2,6-Bis[4-(p-dihexylaminostyryl)styryl]anthracene Derivatives with Large Two-Photon Cross Sections

Synthesis, structure and photophysical properties of truxene and truxene-triindole compounds

Multi-branched π-conjugated organic molecules, due to their fascinating structures and unique optical properties, exhibit potential applications in optoelectronics. Herein, series of 4-N,N-diphenylaminostyryl substituted multi-branched truxene compounds (namely TN1, TN2 and TN3) and a truxene-triindole compound (namely N3T3) were synthesized. These compounds are characterized by effective π-extension and remarkably enhanced two-photon absorption (TPA) compared to their less π-conjugated analogues. For truxene compounds, the more the number of the branch, the large the TPA efficiency, and the C3-symmetric three-branched TN3 exhibit the largest TPA. The aromatic triindole unit proves to be a better two-photon fluorophore compared with truxene. The results conclude that the influence of π-conjugation, molecular planarity and intramolecular charge-transfer (ICT) to TPA is far more pronounced than to linear optical properties.

Two-photon fluorescence imaging of mitochondrial viscosity with water-soluble pyridinium inner salts

Viscosity-induced emission of fluorogenic probes was used to detect intracellular mitochondrial viscosity, even in different tissues and/or zebrafish via TPFM.

Fluorescence Probe for Imaging N-Methyl-d-aspartate Receptors and Monitoring GSH Selectively Using Two-Photon Microscopy

N-Methyl-d-aspartate (NMDA) is an excitotoxic amino acid used to identify a specific subset of glutamate receptors. The activity of NMDA receptors is closely related to the redox level of the biological system. Glutathione (GSH) as an antioxidant plays a key role with regard to modulation of the redox environment. In this work we designed and developed a GSH-specific fluorescent probe with the capability of targeting NMDA receptors, which was composed of a two-photon naphthalimide fluorophore, a GSH-reactive group sulfonamide, and an ifenprodil targeting group for the NMDA receptor. This probe exhibited high selectivity toward GSH in comparison to other similar amino acids. Two-photon fluorescence microscopy allowed this probe to successfully monitor GSH in neuronal cells and hippocampal tissues with an excitation at 750 nm. It could serve as a potential practical imaging tool to explore the function of GSH and related biological processes in the brain.

Photophysical properties and stimuli-responsive crystal-state luminescence switching of morpholine-modified naphthalic anhydride derivative

Stimuli-response organic solid-state luminescence switching materials are attracting increasing interest due to their smart photophysical properties. In this study, a morpholine-modified naphthalic anhydride derivative, 4-(4-morpholinyl)naphthalic anhydride (MBC), was synthesized and studied. Its two crystal polymorphs, MBC-G and MBC-O, were obtained under different crystallization conditions. These two distinct crystals show significantly different solid-state luminescence behaviors: a green emission at 535 nm for MBC-G, and an orange emission at 572 nm for MBC-O. Upon fiercely grinding the MBC-G crystal or melting and then quickly cooling it, a phase transition occurs from MBC-G to MBC-O, accompanied by a fluorescence change from green to orange. The reverse transformation from MBC-O to MBC-G can be achieved by recrystallization. The X-ray single crystal structures show that the green emission should be attributed to molecular J-aggregation in the crystal packing, and the orange emission may originate from molecular H-aggregation. This switchable color nature gives MBC the promising candidate for potential smart anti-counterfeiting and light-emitting materials.

A zwitterionic red-emitting water-soluble conjugated polymer with high resistance to nonspecific binding for two-photon cell imaging and good singlet oxygen production capability

A zwitterionic red-emitting water-soluble conjugated polymer exhibited better two-photon cell imaging and singlet oxygen production capability than its cationic analogue.

PyrPeg, a Blood-Brain-Barrier-Penetrating Two-Photon Imaging Probe, Selectively Detects Neuritic Plaques, Not Tau Aggregates

Amyloid-β (Aβ) tracers have made a significant contribution to the treatment of Alzheimer's disease (AD) by allowing a definitive diagnosis in living patients. Unfortunately, they also detect tau and other protein aggregates that compromise test accuracy. In AD research, there has been a growing need for in vivo Aβ imaging by two-photon microscopy, which enables deep-brain-fluorescence imaging. There is no suitable neuritic Aβ probe for two-photon microscopy. Here we report PyrPeg, a novel two-photon fluorescent probe that can selectively target insoluble Aβ rather than tau and α-synuclein aggregates in the AD model brain and postmortem brain. When injected intravenously, PyrPeg detects the neuritic plaques in the brain and olfactory bulb of the AD model. PyrPeg may serve as a useful blood-brain-barrier-penetrating diagnostic tool for optical and functional monitoring of insoluble forms of Aβ aggregates in the living AD brain.

