The D‐π‐A type phosphonium salts in which electron acceptor (A=‐+PR3) and donor (D=‐NPh2) groups are linked by polarizable π‐conjugated spacers show intense fluorescence that is classically ascribed ...to excited‐state intramolecular charge transfer (ICT). Unexpectedly, salts with π=‐(C6H4)n‐ and ‐(C10H6C6H4)‐ exhibit an unusual dual emission (F1 and F2 bands) in weakly polar or nonpolar solvents. Time‐resolved fluorescence studies show a successive temporal evolution from the F1 to F2 emission, which can be rationalized by an ICT‐driven counterion migration. Upon optically induced ICT, the counterions move from ‐+PR3 to ‐NPh2 and back in the ground state, thus achieving an ion‐transfer cycle. Increasing the solvent polarity makes the solvent stabilization dominant, and virtually stops the ion migration. Providing that either D or A has ionic character (by static ion‐pair stabilization), the ICT‐induced counterion migration should not be uncommon in weakly polar to nonpolar media, thereby providing a facile avenue for mimicking a photoinduced molecular machine‐like motion.
Pair at work: Optically driven charge transfer in nondissociated donor–acceptor ion pairs induces a counterion migration, reminiscent of a molecular machine. This motion leads to relocation of the cation and anion, and results in unusual dual fluorescence through the previously unrecognized properties of ionic D‐π‐A systems in nonpolar media.
Rationally designed cationic phospha‐polyaromatic fluorophores were prepared through intramolecular cyclization of the tertiary ortho‐(acene)phenylene‐phosphines mediated by CuII triflate. As a ...result of phosphorus quaternization, heterocyclic phosphonium salts 1 c–3 c, derived from naphthalene, phenanthrene, and anthracene cores, exhibited very intense blue to green fluorescence (Φem=0.38–0.99) and high photostability in aqueous medium. The structure–emission relationship was further investigated by tailoring the electron‐donating functions to the anthracene moiety to give dyes 4 c–6 c with charge‐transfer character. The latter significantly decreases the emission energy to reach near‐IR region. Thus, the intramolecular phosphacyclization renders an ultra‐wide tuning of fluorescence from 420 nm (1 c) to 780 nm (6 c) in solution, extended to 825 nm for 6 c in the solid state with quantum efficiency of approximately 0.07. The physical behavior of these new dyes was studied spectroscopically, crystallographically, and electrochemically, whereas computational analysis was used to correlate the experimental data with molecular electronic structures. The excellent stability, water solubility, and attractive photophysical characteristics make these phosphonium heterocycles powerful tools in cell imaging.
Bandgap control: A family of PAH‐fused heterocyclic phosphonium dyes, prepared through intramolecular phosphacyclization, demonstrates remarkably rich opportunity to control the optical bandgap.
In the emerging field of intramolecular charge transfer induced counterion migration, we report the new insights into photophysical features of luminescent donor–acceptor phosphonium dyes (D−π−)nA+X− ...(π=−(C6H4)x−). The unique connectivity of the phosphorus atom affords multipolar molecules with a variable number of arms and the electronic properties of the acceptor group. In the ion‐paired form, the transition from dipolar to quadrupolar configuration enhances the low energy migration‐induced band by providing the additional pathways for anion motion. The multipolar architecture, adjustable lengths of the π‐spacers and the nature of counterions allow for efficient tuning of the emission and achieving nearly pure white light with quantum yields around 30 %. The methyl substituent at the phosphorus atom reduces the rate of ion migration and suppresses the red shifted bands, simultaneously improving total emission intensity. The results unveil the harnessing of the multiple emission of phosphonium fluorophores by anion migration via structure and branching of donor–acceptor arms.
The photophysical properties of a series of multipolar donor–acceptor phosphonium fluorophores (D−π−)nA+X− have been investigated. These dyes exhibit steady‐state panchromatic luminescence attributed to the photoinduced counterion migration. The intra‐ion‐pair dynamics and the fluorescence response can be rationally tuned on the molecular level that might broaden the scope of functionalities and stimulate the development of new ionic systems.
Herein, we introduce the cyclic 8π‐electron (C8π) molecule N,N′‐diaryl‐dihydrodibenzoa,cphenazine (DPAC) as a dual‐functional donor to establish a series of new donor–linker–acceptor (D–L–A) dyads ...DLA1–DLA5. The excited‐state bent‐to‐planar dynamics of DPAC regulate the energy gap of the donor, while the acceptors A1–A5 are endowed with different energy gaps and HOMO/LUMO levels. As a result, the rate and efficiency of the excited‐state electron transfer vs. energy transfer can be finely harnessed, which is verified via steady‐state spectroscopy and time‐resolved emission measurements. This comprehensive approach demonstrates, for the first time, the manifold of excited‐state properties governed by bifunctional donor‐based D–L–A dyads, including bent‐to‐planar, photoinduced electron transfer (PET) from excited donor to acceptor (oxidative‐PET), fluorescence resonance energy transfer (FRET), bent‐to‐planar followed by electron transfer (PFET), and PET from donor to excited acceptor (reductive‐PET).
