Nanosecond Transient Absorption Spectra Research Articles

Molten-salt-chemistry-assisted synthesis of heterostructured g-C3N4/BiOI nanocomposites with enhanced sunlight absorption properties for efficient solar-to-chemical energy conversion

The g-C3N4 nanosheets coupled BiOI (g-C3N4/BiOI) nanocomposites with unique heterojunction structure and excellent wide-spectrum sunlight absorption properties have been developed via molten-salt-chemistry-assisted strategy. The heterostructured g-C3N4/BiOI shows enhanced solar to the thermal effect, improved electron mobility and carrier lifetime for efficient photocatalytic dye degradation. The engineered g-C3N4/BiOI catalyst exhibits outstanding photocatalytic activity for methyl orange (MO) removal with 100 % degradation conversion and excellent stability under solar light irradiation, which is respectively 2.69 and 1.27 times higher than those of g-C3N4 and BiOI counterparts. The broad scope toward other dyes degraded by g-C3N4/BiOI and the generality of the molten-salt-chemistry-assisted strategy for synthesizing g-C3N4/BiOCl and g-C3N4/BiOBr with considerable photodegradation efficiency are proved. The analysis of kinetic behaviors, electron spin resonance, and nanosecond transient absorption spectra for structure-activity relationship, key intermediate of •O2− species, and excited state lifetime of g-C3N4/BiOI heterojunction material in the photocatalytic MO degradation processes are also demonstrated.

Electron Spin Dynamics of the Intersystem Crossing in Aminoanthraquinone Derivatives: The Spectral Telltale of Short Triplet Excited States.

We studied the excited state dynamics of two bis-amino substituted anthraquinone (AQ) derivatives, with absorption in the visible spectral region, which results from the attachment of a electron-donating group to the electron-deficient AQ chromophore. Femtosecond transient absorption spectra show that intersystem crossing (ISC) takes place in 190-320 ps, and nanosecond transient absorption spectra demonstrated an unusually short triplet state lifetime (2.06-5.43 μs) for the two AQ derivatives. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show an inversion of the electron spin polarization (ESP) phase pattern of the triplet state at a longer delay time after laser flash. Spectral simulations show faster decay of the Ty sublevel than the other two sublevels (τx = 15.0 μs, τy = 1.50 μs, τz = 15.0 μs); theoretical computation predicts initial overpopulation of the Ty sublevel, and rationalizes the short T1 state lifetime and the ESP inversion. Theoretical computations taking into account the electron-vibrational coupling, i.e., the Herzberg-Teller effect, successfully rationalize the TREPR experimental observations.

Charge Transfer and Intersystem Crossing in Compact Naphthalenediimide-Phenothiazine Triads: Synthesis and Study of the Photophysical Property with Transient Optical and Electron Paramagnetic Resonance Spectroscopic Methods.

In order to obtain a long-lived charge separation (CS) state in compact electron donor-acceptor molecular systems, we prepared a series of naphthalenediimide (NDI)-phenothiazine (PTZ) triads, with phenylene as the linker between the donor and acceptor. Conformation restriction is imposed to control the mutual orientation of the NDI and PTZ units by attaching methyl groups on the phenylene linker to tune the electronic coupling between the donor and the acceptor. Moreover, the PTZ moiety was oxidized to sulfoxide to tune the ordering of the CS state and the 3LE state (LE: locally excited state). UV-vis absorption spectra indicate electronic coupling between NDI with the phenylene linker as well as the PTZ units, manifested by the appearance of a charge-transfer (CT) absorption band, whereas this coupling is devoid in the triads with conformation restriction imposed. Fluorescence is strongly quenched in the triads compared to the reference compound, indicating electron transfer upon photoexcitation. Femtosecond transient absorption spectra indicate that the CS takes 0.8 ps, and then the 3LE state is formed by charge recombination in 83 ps. Nanosecond transient absorption (ns-TA) spectra show that the 3NDI state was observed in nonpolar solvents such as cyclohexane (triplet state lifetime: 95.7 μs), whereas the CS state was observed in more polar solvents. The CS state lifetimes are up to 1.2 μs (in toluene). Time-resolved electron paramagnetic resonance spectra of the triads in toluene consist of two types of signals: CS states (narrower signals, ∼10 mT) and 3LE states (broader signals, ∼50 to 200 mT). In the spectra of the triads containing PTZ, the CS state signals dominate, whereas for the triads containing oxidized PTZ, the 3NDI signals (zero-field splitting D ≈ 2000 MHz) prevail, both observations being in agreement with the ns-TA spectral studies. The electron spin polarization phase pattern of the 3NDI states of the triads indicates that the intersystem crossing (ISC) mechanism is spin-orbit charge-transfer ISC. Considering the 3CS state as ion pairs, the electron-exchange energy (J) is determined to be -39 to -59 MHz, and the electron spin dipolar interaction is 83-92 MHz.

