LE State Research Articles

Efficient and ultra-high luminance orange-red organic light emitting diode (OLED) based on triphenylamine-benzothiadiazole-phenoxazine hybrid molecule with hybrid local and charge-transfer (HLCT) characteristic

Due to the energy gap law, red emission needs relatively narrow energy gap, thus the nonradiative transition rate (knr) grows exponentially and eventually impacts the materials fluorescent efficiency. Here, an orange-red emission donor-acceptor-π-donor (D-A-π-D’) material N, N-diphenyl-4-(7-(4-(10-phenyl-10 H-phenoxazin-3-yl) phenyl) benzo [c] (Huang et al., 2019; Park et al., 2008; Caspar et al., 1982) [1,2,5] thiadiazol-4-yl) aniline (TBphPP) was designed and synthesized. Introducing a benzene π-bridge between the strong donor 10-phenyl-10H-phenoxazin (PPOX) and acceptor 2,1,3-benzothiadiazole (BTZ) not only is able to stretch the molecular conjugation so as to increase the proportion of LE state in hybrid local and charge-transfer (HLCT) state but also increase the overlap area of the electron cloud to improve luminous efficiency. As a result, the non-doped organic light-emitting diode (OLED) of TBphPP exhibited high luminance of 17,886 cd m−2 and relatively high exciton utilization efficiency (EUE) of 44 % with a standard red electroluminescent peak at 620 nm. Importantly, the optimized doped device based on TBphPP achieved high performance that the maximum external quantum efficiency (EQEmax) was up to 9.86 % with a higher EUE of 66.2 % and the efficiency roll-off (ηroll-off) at 100,000 cd m−2 was only 3.55 %. Simultaneously, it has reached ultra-high luminescence of 180,583 cd m−2, high current efficiency of 29.02 cd A−1 and power efficiency of 16.47 l m W−1. To our knowledge, the TBphPP device with such ultra-high luminance and high efficiency is one of the best performance orange-red materials based on HLCT characteristic.

Exploring the synergistic effect of aggregation and hydrogen bonding: a fluorescent probe for dual sensing of phytic acid and uric acid.

We synthesized an unoxidized bis-indolyl methane (BIM) derivative (probe 1) comprising of tetraphenylethylene (TPE) as the signalling moiety. The amphiphilic probe could form self-assembled nanoscopic aggregates in the aqueous medium. The emission of 1 in non-polar solvents originates from the LE state, while in polar solvents, it is dominated by TICT. Moreover, probe 1 exhibited a 'turn-on' fluorescence response for both uric acid (with a blue shift in emission maxima) and phytic acid (with a red shift in emission maxima). Therefore, the present system provides an exceptional opportunity to distinguish between phytic acid and uric acid by considering two different emission channels. Mechanistic investigations revealed that both H-bonding and electrostatic interactions between the probe and analytes could effectively cause restricted intramolecular rotations, leading to a turn-on response. Additionally, in the case of phytic acid, larger aggregates were observed with prominent CT characteristics. The change in the extent of charge transfer interaction in the formed adducts resulted in distinct fluorescence responses with phytic acid and uric acid. Furthermore, we explored the applicability of the present system in the screening of real-life samples, such as uric acid in urine samples and phytic acid in grains. The LOD for phytic acid and uric acid was found to be ∼5.48 nM and 10.4 nM, respectively. The quantitative nature of the system was confirmed, showing promising results in terms of recovery values (between 95.6% and 104.2%) and detection limits. Additionally, we also employed handy paper strips for the on-site monitoring of phytic acid and uric acid, thereby eliminating the need for complex instrumentation or trained technicians.

Contrasting the excited state properties of different conformers of trans- and cis-2,2'-bipyridine oligomers in the gas phase.

In this article, we present conformation-dependent photophysical and excited state properties of trans- and cis- BPY oligomers. Oligomers up to tetramers for three conformers, namely, o-, m-, and p-, are constructed and optimized at the B3LYP-D3/def2-SVPD level. The photophysical and excited state properties are interpreted in terms of UV and CD spectra at the RI-ADC(2)/def2-TZVPD level. The UV spectra of oligomers of the m-conformer show high-intensity and red-shifted UV bands compared to o- and p-oligomers. The CD spectra of p-oligomers show intense CD bands compared to o- and p-oligomers in the case of trans-structures. In contrast, oligomers of each conformer of cis-structures show high-intensity CD bands. The excited states of (BPY)2 and (BPY)4 are also characterized by analysis of one-electron transition density matrix considering three descriptors: ωCT, dexc, and PRNTO. The ωCT values of dimers are in the range of 0.06-0.32, which indicates the excited states are mainly LE states, whereas, for (BPY)4, the ωCT values range from 0.17 to 0.53, indicating the possibility of partial CT in the excited states. These observations are also explained using the NTOs and e-h correlation plots.

