Lowest Triplet State Research Articles

Substituent effects on energetics and crystal morphology modulate singlet fission in 9,10-bis(phenylethynyl)anthracenes.

Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4'-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S1) energy of 4,4'-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps)-1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4'-methoxy-BPEA and 4,4'-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps)-1 and (120 ± 10 ps)-1, and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4'-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids.

Design of a New Mechanism beyond Thermally Activated Delayed Fluorescence toward Fourth Generation Organic Light Emitting Diodes

Usually, development of organic molecules with efficient thermally activated delayed fluorescence (TADF) focuses on minimizing the energy gap between the lowest singlet and triplet state. However, ...

Simplified Treatment of Electronic Structures of the Lowest Singlet and Triplet States of Didehydropyrazines.

Experiments on the lowest lying singlet and triplet states of didehydropyrazine isomers (that focus on energy gaps, geometries, and vibrational frequencies) have been carried out computationally by implementing the improved virtual orbital-based multireference (MR) ab initio methods. Pyrazines possess a reasonable MR nature making their description challenging with the conventional quantum chemical approaches. Although wave functions of the diradicals usually have two dominant configurations, a larger reference space is warranted to consistently and accurately describe pyrazine diradicals indicating the complex nature of the systems. Present calculations predict a singlet ground state for ortho- and para-pyrazines, while a triplet ground state is suggested for the meta isomer. The adiabatic singlet-triplet energy gap for the para form is found to be notably higher (by 28.4 kcal/mol) compared to ortho (2.0 kcal/mol) and meta isomers (-5.1 kcal/mol). Accurate and reliable computations are imperative for forecasting the state-ordering in such diradicals. The structural properties obtained from the present calculations lend strong support toward future experimental explorations on these systems.

Syntheses, crystal structures, and solid-state photoluminescence properties of heterotrinuclear Zn2Ln (Ln: La, Sm, Eu, Tb) complexes derived from 1,4-diaminobutane-based N2O4 compartmental ligand

The one-pot reaction of N,N′-bis(3-methoxysalicylidene)-1,4-diaminobutane (MeOsalbnH2), zinc(II) perchlorate, and lanthanide(III) acetate by a molar ratio of 2:2:1 affords a heterotrinuclear Zn2Ln (Ln: La, Sm, Eu, Tb) complex formulated as [Ln(CH3COO)2{Zn(MeOsalbn)}2]ClO4. In this complex, the central Ln(III) atom is bridged by two {Zn(MeOsalbn)} units and two acetate ligands to form ten-coordination environment. Each Zn(II) atom is coordinated by MeOsalbn2− through the N2O2 site and one oxygen atom from the bridging acetate to afford a five-coordination geometry. In the crystalline state at room temperature, the La(III) complex exhibits a bright blue fluorescence, while the Sm(III) complex shows a reddish-orange colored f–f photoluminescence via an energy transfer from the T1 state localized on the {Zn(MeOsalbn)} units to the acceptor level of the central Sm(III) ion. The results of TD-DFT calculations show that the lowest excited singlet state or triplet state involved in photoluminescence of the La(III) or Sm(III) complex has some intramolecular charge-transfer (CT) character from the phenoxo or methoxy groups to the imino groups in the coordinated MeOsalbn2− moieties.

Accurate CHIPR Potential Energy Surface for the Lowest Triplet State of C3.

We report the first global ab initio-based potential energy surface (PES) for ground-state triplet C3(3A') based on accurate energies extrapolated to the complete basis set (CBS) limit, and using the combined-hyperbolic-inverse-power-representation method for the analytical modeling. By relying on a cost-effective CBS(D,T) protocol, we ensure that the final form reproduces all topographical features of the PES, including its cyclic-linear isomerization barrier, with CBS(5,6)-quality. To partially account for the incompleteness of the N-electron basis and other minor effects, the available accurate experimental data on the relevant diatomics were used to obtain direct-fit curves that replace the theoretical ones in the many-body expansion. Besides describing properly long-range interactions at all asymptotic channels and permutational symmetry by built-in construction, the PES reported here reproduces the proper exothermicities at dissociation regions as well as the spectroscopy of the diatomic fragments. Bound vibrational state calculations in both linear and cyclic isomers have also been carried out, unveiling a good match of the available data on C3(ã 3Πu), while assisting with IR band positions for C3(3A2') that may serve as a guide for its laboratory and astronomical detection.