A Rational Way to Control the Triplet State Wave Function Confinement of Organic Chromophores: Effect of the Connection Sites and Spin Density Distribution-Guided Molecular Structure Design Principles in Bodipy Dimers.

To study the intersystem crossing (ISC) and the spatial confinement of the triplet excited states of organic chromophores, we prepared a series of Bodipy dimers. We found that the connection position of the two units has a significant effect on the absorption and fluorescence. Singlet oxygen quantum yields of 3.8-12.4% were observed for the dimers, which are independent of solvent polarity. Nanosecond transient absorption spectra indicate the population of long-lived triplet excited states with lifetimes (τT) of 45-454 μs. Pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that the T1 triplet states are essentially delocalized, which is different from the case for the previously reported Bodipy dimers. The TREPR spectra of the triplet states imply that the delocalization over the whole dimer essentially depends on the electron density of the carbon atoms at the connection sites. This property may become a universal rule for controlling the T1 state confinement in multichromophore organic molecules.

Ultrafast excited state relaxation dynamics of pyran-based D-π-A systems: solvent polarity controls the triplet state.

The excited state relaxation dynamics of V-shaped D-π-A systems having 4H-pyranylidene appended barbituric acid as an acceptor and diphenylamine (TPAPBA) and diethyl amine (EAPBA) as donors were investigated using steady-state and time-resolved spectroscopy along with theoretical optimization. The steady-state photophysical characterization exhibited the bathochromic shift of the emission maximum (∼6400 cm-1) and large change in the dipole moment (∼24D) with an increase of solvent polarity, reflecting the occurrence of the intramolecular charge transfer state (ICT) in the excited state. The nanosecond and femtosecond transient absorption spectra of these derivatives in a non-polar solvent, toluene, reveal that the excited state relaxation pathway involving a local excited state (LE) decayed to ICT followed by the formation of a twisted ICT state by conformational relaxation, finally leading to the triplet state. The lack of observation of a triplet state in the polar solvent, acetonitrile, signifies that the relaxation dynamics of V-shaped triads in the excited state are influenced by the polarity of the solvent.

Effect of the charge-separated state on the 3MLCT state: Synthesis and study of the photophysics of electron donor-acceptor dyads based on Pt(II)-Schiff base coordination framework and naphthalenediimide chromophore

We prepared Pt(II)-Salen Schiff base complexes with a naphthalenediimide (NDI) unit attached on the coordination framework, to study the intersystem crossing (ISC), the charge separation, and the possible formation of a long-lived triplet charge separated (CS) state. The Pt(II)-Schiff base coordination framework acts as electron donor and it shows ultrafast ISC (<1 ps) and small Stokes shift. NDI acts as electron acceptor. The two units are attached either via a phenyl linker (Pt-NDI) or phenyl ethynyl linker (Pt-CC-NDI). Negligible electronic interaction between the Pt(II)-Schiff base coordination framework and the NDI unit at ground state was observed for both dyads, whereas the phosphorescence of the Pt(II)-Schiff base coordinated framework was quenched to large extent in the dyads, as compared to the reference complexes containing no NDI unit, especially in polar solvents. Electrochemical and optical spectral data show that the CS state energy is 1.93 eV, 1.82 eV, 1.54 eV, 1.43 eV in n-hexane (HEX), toluene (TOL), tetrahydrofuran (THF), acetonitrile (ACN), respectively. In comparison, the 3MLCT state and 3NDI state energy are 1.97 and 2.04 eV, respectively. Nanosecond transient absorption spectra show the formation of the 3MLCT state for Pt-CC-NDI, whose lifetimes is shortened in polar solvents (τT = 3.5 µs, 2.1 µs, 22 ns, and 19 ns in HEX, TOL, THF and ACN, respectively), similar trend was observed for Pt-NDI. Femtosecond transient absorption spectra show that the charge separation takes less than 1 ps both in toluene and acetonitrile. Quenching of the otherwise long-lived triplet excited state (3.5 µs) by a short lived CS state may be developed as a photo-protection mechanism similar to that found in natural photosynthetic apparatus.

