Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution

We present an accidental mechanism for efficient intersystem crossing (ISC) between singlet and triplet states with low spin-orbit coupling (SOC) in molecules having donor-acceptor (D-A) moieties separated by a Sigma bond. Our study shows that SOC between the lowest singlet excited state and the higher-lying triplet states, together with nuclear motion-driven coupling of this triplet state with lower-lying triplet states during the free rotation about a Sigma bond, is one of the possible ways to achieve the experimentally observed ISC rate for a class of D-A type photoredox catalysts. This mechanism is found to be the dominant contributor to the ISC process with the corresponding rate reaching a maximum at a dihedral angle in the range of 72°-78° between the D-A moieties of 10-(naphthalen-1-yl)-3,7-diphenyl-10H-phenoxazine and other molecules included in the study. We have further demonstrated that the same mechanism is operative in a specific spirobis[anthracene]dione molecule, where the D and A moieties are interlocked near to the optimal dihedral angle, indicating the plausible effectiveness of the proposed mechanism. The present finding is expected to have implications in strategies for the synthesis of new generations of triplet-harvesting organic molecules.

The amino-keto tautomer of supersonic jet-cooled cytosine undergoes intersystem crossing (ISC) from the v = 0 and low-lying vibronic levels of its S1((1)ππ(∗)) state. We investigate these ISC rates experimentally and theoretically as a function of S1 state vibrational excess energy Eexc. The S1 vibronic levels are pumped with a ∼5 ns UV laser, the S1 and triplet state ion signals are separated by prompt or delayed ionization with a second UV laser pulse. After correcting the raw ISC yields for the relative S1 and T1 ionization cross sections, we obtain energy dependent ISC quantum yields QISC (corr)=1%-5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates kISC = 0.4-1.5 ⋅ 10(9) s(-1), the corresponding S1⇝S0 internal conversion (IC) rates are 30-100 times larger. Theoretical ISC rates are computed using SCS-CC2 methods, which predict rapid ISC from the S1; v = 0 state with kISC = 3 ⋅ 10(9) s(-1) to the T1((3)ππ(∗)) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of (1)nOπ(∗) character into the S1((1)ππ(∗)) wave function at its non-planar minimum geometry. The combination of experiment and theory implies that (1) below Eexc = 550 cm(-1) in the S1 state, S1⇝S0 internal conversion dominates the nonradiative decay with kIC ≥ 2 ⋅ 10(10) s(-1), (2) the calculated S1⇝T1 ((1)ππ(∗)⇝(3)ππ(∗)) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S1⇝T1 ISC is moderately fast (kISC = 3 ⋅ 10(9) s(-1)), and not ultrafast, as claimed by other calculations, and (4) at Eexc ∼ 550 cm(-1) the IC rate increases by ∼50 times, probably by accessing the lowest conical intersection (the C5-twist CI) and thereby effectively switching off the ISC decay channels.

Assessing and improving the performance of organic light-emitting diode (OLED) materials require quantitative prediction of rate coefficients for the intersystem crossing (ISC) and reverse ISC (RISC) processes, which are determined not only by the energy gap and the direct spin-orbit coupling (SOC) between the first singlet and triplet excited-states at a thermal equilibrium position of the initial electronic state but also by the non-Condon effects such as the Herzberg-Teller-like vibronic coupling (HTVC) and the spin-vibronic coupling (SVC). Here we apply the time-dependent correlation function approaches to quantitatively calculate the vibrationally resolved absorption and fluorescence spectra and ISC/RISC rates of a newly synthesized multiple-resonance-type (MR-type) thermally activated delayed fluorescence (TADF) emitter, 7-phenylquinolino[3,2,1-de]acridine-5,9-dione (7-PhQAD), with the inclusion of the Franck-Condon (FC), HTVC, and Duschinsky rotation (DR) effects. The SVC effect on the rates has also been approximately evaluated. We find that the experimentally measured ISC rates of 7-PhQAD originate predominantly from the vibronic coupling, consistent with the previous reports on other MR-type TADF emitters. The SVC effect on ISC rates is about 10 times larger than the HTVC effect, and the latter increases the ISC rates by more than 1 order of magnitude while it slightly affects the vibrationally resolved absorption and fluorescence spectra. The discrepancy between the theoretical and experimental results is attributed to inaccurately describing excited-states calculated by the time-dependent density functional theory as well as to not fully accounting for the complex experimental conditions. This work provides a demonstration of what proportion of ISC and RISC rate coefficients of a MR-type TADF emitter can be covered by the HTVC effect, and it opens design routes that go beyond the FC approximation for the future development of high-performance OLED devices.

