Photophysical Events Research Articles

NIR-triggered photooxygenation of α-terpinene with upconversion nanohybrids.

Upconversion nanohybrids (UCNHs) consisting of rose bengal (RB) and upconversion nanoparticles (UCNPs) are able to promote terpinene oxidation upon near-infrared irradiation. The photophysical events occurring upon NIR-irradiation of the UCNH correlate well with the synthetic protocol used to prepare the UCNHs (RB loading and aggregation). These results highlight the importance of the optimization of UCNH composition for the photocatalysis outcome.

BNP-Track: a framework for superresolved tracking

Superresolution tools, such as PALM and STORM, provide nanoscale localization accuracy by relying on rare photophysical events, limiting these methods to static samples. By contrast, here, we extend superresolution to dynamics without relying on photodynamics by simultaneously determining emitter numbers and their tracks (localization and linking) with the same localization accuracy per frame as widefield superresolution on immobilized emitters under similar imaging conditions (≈50 nm). We demonstrate our Bayesian nonparametric track (BNP-Track) framework on both in cellulo and synthetic data. BNP-Track develops a joint (posterior) distribution that learns and quantifies uncertainty over emitter numbers and their associated tracks propagated from shot noise, camera artifacts, pixelation, background and out-of-focus motion. In doing so, we integrate spatiotemporal information into our distribution, which is otherwise compromised by modularly determining emitter numbers and localizing and linking emitter positions across frames. For this reason, BNP-Track remains accurate in crowding regimens beyond those accessible to other single-particle tracking tools.

Near-IR Capturing N-Methylbenzene Sulfonamide-Phenothiazine Incorporating Strong Electron Acceptor Push-Pull Systems: Photochemical Ultrafast Carrier Dynamics.

Unraveling the intriguing aspects of the intramolecular charge transfer (ICT) phenomenon of multi-modular donor-acceptor-based push-pull systems are of paramount importance considering their promising applications, particularly in solar energy harvesting and light-emitting devices. Herein, a series of symmetrical and unsymmetrical donor-acceptor chromophores 1-6, are designed and synthesized by the Corey-Fuchs reaction via Evano's condition followed by [2+2] cycloaddition retroelectrocyclic ring-opening reaction with strong electron acceptors TCNE and TCNQ in good yields (~60-85 %). The photophysical, electrochemical, and computational studies are investigated to explore the effect of incorporation of strong electron acceptors 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and dicyanoquinodimethane (DCNQ) with phenothiazine (PTZ) donor. An additional low-lying broad absorption band extended towards the near-infrared (NIR) region suggests charge polarization after the introduction of the electron acceptors in both symmetrical and asymmetrical systems, leading to such strong ICT bands. The electrochemical properties reveal that reduction potentials of 3 and 6 are lower than those of 2 and 5, suggesting DCNQ imparts more on the electronic properties and hence largely contributes to the stabilization of LUMO energy levels than TCBD, in line with theoretical observations. Relative positions of the frontier orbitals on geometry-optimized structures further support accessing donor-acceptor sites responsible for the ICT transitions. Eventually, ultrafast carrier dynamics of the photoinduced species are investigated by femtosecond transient absorption studies to identify their spectral characteristics and target analysis further provides information about different excited states photophysical events including ICT and their associated time profiles. The key findings obtained here related to excited state dynamical processes of these newly synthesized systems are believed to be significant in advancing their prospect of utilization in solar energy conversion and related photonic applications.

Microviscosity-Assisted Disaggregation of a Model Ophthalmic Drug and FRET-Controlled Singlet Oxygen Generation in Lyotropic Liquid Crystals.