Efficient Spin-Orbit Charge-Transfer Intersystem Crossing and Slow Intramolecular Triplet-Triplet Energy Transfer in Bodipy-Perylenebisimide Compact Dyads and Triads.

Bodipy (BDP)-perylenebisimide (PBI) donor-acceptor dyads/triad were prepared to study the spin-orbit charge-transfer intersystem crossing (SOCT-ISC). For BDP-PBI-3, in which BDP was attached at the imide position of PBI, higher singlet oxygen quantum yield (ΦΔ =85 %) was observed than the bay-substituted derivative BDP-PBI-1 (ΦΔ =30 %). Femtosecond transient absorption spectra indicate slow Förster resonance energy transfer (FRET; 40.4 ps) and charge separation (CS; 1.55 ns) in BDP-PBI-3, while for BDP-PBI-1, CS takes 2.8 ps. For triad BDP-PBI-2, ultrafast FRET (149 fs) and CS (4.7 ps) process were observed, the subsequent charge recombination (CR) takes 5.8 ns and long-lived 3 PBI* (179.8 μs) state is populated. Nanosecond transient absorption spectra of BDP-PBI-3 show that the CR gives upper triplet excited state (3 BDP*) and subsequently, via a slow intramolecular triplet energy transfer (14.5 μs), the 3 PBI* state is finally populated, indicating that upper triplet state is involved in SOCT-ISC. Time-resolved electron paramagnetic resonance spectroscopy revealed that both radical pair ISC (RP ISC) and SOCT-ISC contribute to the ISC. A rare electron spin polarization of (e, e, e, e, e, e) was observed for the triplet state formed via the RP ISC mechanism, due to the S-T+1 /T0 states mixing.

The effect of dark states on the intersystem crossing and thermally activated delayed fluorescence of naphthalimide-phenothiazine dyads.

A series of 1,8-naphthalimide (NI)-phenothiazine (PTZ) electron donor-acceptor dyads were prepared to study the thermally activated delayed fluorescence (TADF) properties of the dyads, from a point of view of detection of the various transient species. The photophysical properties of the dyads were tuned by changing the electron-donating and the electron-withdrawing capability of the PTZ and NI moieties, respectively, by oxidation of the PTZ unit, or by using different aryl substituents attached to the NI unit. This tuning effect was manifested in the UV-vis absorption and fluorescence emission spectra, e.g., in the change of the charge transfer absorption bands. TADF was observed for the dyads containing the native PTZ unit, and the prompt and delayed fluorescence lifetimes changed with different aryl substituents on the imide part. In polar solvents, no TADF was observed. For the dyads with the PTZ unit oxidized, no TADF was observed as well. Femtosecond transient absorption spectra showed that the charge separation takes ca. 0.6 ps, and admixtures of locally excited (3LE) state and charge separated (1CS/3CS) states formed (in n-hexane). The subsequent charge recombination from the 1CS state takes ca. 7.92 ns. Upon oxidation of the PTZ unit, the beginning of charge separation is at 178 fs and formation of 3LE state takes 4.53 ns. Nanosecond transient absorption (ns-TA) spectra showed that both 3CS and 3LE states were observed for the dyads showing TADF, whereas only 3LE or 3CS states were observed for the systems lacking TADF. This is a rare but unambiguous experimental evidence that the spin-vibronic coupling of 3CS/3LE states is crucial for TADF. Without the mediating effect of the 3LE state, no TADF is resulted, even if the long-lived 3CS state is populated (lifetime τCS ≈ 140 ns). This experimental result confirms the 3CS → 1CS reverse intersystem crossing (rISC) is slow, without coupling with an approximate 3LE state. These studies are useful for an in-depth understanding of the photophysical mechanisms of the TADF emitters, as well as for molecular structure design of new electron donor-acceptor TADF emitters.