To achieve the effective regulation of organic room temperature phosphorescence (RTP) in supramolecular systems, the elucidation of host-guest interactions in RTP is of vital importance. Herein, we employed two organic dyes (PYCl and PYBr) and their four host-guest complexes with CB[6] and CB[7] and explored the mechanism of host-guest interaction induced RTP enhancement using quantum mechanics/molecular mechanics (QM/MM) approach. For the two organic dyes, we found that the better RTP performance of PYBr than PYCl is attributed to intersystem crossing (ISC) augmentation induced by the heavy atom effect. Binding to CB[6] through host-guest interactions can simultaneously accelerate the radiative decay process by increasing the transition dipole moment of T1 → S0 (μT1→S0), block the nonradiative decay process, and promote the ISC process, eventually leading to a remarkably boosted RTP. Upon complexation, the conversion of S1 from 1(n, π*) to 1(π, π*) is key to μT1→S0 enhancement; reduced reorganization energies reflect the suppression of the nonradiative decay process by restricting the rotation of rings A and B in organic dyes. In addition, the promoted ISC process is due to the activation of more ISC channels between S1 and high-lying triplet states with large spin-orbital coupling constants and small energy gap. The case of CB[7]-type complexes is much different, because of the extremely large cavity size of CB[7] for encapsulation. This work proposes the mechanism of host-guest interaction-induced RTP enhancement of organic dyes, thus laying a solid foundation for the rational design of advanced RTP materials based on supramolecular assemblies.

Weakly bound molecular complexes with more than one well-defined structures provide us with an unique opportunity to investigate dynamic processes induced by intermolecular interactions with specific orientations. The relative orientation of the two interacting molecules or atoms is defined by the complex structure. The effect of the orientation in the spin changing collisions glyoxal(S1)+Ar → glyoxal(T1)+Ar and acetylene (S1)+Ar → acetylene(T)+Ar have been studied by measuring the intersystem crossing (ISC) rates of the glyoxal(S1)·Ar and acetylene(S1)·Ar complexes with different isomeric structures. Results show that there is a strong orientation dependence in the ISC of glyoxal(S1) induced by interaction with the Ar atom: the Ar atom positioned in the molecular plane is much more effective than in the out-of-plane position in inducing the S1 → T1 transition of glyoxal. On the other hand, studies of acetylene(S1)·Ar complexes indicate that the Ar-induced ISC rates are nearly identical for the in-plane and out-of-plane positions. Orientation dependence in the collision induced vibrational relaxation process C2H2(S1,vi)+Ar → C2H2(S1,vf<vi)+Ar is also studied by measuring the vibrational predissociation rates of the acetylene(S1)·Ar complex isomers. The results indicate that collisions of C2H2(S1,v3=3, 4) with Ar at two orthogonal orientations are equally effective in causing vibrational relaxation of C2H2.

1-photon (382 nm) and 2-photon (752 nm) excitations to the S1 state are applied to record and compare transient absorption spectra of a push-pull triphenylamine (TrP) dye in solution. After 1-photon excitation, ultrafast vibrational and structural molecular relaxations are detected on a 0.1 ps time scale in nonpolar hexane, while in polar acetonitrile, the spectral evolution is dominated by dipolar solvation. Upon 2-photon excitation, transient spectra in hexane reveal an unexpected growth of stimulated emission (SE) and excited-state absorption (ESA) bands. The behavior is explained by strong population transfer S1 → Sn due to resonant absorption of a third pump photon. Subsequent Sn → S1 internal conversion (with τ1 = 1 ps) prepares a very hot S1 state which cools down with τ2 = 13 ps. The pump pulse energy dependence proves the 2-photon origin of the bleach signal. At the same time, SE and ESA are strongly affected by higher-order pump absorptions that should be taken into account in nonlinear fluorescence applications. The 2-photon excitation cross sections σ(2) = 32 ⋅ 10(-50) cm(4) s at 752 nm are evaluated from the bleach signal.

