2‑(2-Hydroxyphenyl)-benzothiazole (HBT)-Rhodamine Dyad: Acid-Switchable Absorption and Fluorescence of Excited-State Intramolecular Proton Transfer (ESIPT)

Dyad was prepared by link rhodamine and excited state intramolecular proton transfer (ESIPT) chromophore 2-(2-hydroxyphenyl)-benzothiazole (HBT) using Click reaction, with the goal to switch the absorption/emission property of ESIPT chromophore. The photophysical properties of the dyad were studied with steady state and time-resolved absorption and emission spectroscopy. In the absence of acid, that is, with rhodamine is in spirolactam structure, ESIPT was observed, the enol form emission of HBT unit was observed at 404 nm in protic solvents. In aprotic solvents, emission of the keto form of HBT was observed at 543 nm. With addition of acid such as trifluoroacetic acid, the rhodamine unit transforms to the opened amide structure, intense absorption band at 554 nm developed, as well as a strong fluorescence band at 579 nm; in EtOH, the enol emission of HBT at 406 nm was not quenched by the resonance energy transfer (RET), thus, dual fluorescence was observed. In dichloromethane, however, the fluorescence of the keto form of HBT unit was completely quenched. Thus, the absorption and emission of the ESIPT chromophore were switched by a acid/base-activatable rhodamine chromophore. Such studies will add additional modulability to the ESIPT chromophores

Thiol-Activatable Triplet–Triplet Annihilation Upconversion with Maleimide-Perylene as the Caged Triplet Acceptor/Emitter

Efficient thiol-activated triplet–triplet annihilation (TTA) upconversion system was devised with maleimide-caged perylene (<b>Py-M</b>) as the thiol-activatable triplet acceptor/emitter and with diiodoBodipy as the triplet photosensitizer. The photophysical processes were studied with steady-state UV–vis absorption spectroscopy, fluorescence spectroscopy, electrochemical properties, and nanosecond transient absorption spectroscopy. The triplet acceptor/emitter <b>Py-M</b> shows week fluorescence (Φ_F = 0.8%), and no upconversion (Φ_UC = 0%) was observed. The quenching of fluorescence of <b>Py-M</b> is due to photoinduced electron-transfer (PET) process from perylene to maleimide-caging unit, which quenches the singlet excited state of perylene. The fluorescence of <b>Py-M</b> was enhanced by 200-fold (Φ_F = 97%) upon addition of thiols such as 2-mercaptoethanol, and the Φ_UC was increased to 5.9%. The unique feature of this thiol-activated TTA upconversion is that the activation is based on addition reaction of the thiols with the caged acceptor/emitter, and no side products were formed. The previously reported cleavage approach gives side products which are detrimental to the TTA upconversion. With nanosecond transient absorption spectroscopy, we found that the triplet excited state of <b>Py-M</b> was not quenched by any PET process, which is different from singlet excited state (fluorescence) of <b>Py-M</b>. The results are useful for study of the triplet excited states of organic chromophores and for activatable TTA upconversion

Switching of the Triplet Excited State of Styryl 2,6-Diiodo-Bodipy and Its Application in Acid-Activatable Singlet Oxygen Photosensitizing

IodoBodipy-styrylBodipy dyads triplet photosensitizers were prepared (<b>B-1</b> and <b>B-2</b>) which contain acid-responsive moiety. Both compounds show broadband visible light absorption, due to the resonance energy transfer (RET) between the two different visible light-harvesting Bodipy units. The photophysical properties of the dyads were studied with steady-state and nanosecond time-resolved transient absorption spectroscopy. The production of triplet excited state is switched ON or OFF by protonation/deprotonation of the amino group in the dyads. In the neutral form, the excited state is short-lived (<10 ns) and no singlet oxygen (¹O₂) photosensitizing was observed. Upon protonation, a long-lived triplet excited state was observed (τ_T = 3.1 μs) and the ¹O₂ quantum yield (Φ_Δ) is up to 73.8%. The energy levels of the components of the dyads were changed upon protonation and this energy level tuning exerts significant influence on the triplet state property of the dyad. Acid-activated shuffling of the localization of the triplet excited state between two components of a dyad was observed. Furthermore, we observed a rare example that a chromophore giving shorter absorption wavelength is acting as the singlet energy <i>acceptor</i> in RET. The experimental results were rationalized by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations

Styryl Bodipy-C₆₀ Dyads as Efficient Heavy-Atom-Free Organic Triplet Photosensitizers

C₆₀-styryl Bodipy dyads that show strong absorption of visible light (ε = 64 600 M^–1 cm^–1 at 657 nm) and a long-lived triplet excited state (τ_T = 123.2 μs) are prepared. The dyads were used as heavy-atom-free organic triplet photosensitizers for photooxidation of 1,5-dihydroxynaphthalene via the photosensitizing of singlet oxygen (¹O₂). The photooxidation efficiency of the dyads compared to the conventional Ir(III) complex ¹O₂ photosensitizer increased 19-fold

Switching of the Triplet–Triplet-Annihilation Upconversion with Photoresponsive Triplet Energy Acceptor: Photocontrollable Singlet/Triplet Energy Transfer and Electron Transfer

A photoswitchable fluorescent triad based on two 9,10-diphenylanthracene (<b>DPA</b>) and one dithienylethene (<b>DTE</b>) moiety is prepared for photoswitching of triplet–triplet annihilation upconversion. The <b>DPA</b> and <b>DTE</b> moieties in the triad were connected via Click reaction. The <b>DPA</b> unit in the triad was used as the triplet energy acceptor and upconverted fluorescence emitter. The fluorescence of the triad is switched ON with the <b>DTE</b> moiety in open form [<b>DTE-(o)</b>] (upconversion quantum yield Φ_UC = 1.2%). Upon UV irradiation, photocyclization of the <b>DTE-(o)</b> moiety produces the closed form [<b>DTE-(c)</b>], as a result the fluorescence of <b>DPA</b> moiety was switched off (Φ_UC is negligible). Three different mechanisms are responsible for the upconverted fluorescence photoswitching effect (i.e., the photoactivated fluorescence resonance energy transfer, the intramolecular electron transfer, as well as the photoactivated intermolecular triplet energy transfer between the photosensitizer and <b>DTE-(c)</b> moiety). Previously, the photoswitching of TTA upconversion was accomplished with only one mechanism (i.e., the triplet state quenching of the photosensitizer by <b>DTE-(c)</b> via either the intermolecular or intramolecular energy transfer). The photophysical processes involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV–vis absorption and fluorescence emission spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization, and DFT/TDDFT calculations

Geometry Relaxation-Induced Large Stokes Shift in Red-Emitting Borondipyrromethenes (BODIPY) and Applications in Fluorescent Thiol Probes

2-Thienyl and 2,6-bisthienyl BODIPY derivatives (<b>BS-SS</b> and <b>BS-DS</b>) were prepared that show intense absorption (ε = 65000 M^–1 cm^–1 at 507 nm) and a large Stokes shift (96 nm) vs the small Stokes shift of typical BODIPY (<15 nm). Control compounds with a thienyl unit at the 8-position or phenyl substituents at the 2,6-positions were prepared (<b>BS-1</b> and <b>9</b>). <b>BS-1</b> shows absorption/emission in the blue-shifted range and a small Stokes shift (12 nm). Compound <b>9</b> shows absorption in the red-shifted range, but the Stokes shift (<30 nm) is much smaller than that for <b>BS-SS</b> and <b>BS-DS</b>. DFT calculations propose the large Stokes shifts of <b>BS-SS</b> and <b>BS-DS</b> are due to the remarkable geometry relaxation upon photoexcitation and its substantial effect on the energy levels of molecular orbitals. For the dyes with small Stokes shifts, much smaller geometry relaxations were found. The fluorophores were used for fluorescent thiol probes, with 2,4-dinitrobenzenesulfonyl (DNBS) as the fluorescence switch. Both fluorescence OFF–ON and unprecedented ON–OFF transduction were observed, which are attributed to the different photoinduced intramolecular electron-transfer (PET) profile. All the photophysics were rationalized by DFT calculations based on the concept of “electronic states” instead of the very often used approximation of “molecular orbitals”