Anthracene derivatives as broadband nonlinear optical materials: nonlinear absorption and excited-state dynamics analysis

Two anthracene derivatives, AN-1 and AN-2, with different π-bridge lengths were designed and synthesized to investigate their optical nonlinearities. The nonlinear absorption (NLA) properties of both derivatives were measured via the femtosecond Z-scan technique with the wavelength range from 532 nm to 800 nm. The reverse saturable absorption (RSA) of both compounds results from two-photon absorption induced excited-state absorption (TPA-ESA). At all wavelengths, the reverse saturable absorption of AN-2 is superior to that of AN-1 due to a better molecular planarity for AN-2. Compared with the results of AN-1, the two-photon absorption coefficient of AN-2 can be increased by nearly 8 times (from 0.182 × 10⁻² cm GW⁻¹ for AN-1 to 1.42 × 10⁻² cm GW⁻¹ for AN-2) at 600 nm by extending the π-bridge. The evolution of femtosecond transient absorption (TA) spectra reveals the relaxation process from the singlet local excited-state (LES) to charge transfer state (CTS) for both compounds. The results imply that anthracene derivatives may be potential candidates for applications in future laser photonics.

Expanding the π-bridge to adjust the molecular planarity via increasing the amount of ethylene can modulate the nonlinear optical response.

Two-photon fluorescent probe with enhanced absorption cross section for relay recognition of Zn2+/P2O74- and in vivo imaging

A novel multifunctional probe, L, based on triphenylamine o-hydroxyl Schiff base was constructed for the sequential detection of Zn²⁺/P₂O₇^4-. Interestingly, probe L also showed two-photon fluorescent "off-on" response to Zn²⁺ along with a large effective two-photon absorption cross-section value of 240 GM at 720 nm, a low cytotoxic and a moderate photostability, which made L a good candidate for two-photon fluorescence microscopy imaging and monitoring the fluctuation of exogenous Zn²⁺.

A Two-Photon Fluorescent Probe for Imaging Endogenous ONOO- near NMDA Receptors in Neuronal Cells and Hippocampal Tissues

In this study, we developed a two-photon fluorescent probe for detection of peroxynitrite (ONOO^-) near the N-methyl-d-aspartate (NMDA) receptor. This naphthalimide-based probe contains a boronic acid reactive group and an ifenprodil-like tail, which serves as an NMDA receptor targeting unit. The probe displays high sensitivity and selectivity, along with a fast response time in aqueous solution. More importantly, the probe can be employed along with two-photon fluorescence microscopy to detect endogenous ONOO^- near NMDA receptors in neuronal cells as well as in hippocampal tissues. The results suggest that the probe has the potential of serving as a useful imaging tool for studying ONOO^- related diseases in the nervous system.

Celecoxib Conjugated Fluorescent Probe for Identification and Discrimination of Cyclooxygenase-2 Enzyme in Cancer Cells

Cyclooxygenase-2 (COX-2) is an enzyme overexpressed in most types of cancers and has been used for an excellent targetable biomarker. Celecoxib is an effective inhibitor of COX-2, used in anti-inflammation. Herein we report a one and two-photon fluorescence probe (NP-C6-CXB) for COX-2, based on the conjugation of naphthalamide with Celecoxib, by using flexible hexylene linker. NP-C6-CXB is nonfluorescent in buffer solution and normal cells, while it shows bright fluorescence in solutions and cancer cells in the presence of COX-2 with an excellent selectivity. Interestingly, NP-C6-CXB can discriminate cancer cells (MCF-7) from normal cells (COS-7) in the single culture medium under confocal microscopy. Due to the selective binding affinity of NP-C6-CXB with a COX-2 enzyme, the intensity is proportional to the level of COX-2 enzyme in cancer cells. In vivo and in vitro experiments proved that NP-C6-CXB is a potential tool for identification of tumor and might be used in surgical resection of COX-2 expressed tumors.

Selective and rapid detection of ascorbic acid by a cobalt oxyhydroxide-based two-photon fluorescent nano-platform

A novel and facilitated two-photon approach was developed for efficient ascorbic acid (AA) detection.