Roles of Localized Electronic Structures Caused by π Degeneracy Due to Highly Symmetric Heavy Atom-Free Conjugated Molecular Crystals Leading to Efficient Persistent Room-Temperature Phosphorescence.

Conjugated molecular crystals with persistent room‐temperature phosphorescence (RTP) are promising materials for sensing, security, and bioimaging applications. However, the electronic structures that lead to efficient persistent RTP are still unclear. Here, the electronic structures of tetraphenylmethane (C(C6H5)4), tetraphenylsilane (Si(C6H5)4), and tetraphenylgermane (Ge(C6H5)4) showing blue‐green persistent RTP under ambient conditions are investigated. The persistent RTP of the crystals originates from minimization of triplet exciton quenching at room temperature not suppression of molecular vibrations. Localization of the highest occupied molecular orbitals (HOMOs) of the steric and highly symmetric conjugated crystal structures decreases the overlap of intermolecular HOMOs, minimizing triplet exciton migration, which accelerates defect quenching of triplet excitons. The localization of the HOMOs over the highly symmetric conjugated structures also induces moderate charge‐transfer characteristics between high‐order singlet excited states (Sm) and the ground state (S0). The combination of the moderate charge‐transfer characteristics of the Sm–S0 transition and local‐excited state characteristics between the lowest excited triplet state and S0 accelerates the phosphorescence rate independent of the vibration‐based nonradiative decay rate from the triplet state at room temperature. Thus, the decrease of triplet quenching and increase of phosphorescence rate caused by the HOMO localization contribute to the efficient persistent RTP of Ge(C6H5)4 crystals.

Ab Initio Molecular Orbital Study of the First Four Si/C Alternately Substituted Annulenes.

The ground and low-lying excited states of four alternating Si/C annulenes, H nSi n/2C n/2 with n = 4, 6, 8, and 10, have been investigated by ab initio molecular orbital methods and compared to those of their all-carbon and all-silicon analogues. In the ground state, all of the Si/C-mixed annulenes, except for the largest 10-membered annulene (H10Si5C5), assume equal-bond-length structures by adopting a closed-shell electronic structure in the possible highest symmetry. For the largest H10Si5C5, the trend of the bond delocalization still remains but the circular structure is considerably distorted and nonplanar due to severe angle strain. In the low-lying singlet (S1) and triplet (T1) states, the geometry of the compounds tends to be nonplanar as the excitations produce silyl radical character. Relative energies of the T1 and S1 states of the 6-membered ring, compared to those of the respective ground states (S0), are higher than those of the 4- and 8-membered rings, suggesting a special stability for H6Si3C3. The planar rhombus shape of the formally antiaromatic H4Si2C2 suggests that a synthetic effort is merited. Bonding analyses are given to support the conclusions reached on the basis of geometric structures and excited-state energetics.

An Updated Strategy for Designing Non-Fullerene Acceptors by the Lowest Singlet and Triplet States Excitation: Influence of Periodical Substitution from O, S, and Se to Te for BAE Derivatives

When one is trying to maintain original advantageous properties of molecule without increasing the difficulty of synthesis and processing, simultaneously, adding more predictable properties is a preferred choice for designing promising molecules. Here, we report the photovoltaic performance of bistricyclic aromatic enes (BAE) derivative acceptors as a function of heteroatom substitutions (O, S, Se, Te), and the range they can maintain. Beyond that, by combining singlet–triplet exciton conversion with singlet fission (SF), an additional perspective to characterize molecules performance is predicted by employing T1-excitation. Theoretical calculation results showed that for small molecules (BAEs), heavy atom substitution of the same group could increase conformation stability, electron acceptability, and spin–orbit coupling. After expansion of the molecular size (DPP-BAE-DPP), the difference in molecular properties is mainly due to the conformation type for singlet states excitation. For triplet states excitation, the degree of negative correlation between SF and singlet–triplet exciton conversion decreases, which is conducive to obtaining more T1 excitons, thus improving the organic photovoltaic performances. As a result, A-S-DPP, A-Te-DPP, T-S-DPP, T-Se-DPP and T-Te-DPP not only have superior single-excitation performance but also potential triple-excitation possibilities; they are promising acceptors. These results provided some new evidence for designing nonfullerene acceptors and demonstrating the role of heavy atoms substitution in photovoltaic performance.

