiling the relationship between two-photon absorption cross-section and lifetime of singlet excited state intramolecular charge separation

Two-photon absorption (2 PA) with three-dimensional (3D) resolution and deep penetration of light into absorbing materials hold great promise for many applications such as 3D microscopy, 3D microfabrication, polymerization and optical data storage. We herein designed and synthesized two novel series of arylamine-coumarin based donor-acceptor dyads that showed good two-photon absorption activities. Tested by an open/close aperture Z-scan technique, the largest 2 PA cross-section value is 103.6 GM and the largest two-photon absorption coefficient (β) is 7.84 × 10−12 cm W−1. Fluorometric and electrochemical studies further reveal the formation of intramolecular charge transfer (ICT) among two dyads. Photoinduced singlet excited state intramolecular charge separation states were confirmed by ultrafast laser flash photolysis analyses coupled with TD-DFT excited state calculations. Surprisingly, the experimental measured lifetime of singlet excited state intramolecular charge separation state may be a diagnostic criterion for 2 PA cross-section of donor-acceptor systems. This work provides a new strategy for designing high-efficiency 2 PA materials.

Perylenetetracarboxy-3,4:9,10-diimide derivatives with large two-photon absorption activity

Perylenetetracarboxy-3,4:9,10-diimides with large TPA cross-sections.

A novel multifunctional cyclometallated iridium(III) complex with interesting second-order nonlinear optical properties and two-photon absorption activity

A novel cationic cyclometallated Ir(III) complex, namely [Ir(C^N-ppy-4-CHCHC6H4NEt2)2(N^N-bpy-CHCHC6H4NEt2)][PF6] (ppy = 2-phenylpyridine; bpy = 2,2′-bipyridine), was prepared and well characterized. Its good second-order NLO response, as determined in solution by the EFISH technique, and its interesting two-photon absorption activity, as measured by the Z-scan technique in the femtosecond case, make it a good candidate for application in photonics and biomedicine.

Two Photon Absorption Study of Low-Bandgap, Fully Conjugated Perylene Diimide-Thienoacene-Perylene Diimide Ladder-Type Molecules

A new series of donor–acceptor ladder-type molecules were synthesized via Scholl reaction. These molecules contain up to 25-fused rings but still show good air stability and good solubility. The ring-fusing reaction is found to be sensitive to the nature of the side-chain in donor units. The molecular conformations were investigated by 2D NMR and DFT calculations. The photophysical properties were investigated and an intense intramolecular charge-transfer was observed. All of the molecules exhibit two-photon absorption (TPA) activity, and their TPA cross-section shows a linear relationship with increasing conjugation length of the thienoacene-PDI derivatives.

Small water-soluble pyrimidine hexafluorophosphate derivatives with high two-photon absorption activities in the near-IR region and their biological applications

Two imidazole–pyrimidine hexafluorophosphate derivatives (6P and 10P) are synthesized. The 2PA cross-section values (σ) were obtained by Z-scan and two-photon excited fluorescence methods. 6P and 10P can be used to quantify and image organelles in living cells and tissues.

Enhanced two-photon absorption property of silver nanoparticle aggregates induced by a thioether derivative

A novel thioether derivative with two-photon absorption activity, 4,4'-((4-(dimethylamino)phenyl)methylene)bis (sulfanediyl)dianiline (abbreviated as L), was designed and synthesized, which was used to couple with Ag nanoparticles (Ag NPs, ∼6 nm) to construct L-Ag hybrid particles with L uniformly dispersed on the surface of Ag NPs. The newly-formed hybrid particles self-assembled through L-L interactions between L molecules in one hybrid particle and adjacent particle to from Ag NPs aggregates (100 nm in diameter). By Raman and XPS analysis, the interfacial interaction ‘hot spot’ was determined, which was between thioether group and primary amino group of L molecule and Ag+ ion on the surface of pure Ag NPs. The interfacial interactions between the two components brought about changeable linear optical properties and enhanced nonlinear optical properties, two-photon absorption cross section and two-photon absorption coefficient included. Furthermore, the optical power limiting application of Ag NPs aggregates was also optimized by this means.

Elucidating the Mechanism of Zn(2+) Sensing by a Bipyridine Probe Based on Two-Photon Absorption.

