Internal Conversion Rate Research Articles

Internal conversion rate constant calculations considering Duschinsky, anharmonic and Herzberg-Teller effects.

A novel method for calculating rate constants for internal conversion (kIC) that simultaneously accounts for Duschinsky, anharmonic and Herzberg-Teller effects has been developed and implemented. This method has been applied to robust planar molecules like tetraoxa[8]circulene (4B), free-base porphyrin (H2P) and pyrometene (PM567) with small Duschinsky rotation (i.e. with almost identical normal coordinates in the ground and excited states) and to poly[n]fluorenes (P[n]F) (n = 2-14) with a substantial Duschinsky rotation. The obtained results show that the Duschinsky effect is large in the harmonic approximation, whereas it is in general much smaller in the anharmonic approximation. The Duschinsky effect is found to be large for high frequency vibrational modes with energies of ∼3300 cm-1 such as the X-H (X = C, N and O) stretching modes that mix in the S1 → S0 electronic transition. However, even in this case, the increase in kIC due to the Duschinsky effect does not exceed one order of magnitude. The calculations show that anharmonic contributions to kIC are larger than Herzberg-Teller contributions which in turn are larger than contributions from the Duschinsky effect ANH > HT > Du. We also show that an approximation, where only X-H bonds are considered in the kIC calculation, is accurate even for P[n]F (n = 2-14).

Synthesis and Photophysical Properties of Novel Meta-Conjugated Organic Molecules with 1,3,5-Benzene Branching Units

The synthesis and photophysical investigation of three novel meta-conjugated molecules based on 3,1,2-benzothiadiazole and thiophene-2,5-diyl derivatives linked through 1,3,5-benzene branching units are described. Each of them is a symmetrical molecule with two branching units, four identical lateral thiophene-containing fragments, and one central benzothiadiazole-containing fragment. To study the effect of the chemical structure on their photophysical properties, the molecules with different linearly conjugated lateral and central fragments due to incorporation of additional thiophene rings were synthesized and compared. It was shown that absorption spectra of the meta-conjugated molecules can be represented as a sum of absorption bands of model compounds for their peripheral and central fragments containing a common benzene ring being branched at the 1,3,5-benzene unit in the meta-conjugated molecules. Therefore, they cannot be considered simply as isolated π-conjugated systems of their peripheral and central fragments. Instead, DFT calculations showed that several transitions between the orbitals located in different regions of the meta-conjugated molecule are responsible for the formation of their absorption spectra, and they strongly depend on the degree of their overlapping. Theoretical absorption spectra reconstructed from the DFT data demonstrated a good agreement with the experimental results: the transitions with larger oscillator strength correspond to the bands with higher molar extinction coefficients and vice versa. It was shown that luminescence spectral maxima of the meta-conjugated molecules monotonically shift to the lower energy from 489 to 540 and 613 nm with increasing the number of thiophene rings in the peripheral and central fragments, respectively. However, luminescence quantum yield of the meta-conjugated molecules critically depends on the length of linearly conjugated fragments in its structure decreasing from 24% to 1.3% with increasing the number of thiophene rings in the lateral fragments but increasing to 90% in the molecule with more thiophene rings in both types of the fragments. The results obtained are well correlated to the ratio of radiative and nonradiative deactivation rate constants of the meta-conjugated molecules that indicates a high rate of internal conversion between the excited states corresponding to different fragments of the molecule. The CV measurements allowed estimating the HOMO, LUMO, and bandgap values of the target and model compounds, which confirm the presence of meta-conjugation within the molecules investigated. Thus, connection of linearly conjugated fragments through meta-positions (meta-conjugation) of a benzene ring leads to an intermediate option between fully conjugated and nonconjugated molecules due to partial delocalization of electron density through the 1,3,5-substituted benzene branching center.

Interexcited State Photophysics I: Benchmarking Density Functionals for Computing Nonadiabatic Couplings and Internal Conversion Rate Constants.