Thianthrene and acridan-substituted benzophenone or diphenylsulfone: Effect of triplet harvesting via TADF and phosphorescence on efficiency of all-organic OLEDS

Two new asymetric D–A–D′ type compounds containing benzophenone and diphenylsulfone as electron accepting core, and acridan and thianthrene as electron donating moieties were design as multifunctional materials. The influence of different acceptor and donor units on thermally activated delayed fluorescence and room temperature phosphorescence was investigated and discussed. The combination of different donors with benzophenone unit resulted in the compound with effective frontier orbital separation, as well as the sustainment of high radiative rate. These statements are both experimentally and theoretically supported using time-resolved photoluminescence spectroscopy and DFT calculations. Deoxygenation experiment revealed higher triplet exciton contribution in the emission of benzophenone derivative, as compared to that of diphenylsulfone-based compound. The films of molecular mixtures of these emitters with 1,3-bis(N-carbazolyl)benzene demonstrated temperature-dependent photoluminescence. The estimated energy gaps between the lowest singlet and triplet states were found to be 0.06 eV and 0.27 eV for benzophenone and diphenylsulfone derivatives, respectively. Due to the relatively high singlet-triplet energy splitting of the diphenylsulfone-based compound, triplet harvesting was detected not only via temperature-stimulated fluorescence but also via room temperature phosphorescence. Displaying effect of triplet harvesting on efficiency of all-organic blue and green OLEDs based on thianthrene and acridan-substituted benzophenone or diphenylsulfone derivatives demonstrated very different maximum external quantum efficiencies of 1.4 and 22.2% respectively.

TADF activation by solvent freezing: The role of nonradiative triplet decay and spin-orbit coupling in carbazole benzonitrile derivatives

Thermally activated delayed fluorescence (TADF) materials have attracted considerable attentions as a new kind of emitters in organic light-emitting diodes. While it is requisite to minimize an energy difference between the lowest excited triplet state (T1) and lowest excited singlet state (S1), so-called ΔEST, a deeper understanding of the emission mechanism is desirable to clarify the comprehensive molecular design. In this paper, we present that the TADF ability and efficiency of (rare-)metal free organic molecules are surely influenced by both the nonradiative decay of T1 and spin-orbit coupling. By investigating a temperature dependent photoluminescence of carbazole benzonitrile derivatives in toluene solutions using a newly developed liquid nitrogen cryostat, we demonstrate the activation of TADF by solvent freezing for room temperature-TADF inactive molecules. Transient photoluminescence measurements of the frozen samples show a significant increase of a lifetime of T1, probing the suppression of nonradiative decay path of T1. A magnitude of the TADF activation by the solvent freezing is closely related to the degree of spin-orbit coupling of the molecules. The present results emphasize the importance of suppression of nonradiative decay of T1 and an increase of spin-orbit coupling together with reducing ΔEST to achieve a high TADF emission efficiency.

Recent Advances on Metal-Based Near-Infrared and Infrared Emitting OLEDs.

During the past decades, the development of emissive materials for organic light-emitting diodes (OLEDs) in infrared region has focused the interest of numerous research groups as these devices can find interest in applications ranging from optical communication to defense. To date, metal complexes have been most widely studied to elaborate near-infrared (NIR) emitters due to their low energy emissive triplet states and their facile access. In this review, an overview of the different metal complexes used in OLEDs and enabling to get an infrared emission is provided.