Different phases of lyotropic liquid crystals (LLCs), made up of mesogen-like sodium dodecyl sulfate (SDS), mainly bestow different bulk viscosities. Along with this, the role of microviscosities of the individual LLC phases is of immense interest because a minute change in it due to guest incorporation can cause significant alteration in their property as a potential energy transfer scaffold. Recently, LLCs have been identified as plausible drug delivery agents for ocular treatments. In this direction, the present work illustrates photophysical modulations of an important laser dye as well as an ophthalmic medicine, coumarin 6 (C6), inside different LLC phases in an aqueous medium. C6 molecules spontaneously accumulate in water, leading to aggregation-caused quenching (ACQ) of fluorescence. However, the different phases of the LLCs prepared from SDS and water helped in disintegrating the C6 colonies to various extents depending upon the microviscosity. The heterogeneity in the LLC phases, in turn, could modulate the Förster resonance energy transfer (FRET) between C6 and the LLC incorporated with N-doped carbon nanoparticles (N-CNPs). The N-CNPs act as potential photosensitizers and generate singlet oxygen (1O2), a reactive oxygen species (ROS), to different extents. Microviscosities of the prepared LLCs were calculated by using fluorescence correlation spectroscopy (FCS). The different phases of the LLCs, viz., lamellar and hexagonal, with different microviscosities controlled the extent of C6 disaggregation and hence the FRET and the ROS generation. The results are encouraging since ROS generation has a significant role in the vision mechanism and PDT-based applications. LLC-based drug administration with potential FRET to control ROS generation may become handy in ophthalmology. The LLC phases used in this experiment not only served the purpose of drug delivery but also the photophysical events therein are compatible with the ocular environment.

Deciphering intersystem crossing and energy transfer mechanisms in a nonacoordinated ternary europium(iii) complex: a combined spectroscopic and theoretical study.

A monochromatic red emitting nonacoordinate organoeuropium complex with the formula [Eu(hfaa)3(Ph-TerPyr)] (Eu-1) incorporating hexafluoroacetylacetone (hfaa) primary ligands and a tridentate 4'-phenyl-2,2':6',2''-terpyridine (Ph-TerPyr) ancillary ligand has been synthesized. The complex was characterized by analytical and spectroscopic methods, and its structure was established by single crystal X-ray diffraction (SC-XRD) analysis at low temperature, which explicitly confirms that the coordination sphere is composed of a EuO6N3 core. Under the UV excitation, Eu-1 displayed typical red emission in solution with a long-excited state lifetime (τ obs = 1048.06 ± 9.39 μs) with a good photoluminescence quantum yield (Q L Eu = 41.14%). We have utilized pump-probe ultrafast transient absorption spectroscopy in tandem with the time-dependent density functional theory (TD-DFT) and the Lanthanide Luminescence Software Package (LUMPAC) to explore the intricate photophysical event that occurs in the vicinity of the ligands of Eu-1 sensitized photoluminescence (PL).

Peptide-bridged bis-porphyrin compounds: A photophysical and molecular dynamics study

Covalently linked peptide-porphyrin compounds are most suitable systems for fundamental studies aiming to the comprehension of the mechanisms driving photoinduced energy/electron transfer processes. Mimicking photosynthetic units, the porphyrin groups act as antenna moieties while the peptide chain is the active medium through which energy and/or electron funneling occur. In this contribution we studied the transfer of excitation between two identical tetraphenylporphyrin groups connected by short peptide chains of different length formed by non-coded conformationally constrained α-amino acids, i.e., Cα-methylvaline. The photophysical events following porphyrin photoexcitation were characterized from the microsecond to the picosecond time region by time-resolved spectroscopy techniques. Ultrafast transient absorption measurements revealed the presence of a transient species that we assign to a self-trapped exciton migrating through the peptide chain. The exciton species propagates the electronic coupling between the two porphyrin groups giving rise to a characteristic bisignate band measured by circular dichroism experiments. Molecular dynamics simulations strongly suggest that the long lifetime (hundreds of picoseconds) of the exciton species is determined by the rigidity of the Cα-methylvaline residues, that inhibited energy relaxation pathways coupled to torsional motions of the peptide chain.

Effectiveness of Light-Emitting Diode Epilation on Different Skin Types: A Pilot Study.