We present the first benchmarking study of nonadiabatic matrix coupling elements (NACMEs) calculated using different density functionals. Using the S1 → S0 transition in perylene solvated in toluene as a case study, we calculate the photophysical properties and corresponding rate constants for a variety of density functionals from each rung of Jacob's ladder. The singlet photoluminescence quantum yield (sPLQY) is taken as a measure of accuracy, measured experimentally here as 0.955. Important quantum chemical parameters such as geometries, absorption, emission, and adiabatic energies, NACMEs, Hessians, and transition dipole moments were calculated for each density functional basis set combination (data set) using density functional theory based multireference configuration interaction (DFT/MRCI) and compared to experiment where possible. We were able to derive simple relations between the TDDFT and DFT/MRCI photophysical properties; with semiempirical damping factors of ∼0.843 ± 0.017 and ∼0.954 ± 0.064 for TDDFT transition dipole moments and energies to DFT/MRCI level approximations, respectively. NACMEs were dominated by out-of-plane derivative components belonging to the center-most ring atoms with weaker contributions from perturbations along the transverse and longitudinal axes. Calculated theoretical spectra compared well to both experiment and literature, with fluorescence lifetimes between 7.1 and 12.5 ns, agreeing within a factor of 2 with experiment. Internal conversion (IC) rates were then calculated and were found to vary wildly between 106-1016 s-1 compared with an experimental rate of the order 107 s-1. Following further testing by mixing data sets, we found a strong dependence on the method used to obtain the Hessian. The 5 characterized data sets ranked in order of most promising are PBE0/def2-TZVP, ωB97XD/def2-TZVP, HCTH407/TZVP, PBE/TZVP, and PBE/def2-TZVP.

FCclasses3: Vibrationally-resolved spectra simulated at the edge of the harmonic approximation.

We introduce FCclasses3, a code to carry out vibronic simulations of electronic spectra and nonradiative rates, based on the harmonic approximation. Key new features are: implementation of the full family of vertical and adiabatic harmonic models, vibrational analysis in curvilinear coordinates, extension to several electronic spectroscopies and implementation of time-dependent approaches. The use of curvilinear valence internal coordinates allows the adoption of quadratic model potential energy surfaces (PES) of the initial and final states expanded at arbitrary configurations. Moreover, the implementation of suitable projectors provides a robust framework for defining reduced-dimensionality models by sorting flexible coordinates out of the harmonic subset, so that they can then be treated at anharmonic level, or with mixed quantum classical approaches. A set of tools to facilitate input preparation and output analysis is also provided. We show the program at work in the simulation of different spectra (one and two-photon absorption, emission and resonance Raman) and internal conversion rate of a typical rigid molecule, anthracene. Then, we focus on absorption and emission spectra of a series of flexible polyphenyl molecules, highlighting the relevance of some of the newly implemented features. The code is freely available at http://www.iccom.cnr.it/en/fcclasses/.

Light Harvesting Antenna Properties of Framework Solids

ConspectusEnergy transport within organic solids typically takes place via the movement of localized excitation or excitons, i.e., tightly bound electron–hole pairs. Exciton diffusion dynamics are crucial to applications like solar fuels production through photocatalysis and organic optoelectronics. Covalent organic frameworks (COFs) and metal–organic frameworks (MOFs) are classes of framework solids that integrate ordered assemblies of chromophores. These materials possess properties desirable for the aforementioned applications. Their absorption cross sections across the ultraviolet–visible spectrum can be tuned via linker design. They are often crystalline and provide control over the framework topology in terms of spatial separation and relative orientation of chromophores. These properties allow for detailed understanding and tuning of structure–activity relationships.In this Account, we highlight our recent studies on photophysics and photonics of MOFs and COFs. Three materials are discussed with the focus on structural factors that govern the exciton properties in terms of diffusion, delocalization, and transport. The first is COF-5, a layered, two-dimensional COF with boronate ester linkages. We show that the molecular excitons in COF-5 are delocalized over multiple constituent building blocks with optical signatures of mixed (e.g., H- and J-type) excitonic interactions. The exciton of the S1 state decays and evolves into an excimer trap state in a few picoseconds. The excitons in COF-5 possess a diffusivity of about 0.02 cm2/s and a diffusion radius of about 5 nm based on exciton–exciton annihilation studies. To investigate the range of exciton diffusion, parallel exciton dynamics measurements were performed on COF-5 colloids with varying crystallite size, revealing that exciton diffusion is limited by crystallite size. The second system studied is an imine-linked two-dimensional COF with a hexagonal structure, the TAPB-DiOMe COF. To understand the photophysics of this material, we extended the study toward isomeric materials with a lower dimensionality: cofacially stacked nanotubes and free macrocycles assembled from the same primary building blocks. In-plane exciton delocalization and movement cannot occur in these lower dimensional species. We find that the exciton lifetime is prolonged significantly in the π-stacked nanotubes where the macrocycles undergo planarization that restricts atomic movements of the imine bonds in the peripheral groups, which may reduce the rate of internal conversion. In addition, orientation-dependent anisotropic energy transfer through the nanotubes was detected via optical transient absorption anisotropy. The third system discussed is the MOF NDC-NU-1000 decorated with single-atom metal-oxy (i.e., mix of oxo, hydroxo, and/or aqua) TiIV, CoIII, and NiII species mimicking single-atom catalysts (SACs). These composite materials display photoinduced charge separation. We show the formation of charge-separated states in these M-oxy decorated MOF samples upon photoexcitation with an electron transferred from the MOF linker to the M-oxy moiety. We compare and rationalize the differences in photoinduced charge separation kinetics among these M-oxy decorated MOFs and hope to identify energetic and structural factors that make the material an effective light harvesting platform and facilitate formation of the charge-separated state. Finally, we review our progress in context of the research published previously in the field and provide our perspective on the future of crystalline porous assemblies serving as light-harvesting antennae.