Bonding aromatic ring to carbonyl group is recognized as crucial for promoting intersystem crossing (ISC) owing to allowed 1nπ* → 3ππ* or 1ππ* → 3nπ* transition. The effect of charge transfer (CT) in such systems, however, is largely ignored for understanding ISC and phosphorescence efficiency. Here, we unveiled a principle for producing highly-efficient room-temperature phosphorescence (RTP) polymer where the role of CT in diarylketone phosphors was a prerequisite. Designed diarylketones with carbonyl group substituted by aromatic rings unsymmetrically achieved superior RTP quantum yield of >30% in copolymer. Detailed investigation indicated that 1CT→3LE (locally excited triplet state) transition with small energy gap (∆EST) and strong spin-orbit coupling underlies their ultrafast ISC, as recorded by femtosecond transient absorption (fs-TA) spectroscopy. When single aromatic ring was bonded to carbonyl group, singlet (S1) and triplet (T1) states remained LE with large ∆EST and weak spin-orbit coupling, leading to low RTP efficiency. Finally, manufacturing organic glass in presence of trace diarylketone monomers could generate shape-rich three-dimensional (3D) objects with intense afterglow under room lighting, even at high-temperature and wet conditions. This work advances understanding on structure-property relationship in popular aromatic ketone phosphors, and paves the way for producing afterglow materials with high brightness and stability.

Pentaerythritol tetranitrate (PETN) is a common secondary explosive and has been used extensively to study shock initiation and energy propagation in energetic materials. We report 2D IR measurements of PETN thin films that resolve vibrational energy transfer and relaxation mechanisms. Ultrafast anisotropy measurements reveal a sub-500 fs reorientation of transition dipoles in thin films of vapor-deposited PETN that is absent in solution measurements, consistent with intermolecular energy transfer. The anisotropy is frequency dependent, suggesting spectrally heterogeneous vibrational relaxation. Cross peaks are observed in 2D IR spectra that resolve a specific energy transfer pathway with a 2 ps time scale. Transition dipole coupling calculations of the nitrate ester groups in the crystal lattice predict that the intermolecular couplings are as large or larger than the intramolecular couplings. The calculations match well with the experimental frequencies and the anisotropy, leading us to conclude that the observed cross peak is measuring energy transfer between two eigenstates that are extended over multiple PETN molecules. Measurements of the transition dipole strength indicate that these vibrational modes are coherently delocalized over at least 15-30 molecules. We discuss the implications of vibrational relaxation between coherently delocalized eigenstates for mechanisms relevant to explosives.

Photodynamic therapy (PDT) is a clinically approved therapeutic modality that has demonstrated significant potential for cancer treatment, and triplet photosensitizers (PSs) play a key role in its efficacy. Despite deep learning having emerged as a next-generation tool for material discovery, existing methods mainly target a limited subset of triplet PSs, such as thermally activated delayed fluorescence (TADF) materials, neglecting the critical intersystem crossing (ISC) between the high-lying singlet and triplet states (ΔE S n T n ). To overcome this limitation, we compiled a comprehensive dataset (∼1.90 × 109) of triplet PSs encompassing various ISC mechanisms. Then, we proposed a novel strategy that incorporates two models: a fragment-based model (Frag-MD) and a character-based model (MD), both integrating a conditional transformer, recurrent neural networks, and reinforcement learning. In silico experiments revealed that the Frag-MD model outperforms the MD model in generating larger conjugated motifs with higher average ring numbers and atom counts; while the MD model generates twice as many unique motifs and excels in novelty and diversity, as evaluated by conditional and MOSES metrics. Therefore, our approach is highly effective for modifying conjugated motifs and designing novel triplet PSs. Notably, the recently reported high-efficiency triplet PSs have been re-identified through ablation experiments using our proposed models, which target ΔE S n T n and significantly outperform traditional baselines, achieving a prediction accuracy of 73% versus 4%. Our approach holds the potential to establish a new paradigm for discovering novel PSs applicable in PDT.