A Dual-Signaling Ferrocene-Pyrene Dyad: Triple-Mode Recognition of the Cu(II) Ions in Aqueous Medium

We report a structure of ferrocene-pyrene conjugate (1) comprising electro and photo-active dual-signaling units. In particular, 1 upon interaction with Cu(II), displays selectively one-photon fluorescence quenching, but it shows two-photon absorption (TPA) cross-section 1230 GM (at 780 nm). Further, 1 displayed two irreversible oxidative waves at 0.39 V and 0.80 V (vs Ag/AgCl), in the electrochemical analysis which upon addition of Cu²⁺, led to the negative potential shift in both the oxidative waves to appear at 0.25 V and 0.68 V. The triple mode changes in presence of Cu(II) suggesting the possible application of 1 for the detection of Cu(II) in aqueous media. Graphical Abstract.

Symmetry Breaking in Pyrrolo[3,2-b]pyrroles: Synthesis, Solvatofluorochromism and Two-photon Absorption

Five centrosymmetric and one dipolar pyrrolo[3,2-b]pyrroles, possessing either two or one strongly electron-withdrawing nitro group have been synthesized in a straightforward manner from simple building blocks. For the symmetric compounds, the nitroaryl groups induced spontaneous breaking of inversion symmetry in the excited state, thereby leading to large solvatofluorochromism. To study the origin of this effect, the series employed peripheral structural motifs that control the degree of conjugation via altering of dihedral angle between the 4-nitrophenyl moiety and the electron-rich core. We observed that for compounds with a larger dihedral angle, the fluorescence quantum yield decreased quickly when exposed to even moderately polar solvents. Reducing the dihedral angle (i.e., placing the nitrobenzene moiety in the same plane as the rest of the molecule) moderated the dependence on solvent polarity so that the dye exhibited significant emission, even in THF. To investigate at what stage the symmetry breaking occurs, we measured two-photon absorption (2PA) spectra and 2PA cross-sections (σ_2PA ) for all six compounds. The 2PA transition profile of the dipolar pyrrolo[3,2-b]pyrrole, followed the corresponding one-photon absorption (1PA) spectrum, which provided an estimate of the change of the permanent electric dipole upon transition, ≈18 D. The nominally symmetric compounds displayed an allowed 2PA transition in the wavelength range of 700-900 nm. The expansion via a triple bond resulted in the largest peak value, σ_2PA =770 GM, whereas altering the dihedral angle had no effect other than reducing the peak value two- or even three-fold. In the S₀ →S₁ transition region, the symmetric structures also showed a partial overlap between 2PA and 1PA transitions in the long-wavelength wing of the band, from which a tentative, relatively small dipole moment change, 2-7 D, was deduced, thus suggesting that some small symmetry breaking may be possible in the ground state, even before major symmetry breaking occurs in the excited state.

A tumor-targeting and lysosome-specific two-photon fluorescent probe for imaging pH changes in living cells

Lysosomal pH is closely related to the metastasis and apoptosis of cancer cells. Detecting lysosomal pH changes in cancer cells could be helpful for analyzing tumor progressions and in-depth study of the roles of lysosomes in tumor invasion and metastasis. Herein, we describe a novel tumor-targeting and lysosome-specific two-photon fluorescent probe (BN-lys) for imaging pH changes for the first time. Biotin was employed as the tumor-targeting module, and morpholine was selected as the lysosome-specific group and the pH site to control the fluorescence by photoinduced electron transfer (PET) mechanism. With a pK_a value of 5.36, BN-lys showed a fast and reversible fluorescence response to pH. Under the guidance of the biotin group, BN-lys displayed strong one-photon and two-photon fluorescence responses to lysosomal pH in cancer cells, while it displayed weak fluorescence in normal cells. Furthermore, BN-lys could be applied for the imaging of chloroquine-stimulated lysosomal pH changes in living cells. These features demonstrate that this probe could have practical applications in biological research.

Highly Selective and Sensitive One- and Two-Photon Ratiometric Fluorescent Probe for Intracellular Hydrogen Polysulfide Sensing

Hydrogen polysulfide (H2Sn) has attracted increasing attention due to the fact that it is actually the key signaling molecule rather than hydrogen sulfide (H2S). Therefore, developing a sensitive and accurate assay to investigate the biosynthetic pathways of H2Sn is of physiological and pathological significance. In this work, based on the commonly used two-photon fluorophore, 1,8-naphthalimide, a new probe, NRT-HP, has been designed and synthesized that displayed both one- and two-photon ratiometric fluorescence changes toward H2Sn via H2Sn-mediated benzodithiolone formation. NRT-HP exhibits excellent pH stability, high selectivity and low detection limit (0.1 μM) in aqueous media. Furthermore, two-photon fluorescence microscopy experiments have demonstrated that NRT-HP could be used for the H2Sn detection in live cells as well as tissue slices.