Experimental and theoretical investigation on photodegradation mechanisms of naproxen and its photoproducts

The photochemical degradation of the pharmaceuticals and personal care products (PPCPs) has attracted increasing attention. In this study, a deep inspection of the photolysis mechanisms of naproxen and its photoproducts has been performed by employing experimental and theoretical methods. Contributions of different reactive oxygen species (ROS, such as OH, 1O2, and O2−) in the photolysis reaction also have been clarified. Based on the detected intermediates and DFT calculations, several photodegradation pathways of naproxen and its photoproducts have been proved. Furthermore, the deprotonated form of naproxen has been confirmed to be more reactive than the protonated one, and the lowest triplet state of naproxen is the reactive state. The decarboxylation mechanism of naproxen has been fully discussed. Meanwhile, the free energy barriers of OH-induced photolysis reactions (ΔG‡eff(1a) = 7.6 kcal mol−1, ΔG‡eff(4a) = 7.0 kcal mol−1) are much lower than the free energy barriers induced by O2− and 1O2. It proves that OH is the most favourable one among the three ROS. The similar inhabition rates and free energy barriers of reactions induced by O2− and 1O2, respectively, have demonstrated that O2− and 1O2 equally contribute to the degradation. Additionally, the computational results are coincident with the observed experimental findings. Hence, this work has verified a part of naproxen photodegradation mechanism under UV irradiation and brought about a rational way to investigate contributions of different ROS in the complex photochemical system of PPCPs.

Predicting Phosphorescence Quantum Yield for Pt(II)-Based OLED Emitters from Correlation Function Approach

We report a new formulation for Golden Rule-based predictions of photoluminescence quantum yields (PLQY) of phosphorescent emitters containing a heavy element, and its implementation compatible with first-principles computation frameworks. The main components of PLQY (i.e., phosphorescent rate and intersystem crossing rate from the lowest triplet state to the ground singlet state) are obtained through correlation functions in time domain, and the relativistic effects are also considered using the relativistic effective core potentials. The spin–orbit coupling is treated in a perturbative manner to generate spin–orbit-corrected, two-component T1 substates within single-excitation theory, where the electronic transition dipole moments and the non-Born–Oppenheimer coupling matrix elements to the S0 state are computed. We applied this new approach to the photophysical properties of 34 Pt(II) complexes designed for the organic light-emitting diode (OLED) applications and observed a good agreement between predictions and experiments over diverse scaffolds. For the two representative complexes, further analysis on the nonradiative characteristics was performed based on the decomposition of the non-Born–Oppenheimer coupling into contributions from the nuclear vibrations and from the excited-state electronic structures.

QM/MM nonadiabatic dynamics simulation on ultrafast excited-state relaxation in osmium(II) compounds in solution

Photoinduced excited-state relaxation dynamics of organometallic compounds, due to their potential applications in solar cells, organic light-emitting diodes, photocatalysis, etc., have been recently explored by a wealth of ultrafast experimental techniques. However, the corresponding nonadiabatic dynamics simulations are rarely reported. In this work, the early-time excited-state relaxation dynamics of two Os(II) compounds i.e. Os(bpy)3 and Os(bpy)2(dpp) (Os1 and Os2) have been investigated by our recently implemented TD-DFT based generalized surface-hopping dynamics simulation method in combination with the quantum mechanics/molecular mechanics (QM/MM) approach. The metal-to-ligand charge transfer (MLCT) excited singlet states from the Os atom to the bpy and dpp ligands are first populated in the Franck-Condon region. These initially populated excited singlet states will be converted through a series of ultrafast internal conversion and intersystem crossing processes to the lowest triplet state as a result of significant nonadiabatic and spin-orbit couplings among singlet and triplet electronic excited states. The intersystem crossing rates are estimated to be 72 and 53 fs for Os1 and Os2, respectively, which are consistent with experimental measured 100 fs and less than 50 fs. The time-dependent transition density analysis reveals that the MLCT excited states are dominant in the entire relaxation dynamics for either Os1 or Os2; however, the MLCT character changes in these processes. In Os1, the initially populated MLCT state is mainly from the Os atom to one bpy ligand, which is changed to the other two MLCT states. In Os2, the MLCT state that is mainly from the Os atom to the two bpy ligands is converted to the MLCT state from the Os to the dpp ligand. Finally, we have found that inter-ligand electron transfer plays a major role in the excited-state relaxation dynamics of Os1 and Os2; while, the hole transfer is minor. Our results also show that the ligand properties have significant influence on the excited-state relaxation dynamics of organometallic compounds. The insights gained in the present study provide atomistic insights for early-time excited-state relaxation dynamics of Osmium(II) compounds.