Rational design of hybridized local and charge transfer emitters towards deep blue emission by incorporating extra cyano-based acceptor moieties

The molecular excited-state manipulation from LE state to HLCT state via introducing cyano groups resulted in a PLQY of 100% in solution and augmented the radiative transition rate, and achieved an EQE of 4.43% and an emission peak at 438 nm.

High-efficiency multicolor fluorescent indoleninonaphthooxazole derivatives with adjustable excitation

Solvatochromic fluorophores are one of the most investigated dyes because they exhibit multi-color luminescence properties as the polarity of the solvent changes. It remains a challenge to regulate solvatochromism with a large emission shift and maintain high fluorescence efficiency from apolar to polar solvents. Herein, we report a new type of solvatochromic fluorophore that exhibits both multicolor solvatochromism and stable high fluorescence efficiency in polar solvents. The unique solvatochromic properties of indoleninonaphthooxazole derivatives are realized by constructing a single carbon-bond-linked coplanar scaffold containing indolenine and naphthooxazole components with different substituents. We propose a possible mechanism to explain the unique solvatochromism. Excited USTS-3 and USTS-4 exhibit LE state in non-polar solvent and different HLCT states in polar solvents with both high luminescence efficiency. And the excitation of USTS-3 and USTS-4 is determined by aggregation states of molecular USTS-3 and USTS-4 under different concentrations, which exhibits different relaxations but share the same luminescence decay pathway. Benefitting from the unique luminescence properties of indoleninonaphthooxazole derivatives, they exhibit potential applications in solvent polarity detection, bioimaging, anticounterfeiting, electroluminescence, and lighting in different matrix. Our results also provide a new perspective on designing novel fluorophores based on indolenine and naphthoxazole moieties.

Understanding of Intramolecular Charge Transfer Dynamics of a Push–Pull Dimethylamino-phenylethynylphenyl-dicyanoimidazole by Steady-State and Ultrafast Spectroscopic Studies

Photophysical behaviors of D−π–A compound 2-{4-[4-(N,N-dimethylamino)phenylethynyl]phenyl-1-methyl-1H-imidazole-4,5-dicarbonitrile (DMAPPIDCN) were explored using steady-state absorption, fluorescence emission, and femtosecond time-resolved absorption and emission spectroscopic techniques at room temperature along with computational time-dependent density functional theory (TD-DFT) calculation. The spectroscopic studies were carried out in different solvents of varying polarities including binary solvent mixtures. The role of the solvent polarity, viscosity, and temperature on the relaxation mechanism of DMAPPIDCN is disclosed. The observed steady-state and time-resolved spectroscopic features were attributed to intramolecular charge transfer (ICT) dynamics. The ICT in DMAPPIDCN is rationalized to a sequential twisted motion of both N(CH3)2 and whole N,N-dimethylaminophenyl moieties around the molecular axis interconnecting the adjacent imidazolephenyl moiety leading to the TICT1 and TICT2 (σ*) states. The increased solvent polarity affected mostly the fluorescence emission spectra pointing to a significant increase in the excited state dipole moment. This result clearly reveals formation of the TICT2 (σ*) involving efficient charge transfer from the (N,N-dimethylamino)phenyl (DMAP) donor to the phenyl-1-methyl-1H-imidazole-dicarbonitrile (PIDCN) acceptor in the excited state in a polar environment. In the TICT2 (σ*) state, the planes of electron-withdrawing and electron-donating moieties are perpendicular with the angle (DMAP)C–C≡C being 141.1°. This nonplanar arrangement accounts for the observed large Stokes shift. Time-resolved fluorescence spectroscopic studies unveil the excited state relaxation processes confirming the increase in the nonradiative decay rate in aprotic medium with increase in the solvent dielectric constants. Femtosecond transient spectroscopic studies unambiguously confirmed the existence of well separated LE and TICT states and their ensuing kinetics in polar medium. In nonpolar solvents, DMAPPIDCN shows strong fluorescence which emits from the LE (ππ*) state, whereas in polar solvents, formation of two consecutive TICT states occurs from the LE (ππ*) in a sub picosecond to few picosecond time domain depending on polarity of the solvents and the non-radiative decay from the TICT states.