We show that a vanadium-chromium Prussian blue analogue, which is a room-temperature molecule-based magnet, displays a fast magnetic response on a femtosecond timescale that is attributed to the super-exchange interaction between the metal ions. These dynamics are obtained from femtosecond Faraday magneto-optical (MO) measurements, performed at 50 and 300 K. Exciting at the ligand-to-metal charge-transfer (LMCT) band results in the formation of the 2E excited state on the Cr ion via intersystem crossing (ISC) from the 4LMCT state in less than 250 fs. Subsequent vibrational relaxation in the 2E state occurs on a 0.78 ± 0.05 ps timescale at 50 K and 1.1 ± 0.1 ps at 300 K. The MO measurements can detect the formation of the 2E state on the Cr ion from the change in the super-exchange interaction taking place as a result of the corresponding spin flip associated with the formation of the 2E state. These results open up a new avenue to study molecular magnets using a powerful method that is capable of directly probing spin dynamics on a sub-picosecond timescale in thin film environments.

Relaxation dynamics following photoexcitation of a calcium atom deposited on an icosahedral-like argon cluster Ar(n) (n approximately = 55) is investigated through theoretical simulations. Based on ab initio calculations of the CaAr molecule, a diatomics-in-molecules model is set up to efficiently describe the electronic excited states of the system. The excited state dynamics is studied using molecular dynamics with electronic transitions (Tully, J. C. J. Chem. Phys. 1990, 93, 1061). The signature of this dynamics in the time-resolved photoelectron spectra is investigated, to assess the possibility of detecting competing vibrational and electronic relaxations through pump-probe experiments. The vibrational relaxation, influenced by nonadiabatic transitions, can clearly be seen in the time-resolved photoelectron spectra. The details of the electronic relaxation, as well as the possible ejection of the chromophore, are found to be sensitive to the local environment of the calcium atom deposited on the argon cluster.

Concentration and temperature dependence of electronic and vibrational energy relaxation of O 2 in liquid mixtures

In this work, we reported a universal molecular design method for anthraquinone (AQ) flexible linking with phenol, thiophenol, and benzene selenophenol to achieve effective thermally activated delayed fluorescence (TADF). The effects of the substituent, temperature, and solvent on the emission efficiency and lifetime of different aminoanthraquinone (AAQ) derivatives are further investigated. The excited-state absorption (ESA) can be observed in femtosecond transient absorption (FTA) and nanosecond transient absorption (NTA) spectroscopy. The nonadiabatic conformation distortion-coupled intramolecular charge transfer (CD-ICT) reduces the difference between the excited singlet and excited triplet conformations, which is conducive to intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes. The results show that the ISC is significantly enhanced with the promotion of a smaller energy barrier (ΔEST = 0.05 eV) and spin–orbit coupling (81.56 cm–1). The nonadiabatic conformation distortion-coupled intramolecular charge transfer opens a new avenue for the development of purely organic light-emitting materials.