Hydrogen bond configuration and protonation of ground and lowest excited triplet states of 4‑amino‑4'‑nitrobiphenyl based on nanosecond transient absorption spectroscopy.

Intramolecular charge transfer (ICT) is an important photochemical process. In contrast to those in singlet manifold, triplet ICT states were less studied. In this paper, the lowest excited triplet state (T1) of 4‑amino‑4'‑nitrobiphenyl (NH2-Bp-NO2) was recorded with nanosecond transient absorption spectroscopy in acidic acetonitrile and alcoholic solutions. By employing the Kamlet-Taft model to analyze the correlation between absorption maxima and alcohol solvent properties including polarity/polarizability, abilities of hydrogen bond donating and hydrogen bond accepting, hydrogen bond configuration in the ground state (S0) and T1 was resolved. The results suggest that the hydrogen bond between amino H and alcohol is dominant in S0, while in T1, hydrogen bonds between amino H and alcohol, between nitro O and alcohol have comparable contribution. By examination of the 1‑naphthol quench effect on T1, the hydrogen bond between nitro O and alcohol was confirmed present. Theoretical calculation results on the model of NH2-Bp-NO2-(MeOH)3 also indicate that hydrogen bonds between amino H and alcohol, between nitro O and alcohol are both much stronger in T1 than in S0. In acidic acetonitrile solution, in S0 of NH2-Bp-NO2 the amino group is protonated with pKa of 4.5, meanwhile in T1 the nitro group is much easier to be protonated than in S0. Its conjugated acid was measured to have a pKa of 3.1.

Photosubstitution Reaction of cis-[Ru(bpy)2(CH3CN)2]2+ and cis-[Ru(bpy)2(NH3)2]2+ in Aqueous Solution via Monoaqua Intermediate.

The photoinduced ligand exchange reaction of Ru(II) complexes in aqueous solution was studied using density functional theory (DFT). The optimized structures of the lowest triplet state of cis-[Ru(bpy)2(CH3CN)2]2+ (bpy = bipyridine), cis-[Ru(bpy)2(NH3)2]2+, and their monoaqua complexes were analyzed. The metal-centered (3MC) structure was lower than the metal-to-ligand charge transfer (3MLCT) structure for cis-[Ru(bpy)2(CH3CN)2]2+, whereas the 3MLCT structure was lower than the 3MC structure for cis-[Ru(bpy)2(NH3)2]2+. Such a difference would correlate with the higher quantum yield of the former complex. For the monoaqua complexes, the most stable local minimum structure was the 3MC structure, in which the Ru-OH2O and Ru-Nbpy ( trans to the oxygen) bonds were elongated. Therefore, the dissociation of the H2O ligand would be preferred to that of the CH3CN (or NH3) ligand from the monoaqua intermediate, which might result in the reformation of the monoaqua intermediate, and thus, the formation of the bis-aqua product would take a longer time than that of the monoaqua intermediate.

Excited State Intramolecular Proton Transfer Dynamics for Triplet Harvesting in Organic Molecules.

Thermally activated delayed fluorescence (TADF) has shown great potential as a mechanism for harvesting low-lying triplet excited states in organic molecules and is therefore of great interest in the context of organic electronics, especially organic light emitting diodes (OLEDs). Herein we study the mechanism for triplet harvesting in triquinolonobenzene (TQB), which instead of relying upon the well-established donor-acceptor (D-A) scheme uses excited-state intramolecular proton transfer (ESIPT). We demonstrate that upon photoexcitation into the lowest singlet excited state the proton is transferred within 20 fs, suggesting it plays little role in triplet harvesting, which occurs on the nano- to microsecond time scale. However, TQB exhibits multiple low-lying triplet states that are strongly coupled along this proton transfer coordinate. The majority of these states favor the structure prior to proton transfer (TQB-TA) and this means that the proton transfer dynamics (3TQB-TA → 1TQB-TB) plays a crucial role in triplet harvesting. This mechanism yields an energy gap in good agreement with that reported experimentally and is consistent with previous photophysical characterization. Finally, a discussion upon extending this understanding into a device context is also presented.

Highly Efficient Photo- and Electroluminescence from Two-Coordinate Cu(I) Complexes Featuring Nonconventional N-Heterocyclic Carbenes.