Energy balance and momentum budgets due to the internal tides generated by a group of submarine canyons

A field multibeam survey discovered a group of regularly-spaced submarine canyons on the continental shelf in the northern South China Sea. However, they are not included in the bathymetry datasets commonly used by the oceanographic community. In this study, we employ a very-high-resolution (as fine as 54 m) numerical model that resolves these submarine canyons to investigate their impacts on the local internal tide generation and propagation. Results show that the barotropic to internal tidal conversion rate exhibits an alternating distribution on each flank of submarine canyons. Rotational direction of the tidal flow determines a positive conversion rate on the west flank and a negative rate on the east flank. The rough seafloor due to the submarine canyons does not appreciably change the dissipation of barotropic tides, but significantly affects the dissipation of internal tides, leading to an averaged dissipation rate of 1.2 × 10−9 W kg −1. Internal tides also induce a bottom intensified mean flow, which causes westward mean transport on the continental slope and a clockwise circulation inside each submarine canyon. The mean flow satisfies a momentum budget between the Coriolis force, pressure gradient and divergence of momentum flux of barotropic and internal tides. We highlight that the divergence of the pseudo-momentum flux of internal tides acts as a stress and is especially reinforced near the sea bottom and surface. Its effect on the sea surface is even comparable with moderate winds (∼13 m s−1).

Energy Transfer Mechanism and Quantitative Modeling of Rate from an Antenna to a Lanthanide Ion.

The excitation energy transfer (ET) pathway and mechanism from an organic antenna to a lanthanide ion has been the subject of discussion for many decades. In the case of europium (Eu3+), it has been suggested that the transfer originates from the ligand singlet state or a triplet state. Taking the lanthanide complex Eu(TTA)3(H2O)2 as an example, we have investigated the spectra and luminescence kinetics, mainly at room temperature and 77 K, to acquire the necessary experimental data. We put forward an experimental and theoretical approach to measure the energy transfer rates from the antenna to different Eu3+ levels using the Dexter formulation. We find that transfer from the ligand singlet state to Eu3+ may account for the ET pathway, by combined electric dipole–electric dipole (ED–ED) and ED-electric quadrupole (EQ) mechanisms. The contributions from the triplet state by these mechanisms are very small. An independent systems rate equation approach can effectively model the experimental kinetics results. The model utilizes the cooperative processes that take place on the metal ion and ligand and considers S0, S1, and T1 ligand states in addition to 7F0,1, 5D0, 5D1, and 5DJ (=5L6, 5D3, 5D2 combined) Eu3+ states. The triplet exchange ET rate is estimated to be of the order 107 s–1. The observation of a nanosecond risetime for the Eu3+ 5D1 level does not enable the assignment of the ET route or the mechanism. Furthermore, the 5D1 risetime may be contributed by several processes. Observation of its temperature dependence and also that of the ground-state population can supply useful information concerning the mechanism because the change in metal-ion internal conversion rate has a greater effect than changes in singlet or triplet nonradiative rates. A critical comparison is included for the model of Malta employed in the online software LUMPAC and JOYSpectra. The theoretical treatment of the exchange mechanism and its contribution are now being considered.