&lt;p&gt;Solar ultraviolet (UV) radiation is a highly toxic carcinogen prevalent in our environment. Eumelanin pigment is a photo-stable biopolymer naturally produced in the skin's pigmentary system, providing the skin a unique photo-protection mechanism against exposure to UV radiation. The large macro-molecule rapidly dissipates 99 % of incident UV photons as thermal energy on ultra-fast femtosecond (fs) - picosecond (ps) time scales, before damage can occur to the underlying cells. The fundamental nature of eumelanin's structure and its vibrational energy dissipation mechanism is not yet fully understood, with complexities in the molecule's highly disordered chemical structure, and the ultrafast time-scale on which the energy dissipation occurs, rendering its characterisation elusive. &lt;/p&gt;&lt;p&gt;&lt;br&gt;&lt;/p&gt; &lt;p&gt;Indeed, the absorption spectrum of eumelanin gives little away, rising monotonically in wavelength towards the UV - quite unusual in organic polymers. It is proposed that due to the highly disordered structure of the molecule, multiple chromophores of overlapping energies may be selectively excited with differing irradiation wavelengths. This theory is further supported by eumelanin’s transient absorption signatures and its wavelength dependant photo-luminescence spectra, however the fundamental non-radiative relaxation pathways are not yet understood. &lt;/p&gt; &lt;p&gt;&lt;br&gt;&lt;/p&gt;&lt;p&gt;To bridge this knowledge gap, we present here the application of femtosecond stimulated Raman spectroscopy (FSRS) to eumelanin pigment. FSRS reveals ultra-fast vibrational dynamics on fs – ps time scales, allowing excited state vibrational pathways to be mapped, providing essential structural information of this intriguing molecule. &lt;/p&gt; &lt;p&gt;&lt;br&gt;&lt;/p&gt;&lt;p&gt;Following the introduction of eumelanin’s known photo-physical and structural properties as presented in chapter 1, an introduction to FSRS is presented in chapter 2. The build method and optical construction of the FSRS experiment are presented in chapter 4 including a novel bandwidth compression method used to generate a narrow-band Raman pump using frequency-domain nonlinear optics, presented in chapter 3. Here, using a 1.5 kHz, 800 nm Ti:Sapphire pulsed laser at a power of 3 W, conversion efficiencies of up to 30 % are achieved, generating intense second harmonic Raman pump pulses centred at 400 nm with &lt;20 cm-1 bandwidths. Additionally, the well-known spatial filtering technique is used to generate tuneable, narrow-band pulses centered at the fundamental 800 nm laser pulse. &lt;/p&gt; &lt;p&gt;&lt;br&gt;&lt;/p&gt;&lt;p&gt;The application of FSRS to the study of eumelanin’s indole subunits, 5-6-dihydroxyindole (DHI) and its carboxylated form, 5-6-dihydroxyindole-2-carboxylic acid (DHICA) (and their oligomers) are presented in chapter 5. These studies provide direct evidence for excited state proton transfer (ESPT) in DHICA, and a reference for the interpretation of the complex eumelanin macro-molecule, as well as its dynamic vibrational signatures, which are discussed in chapter 6. Here, vibrational dynamics are resolved on From these FSRS studies, ESPT – proposed as a relaxation pathway in eumelanin’s subunit DHICA – is demonstrated. Direct vibrational relaxation measurement mapped using density functional theory (DFT) provides strong evidence of excited state de-activation of DHICA via this mechanism. Further, eumelanin’s vibrational mode deactivation pathways are presented, providing evidence of specific mode excitations of the macro-molecule upon photo-excitation in both the UV (267 nm) and visible (400 nm) regions. &lt;/p&gt;&lt;p&gt;&lt;br&gt;&lt;/p&gt;&lt;p&gt;Resolving eumelanin’s excited state vibrational modes and kinetics using FSRS provides dynamic structural information of eumelanin's thermal energy transfer system, including that of its indole subunits. Determining FSRS signatures as a function of excitation wavelength in both the visible and UV regions, reveals insights into this highly efficient UV absorbing material. &lt;/p&gt;

The peridinin chlorophyll-a protein (PCP) is a water-soluble, trimeric light harvesting complex found in marine dinoflagellates that binds peridinin and Chl-a in an unusual stoichiometric ratio of 4:1. In this paper, the pathways of excited-state energy transfer and relaxation in PCP were identified by means of femtosecond visible-pump, mid-infrared probe spectroscopy. In addition, excited-state relaxation of peridinin dissolved in organic solvent (CHCl(3) and MeOH) was investigated. For peridinin in solution, the transient IR signatures of the low-lying S(1) and intramolecular charge transfer (ICT) states were similar, in line with a previous ultrafast IR study. In PCP, excitation of the optically allowed S(2) state of peridinin results in ultrafast energy transfer to Chl-a, in competition with internal conversion to low-lying optically forbidden states of peridinin. After vibrational relaxation of the peridinin hot S(1) state in 150 fs, two separate low-lying peridinin singlet excited states are distinguished, assigned to an ICT state and to a slowly transferring, vibrationally relaxed S(1) state. These states exhibit different lactone bleaches, indicating that the ICT and S(1) states localize on distinct peridinins. Energy transfer from the peridinin ICT state to Chl-a constitutes the dominant energy transfer channel and occurs with a time constant of 2 ps. The peridinin S(1) state mainly decays to the ground state through internal conversion, in competition with slow energy transfer to Chl-a. The singlet excited state of Chl-a undergoes intersystem crossing (ISC) to the triplet state on the nanosecond timescale, followed by rapid triplet excitation energy transfer (TEET) from Chl-a to peridinin, whereby no Chl-a triplet is observed but rather a direct rise of the peridinin triplet. The latter contains some Chl-a features due to excitonic coupling of the pigments. The peridinin triplet state shows a lactone bleach mode at 1748 cm(-1), while that of the peridinin ICT state is located at 1745 cm(-1), indicating that the main channels of singlet and triplet energy transfer in PCP proceed through distinct peridinins. Our results are consistent with an energy transfer scheme where the ICT state mainly localizes on Per621/611 and Per623/613, the S(1) state on Per622/612 and the triplet state on Per624/614.