A carboxylesterase-selective ratiometric fluorescent two-photon probe and its application to hepatocytes and liver tissues

Carboxylesterases (CEs) are widely distributed enzymes in the human body that catalyze hydrolysis of various endogenous and exogenous substrates. They are directly linked to hepatic drug metabolisms and steatosis, and their regulations are important issues in pharmacological and clinical applications. In this work, we have developed an emission ratiometric two-photon probe (SE1) for quantitatively detecting CE in situ. This probe is based on a translation of intramolecular charge transfer character upon reaction with CE. It shows a sensitive blue-to-yellow emission change in response to human CE activity, easy loading into cells, insensitivity to pH and other metabolites including ROS and RNS, high photostability, and low cytotoxicity. Using live hepatocytes and liver tissues, we found that ratiometric two-photon microscopic imaging with SE1 is an effective tool for monitoring CE activities at the subcellular level in live tissues. This probe will find useful applications in biomedical research, including studies of hepatic steatosis and drug developments.

A quadrupolar two-photon fluorescent probe for in vivo imaging of amyloid-β plaques

A quadrupolar two-photon fluorescent probe for in vivo imaging of amyloid-β plaques is reported.

The formation of beta amyloid (Aβ) plaques in specific brain regions is one of the early pathological hallmarks of Alzheimer's disease (AD). To enable the early detection of AD and related applications, a method for real-time, clear 3D visualization of Aβ plaques in vivo is highly desirable. Two-photon microscopy (TPM) which utilizes two near-infrared photons is an attractive tool for such applications. However, this technique needs a sensitive and photostable two-photon (TP) probe possessing bright TP exited fluorescence to impart high signal-to-noise (S/N) visualization of Aβ plaques. Herein, we report a quadrupolar TP fluorescent probe (QAD1) having large TP action cross section (Φδ_max = 420 GM) and its application for in vivo TPM imaging of Aβ plaques. This probe, designed with a centrosymmetric D–A–D motif with a cyclic conjugating bridge and solubilizing unit, displays bright TP excited fluorescence, appreciable water solubility, robust photostability, and high sensitivity and selectivity for Aβ plaques. Using the real-time TPM imaging of transgenic 5XFAD mice after intravenous injection of QAD1, we show that this probe readily enters the blood brain barrier and provides high S/N ratio images of individual Aβ plaques in vivo. We also used QAD1 in dual-color TPM imaging for 3D visualization of Aβ plaques along with blood vessels and cerebral amyloid angiopathy (CAA) inside living mouse brains. These findings demonstrate that this probe will be useful in biomedical applications including early diagnosis and treatments of AD.

A ratiometric two-photon probe for quantitative imaging of mitochondrial pH values

Mitochondrial pH (pHmito) is known to be alkaline (near 8.0) and has emerged as a potential factor for mitochondrial function and disorder. We have developed a ratiometric two-photon probe (CMP1) for quantitative analysis of pHmito in live cells and tissues. This probe is designed to function by controlling the intramolecular charge transfer from 2-naphthol, having an ideal pKa value (7.86 ± 0.05) in the cells to monitor pHmito. This transition results in a marked yellow to red emission color change in response to pH alterations from 6.0 to 9.0. CMP1 exhibits easy loading, selective and robust staining ability of mitochondria, low cytotoxicity, and bright two-photon excited fluorescence in situ, thereby allowing quantitative imaging of the pHmito in live cells and tissues. The ratiometric TPM imaging clearly reveals that subcellular distribution of the pHmito values is heterogeneous, with the pHmito values in the perinuclear region being higher than those at the periphery of the cells. The changes of pHmito values on carbonyl cyanide m-chlorophenyl hydrazone (CCCP) treatment and autophagic processes were also investigated along with their morphological alterations at specific subcellular positions. We also used CMP1 to visualize the pHmito values of Parkinson's disease model astrocytes as well as living hippocampal tissues. Our results demonstrate that CMP1 will be useful as a quantitative imaging probe to study pHmito in biomedical research.

Symmetric and unsymmetric thienyl-substituted fluorenone dyes: static excimer-induced emission enhancement

In this work, we designed and synthesized four symmetric and asymmetric thienyl-substituted fluorenone compounds, which all exhibited typical AIE properties.