Anti-Kasha emissions of single molecules in a plasmonic nanocavity.

Kasha's rule generally holds true for solid-state molecular systems, where the rates of internal conversion and vibrational relaxation are sufficiently higher than the luminescence rate. In contrast, in systems where plasmons and matter interact strongly, the luminescence rate is significantly enhanced, leading to the emergence of luminescence that does not obey Kasha's rule. In this work, we investigate the anti-Kasha emissions of single molecules, free-base and magnesium naphthalocyanine (H2Nc and MgNc), in a plasmonic nanocavity formed between the tip of a scanning tunneling microscope (STM) and metal substrate. A narrow-line tunable laser was employed to precisely reveal the excited-state levels of a single molecule located under the tip and to selectively excite it into a specific excited state, followed by obtaining a STM-photoluminescence (STM-PL) spectrum to reveal the energy relaxation from the state. The excitation to higher-lying states of H2Nc caused various changes in the emission spectrum, such as broadening and the appearance of new peaks, implying the breakdown of Kasha's rule. These observations indicate emissions from the vibrationally excited states in the first singlet excited state (S1) and second singlet excited state (S2), as well as internal conversion from S2 to S1. Moreover, we obtained direct evidence of electronic and vibronic transitions from the vibrationally excited states, from the STM-PL measurements of MgNc. The results obtained herein shed light on the energy dynamics of molecular systems under a plasmonic field and highlight the possibility of obtaining various energy-converting functions using anti-Kasha processes.

Lesion Quantification Accuracy of Digital 90Y PET Imaging in the Context of Dosimetry in Systemic Fibroblast Activation Protein Inhibitor Radionuclide Therapy.

Therapy with 90Y-labeled fibroblast activation protein inhibitors (90Y-FAPIs) was recently introduced as a novel treatment concept for patients with solid tumors. Lesion and organ-at-risk dosimetry is part of assessing treatment efficacy and safety and requires reliable quantification of tissue uptake. As 90Y quantification is limited by the low internal positron-electron pair conversion rate, the increased effective sensitivity of digital silicon photomultiplier-based PET/CT systems might increase quantification accuracy and, consequently, allow for dosimetry in 90Y-FAPI therapy. The aim of this study was to explore the conditions for reliable lesion image quantification in 90Y-FAPI radionuclide therapy using a digital PET/CT system. Methods: Two tumor phantoms were filled with 90Y solution using different sphere activity concentrations and a constant signal-to-background ratio of 40. The minimum detectable activity concentration was determined, and its dependence on acquisition time (15 vs. 30 min per bed position) and smoothing levels (all-pass vs. 5-mm gaussian filter) was investigated. Quantification accuracy was evaluated at various activity concentrations to estimate the minimum quantifiable activity concentration using contour-based and oversized volume-of-interest-based quantification approaches. A ±20% deviation range between image-derived and true activity concentrations was regarded as acceptable. Tumor dosimetry for 3 patients treated with 90Y-FAPI is presented to project the phantom results to clinical scenarios. Results: For a lesion size of 40 mm and a clinical acquisition time of 15 min, both minimum detectable and minimum quantifiable activity concentrations were 0.12 MBq/mL. For lesion sizes of greater than or equal to 30 mm, accurate quantification was feasible for detectable lesions. Only for the smallest 10-mm sphere, the minimum detectable and minimum quantifiable activity concentrations differ substantially (0.43 vs. 1.97 MBq/mL). No notable differences between the 2 quantification approaches were observed. For the investigated tumors, absorbed dose estimates with reliable accuracy were achievable. Conclusion: For lesion sizes and activity concentrations that are expected to be observed in patients treated with 90Y-FAPI, quantification with reasonable accuracy is possible. Further dosimetry studies are needed to thoroughly investigate the efficacy and safety of 90Y-FAPI therapy.

