37 research outputs found
2ā(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 TripletāTriplet Annihilation Upconversion with Maleimide-Perylene as the Caged Triplet Acceptor/Emitter
Efficient
thiol-activated tripletātriplet 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 UVāvis absorption
spectroscopy, fluorescence spectroscopy, electrochemical properties,
and nanosecond transient absorption spectroscopy. The triplet acceptor/emitter <b>Py-M</b> shows week fluorescence (Ī¦<sub>F</sub> = 0.8%),
and no upconversion (Ī¦<sub>UC</sub> = 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 (Ī¦<sub>F</sub> = 97%) upon addition of thiols such as 2-mercaptoethanol, and the
Ī¦<sub>UC</sub> 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 (<sup>1</sup>O<sub>2</sub>) photosensitizing
was observed. Upon protonation, a long-lived triplet excited state
was observed (Ļ<sub>T</sub> = 3.1 Ī¼s) and the <sup>1</sup>O<sub>2</sub> quantum yield (Ī¦<sub>Ī</sub>) 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-C<sub>60</sub> Dyads as Efficient Heavy-Atom-Free Organic Triplet Photosensitizers
C<sub>60</sub>-styryl Bodipy dyads that show strong absorption of visible light (Īµ = 64ā600 M<sup>ā1</sup> cm<sup>ā1</sup> at 657 nm) and a long-lived triplet excited state (Ļ<sub>T</sub> = 123.2 Ī¼s) 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 (<sup>1</sup>O<sub>2</sub>). The photooxidation efficiency of the dyads compared to the conventional Ir(III) complex <sup>1</sup>O<sub>2</sub> photosensitizer increased 19-fold
Switching of the TripletāTriplet-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 tripletātriplet 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 Ī¦<sub>UC</sub> = 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 (Ī¦<sub>UC</sub> 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
UVāvis 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 (Īµ = 65000 M<sup>ā1</sup> cm<sup>ā1</sup> 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 OFFāON and unprecedented ONāOFF 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 āelectronic
statesā instead of the very often used approximation of āmolecular
orbitalsā
Efficient Enhancement of the Visible-Light Absorption of Cyclometalated Ir(III) Complexes Triplet Photosensitizers with Bodipy and Applications in Photooxidation and TripletāTriplet Annihilation Upconversion
We report molecular designing strategies to enhance
the effective visible-light absorption of cyclometalated IrĀ(III) complexes.
Cationic cyclometalated IrĀ(III) complexes were prepared in which boronādipyrromethene
(Bodipy) units were attached to the 2,2ā²-bipyridine (bpy) ligand
via āCī¼Cā bonds at either the <i>meso</i>-phenyl (<b>Ir-2</b>) or 2 position of the Ļ core of
Bodipy (<b>Ir-3</b>). For the first time the effect of Ļ
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. IrĀ(ppy)<sub>2</sub>(bpy) (<b>Ir-1</b>; ppy = 2-phenylpyridine) was used as model complex, which
gives the typical weak absorption in visible range (Īµ < 4790
M<sup>ā1</sup> cm<sup>ā1</sup> in region > 400 nm). <b>Ir-2</b> and <b>Ir-3</b> showed much stronger absorption
in the visible range (Īµ = 71ā400 M<sup>ā1</sup> cm<sup>ā1</sup> at 499 nm and 83ā000 M<sup>ā1</sup> cm<sup>ā1</sup> at 527 nm, respectively). Room-temperature
phosphorescence was only observed for <b>Ir-1</b> (Ī»<sub>em</sub> = 590 nm) and <b>Ir-3</b> (Ī»<sub>em</sub> =
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> (Ļ<sub>T</sub> = 23.7 Ī¼s) and <b>Ir-3</b> (87.2
Ī¼s). <b>Ir-1</b> gives a much shorter triplet-state lifetime
(0.35 Ī¼s). Complexes were used as singlet oxygen (<sup>1</sup>O<sub>2</sub>) photosensitizers in photooxidation. The <sup>1</sup>O<sub>2</sub> quantum yield of <b>Ir-3</b> (Ī¦<sub>Ī</sub> = 0.97) is ca. 2-fold of <b>Ir-2</b> (Ī¦<sub>Ī</sub> = 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 (Īµ = 65000 M<sup>ā1</sup> cm<sup>ā1</sup> 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 OFFāON and unprecedented ONāOFF 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 āelectronic
statesā instead of the very often used approximation of āmolecular
orbitalsā
Light-Harvesting Fullerene Dyads as Organic Triplet Photosensitizers for TripletāTriplet Annihilation Upconversions
Visible light-harvesting C<sub>60</sub>ābodipy
dyads were
devised as universal organic triplet photosensitizers for tripletātriplet
annihilation (TTA) upconversion. The antennas in the dyad were used
to harvest the excitation energy, and then the singlet excited state
of C<sub>60</sub> will be populated via the intramolecular energy
transfer from the antenna to C<sub>60</sub> unit. In turn with the
intrinsic intersystem crossing (ISC) of the C<sub>60</sub>, the triplet
excited state of the C<sub>60</sub> will be produced. Thus, without
any heavy atoms, the triplet excited states of organic dyads are populated
upon photoexcitation. Different from C<sub>60</sub>, the dyads show
strong absorption of visible light at 515 nm (<b>C-1</b>, Īµ
= 70400 M<sup>ā1</sup> cm<sup>ā1</sup>) or 590 nm (<b>C-2</b>, Īµ = 82500 M<sup>ā1</sup> cm<sup>ā1</sup>). Efficient intramolecular energy transfer from the bodipy moieties
to C<sub>60</sub> unit and localization of the triplet excited state
on C<sub>60</sub> 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 C<sub>60</sub>āorganic
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 TripletāTriplet Annihilation Upconversions
Naphthalenediimide (NDI) derivatives with 2,6- or 2,3,6,7-tetrabromo
or amino substituents were prepared. <i>N</i>,<i>Nā²</i>-dialkyl-2,6-dibromo NDI (compound <b>2</b>) and <i>N</i>,<i>Nā²</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>Nā²</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 (<sup>1</sup>O<sub>2</sub>) photosensitizers for the photooxidation of 1,5-dihydroxylnaphthalene
(DHN). We found that <b>5</b> is more efficient than the conventional
photosensitizer, such as IrĀ(ppy)<sub>2</sub>(bpy)Ā[PF<sub>6</sub>].
The compounds were also used as organic triplet photosensitizers for
tripletātriplet annihilation based upconversions. An upconversion
quantum yield up to 18.5% was observed