A combinatorial effect of TICT and AIE on bisulfate detection using a pyrenylated charge-transfer luminogen

A donor-acceptor based fluorogenic probe, 4-(2-pyren-1-yl-vinyl) pyridine, 1 is synthesized that can form pH-sensitive nanoscopic aggregates in the aqueous medium. The fluorescence response of 1 in apolar solvents originates from LE state, while in polar solvents, it is dominated by TICT state. On the contrary, emissions from both TICT and AIE states is observed in water. A change in solution color from pale yellow to deep yellow is observed specifically upon the addition of HSO4¯ ions with a concomitant change in fluorescence color. The mechanistic investigations indicate the hydrogen bonding interaction of HSO4¯ ions with the pyridine nitrogen end, which can effectively alter the extent of charge transfer. Moreover, the interaction with HSO4¯, leading to larger aggregate formation. Further, the current system is involved in the analysis of water samples and screening care-kit products. Finally, the dye-coated paper strips are designed for cost-effective, on-location detection of HSO4¯.

Photophysics and charge transfer in oligo(thiophene) based conjugated diblock oligomers.

This paper reports an investigation of the electronic structure and photophysical properties of two "diblock" π-conjugated oligomers (T4-TBT and T8-TBT) that feature electron rich tetra(thiophene) (T4) or octa(thiophene) (T8) segments linked to an electron poor 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) moiety. Electrochemistry and UV-visible absorption spectroscopy reveals that the diblock oligomers display redox and absorption features that can be attributed to the Tn and TBT units. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations support the experimental electrochemistry and optical spectroscopy results, suggesting that the frontier orbitals on the diblock oligomers retain characteristics of the individual π-conjugated segments. However, low energy optical transitions are anticipated to arise from Tn to TBT charge transfer. Fluorescence spectroscopy on the diblock oligomers reveals that the oligomers feature strongly solvent dependent fluorescence. In non-polar solvents (hexane, toluene), the emission is structured with a moderate Stokes shift; however, in more polar solvents the emission becomes broader, and red-shifts significantly. Transient absorption spectroscopy on timescales from femtoseconds (fs) to microseconds (μs) reveals that in non-polar solvents excitation produces a singlet excited state (LE) that decays uniformly to the ground state in parallel with intersystem crossing to a triplet state. By contrast, in more polar solvents, excitation produces a very short-lived excited state (1-3 ps) which evolves rapidly into a second excited state that is attributed to the charge transfer (CT) state. The fast dynamics are associated with crossing from the LE state, which is populated initially by photoexcitation, into the CT state, which then decays to the ground state. The photophysical properties and dynamics of the LE and CT excited states are very similar for T4-TBT and T8-TBT, suggesting that the length of the oligo(thiophene) segment does not have a strong influence on the energy, structure or dynamics of the LE and CT excited states.

Theoretical investigation of steric effects on the S1 potential energy surface of o-carborane-anthracene derivatives.

TDDFT scan calculations were performed for s-carborane-anthracene derivatives (o-CB-X-Ant where X=-H, -CH3, -C2H5 and tert-butyl or -tBu) in order to understand the interplay between the steric effects, S1 potential energy surface (PES) and photophysical properties. The results show that all systems exhibit three local minima on the S1 PES, which correspond to the emissive LE and TICT state, along with the nonemissive CT state respectively. In the case of the unsubstituted system (o-CB-H-Ant), and -CH3 and -C2H5 substituted cases, S1 PES is predicted to be quite flat for certain conformations indicating that it is possible for these systems to reach the nonemissive CT state without a large energy penalty. In comparison, conformational pathways for the nonemissive CT state are predicted to be energetically unfavorable for o-CB-tBu-Ant as a result of both steric and electronic effects. These results provide a mechanism for the enhanced emission of σ-CB-fluorophore molecules with bulky ligands.

X-ray Reflectivity Study of Polylysine Adsorption on the Surface of DMPS Monolayers.