Anomalous emission of an ESIPT-capable zinc(II) complex: An interplay of TADF, TICT and anti-Kasha behaviour

In contrast to the vast majority of organic ESIPT-emitters (ESIPT – excited state intramolecular proton transfer), a pyrimidine-based ESIPT-type ligand HL can bind metal ions without its deprotonation due to the spatial separation of metal-binding and ESIPT cores. The photophysical and photochemical properties of HL and the corresponding ESIPT-capable zinc(II) complex [Zn(HL)Cl2] were theoretically investigated using single-reference (DFT) and multi-reference (CASSCF, NEVPT2) quantum chemical methods. The ESIPT process in both compounds is barrierless and thermodynamically favorable in the S1 and S2 states. HL and [Zn(HL)Cl2] demonstrate abnormal anti-Kasha fluorescence (S2 → S0) and anti-Kasha phosphorescence (T2 → S0) associated with relatively low S2 → S1 and T2 → S1 internal conversion rates. Energetically achievable S1/S0 and T1/S0 intersections are responsible for the fast non-radiative decay of the S1 and T1 states. Quasi-intersections of the S2 and T2 state potential energy surfaces facilitate thermally activated delayed fluorescence (TADF, T2 → S2 → S0) for both compounds. The molecules of [Zn(HL)Cl2] can be trapped and photoexcited from the local minimum of the S0 state in the tautomeric keto form, which results in an anomalously small apparent Stokes shift of 90 nm. The absence of such minimum on the ground state potential energy surface of HL leads to a considerably larger Stokes shift of 165 nm.

Excited State Properties of Aggregation‐Induced Delayed Fluorescence Molecules: A Microscopic Insight

AbstractPurely organic fluorescence emitters with ultrahigh exciton utilization, such as thermally‐activated delayed fluorescence (TADF) materials, are pursued as the cornerstone of the new‐generation organic light‐emitting diodes (OLEDs). However, most TADF emitters suffer from the dilemma of the aggregation‐caused fluorescence quenching effects, severely limiting their applications. Excitingly, the recently proposed aggregation‐induced delayed fluorescence (AIDF) strategy holds the potential to tackle this problem thoroughly. Here, the recently reported AIDF emitters CP‐BP‐PXZ and its halogenated counterparts, 3‐CCP‐BP‐PXZ and 3‐BCP‐BP‐PXZ, are investigated with a united theoretical and experimental insight. Based on first‐principles calculations combined with the polarizable continuum model (PCM) and the quantum mechanics/molecular mechanics (QM/MM) model, the photophysical mechanisms of these novel materials are proposed. It is found that both reverse intersystem crossing (RISC) and the subsequent radiative transition of CP‐BP‐PXZ are promoted via aggregation, owing to the increased effective RISC channels and boosted oscillator strength. Meanwhile, the unfavorable internal conversion rate is greatly slumped, ascribed to the suppressed Duschinsky rotation effect (DRE). The unique heavy‐atom effect is disclosed in 3‐CCP‐BP‐PXZ and 3‐BCP‐BP‐PXZ that the halogens act as steric blockers in crystal, rather than spin–orbit coupling enhancers. It is hoped that this work can shed new light on the designation of high‐efficient solid‐state fluorescence emitters.

(Invited) Development of Highly Luminescent Radical Core Carbazole Dendrimer

The development of emitting materials for OLEDs has started with fluorescence, moved to phosphorescence, and reached TADF. These materials have singlet ground state, but recently some ground state doublet (radical) emitting materials have been reported.1,2 The excited state of doublet emitters also has a doublet character, and theoretically, can achieve 100% exciton utilization efficiency in an OLED device. Triphenylmethyl radicals such as PTM (perchlorotriphenylmethyl) radical were recognized as emitters from the 1980s. The photostability of these radicals are extremely low, but in 2006 the introduction of donor (carbazole) unit was reported to improve the photostability and opened the door to utilize them as a luminescent materials.3 Carbazole dendrimers are widely used as solution-processable OLED material including TADF emitters.4 The dendron has a highly polarized unique electronic structure due to the summation of the inductive electron-withdrawing effect of carbazole units.5 The attachment of carbazole dendrons to luminescent radical is expected to increase the stability through the steric and electronic effects.A carbazole dendron (G2 to G4) attached TTM (tris(2,4,6-trichlorophenyl)-methyl) radicals were successfully synthesized and photophysical parameters were determined. All dendrimers showed red emission. PLQY (photoluminescence quantum yield) increased when the generation increased and reached 80%. The major reason was the decrease in the internal conversion rate (non-radiative decay). The photostability (PL intensity half-life measured under continuous laser irradiation) in solution has increased 100 to 1000 times compared to simple TTM radicals. These results indicate that the carbazole dendron attached TTM radicals are potential material for efficient OLEDs.