Preparation of Xanthene-TEMPO Dyads: Synthesis and Study of the Radical Enhanced Intersystem Crossing.

We prepared a rhodamine-TEMPO chromophore-radical dyad (RB-TEMPO) to study the radical enhanced intersystem crossing (REISC). The visible light-harvesting chromophore rhodamine is connected with the TEMPO (a nitroxide radical) via a C-N bond. The UV-vis absorption spectrum indicates negligible electron interaction between the two units at the ground state. Interestingly, the fluorescence of the rhodamine moiety is strongly quenched in RB-TEMPO, and the fluorescence lifetime of the rhodamine moiety is shortened to 0.29 ns, from the lifetime of 3.17 ns. We attribute this quenching effect to the intramolecular electron spin-spin interaction between the nitroxide radical and the photoexcited rhodamine chromophore. Nanosecond transient absorption spectra confirm the REISC in RB-TEMPO, indicated by the detection of the rhodamine chromophore triplet excited state; the lifetime was determined as 128 ns, which is shorter than the native rhodamine triplet state lifetime (0.58 μs). The zero-field splitting (ZFS) parameters of the triplet state of the chromophore were determined with the pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra. RB-TEMPO was used as a photoinitiator for the photopolymerization of pentaerythritol triacrylate (PETA). These studies are useful for the design of heavy atom-free triplet photosensitizers, the study of the ISC, and the electron spin dynamics of the radical-chromophore systems upon photoexcitation.

Electron transfer and intersystem crossing in the coumarin-anthracene electron donor-acceptor dyads

A series of compact coumarin (Cou)-anthracene (An) electron donor-acceptor dyads were prepared, in which the two units are directly connected at either 2- or 9- position of the An unit and the 3-position of the Cou unit to study the relationship between the molecular configuration, electronic coupling and the intersystem crossing (ISC) efficiency. Single crystal X-ray diffraction results indicate that the dihedral angles are 88.0° and 85.7°, respectively, in the dyads, in which the donor and acceptor are connected by 9- position of the An unit (Cou-9An and Cou-9AnCN). However, for Cou-2An the optimized ground state geometry indicates a more coplanar configuration with a dihedral angle of 38.6°. The solvent polarity-dependent singlet oxygen quantum yields (ΦΔ) of the orthogonal dyads (Cou-9An and Cou-9AnCN) are 39% (in acetonitrile) and 64% (in dichloromethane), respectively. whereas ΦΔ is negligible for the coplanar dyad (Cou-2An). These results indicated that orthogonal geometry is beneficial to spin−orbit charge transfer ISC (SOCT-ISC) mechanism. Nanosecond transient absorption spectra demonstrated that the lowest triplet state was mainly localized on An unit in Cou-9An and Cou-9AnCN, with the intrinsic triplet state lifetime of 234 and 207 μs, respectively. Triplet state delocalization was observed for Cou-2An, which prolongs the triplet state lifetime to 742 μs. Femtosecond transient absorption spectra results indicated fast charge separation (CS, 0.36−0.41 ps) and slow charge recombination (CR, 2.1−2.8 ns) in the orthogonal dyads. Time-resolved electron paramagnetic resonance (TREPR) spectra further confirmed the SOCT-ISC mechanism in the orthogonal dyads (Cou-9An and Cou-9AnCN), however, the SOCT-ISC and spin orbit ISC (SO-ISC) mechanism are both responsible for the triplet state formation in the coplanar dyad (Cou-2An). Electron spin polarization (ESP) pattern of the triplet state TREPR spectra of Cou-9An and Cou-9AnCN is (a,e,a,e,a,e), however, ESP phase pattern of (e,a,e,a,e,a) and (e,e,e,a,a,a) were observed for the two triplet states required for the simulation of the TREPR spectrum of Cou-2An, which indicated that ESP patterns of triplet state not only depend on the molecular geometry, but also on the structure of the electron donor and acceptor.