In this work, we examine, by means of computational methods, the mechanism of Zn(2+) sensing by a bipyridine-centered, D-π-A-π-D-type ratiometric molecular probe. According to recently published experimental data [Divya, K. P.; Sreejith, S.; Ashokkumar, P.; Yuzhan, K.; Peng, Q.; Maji, S. K.; Tong, Y.; Yu, H.; Zhao, Y.; Ramamurthy, P.; Ajayaghosh, A. A ratiometric fluorescent molecular probe with enhanced two-photon response upon Zn(2+) binding for in vitro and in vivo bioimaging. Chem. Sci. 2014, 5, 3469-3474], after coordination to zinc ions the probe exhibits a large enhancement of the two-photon absorption cross section. The goal of our investigation was to elucidate the mechanism behind this phenomenon. For this purpose, linear and nonlinear optical properties of the unbound (cation-free) and bound probe were calculated, including the influence of solute-solvent interactions, implicitly using a polarizable continuum model and explicitely employing the QM/MM approach. Because the results of the calculations indicate that many conformers of the probe are energetically accessible at room temperature in solution and hence contribute to the signal, structure-property relationships were also taken into account. Results of our simulations demonstrate that the one-photon absorption bands for both the unbound and bound forms correspond to the bright π → π* transition to the first excited state, which, on the other hand, exhibits negligible two-photon activity. On the basis of the results of the quadratic response calculations, we put forward a notion that it is the second excited state that gives the strong signal in the experimental nonlinear spectrum. To explain the differences in the two-photon absorption activity for the two lowest-lying excited states and nonlinear response enhancement upon binding, we employed the generalized few-state model including the ground, first, and second excited states. The analysis of the optical channel suggests that the large two-photon response is due to the coordination-induced increase of the transition moment from the first to the second excited state.

Small molecules of chalcone derivatives with high two-photon absorption activities in the near-IR region

Six thiophene-based chalcone derivatives were synthesized, and their structure–property relationship was investigated systematically. Small molecules as they are, they have showed favorable Φ and σ values in the near-IR region.

π-Extended diketopyrrolopyrrole-porphyrin arrays: one- and two-photon photophysical investigations and theoretical studies.

A complete one- and two-photon spectroscopic and photophysical characterization of three diketopyrrolopyrrole (DPP)-porphyrin conjugates is reported. The increased conjugation introduced by the incremental addition of one, two and four DPP units on the meso porphyrin positions strongly affects the optical properties of the systems. Ground and triplet excited state absorption spectra show a gradual broadening and bathochromic shift and a trend to lower energies is also observed for both fluorescence and phosphorescence emission. Interestingly, the fluorescence quantum yield increases along the series, leading to remarkable NIR emission properties for the larger derivatives. Unlike the model porphyrin, all derivatives exhibit high two-photon absorption activity. An increase in two-photon absorption cross-section in the regions 800-840 nm and 910-930 nm is observed moving from one DPP to two DPP appended units, with a value of the order of 4000 GM at 910 nm for the latter system. The four compounds show high efficiency in generating singlet oxygen, with yields ranging from 0.7 to 0.5, envisaging favourable applications in both one- and two-photon photodynamic therapies. A detailed theoretical exploration of both linear (absorption and emission) and non-linear (two-photon absorption) properties proposes an analysis of the experimental spectra and a comprehensive interpretation of the two-photon activity within the series of compounds.

Novel colorimetric detection probe for copper(II) ions based on triphenylamine mixed-valence chromophores bearing prodigious two-photon absorption activity

Four conjugated organic triphenylamine-based mixed-valence chromophores, T1–T4, with large two-photon absorption (2PA) across sections were designed, synthesized and fully characterized by IR, MS, EI and NMR spectroscopy. Photophysical properties and mixed-valence characters of them were investigated. It was found that T4 displayed prodigious 2PA response when extending conjugation system and enriching electron density within the molecule. The enriched electron character of T4 makes it to be selectively oxidized with copper salt in solution, forming stable mixed-valence state and then used for colorimetric detection of Cu2+ ions. A simple device was fabricated using T4 for “naked eye” detection of Cu2+. The device can detect Cu2+ as low as 0.01μM.

A novel two-photon absorbing dye derived from a V-shaped chromophore containing triphenylamine and dipicolinate: Synthesis and application as probe for proton, water, mercury ion and cell imaging

A novel two-photon absorbing dye (TP-Py) was synthesized by introduction of a formyl group into the triphenylamine fragment of a V-shaped compound (compound 1). Compared to the corresponding compound 1, TP-Py exhibits enhanced two-photon absorption activity. When measured by Z-scan technique at 800nm, the two-photon cross-section of TP-Py in THF was 1380 GM. Meanwhile, the introduction of a formyl group in compound 1 greatly improves the potential application of the molecule. TP-Py exhibits significant absorption and fluorescence spectral sensitivities in response to changes in pH. Especially, the pKa value of the pH probe is in the range of 4.5–6.5, which is critical for precisely assessing the cellular acidic environments. The fluorescence of TP-Py can be made to measure water content of organic solvents. The detection limit was calculated to be 0.04% for THF. The probing behaviors of TP-Py toward metal ions were investigated via absorption spectra as well as one- and two-photon fluorescence changes. The results indicated that TP-Py exhibits high sensitivity and selectivity for mercury ion. TP-Py was also chosen for two-photon cell imaging. The two-photon scanning microscopy experiment suggests that TP-Py is a promising label potentially applicable for the tracking of biomolecules.