Efficient Enhancement of the Visible-Light Absorption of Cyclometalated Ir(III) Complexes Triplet Photosensitizers with Bodipy and Applications in Photooxidation and Triplet–Triplet Annihilation Upconversion

We report molecular designing strategies to enhance the effective visible-light absorption of cyclometalated Ir­(III) complexes. Cationic cyclometalated Ir­(III) complexes were prepared in which boron–dipyrromethene (Bodipy) units were attached to the 2,2′-bipyridine (bpy) ligand via −CC– bonds at either the <i>meso</i>-phenyl (<b>Ir-2</b>) or 2 position of the π core of Bodipy (<b>Ir-3</b>). For the first time the effect of π conjugating (<b>Ir-3</b>) or tethering (<b>Ir-2</b>) of a light-harvesting chromophore to the coordination center on the photophysical properties was compared in detail. Ir­(ppy)₂(bpy) (<b>Ir-1</b>; ppy = 2-phenylpyridine) was used as model complex, which gives the typical weak absorption in visible range (ε < 4790 M^–1 cm^–1 in region > 400 nm). <b>Ir-2</b> and <b>Ir-3</b> showed much stronger absorption in the visible range (ε = 71 400 M^–1 cm^–1 at 499 nm and 83 000 M^–1 cm^–1 at 527 nm, respectively). Room-temperature phosphorescence was only observed for <b>Ir-1</b> (λ_em = 590 nm) and <b>Ir-3</b> (λ_em = 742 nm). <b>Ir-3</b> gives RT phosphorescence of the Bodipy unit. On the basis of the 77 K emission spectra, nanosecond transient absorption spectra, and spin density analysis, we proposed that Bodipy-localized long-lived triplet excited states were populated for <b>Ir-2</b> (τ_T = 23.7 μs) and <b>Ir-3</b> (87.2 μs). <b>Ir-1</b> gives a much shorter triplet-state lifetime (0.35 μs). Complexes were used as singlet oxygen (¹O₂) photosensitizers in photooxidation. The ¹O₂ quantum yield of <b>Ir-3</b> (Φ_Δ = 0.97) is ca. 2-fold of <b>Ir-2</b> (Φ_Δ = 0.52). Complexes were also used as triplet photosensitizer for TTA upconversion; upconversion quantum yields of 1.2% and 2.8% were observed for <b>Ir-2</b> and <b>Ir-3</b>, respectively. Our results proved that the strong absorption of visible light of <b>Ir-2</b> failed to enhance production of a triplet excited state. These results are useful for designing transition metal complexes that show <i>effective</i> strong visible-light absorption and long-lived triplet excited states, which can be used as ideal triplet photosensitizers in photocatalysis and TTA upconversion

Geometry Relaxation-Induced Large Stokes Shift in Red-Emitting Borondipyrromethenes (BODIPY) and Applications in Fluorescent Thiol Probes