Does Twisted Molecular Structure Always Induce Intersystem Crossing? A Case Study with Near-IR Absorbing 1,8-Diazabicyclo[5.4.0]undec-7-ene-fused Naphthaldiimide.

Heavy atom-free organic chromophores showing absorption in the near-IR region with intersystem crossing (ISC) ability are important for applications in various fields, e.g., photocatalysis and photodynamic therapy. Herein, we studied the photophysical property of a naphthalenediimide (NDI) derivative, in which the NDI chromophore is fused with pentacyclic 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which shows a strong charge-transfer (CT) absorption band (S0 → 1CT transition) in the near-IR region of 600-740 nm. The effect of extended π-conjugation framework in NDI-DBU compared with the derivative of mono-amino substitution (NDI-NH-Br) was studied by steady-state and nanosecond transient absorption (ns-TA) spectra, electron paramagnetic resonance (EPR) spectroscopy, and theoretical computations. The fluorescence is almost completely quenched for NDI-DBU (ΦF = 1.0%) as compared with NDI-NH-Br (ΦF = 24% in toluene). However, the ISC of NDI-DBU is poor, and the singlet oxygen quantum yield was determined as ΦΔ = 9% versus ΦΔ = 57% for NDI-NH-Br, although the compound has significantly twisted molecular structure. The ns-TA spectral study showed a long-lived triplet excited state (τT = 132 μs) in NDI-DBU, with T1 energy of 1.20-1.44 eV, and the ISC is via the S2 → T3 path, which is verified by theoretical calculations. This study displayed that the twisting of molecular geometry does not always assure efficient ISC.

Ultrafast Intermolecular Energy Transfer in OLED Materials: Excited-State Dynamics of a Blend of Poly(vinylcarbazole) and Oxadiazole Derivative in Solution and Film States

Intermolecular energy transfer dynamics between layers of organic materials controls the efficiency of organic light-emitting diode (OLED) devices. Here, to understand the ultrafast intermolecular energy transfer dynamics occurring in widely used UV-OLED materials of poly(vinylcarbazole) (PVK, donor: hole-transporting material) and synthesized oxadiazole derivative (OXD, acceptor: electron-transporting material), their excited-state relaxation pathways are investigated in tetrahydrofuran (THF) and film. Upon addition of OXD, the nanosecond fluorescence lifetime studies of PVK exhibited efficient quenching of dynamics, supporting the occurrence of Förster resonance energy transfer from PVK to OXD with an efficiency of ∼90%. The femtosecond time-resolved absorption measurements of the mixture of PVK and OXD revealed the intermolecular resonance energy transfer with a time constant of ∼1.32 ps from all of the intermediates in the excited states of PVK to OXD. Interestingly, in the film state, the formation of exciplex beneficial for devices with a lifetime of ∼32.66 ns was observed at ∼516 nm upon excitation of PVK in a blend of OXD and PVK. Nanosecond transient absorption spectra confirmed the occurrence of energy transfer also from the triplet state of PVK. Understanding of these excited-state dynamics occurring in solution and film states is a prerequisite to design and improve the performance of OLED materials.

Long‐lived Triplet Excited State of Perylenemonoimide (PMI) in a Diimine Pt(II) Bis‐acetylide Complex: Preparation and Study of the Photophysical Property with Transient Spectra

AbstractWe prepared a N^N Pt(II) bisacetylide complex that has strong absorption of visible light (molar absorption coefficients ϵ=6.7×104 M−1 cm−1 at 570 nm), and the singlet oxygen quantum yield (ΦΔ) is up to 78 %. Femtosecond transient absorption spectra show the intersystem crossing (ISC) of the complex takes 81.8 ps, nanosecond transient absorption spectra show the triplet excited state lifetime is 7.6 μs. Density functional theory (DFT) computation demonstrated that the S1 and T1 states are mainly localized on the perylenemonoimide (PMI) ligands, although the involvement of the Pt(II) centre is noticeable. The complex was used as triplet photosensitizer to generate delayed fluorescence with perylenebisimide (PBI) as the triplet state energy acceptor and emitter, via the intermolecular triplet‐triplet energy transfer (TTET) and triplet‐triplet annihilation (TTA), the delayed fluorescence lifetime is up to 52.5 μs under the experimental conditions.