Theoretical study for the TADF nature of through-space conjugated [2.2]paracyclophane derivatives and the design for red-light emission

The thermally activated delayed fluorescence (TADF) nature of a series of [2.2]paracyclophane derivatives with spatially conjugated structures were studied by DFT/TD-DFT method. Through-space charge transfer, rather than the usual through-bond charge transfer exist between the frontier molecular orbitals of these molecules. The singlet-triplet energy gaps which affect the reverse intersystem crossing (RISC) process were compared among the molecules reported in the experimental study to explain why 2 and 4 are more likely to show TADF emission than 1 and 3. Especially, when discussing the intersystem crossing (ISC) process according to the excited states properties and the spin-orbit coupling matrix elements, the influence of the spatially conjugated structure was also analyzed detailedly. Transition dipole moments of the absorption states were greatly enhanced in the conjugated structure and the ISC between some high-energy states was promoted in consequence, indirectly leading to the promotion of the RISC process which generates S1 excitons. In addition, we designed 5–8 by replacing the benzophenone group with the thianthrene-9,9′,10,10′-tetraoxide group, in order to obtain red-light emission through decreasing the LUMO energy level. The calculation results show that the new molecules have larger radiation rate and smaller internal conversion rate, which indicates that the designed molecules are potential efficient red-light TADF materials.

Gas-Phase Fluorescence of Proflavine Reveals Two Close-Lying, Brightly Emitting States.

Surprising excitation-dependent, dual emission from a small organic model fluorophore is reported. Gas-phase fluorescence spectra of proflavine (a diaminoacridine) ions reveal two long-lived emitting states, with distinct bands separated by just 1700 cm-1. The relative intensities of these two bands depend on the excitation wavelength. Time-dependent density functional theory (TD-DFT) calculations support the existence of two close-lying singlet electronic states, with excitation into S2 predicted to be >1000-fold more likely than into S1. These data strongly suggest that internal conversion (IC) rates are suppressed relative to solvated proflavine, and that IC is competitive with intramolecular vibrational relaxation (IVR). This work offers an in-depth assessment of the gas-phase photophysics of a simple fluorophore that could open a new pathway to understanding dual emission in fluorophores.

Controlling the fluorescence quantum yields of benzothiazole-difluoroborates by optimal substitution.

Precise tuning of the fluorescence quantum yield, vital for countless applications of fluorophores, remains exceptionally challenging due to numerous factors affecting energy dissipation phenomena often leading to its counterintuitive behavior. In contrast to the absorption and emission wavelength which can be precisely shifted to the desired range by simple structural changes, no general strategy exists for controllable modification of the fluorescence quantum yield. The rigidification of the molecular skeleton is known to usually enhance the emission and can be practically realized via the limiting molecular vibrations by aggregation. However, the subtle balance between the abundant possible radiative and non-radiative decay pathways makes the final picture exceptionally sophisticated. In the present study, a series of nine fluorophores obtained by peripheral substitution with two relatively mild electron donating and electron withdrawing groups are reported. The obtained fluorescence quantum yields range from dark to ultra-bright and the extreme values are obtained for the isomeric molecules. These severe changes in emission efficiency have been shown to arise from the complex relationship between the Franck-Condon excited state and conical intersection position. The experimental findings are rationalized by the advanced quantum chemical calculations delivering good correlation between the measured emission parameters and theoretical radiative and internal conversion rate constants. Therefore, the described substituent exchange provides a method to rigorously adjust the properties of molecular probes structurally similar to thioflavin T.

A Multiple Excited-State Engineering of Boron-Functionalized Diazapentacene Via a Tuning of the Molecular Orbital Coupling.

Harvesting high-energy excited-state energy is still challenging in organic chromophores. An introduction of boron atoms along the short axis of the diazapentacene backbone induces multiple emission characteristics. Our studies reveal that the weak molecular orbital (MO) coupling of the S3-S1 transition is responsible for the slow internal conversion rates. Such MO coupling-regulated anti-Kasha emission is different from the large band gap-induced anti-Kasha emission character of classical azulene derivatives. Theoretical studies reveal that a strong MO coupling of the S3-S0 transition is responsible for the higher photoluminescence quantum yield of the anti-Kasha emission in a more polar solution (tetrahydrofuran: 11%; cyclohexane: 0%). Such an MO coupling factor is generally overlooked in anti-Kasha emitters reported previously. Furthermore, the multiple emission can be regulated by solvent polarity, solvent temperature, and fluoride anion binding. As a proof of concept of harvesting high-energy emission, the multiple emission character has allowed us to design single-molecule white-light-emitting materials.