The results of a systematic study on the adsorption of polylysine molecules of different lengths on the surface of a 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) monolayer in the liquid (LE) and condensed (LC) states are presented. A compressibility diagram and the Volta potential were recorded with the Langmuir monolayer technique and further analyzed with the empirical approach. The structure of the monolayer films with adsorbed polypeptides was studied with synchrotron X-ray reflectometry. Two- and three-layer slab models describe the reflectivity data fairly well and reveal both the significant structural changes and the dehydration of the polar groups induced by all polylysines used at the maximal coverage of the monolayer interface in both the LE and LC states. On the one hand, in the LE phase of the monolayer (area per molecule A ≅ 70 Ǻ2), the integrated electron density of the lipid headgroup region is approximately half the density contained in the clean monolayer. This indicates both significant compaction and dehydration in the polar groups of the lipids, caused by the adsorption of polypeptides. On the other hand, in the LC state (A ≅ 40 Ǻ2), the degree of the hydration of the polar region is similar to that for the initial DMPS monolayer. However, both the electron density and the thickness of the head group region differ significantly from the values of these parameters for the clean monolayer in the LC state.

Naphthalimide-phenothiazine dyads: effect of conformational flexibility and matching of the energy of the charge-transfer state and the localized triplet excited state on the thermally activated delayed fluorescence.

In order to investigate the joint influence of the conformation flexibility and the matching of the energies of the charge-transfer (CT) and the localized triplet excited (3LE) states on the thermally activated delayed fluorescence (TADF) in electron donor–acceptor molecules, a series of compact electron donor–acceptor dyads and a triad were prepared, with naphthalimide (NI) as electron acceptor and phenothiazine (PTZ) as electron donor. The NI and PTZ moieties are either directly connected at the 3-position of NI and the N-position of the PTZ moiety via a C–N single bond, or they are linked through a phenyl group. The tuning of the energy order of the CT and LE states is achieved by oxidation of the PTZ unit into the corresponding sulfoxide, whereas conformation restriction is imposed by introducing ortho-methyl substituents on the phenyl linker, so that the coupling magnitude between the CT and the 3LE states can be controlled. The singlet oxygen quantum yield (ΦΔ) of NI-PTZ is moderate in n-hexane (HEX, ΦΔ = 19%). TADF was observed for the dyads, the biexponential luminescence lifetime are 16.0 ns (99.9%)/14.4 μs (0.1%) for the dyad and 7.2 ns (99.6%)/2.0 μs (0.4%) for the triad. Triplet state was observed in the nanosecond transient absorption spectra with lifetimes in the 4–48 μs range. Computational investigations show that the orthogonal electron donor–acceptor molecular structure is beneficial for TADF. These calculations indicate small energetic difference between the 3LE and 3CT states, which are helpful for interpreting the ns-TA spectra and the origins of TADF in NI-PTZ, which is ultimately due to the small energetic difference between the 3LE and 3CT states. Conversely, NI-PTZ-O, which has a higher CT state and bears a much more stabilized 3LE state, does not show TADF.

Reversible thermochromism, two-way mechanofluorochromism and tunable wavelength mechanoluminescence on tetraarylimidazole polymorphs

1, 2, 4, 5-tetraarylimidazole with 4-dimethylaminobiphenyl and 4-cyanophenyl (IMP) respectively introduced at N1 and C2 atom on imidazole core is prepared and two polymorphs correspondingly emit blue (IMP-B) and green (IMP-G) fluorescence are obtained by slow volatilization from solution. The solution-state emission displays the heavily solvatochromic effect as well as the unique solvent- and temperature-dependent LE, ICT and TICT emission equilibrium. X-ray crystallographic analysis, theoretic calculations and temperature-dependent emission decay spectra consistently support the assumption that the emission of IMP-B and IMP-G should respectively originate from LE and TICT state in.crystalline phase. For IMP-G crystal, it displays a reversible theromochromism by annealing-cooling cycles. Under force stimuli, IMP-B and IMP-G crystals present red- and blue-shifted MFC activity. The ground sample selectively pass through the two different routes to be recovered into the original crystalline phase, including transformation into IMP-B by annealing and into IMP-G by vapor fumigation. In spite of centrosymmetric space groups in both crystals, they still exhibit bright ML emission whose wavelength is nearly identical to that of fluorescence on each crystal. It is deduced that polar couples packed by molecules in two crystals may well account for the ML effect on the double polymorphs.