Molecular torsion controls the excited state relaxation pathways of multibranched tetraphenylpyrazines: effect of substitution of morpholine vs. phenoxazine.

Multibranched donor-acceptor derivatives exhibiting desirable photophysical properties are efficiently used in optoelectronic devices, in which the excited state relaxation dynamics of the derivatives control the efficiency of the devices. Here, the effect of intramolecular torsion on the excited state relaxation dynamics of tetraphenylpyrazine (TPP) derivatives in non-polar (toluene) and polar (THF) solvents is investigated by substituting the electron donor of morpholine (TPP-4MOP) and phenoxazine (TPP-4PHO) leading to the planar and twisted configurations, respectively, using femtosecond and nanosecond transient absorption spectroscopy. In the steady state, TPP-4MOP showed feeble emission (ΦF ∼0.03) due to the weak donor by the delocalization of electron density supported by theoretical optimization. The TPP-4PHO exhibited strong emission (ΦF ∼0.18) in toluene compared to that in THF, in which it showed a large Stokes shift (∼9691 cm-1) with low fluorescence quantum yield (ΦF ∼0.01). The observation of large Stokes shifts, inherent nature and theoretical calculations of TPP-4PHO suggest the twisting of the dihedral angle between tetraphenylpyrazine and phenoxazine in the excited state leading to the twisted intramolecular charge transfer state (TICT). The femtosecond and nanosecond transient absorption and picosecond time-resolved emission spectra of TPP-4PHO revealed the signature of the existence of both the partial TICT and TICT states in THF leading to the triplet state. Whereas in the case of TPP-4MOP, the transient absorption spectra showed the formation of the triplet state from the local excited state without the involvement of the TICT state. Aggregation studies of TPP-4PHO in a THF and water mixture reflect the elimination of the TICT state by the restriction of intramolecular torsion in the aggregates leading to an increase of 12-fold of the fluorescence intensity along with shifting of the maximum towards the blue region. These studies revealed that the excited state relaxation pathways of the derivatives are controlled by polarity-dependent torsional motion.

Truxene-linked coumarin-corrole triad: Synthesis, photophysical properties and application in Triplet−Triplet annihilation upconversion

It is crucial to develop organic triplet photosensitizers (PSs) with excellent performance for upconversion applications. Herein, we synthesized a novel truxene-linked coumarin-corrole triads (C-Trux-Corr) and investigated the photophysical properties by steady-state UV–Vis absorption and fluorescence spectroscopies as well as nanosecond time-resolved transient absorption spectroscopy. The triad displays an efficient intramolecular singlet energy transfer from coumarin moiety to the corrole moiety. Nanosecond time-resolved transient difference absorption spectra indicate that the triplet excited states of C-Trux-Corr are distributed on the corrole moiety. The triplet lifetimes of C-Trux-Corr and Trux-C are 195.2 μs and 45.9 μs, respectively. The triplet state lifetime of C-Trux-Corr is significantly prolonged and it was applied as an organic triplet photosensitizer in the triplet-triplet upconversion system with an upconversion quantum yield of 2.93%, which is higher than that the corrole itself (1.27%) by more than 2-fold.

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.

Charge Separation/Recombination, Intersystem Crossing, and Unusually Slow Intramolecular Triplet-Triplet Energy Transfer in Naphthalenediimide-Anthracene Compact Energy Donor-Acceptor Dyads.

Three anthracene (An)-naphthalenediimide (NDI) compact electron donor-acceptor dyads were prepared. Femtosecond transient absorption (fs-TA) spectra show fast charge separation (ca. 0.9-1.7 ps) and relatively slow charge recombination (ca. 8-565 ps) upon photoexcitation; moreover, the 3An state was observed for 9-An-NDI, whereas the final state is 3NDI for both 9-An-Ph-NDI and 2-An-Ph-NDI, which have an intervening phenyl linker between the An and NDI units. Nanosecond transient absorption (ns-TA) spectra indicate that the lowest triplet state of all the dyads is 3An, with triplet lifetimes of 139-354 μs. An unusually slow intramolecular triplet-triplet energy transfer (TTET) was observed for 9-An-Ph-NDI and 2-An-Ph-NDI (32-85 ns). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy confirms that the intersystem crossing (ISC) mechanism is spin orbit charge transfer ISC (SOCT-ISC) for all the dyads; for 9-An-NDI, only the 3An state was observed, while for the other two dyads, both 3NDI and 3An states were observed, with their relative population changing with increasing delay time, which supports TTET.