A series of six luminescent two-coordinate Cu(I) complexes were investigated bearing nonconventional N-heterocyclic carbene ligands, monoamido-aminocarbene (MAC*) and diamidocarbene (DAC*), along with carbazolyl (Cz) as well as mono- and dicyano-substituted Cz derivatives. The emission color can be systematically varied over 270 nm, from violet to red, through proper choice of the acceptor (carbene) and donor (carbazolyl) groups. The compounds exhibit photoluminescent quantum efficiencies up to 100% in fluid solution and polystyrene films with short decay lifetimes (τ ≈ 1 μs). The radiative rate constants for the Cu(I) complexes ( kr = 105-106 s-1) are comparable to state of the art phosphorescent emitters with noble metals such as Ir and Pt. All complexes show strong solvatochromism due to the large dipole moment of the ground states and the transition dipole moment that is in the opposite direction. Temperature-dependent studies of (MAC*)Cu(Cz) reveal a small energy separation between the lowest singlet and triplet states (Δ ES1-T1 = 500 cm-1) and an exceptionally large zero-field splitting (ZFS = 85 cm-1). Organic light-emitting diodes (OLEDs) fabricated with (MAC*)Cu(Cz) as a green emissive dopant have high external quantum efficiencies (EQE = 19.4%) and brightness of 54 000 cd/m2 with modest roll-off at high currents. The complex can also serve as a neat emissive layer to make highly efficient OLEDs (EQE = 16.3%).

Study of energy transfer mechanism in the EuIII and GdIII homobimetallic complexes containing the anti-inflammatory drug naproxen and N,N-donors ligands

Abstract In this work, we present the synthesis, solid state characterization and complete photoluminescence study of new important homobimetallic lanthanide complexes containing the non-steroidal anti-inflammatory drug (NSAID) naproxen. The analytical and spectroscopic techniques reveals the formation of eight compounds of general formula [Ln2(nap)6(H2O)4] (Eu 1 and Gd 2), [Ln2(nap)6(bpy)2] (Eu 3 and Gd 4), [Ln2(nap)6(4,4'-dmbpy)2] (Eu 5 and Gd 6) and [Ln2(nap)6(phen)2] (Eu 7 and Gd 8), where: nap = naproxen ligand, bpy =2,2'-bipyridine, 4,4'-dmbpy = 4,4'-dimethyl-2,2'-bipyridine and phen = 1,10-phenanthroline. Using the RM1 model, the molecular structures of the EuIII complexes were calculated, with your optimized ground state geometries used to obtain all details involved in the energy transfer process. From the respective GdIII complexes were obtained the lowest ligand triplet states, proving that the photoluminescence in the EuIII naproxen complexes is proposed to be a ligand sensitized luminescence process. On the other hand, the position of the triplet states also explains the non-effective energy transfer in the TbIII naproxen complexes. The presence of N,N-donors ligands (bpy, 4,4'-dmbpy and phen) results in an 3–4-fold increase in the quantum efficiency when compared with the EuIII complex without nitrogen ligands. The high values of emission quantum efficiency (η ~ 70 - 98%) show the EuIII complexes can be potential candidates as emitters in biologic assays.

Chemical Stabilities of the Lowest Triplet State in Aryl Sulfones and Aryl Phosphine Oxides Relevant to OLED Applications

Aryl sulfones and phosphine oxides are widely used as molecular building blocks for host materials in the emissive layers of organic light-emitting diodes. In this context, the chemical stability of such molecules in the triplet state is of paramount concern to long-term device performance. Here, we explore the triplet excited-state (T1) chemical stabilities of aryl sulfonyl and aryl phosphoryl molecules by means of UV absorption spectroscopy and density functional theory calculations. Both the sulfur–carbon bonds of the aryl sulfonyl molecules and the phosphorus–carbon bonds of aryl phosphoryl derivatives are significantly more vulnerable to dissociation in the T1 state when compared to the ground (S0) state. Although the vertical S0 → T1 transitions correspond to nonbonding → π-orbital transitions, geometry relaxations in the T1 state lead to σ–σ* character over the respective sulfur–carbon or phosphorus–carbon bond, a result of significant electronic state mixing, which facilitates bond dissociation. B...