The Observation of Interchain Motion in Self-Assembled Crystalline Platinum(II) Complexes: An Exquisite Case but By No Means the Only One in Molecular Solids.

In organic and organometallic solids, upon electronic excitation, most intermolecular structural relaxations follow a pathway along the π-π stacking direction or metal-metal bond with significant coupling strength. Differently, we discovered that the self-assembled platinum(II) complexes, Pt(fppz)2, exhibit an unusual interchain contraction. The ground-state and excited-state multiple local minima were distinguished by temperature-dependent excitation/emission spectra, indicating the existence of multiple local minima. The time-resolved emission decay revealed the excited-state structural relaxation lifetime with τobs = 41 ns at 298 K. Temperature-dependent X-ray diffraction analysis showed that the packing geometries contract 0.6 Å along the interchain direction (a-axis) at 50 K compared to the geometries at 298 K. Such structural displacements render the slow internal conversion rate in the excited states. We thus demonstrate the correlation between the packing geometries and the excited-state dynamics of the self-assembled Pt(II) complexes, shedding light on the unique direction of interchain structural deformation of the molecular aggregates.

Dual-Responsive Thermally Activated Delayed Fluorescence of Spiropyran Derivatives

Dual-Responsive Thermally Activated Delayed Fluorescence of Spiropyran Derivatives

Evaluating the anharmonicity contributions to the molecular excited state internal conversion rates with finite temperature TD-DMRG.

In this work, we propose a new method to calculate molecular nonradiative electronic relaxation rates based on the numerically exact time-dependent density matrix renormalization group theory. This method could go beyond the existing frameworks under the harmonic approximation (HA) of the potential energy surface (PES) so that the anharmonic effect could be considered, which is of vital importance when the electronic energy gap is much larger than the vibrational frequency. We calculate the internal conversion (IC) rates in a two-mode model with Morse potential to investigate the validity of HA. We find that HA is unsatisfactory unless only the lowest several vibrational states of the lower electronic state are involved in the transition process when the adiabatic excitation energy is relatively low. As the excitation energy increases, HA first underestimates and then overestimates the IC rates when the excited state PES shifts toward the dissociative side of the ground state PES. On the contrary, HA slightly overestimates the IC rates when the excited state PES shifts toward the repulsive side. In both cases, a higher temperature enlarges the error of HA. As a real example to demonstrate the effectiveness and scalability of the method, we calculate the IC rates of azulene from S1 to S0 on the ab initio anharmonic PES approximated by the one-mode representation. The calculated IC rates of azulene under HA are consistent with the analytically exact results. The rates on the anharmonic PES are 30%-40% higher than the rates under HA.

Tracking Ultrafast Structural Dynamics in a Dual-Emission Anti-Kasha-Active Fluorophore Using Femtosecond Stimulated Raman Spectroscopy.

The anti-Kasha process provides the possibility of using high-energy excited states to develop novel applications. Our previous research (Nature communications, 2020, 11, 793) has demonstrated a dual-emission anti-Kasha-active fluorophore for bioimaging application, which exhibits near-infrared emissions from the S1 state and visible anti-Kasha emissions from the S2 state. Here, we applied tunable blue-side femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy, assisted by quantum calculations, to reveal the anti-Kasha dual emission mechanism, in which the emergence of two fluorescing states is due to the retardation of internal conversion from the S2 state to the S1 state. It has been demonstrated that the facts of anti-Kasha high-energy emission are commonly attributed to a large energy gap between the two excited states, leading to a decrease in the internal conversion rate due to a poor Franck-Condon factor. In this study, analysis of the calculation and FSRS experimental results provide us further insight into the dual-emission anti-Kasha mechanism, where the observation of hydrogen out-of-plane Raman modes from FSRS suggested that, in addition to the energy-gap law, the initial photoinduced molecular conformational change plays a key role in influencing the rate of internal conversion.