A bent/planar dual‐channel donor was introduced to establish new donor–linker–acceptor (D–L–A) dyads in which the excited‐state bent‐to‐planar structure relaxation of the donor can be used as a clock to harness the electron/energy‐transfer processes. Several new D–L–A structures were designed and synthesized by tuning the energy gaps and frontier‐orbital levels of the acceptors.
Three groups of luminescent platinum complexes Pt(C^N)(L)(Y) C^N=benzothienyl‐pyridine (1), bezofuryl‐pyridine (2), phenyl‐pyridine (3); L/Y=DMSO/Cl (a), PPh3/Cl (b), PPh3/CN (c) have been probed as ...halogen‐bond (XB) acceptors towards iodofluorobenzenes (IC6F5 and I2C6F4). Compounds 1 a and 2 a (L/Y=DMSO/Cl) afford the adducts 1 a⋅⋅⋅I2C6F4 and 2 a⋅⋅⋅I2C6F4, which feature I⋅⋅⋅Sbtpy/I⋅⋅⋅πbtpy and I⋅⋅⋅ODMSO/I⋅⋅⋅Cl short contacts, respectively. The phosphane‐cyanide derivatives 1 c and 2 c (L/Y=PPh3/CN) co‐crystallise with both IC6F5 and I2C6F4. None of the phpy‐based species 3 a–3 c participated in XB interactions. Although the native complexes are rather poor luminophores in the solid state (Φem=0.023–0.089), the adducts exhibit an up to 10‐fold increase of the intensity with a minor alteration of the emission energy. The observed gain in the quantum efficiency is mainly attributed to the joint influence of non‐covalent interactions (halogen/hydrogen bonding, π–π stacking), which govern the crystal‐packing mode and diminish the radiationless pathways for the T1→S0 transition by providing a rigid environment around the chromophore.
Special addition: Cyclometallated platinum complexes, equipped with a range of electron‐rich functionalities, undergo halogen‐bond‐driven co‐crystallisation with fluorinated iodoarenes to give adducts with different intermolecular arrangements, which display up to ten times higher intensities of phosphorescence than the parent compounds.
A new series of molecules bearing a 2,11-dihydro-1
H
-cyclopenta
de
indeno1,2-
b
quinoline (
CPIQ
) chromophore with the N-H N type of intramolecular hydrogen bond are strategically designed and ...synthesized, among which
CPIQ-OH
,
CPIQ-NHAc
and
CPIQ-NHTs
in solution exhibit a single emission band with an anomalously large Stokes shift, whereas
CPIQ-NH
2
and
CPIQ-NHMe
show apparent dual-emission property. This, in combination with time-resolved spectroscopy and the computational approach, leads us to conclude that
CPIQ-OH
,
CPIQ-NHAc
and
CPIQ-NHTs
undergo ultrafast, highly exergonic excited-state intramolecular proton transfer (ESIPT), while a finite rate of ESIPT is observed for
CPIQ-NH
2
and
CPIQ-NHMe
with a time constant of 117 ps and 39 ps, respectively, in acetonitrile at room-temperature. Further temperature-dependent studies deduce an appreciable ESIPT barrier for
CPIQ-NH
2
and
CPIQ-NHMe
. Different from most of the barrier associated ESIPT molecules that are commonly in the thermodynamic-control regime,
i.e.
found in the thermal pre-equilibrium between excited normal and proton-transfer tautomer states,
CPIQ-NH
2
and
CPIQ-NHMe
cases are in the kinetic-control regime where ESIPT is irreversible with a significant barrier. The barrier is able to be tuned by the electronic properties of the -R group in the NR-H proton donor site, resulting in ratiometric fluorescence for normal
versus
tautomer emission.
A new series of ESIPT molecules has been investigated. We found that these compounds undergo kinetic-control ESIPT and the reaction barriers along proton transfer coordinate can be finetuned by different substitution on amino group.
Phosphonium‐based compounds gain attention as promising photofunctional materials. As a contribution to the emerging field, we present a series of donor‐acceptor ionic dyes, which were constructed by ...tailoring phosphonium (A) and extended π‐NR2 (D) fragments to an anthracene framework. The alteration of the π‐spacer of electron‐donating substituents in species with terminal −+PPh2Me groups exhibits a long absorption wavelength up to λabs=527 nm in dichloromethane and shifted the emission to the near‐infrared (NIR) region (λ=805 nm for thienyl aniline donor), although at low quantum yield (Φ<0.01). In turn, the introduction of a P‐heterocyclic acceptor substantially narrowed the optical bandgap and improved the efficiency of fluorescence. In particular, the phospha‐spiro moiety allowed to attain NIR emission (797 nm in dichloromethane) with fluorescence efficiency as high as Φ=0.12. The electron‐accepting property of the phospha‐spiro constituent outperformed that of the monocyclic and terminal phosphonium counterparts, illustrating a promising direction in the design of novel charge‐transfer chromophores.