Red Light-Emitting Thermally-Activated Delayed Fluorescence of Naphthalimide-Phenoxazine Electron Donor-Acceptor Dyad: Time-Resolved Optical and Magnetic Spectroscopic Studies.

We prepared an orthogonal compact electron-donor (phenoxazine, PXZ)-acceptor (naphthalimide, NI) dyad (NI-PXZ), to study the photophysics of the thermally-activated delayed fluorescence (TADF), which has a luminescence lifetime of 16.4 ns (99.2 %)/17.0 μs (0.80 %). A weak charge transfer (CT) absorption band was observed for the dyad, indicating non-negligible electronic coupling between the donor and acceptor at the ground state. Femtosecond transient absorption spectroscopy shows a fast charge separation (CS) (ca. 2.02∼2.72 ps), the majority of the singlet CS state is short-lived, especially in polar solvents (τCR = 10.3 ps in acetonitrile, vs. 1.83 ns in toluene, 7.81 ns in n-hexane). Nanosecond transient absorption spectroscopy detects a long-lived transient species in n-hexane, which is with a mixed triplet local excited state (3 LE) and charge separated state (3 CS), the lifetime is 15.4 μs. In polar solvents, such as tetrahydrofuran and acetonitrile, a neat 3 CS state was observed, whose lifetimes are 226 ns and 142 ns, respectively. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of strongly spin exchanged 3 LE/3 CT states, with the effective zero field splitting (ZFS) |D| and |E| parameters of 1484 MHz and 109 MHz, respectively, much smaller than that of the native 3 NI state (2475 and 135 MHz). It is rare but solid experimental evidence that a closely-lying 3 LE state is crucial for occurrence of TADF and this 3 LE state is an essential intermediate state to facilitate reverse intersystem crossing in TADF systems.

A facile strategy for enhancing reverse intersystem crossing of red thermally activated delayed fluorescence emitters

By enhancing electron-withdrawing abilities of the secondary acceptor attachments into the main acceptor core, their charge transfer triplet ( 3 CT) energy levels are gradually reduced, while the locally excited triplet ( 3 LE A ) levels are well maintained. The well-aligned excited states, consequently, results in the enhancement of the rates of reverse intersystem crossing (RISC), and boosting the device efficiencies in red OLEDs. • A facile strategy to modulate CT states while maintaining LE state is proposed. • CT energy levels are gradually reduced, while the 3 LE A ones are well maintained. • Fast k RISC , small ΔE ST and high PLQY can be realized simultaneously. • mDPBPZ-DPXZ-based red OLED exhibits much higher EQE compared to its counterparts. Realizing highly efficient reverse intersystem crossing (RISC) process for red thermally activated delayed fluorescence (TADF) emitters remains a formidable challenge. Herein we demonstrate that charge transfer (CT) energy levels are gradually reduced with enhancing electron-withdrawing abilities of the secondary acceptor (A) attachments. Meanwhile, their local excited triplet ( 3 LE A ) state levels are well maintained due to the conjugations between the main framework and the attachments are all similar for these A segments. The optimized molecular energy level alignments are obtained in the case of mDPBPZ-DPXZ, in which 3 CT and the 3 LE A states are almost degenerate, and gives rise to a small singlet–triplet energy difference ( ΔE ST ) of 0.02 eV, a fast RISC rate of 1.54 × 10 6 s −1 and a high photoluminescence quantum yield of 93%, simultaneously. And the corresponding device achieves an external quantum efficiency of 21.6%, significantly higher than the other counterparts. This work demonstrates that the energy alignment between the CT and 3 LE A state can be easily managed by introducing secondary A attachments on original A framework, which shifts the 1 CT and 3 CT state to lower energies without affecting the 3 LE A energy. It provides a facile design strategy to promote the RISC process, which is particular useful in designing highly efficient red TADF emitters.