New zinc(ii) dyes with enhanced two-photon absorption cross sections based on the imidazolyl ligand

The effective TPA cross sections of Dyes 1–3 revealed that the exceeding TPA behavior may attributed to form the new D–π–A type compound that enhanced the intramolecular ICT progress. Dyes 1–3 were successfully applied in bio-imaging.

A new water-soluble heteronuclear PdII–AuI pincer complex as two-photon luminescent probe for biological Co2+ detection

A new water-soluble heteronuclear Pd^II–Au^I pincer complex was synthesized and investigated for biological Co²⁺ detection.

A two excited state model to explain the peculiar photobehaviour of a flexible quadrupolar D–π–D anthracene derivative

The excited state deactivation of the investigated compound involves two distinct fluorescent states. ¹A* → ¹B* transition is accompanied by structural motions, with the ¹B* state exhibiting a certain ICT character.

Rationally designed two-photon absorption compounds based on benzoxazole derivatives: structure–property relationships and bio-imaging applications

Five rationally designed benzoxazole-based two photon absorption compounds were reported and their structure–property relationships were summarized clearly.

An internal charge transfer-dependent solvent effect in V-shaped azacyanines

New V-shaped non-centrosymmetric dyes, possessing a strongly electron-deficient azacyanine core, have been synthesized based on a straightforward two-step approach. The key step in this synthesis involves palladium-catalysed cross-coupling of dibromo-N,N'-methylene-2,2'-azapyridinocyanines with arylacetylenes. The resulting strongly polarized π-expanded heterocycles exhibit green to orange fluorescence and they strongly respond to changes in solvent polarity. We demonstrate that differently electron-donating peripheral groups have a significant influence on the internal charge transfer, hence on the solvent effect and fluorescence quantum yield. TD-DFT calculations confirm that, in contrast to the previously studied bis(styryl)azacyanines, the proximity of S1 and T2 states calculated for compounds bearing two 4-N,N-dimethylaminophenylethynyl moieties establishes good conditions for efficient intersystem crossing and is responsible for its low fluorescence quantum yield. Non-linear properties have also been determined for new azacyanines and the results show that depending on peripheral groups, the synthesized dyes exhibit small to large two-photon absorption cross sections reaching 4000 GM.

Measurement of the Nucleus Area and Nucleus/Cytoplasm and Mitochondria/Nucleus Ratios in Human Colon Tissues by Dual-Colour Two-Photon Microscopy Imaging

We developed two-photon (TP) probes for DNA (ABI-Nu), cytoplasm (Pyr-CT), and mitochondria (BF-MT). We found that ABI-Nu binds to AT in the minor groove, while ABI-Nu and BF-MT are effective for tracking in the cytoplasm and mitochondria, respectively. These probes showed very large effective two-photon action cross section values of 2230, 1555, and 790 Göppert-Mayer units (1 GM  =  10(-50) cm(4) s photon(-1) molecule(-1)) at 740 nm with emission maxima at 473, 561, and 560 nm, respectively, in each organelle. Using these probes, we quantitatively estimated the mean nuclear area and the ratios of nuclei to cytoplasm and mitochondria to nuclei in human colon tissues by dual-colour two-photon microscopy imaging within 2  h after biopsy. The mean nuclear area and the nuclei to cytoplasm and mitochondria to cytoplasm ratios increased in the following order: normal colon mucosa Collapse

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MESH Headings

• Cell Nucleus/metabolism
• Colon/metabolism
• Colonic Neoplasms/diagnosis
• Colonic Neoplasms/metabolism
• Cytoplasm/metabolism
• DNA/metabolism
• Diagnostic Imaging/methods
• Fluorescent Dyes/chemistry
• Fluorescent Dyes/metabolism
• HeLa Cells
• Humans
• Microscopy, Fluorescence, Multiphoton/methods
• Mitochondria/metabolism
• Molecular Structure
• Reproducibility of Results
• Sensitivity and Specificity
• Spectrometry, Fluorescence/methods

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Affiliation(s)