2-Thienyl and 2,6-bisthienyl BODIPY derivatives (<b>BS-SS</b> and <b>BS-DS</b>) were prepared that show intense absorption (ε = 65000 M^–1 cm^–1 at 507 nm) and a large Stokes shift (96 nm) vs the small Stokes shift of typical BODIPY (<15 nm). Control compounds with a thienyl unit at the 8-position or phenyl substituents at the 2,6-positions were prepared (<b>BS-1</b> and <b>9</b>). <b>BS-1</b> shows absorption/emission in the blue-shifted range and a small Stokes shift (12 nm). Compound <b>9</b> shows absorption in the red-shifted range, but the Stokes shift (<30 nm) is much smaller than that for <b>BS-SS</b> and <b>BS-DS</b>. DFT calculations propose the large Stokes shifts of <b>BS-SS</b> and <b>BS-DS</b> are due to the remarkable geometry relaxation upon photoexcitation and its substantial effect on the energy levels of molecular orbitals. For the dyes with small Stokes shifts, much smaller geometry relaxations were found. The fluorophores were used for fluorescent thiol probes, with 2,4-dinitrobenzenesulfonyl (DNBS) as the fluorescence switch. Both fluorescence OFF–ON and unprecedented ON–OFF transduction were observed, which are attributed to the different photoinduced intramolecular electron-transfer (PET) profile. All the photophysics were rationalized by DFT calculations based on the concept of “electronic states” instead of the very often used approximation of “molecular orbitals”

Light-Harvesting Fullerene Dyads as Organic Triplet Photosensitizers for Triplet–Triplet Annihilation Upconversions

Visible light-harvesting C₆₀–bodipy dyads were devised as universal organic triplet photosensitizers for triplet–triplet annihilation (TTA) upconversion. The antennas in the dyad were used to harvest the excitation energy, and then the singlet excited state of C₆₀ will be populated via the intramolecular energy transfer from the antenna to C₆₀ unit. In turn with the intrinsic intersystem crossing (ISC) of the C₆₀, the triplet excited state of the C₆₀ will be produced. Thus, without any heavy atoms, the triplet excited states of organic dyads are populated upon photoexcitation. Different from C₆₀, the dyads show strong absorption of visible light at 515 nm (<b>C-1</b>, ε = 70400 M^–1 cm^–1) or 590 nm (<b>C-2</b>, ε = 82500 M^–1 cm^–1). Efficient intramolecular energy transfer from the bodipy moieties to C₆₀ unit and localization of the triplet excited state on C₆₀ were confirmed by steady-state and time-resolved spectroscopy as well as DFT calculations. The dyads were used as triplet photosensitizers for TTA upconversion, and an upconversion quantum yield up to 7.0% was observed. We propose that C₆₀–organic chromophore dyads can be used as a general molecular structural motif for organic triplet photosensitizers, which can be used for photocatalysis, photodynamic therapy, and TTA upconversions

Room-Temperature Long-Lived Triplet Excited States of Naphthalenediimides and Their Applications as Organic Triplet Photosensitizers for Photooxidation and Triplet–Triplet Annihilation Upconversions

Naphthalenediimide (NDI) derivatives with 2,6- or 2,3,6,7-tetrabromo or amino substituents were prepared. <i>N</i>,<i>N′</i>-dialkyl-2,6-dibromo NDI (compound <b>2</b>) and <i>N</i>,<i>N′</i>-dialkyl-2,3,6,7-tetrabromo NDI (compound <b>4</b>) show phosphorescence emission at 610 or 667 nm, respectively. Phosphorescence was never observed for NDI derivatives. Conversely, <i>N</i>,<i>N′</i>-dialkyl-2,6-dibromo-3,7-diamino NDI (compound <b>5</b>) shows strong absorption at 526 nm and fluorescence at 551 nm, and no phosphorescence was observed. However, nanosecond time-resolved transient difference absorption spectroscopy confirmed that the triplet excited state of <b>5</b> was populated upon photoexcitation. 2,3,6,7-Tetraamino NDI (<b>6</b>) shows fluorescence, and no triplet excited state was populated upon excitation. The compounds were used as singlet oxygen (¹O₂) photosensitizers for the photooxidation of 1,5-dihydroxylnaphthalene (DHN). We found that <b>5</b> is more efficient than the conventional photosensitizer, such as Ir­(ppy)₂(bpy)­[PF₆]. The compounds were also used as organic triplet photosensitizers for triplet–triplet annihilation based upconversions. An upconversion quantum yield up to 18.5% was observed