Multiple charge transfer disk-like emitters with fast fluorescence radiation rate and high horizontal dipole orientation for pure blue organic light-emitting diodes

• Multiple CT channels and 3 LE states contribute to the speedy k r and RISC processes. • High Θ and small FWHM are realized due to the rigid disk-like structure. • η r and η out above 30% are achieved owing to triplet exciton utilization and high Θ . • EQE above 10% was realized for the standard blue device based on BO3N. The efficiency of pure blue Organic light-emitting diodes (OLEDs) still needs to be improved, which is limited by the utilization of the forbidden transition triplet excitons and low light-out-coupling efficiency in devices. Herein, two blue emitters were developed through combining multiple diphenylamine (DPA) donors with an oxygen-bridged boron (DBA) or triphenyl triazine (TRZ) acceptor. Notably, a narrow-band deep blue emission with a peak of 423 nm and a Full-width at half-maximum (FWHM) of 34 nm is realized for the boron-based emitter in toluene solution. Thanks to their multiple Charge transfer (CT) channels and high orientated disk-like structure, high radiative transition rates ( k r > 10 8 s −1 ) and high horizontal emitting dipole ratios ( Θ > 80%) can be realized simultaneously. Meanwhile, the introduction of the peripheral DPA units can also provide multiple localized triplet ( 3 LE) states for spin up-conversion of forbidden transition triplet excitons. Owing to the effective up-conversion of triplet excitons and high light-out-coupling efficiency, maximum external quantum efficiency of more than 10% is achieved for the boron emitter-based pure blue OLEDs with a Commission Internationale de l’Eclairage (CIE) coordinates of (0.14, 0.08).

Anthracene-containing complexes of boron difluoride. Dual luminescence, formation of excimers, and mechanochromism

A comparative study of the luminescence properties and crystal structure of boron difluoride 1-(anthracen-9-yl)butane-1,3-dionate (1) and their nitrogen-containing analogs with hydrogen and methyl substituents (2 and 3) has been performed. For boron difluoride beta-diketonate (1) and beta-ketoiminate with a hydrogen substituent (2), which does not create significant steric difficulties, the luminescence of crystals is determined by that of excimers and aggregates based on them. For the compounds 1–3, mechanofluorochromism has been observed, the spectral manifestation of which is different: at grinding the crystals of 1 and 2, a hypsochromic shift of the excitation and luminescence bands occurs, whereas in the case of crystals of 3, a bathochromic shift takes place. Dual luminescence has been detected for the solutions of 1. During the relaxation in the excited state of a molecule of 1 (S1 → Sʹ1), TICT is formed. It was possible to register the short-wavelength luminescence from the LE state in the dilute solutions of 1. TICT or LE luminescence can be excited, in the case of 1, by varying the wavelength of the exciting light.

Polarity-active NIR probes with strong two-photon absorption and ultrahigh binding affinity of insulin amyloid fibrils.

Amyloid fibrils are associated with many neurodegenerative diseases. In situ and in vivo visualization of amyloid fibrils is important for medical diagnostics and requires fluorescent probes with both excitation and emission wavelengths in the far-red and NIR region, and simultaneously with high binding-affinity to amyloid fibrils and the ability to cross the blood–brain barrier, which, however, remain a challenge. Here, we rationally design and synthesize an excellent polarity-sensitive two-photon excited NIR fluorophore (TZPI) based on a donor (D)–acceptor (A)-ion compound. The electron-rich carbazole group and the ionic pyridinium bromide group, linked by an electron-poor π-conjugated benzothiadiazole group, ensure strong near infrared (NIR) emission. Furthermore, the lipophilic carbazole together with the benzothiadiazole group facilitates docking of the probe in the hydrophobic domains of amyloid aggregates with the dissociation constant Kd = 20 nM and 13.5-fold higher binding affinity to insulin fibrils than the commercial probe ThT. On association with the amyloid fibrils, the tiny decrease in polarity leads to a large increase in its NIR emission intensity with an on–off ratio > 10; meanwhile, the TZPI probe exhibits a quantum yield of up to 30% and two-photon absorption cross-section values of up to 467.6 GM at 890 nm. Moreover, the application of TZPI in two-photon imaging is investigated. The ultrahigh binding affinity, the strong NIR emission, the good two-photon absorption properties, the high photo-stability, the appropriate molecular mass of 569 Da and the lipophilicity with log P = 1.66 ± 0.1 to cross the BBB make TZPI promising as an ideal candidate for detecting amyloid plaques in vivo.

Absorption and fluorescence properties of non-symmetric benzo-, furo-, and thieno-fused structures at the b bonds in the BODIPY frame.