Chang Su Lim Department of Chemistry, Korea University, 145 Anam-ro, Sungbuk-gu, Seoul 136-713, Korea
Eun Sun Kim Department of Internal Medicine, Korea University College of Medicine, 73 Inchon-ro, Seoul, 136-705, Korea
Ji Yeon Kim Department of Chemistry, Korea University, 145 Anam-ro, Sungbuk-gu, Seoul 136-713, Korea
Seung Taek Hong KU-KIST Graduate School of Converging Science and Technology, Korea University, 145 Anam-ro, Sungbuk-gu, Seoul 136-713, Korea
Hoon Jai Chun Department of Internal Medicine, Korea University College of Medicine, 73 Inchon-ro, Seoul, 136-705, Korea
Dong Eun Kang Department of Chemistry, Korea University, 145 Anam-ro, Sungbuk-gu, Seoul 136-713, Korea
Bong Rae Cho Department of Chemistry, Korea University, 145 Anam-ro, Sungbuk-gu, Seoul 136-713, Korea

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Pyrrolo[3,2-b]pyrroles-From Unprecedented Solvatofluorochromism to Two-Photon Absorption

A combined experimental and theoretical study of the two-photon absorption (2PA) properties of a series of quadrupolar molecules possessing a highly electron-rich heterocyclic core, pyrrolo[3,2-b]pyrrole, is presented. In agreement with quantum-chemical calculations, large 2PA cross-section values, σ2PA ≈10(2) -10(3)  GM (1 GM=10(50)  cm(4)  s photon(-1) ), are observed at wavelengths of 650-700 nm, which correspond to the two-photon allowed but one-photon forbidden transitions. The calculations also predict that increased planarity of this molecule through removal of two N-substituents leads to further increase in the σ2PA values. Surprisingly, the most quadrupolar pyrrolo[3,2-b]pyrrole derivative, containing two 4-nitrophenyl substituents at positions 2 and 5, demonstrates a very strong solvatofluorochromic effect, with a fluorescence quantum yield as high as 0.96 in cyclohexane, whereas the fluorescence vanishes in DMSO.

Two-photon fluorescent probe for detection of exogenous and endogenous hydrogen persulfide and polysulfide in living organisms

Hydrogen persulfide and polysulfide (H2S(n)) are newly discovered intracellular reactive species considered to have high protein S-sulfhydration efficiency. The detection of H2S(n) in living systems is essential for studying their functions but is quite challenging. In this work, we report a two-photon excited fluorescent probe, QS(n), capable of tracking H2S(n) in living organisms. QS(n) exhibited turn-on two-photon fluorescence response upon reaction with H2S(n). With a favorable photophysical property, high specificity, and low cytotoxicity, QS(n) was able to recognize exogenous H2S(n) in living cells. More importantly, it realized for the first time the visualization of endogenous H2S(n) generated in cells overexpressing cystathionine β-synthase and cystathionine γ-lyase, the enzymes responsible for producing endogenous H2S(n). Taking advantage of two-photon microscopy, the probe was also applied to achieve H2S(n) detection in zebrafish embryos and to observe H2S(n) distribution in living organisms.

Design and synthesis of fluorenone-based dyes: two-photon excited fluorescent probes for imaging of lysosomes and mitochondria in living cells

Three fluorenone-based two-photon fluorescent probes for specific targeting of lysosomes and mitochondria in cancer cells.

Formation and nonlinear optical properties of Ag nanocrystals capped with the conjugated ligand carbazolyl styryl terpyridine

NLO (non-linear optics) properties of the optical organic molecules were significantly improved when capped with nano-Ag.

A novel pyrene-based two-photon active fluorescent dye efficiently excited and emitting in the 'tissue optical window (650-1100 nm)'

The development of two-photon (TP) active fluorophores remains an important issue. Dyes that can be excited and fluoresce efficiently in the 'tissue optical window' (650-1100 nm) are especially in demand to maximize the underlying performance of two-photon fluorescence microscopy (TPFM) as an advanced optical technique. Ideally, such dyes would be compatible with the 1050 nm femtosecond fibre laser, which has recently been developed as an inexpensive excitation source to make the TPFM technique universal. In this work, we designed and synthesized a novel pyrene-based acceptor-π-acceptor (A-π-A) dye, PY, which exhibited outstanding properties such as bright fluorescence (λ_em = 650 nm and Φ_FL = 0.80) and a large two-photon absorption cross-section (1100 GM (1 GM = 10^-50 cm⁴ per photon per molecule) at 950 nm and 380 GM at 1050 nm) in the tissue optical window. In living mitochondria, PY provided more sensitive microscopic images than current dyes and showed great potential to be a building block of TP active fluorescent probes for the 1050 nm fibre laser. We believe that the exceptional properties of PY will be extended to other fluorescent probes through further chemical modification.