Spiro rhodamine-coumarin compact electron donor-acceptor dyads: synthesis and spin-orbit charge transfer intersystem crossing.

We prepared spiro rhodamine (RB)-coumarin (Cou) compact electron donor-acceptor dyads (RB-Cou-CF3 and RB-Cou-CN), to study the charge transfer (CT) and spin-orbit CT intersystem crossing (SOCT-ISC). The π-conjugation planes of the rhodamine and coumarin units in both dyads are in nearly orthogonal geometry (dihedral angle: 86.3°). CT state emission was observed for RB-Cou-CF3 (at 550nm) and RB-Cou-CN (at 595nm). Although the fluorescence of the pristine coumarin units (fluorescence quantum yields ΦF = 59%) was quenched in the dyads (ΦF = 0.5 ~ 1.1% in n-hexane), the triplet state quantum yields of the dyads are also low (singlet oxygen quantum yield, ΦΔ = 2.3-7.5% in n-hexane). Nanosecond transient absorption spectra show that the 3Cou* state was formed, which shows a triplet state lifetime of 11-15.6μs. The proposed photophysical path for the dyads is as follows: RB-1Cou* → RB+•-Cou-• → RB-3Cou*. The low SOCT-ISC yield is attributed to the slightly lower charge-transfer state energy (1.94eV in toluene) as compared to the 3Cou* state energy (2.23eV) and the shallow potential energy curve (PEC) at energy minima of the dyads. This work indicates that orthogonal conformation of donor-acceptor units is inadequate for achieving efficient SOCT-ISC. These results are useful for studying charge separation and intersystem crossing of electron donor/acceptor dyads.

Photophysical Properties of Naphthalene-oxacalix[m]arene and Recognition of Fullerene C60.

Three different pore sizes of oxacalix[m]arene[n]pyrimidines modified with a naphthalene substituent were synthesized and characterized by HRMS, 1H NMR, and single-crystal analysis (8OA and 8OA-N). Steady-state spectroscopy indicates these naphthalene-oxacalix[m]arenes exhibit good fluorescence properties, which isattributed to the locally excited (LE) state emission, and electrochemical results show that the photoinduced electron transfer (PET) process occurs from the naphthalene substituent to the linked pyrimidine. Nanosecond transient absorption spectra, singlet oxygen quantum yields (ΦΔ4OA-N = 45.1%, ΦΔ6OA-N = 56.6%, and ΦΔ8OA-N = 65.7%) and theoretical calculations demonstrate that the torsion angle between the donor (naphthalene) and the acceptor (pyrimidine) promotes intersystem crossing (ISC), and the lifetime of the triplet state reaches ca. 8 ms. Interestingly, all three host molecules (4OA-N, 6OA-N, and 8OA-N) showed a high affinity for fullerene C60, and significant binding constants in the range of 4.10–6.68 × 104 M–1 were obtained by fluorescence titration; in contrast, previous reports indicated that the similar oxacalix[m]arene[n]pyrimidine scaffold could not efficiently complex with C60. In the frontier molecular orbital theory calculations of the supramolecular system of 4OA-N@C60, the HOMO is distributed on 4OA-N and the LUMO is localized on fullerene. The calculation results further demonstrated that there are strong interactions between the host and the fullerene guest, which is consistent with the result of the experiments. The characteristic photophysical properties of these novel naphthyl-decorated oxacalix[m]arene[n]pyrimidines broaden their application field, and the stable host–guest system with fullerene can be applied to supramolecular chemistry.

Long-Lived Charge-Transfer State in Spiro Compact Electron Donor-Acceptor Dyads Based on Pyromellitimide-Derived Rhodamine: Charge Transfer Dynamics and Electron Spin Polarization.

We observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a 3CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the 3CT state (0.29 μs).