The influence of furo-, thieno-, and benzo-fused structures at the b bonds in the BODIPY frame on the optical properties is investigated by TD-CAM-B3LYP and RI-CC2 calculations. The most important result is that substituents at the b bond of the BODIPY core affect strongly the S1-S2 gaps. In contrast to the S1 (local excited (LE-type)) state, energy of which is nearly the same for all the substituents at the b bond, energy of the S2 (charge transfer (CT-type)) state depends strongly on the nature of the substituent and decreases in the following order: furo-fused > thieno-fused > benzo-fused. In the last case the inversion of S1 and S2 levels occurs. No red shift of the main long-wave absorption transition and no substantial changes in its intensity can be predicted by the calculations for benzo[b]-derivative (vertical energy (Ev) is 2.95eV, oscillator strength (f) is 0.80) relative to furo- (Ev=2.97eV, f=0.69) and thieno-derivatives (Ev=2.95eV, f=0.65). However, the dramatic decrease of the fluorescence quantum yield is expected owing to positions of their LE-type (Ev=2.95eV, f=0.80) and CT-type (Ev=2.79eV, f=0.01) transitions. In the case of thieno-fused BODIPY, owing to the approach of the energy levels of the vertical S2 and S1 states, the energy equilibrium of the [1]CT-type state becomes lower than that of the [1]LE state, and Φf of the thieno-derivative may be substantially lower than Φf of the furo-derivative.

Spin-Orbit Charge-Transfer Intersystem Crossing (ISC) in Compact Electron Donor-Acceptor Dyads: ISC Mechanism and Application as Novel and Potent Photodynamic Therapy Reagents.

Spin-orbit charge-transfer intersystem crossing (SOCT-ISC) is useful for the preparation of heavy atom-free triplet photosensitisers (PSs). Herein, a series of perylene-Bodipy compact electron donor/acceptor dyads showing efficient SOCT-ISC is prepared. The photophysical properties of the dyads were studied with steady-state and time-resolved spectroscopies. Efficient triplet state formation (quantum yield ΦT =60 %) was observed, with a triplet state lifetime (τT =436 μs) much longer than that accessed with the conventional heavy atom effect (τT =62 μs). The SOCT-ISC mechanism was unambiguously confirmed by direct excitation of the charge transfer (CT) absorption band by using nanosecond transient absorption spectroscopy and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The factors affecting the SOCT-ISC efficiency include the geometry, the potential energy surface of the torsion, the spin density for the atoms of the linker, solvent polarity, and the energy matching of the 1 CT/3 LE states. Remarkably, these heavy atom-free triplet PSs were demonstrated as a new type of efficient photodynamic therapy (PDT) reagents (phototoxicity, EC50 =75 nm), with a negligible dark toxicity (EC50 =78.1 μm) compared with the conventional heavy atom PSs (dark toxicity, EC50 =6.0 μm, light toxicity, EC50 =4.0 nm). This study provides in-depth understanding of the SOCT-ISC, unveils the design principles of triplet PSs based on SOCT-ISC, and underlines their application as a new generation of potent PDT reagents.

TICT excited states of o‐ and p‐N,N‐dimethylamino analogues of GFP chromophore

AbstractEven though all the p‐N,N‐dimethylaminobenzonitrile (p‐DMABN), cis‐o‐DMABDI, and cis‐p‐DMABDI (the N,N‐dimethylamino analogues of green fluorescence protein chromophore) have the same electron‐donating N,N‐dimethylamino group, unlike the dual fluorescence of p‐DMABN, both cis‐o‐DMABDI and cis‐p‐DMABDI display single fluorescence. To figure out the interesting phenomena, the CAM‐TD‐B3LYP method and the cc‐pVDZ basis set were used to explore geometries, molecular orbitals, electronic transition, dipole moment, and potential energy surfaces of the S1 excited states of cis‐o‐DMABDI and cis‐p‐DMABDI. We found that the S1 excited states of cis‐o‐DMABDI and cis‐p‐DMABDI are 1(π, π*) charge transfer excited states with twisted structures, where the N,N‐dimethylaminobenzene moiety functions as an electron donor, the methyleneimidazolone moiety serves as an electron acceptor, and the electron donor is linked with the electron acceptor by the C─C single bond (P‐bond). The fluorescent emissions of cis‐o‐DMABDI and cis‐p‐DMABDI predicted by the CAM‐TD‐B3LYP/cc‐pVDZ level are quite consistent with the experimental results. For the cis‐o‐DMABDI and cis‐p‐DMABDI, the S1 locally excited state is less stable than the S1 twisted intramolecular charge transfer state, and the S1 LE state is not a stationary point (global minimum). That is why both cis‐o‐DMABDI and cis‐p‐DMABDI display single fluorescence.