Low energy absorption and emission were realized in a series of anthracene‐derived donor‐acceptor phosphonium dyes. The electron deficiency of the rare phospha‐spiro architecture allowed to achieve fluorescence at 797 nm in dichloromethane and at 860 nm in the solid state.
We demonstrate directed translocation of ClO4− anions from cationic to neutral binding site along the synthetized BPym‐OH dye molecule that exhibits coupled excited‐state intramolecular ...proton‐transfer (ESIPT) and charge‐transfer (CT) reaction (PCCT). The results of steady‐state and time‐resolved spectroscopy together with computer simulation and modeling show that in low polar toluene the excited‐state redistribution of electronic charge enhanced by ESIPT generates the driving force, which is much stronger than by CT reaction itself and provides more informative gigantic shifts of fluorescence spectra signaling on ultrafast ion motion. The associated with ion translocation red‐shifted fluorescence band (at 750 nm, extending to near‐IR region) appears at the time ~83 ps as a result of electrochromic modulation of PCCT reaction. It occurs at substantial delay to PCCT that displayed fluorescence band at 640 nm and risetime of <200 fs. Thus, it becomes possible to visualize the manifestations of light‐triggered ion translocation and of its driving force by fluorescence techniques and to separate them in time and energy domains.
The light‐triggered translocation of ClO4− anion from cationic to neutral‐binding site along BPym‐OH dye is visualized by emission. The combination of time‐resolved fluorescence and molecular dynamic simulation allows its description in time and energy domains. This reaction generates dramatically red‐shifted fluorescence extending to near‐IR that appears at ~83 ps, resulting from electrochromic modulation of coupled proton‐electron transfer.
The triad types of molecules with various combinations of electron donors (D) and acceptors (A) have been widely explored in optoelectronics. However, their photophysical and photochemical ...properties, which are frequently unconventional, are relatively unexplored. In this study, a donor–donor–acceptor (D–D–A)-type triad, CTPS, consisting of the donor moiety of triphenylamine (D1) and the acceptor moiety of dibenzothiophene sulfone (A) bridging through the second donor carbazole (D2) into a U-shape configuration, was synthesized. CTPS exhibited dual emission bands, both of which reveal solvent-polarity-dependent solvatochromism and unusual excitation-wavelength-dependent ratiometric emission. Comprehensive studies clarified that two emissions originate from two different D–A charge-transfer (CT) states. The lower-energy CT(S) state possesses D1 → A through-space CT nature with optically forbidden transition, whereas the higher-lying CT(B) state is associated with optically allowed D2 → A CT through the π-conjugation transition. Upon S0 → CT(B) excitation, the charge transfer creates D2δ+Aδ− dipolar changes and Aδ−–D1 repulsion, leading to structural relaxation of the CT(B) state that competes with fast CT(B) → CT(S) internal conversion. Therefore, despite the fact that they originate from the same Franck–Condon excited state, both energy-stabilized CT(B) and CT(S) states are populated through two independent channels. The stabilized CT(B) and CT(S) states possess different optimized geometries and do not interconvert during their lifespans, rendering different population decay time constants. The slim highest occupied molecular orbital/lowest unoccupied molecular orbital overlapped D1–A CT(S) state exhibits thermally activated delayed fluorescence (TADF), the character of which was further exploited as a host in organic light-emitting diode. The results gain new insights into the properties of the bending-type D–D–A TADF triads. CTPS should not be a unique case. Bizarre photophysical behavior encountered in molecules comprising multiple D and A groups may involve the interplay among various local CT states, which might have been overlooked.
Abstract
Excited-state intramolecular proton transfer (ESIPT) and excimer formation are unimolecular and bimolecular reactions, respectively. The coupling between these two reactions has been rarely ...observed. Here we show the interplay between ESIPT and excimer formation in
CF
3
-HTTH
(2,2′-(thiazolo5,4-dthiazole-2,5-diyl)bis(4-(trifluoromethyl)phenol) solid crystal, resulting in the extensively broad multiple emissions spanning from 420 to 750 nm. Comprehensive structural, time-resolved spectroscopic, and two-photon microscopic studies of
CF
3
-HTTH
in crystal lattice unveil the pre-equilibrium ESIPT between the normal (N*) and tautomer (T*) form, for which the slow population decay and well-ordered lattice packing facilitate excimeric (E*) formation, rendering N* (460 nm), T* (520 nm) and E* (600 nm) triple emissions. In contrast, the lower degree of packing on the solid surface prohibits excimer formation, showing only the ESIPT process. The correlation between luminescence properties and packing structure sheds light on the corresponding mechanochromic effect based on molecular solid architecture and provide new insight into the aggregation-induced properties.
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