Objective: This study analyzed the histological and immunohistochemical changes in hair follicles submitted to epilation with light-emitting diode (LED). Background: The use of specific wavelengths of LED leads to the absorption of photons by chromophore tissues, enabling different photophysical and photochemical events, bringing therapeutic benefits such as removing body hair. Methods: The sample included five participants, with phototypes II-V, divided into two groups. The volunteers received a session of epilation with the Holonyak® device on the pubic region and right groin, whereas the contralateral side was kept as a control. An energy of 10 J and a cooling temperature of -5°C were used, after which the pain provoked by the equipment was questioned using the analogue pain scale. After 45 days, the punching procedure was performed in the region where skin samples were taken for histological and immunohistochemical analysis. Results: For all phototypes, in the treated area, the follicles and sebaceous glands were in a stage of involution, showing perifollicular inflammatory infiltrate with changes suggestive of apoptosis. The apoptosis process was confirmed by the increase in markers cytokeratin-18 and cleaved caspase 3, in addition to the reduced expression of Blc-2, and the lower cell proliferation (Ki67), reinforcing the action of LED based on the definite involution and resorption of the follicle, through macrophages (CD68) triggered by the inflammatory process. Conclusions: The preliminary results of this study found relevant histological changes and immunohistochemical markers in the epilation process, which may indicate the efficacy of LED in permanent hair removal.

Single-photon smFRET. I: Theory and conceptual basis

We present a unified conceptual framework and the associated software package for single-molecule Förster resonance energy transfer (smFRET) analysis from single-photon arrivals leveraging Bayesian nonparametrics, BNP-FRET. This unified framework addresses the following key physical complexities of a single-photon smFRET experiment, including: 1) fluorophore photophysics; 2) continuous time kinetics of the labeled system with large timescale separations between photophysical phenomena such as excited photophysical state lifetimes and events such as transition between system states; 3) unavoidable detector artefacts; 4) background emissions; 5) unknown number of system states; and 6) both continuous and pulsed illumination. These physical features necessarily demand a novel framework that extends beyond existing tools. In particular, the theory naturally brings us to a hidden Markov model with a second-order structure and Bayesian nonparametrics on account of items 1, 2, and 5 on the list. In the second and third companion articles, we discuss the direct effects of these key complexities on the inference of parameters for continuous and pulsed illumination, respectively.

(Invited) Untethering the Tether Towards Spontaneously Self-Assembled Donuts

Functional materials comprised of spontaneously self-assembled electron donor and acceptor entities capable of generating long-lived charge-separated states upon photo-illumination are in great demand as they are key in building the next generation of light energy harvesting devices. However, creating such well-defined architectures is challenging due to the intricate molecular design, multi-step synthesis, and issues associated in demonstrating long-lived electron transfer.Here, we have newly synthesized, by tether-directed functionalization, a [60]fullerene e-bisadduct carrying two Zn-porphyrins (Figure), and demonstrate supramolecular organization and photophysical events. Remarkably, the supramolecular assembly of the present bisporphyrin-C60 forms donut-shape aggregates, primarily via π-π type charge transfer interactions. Upon photoexcitation, the supramolecular assembly generates long-lived charge-separated states of »1−40 μs lifetime due to electron/hole delocalization within the supramolecular structureReference Caballero, M. Barrejón, J. Cerdá, J. Aragó, S. Seetharaman, P. de la Cruz, E. Ortí, F. D’Souza, F. Langa. J. Am. Chem. Soc. 2021, 143, 11199. Figure 1

Photochemistry and photophysics of cholesta-5,7,9(11)-trien-3β-ol: a fluorescent analogue of cholesterol

Cholesta-5,7,9(11)-trien-3β-ol (9,11-dehydroprovitamin D3, CTL) is used as a fluorescent probe to track the presence and migration of cholesterol in vivo. CTL is known to be photochemically active, but little consideration has been given to the formation efficiency and possible toxicity of its photoproducts. In degassed tetrahydrofuran (THF) solution, we isolated the photoproduct of CTL and of its 25-hydroxy derivative (HOCTL), and X-ray crystal structures were obtained for HOCTL and the photorearrangement product. The X-ray crystal structure and its 1H NMR spectrum confirm the product structure as a pentacyclic HOCTL isomer. In the presence of air in THF, endoperoxide formation via [2+4] addition of 1O2* across the B ring of CTL or HOCTL becomes the dominant photoreaction. The UV spectrum and decay kinetics of the triplet state of HOCTL, the precursor of 1O2*, are determined by transient absorption spectroscopy. We confirm the proposed structure of the endoperoxide by X-ray crystallography. Kinetics analysis of quantum yields provides rate constants for photophysical and photochemical events.

Photophysical Deactivation Mechanisms of the Pyrimidine Analogue 1-Cyclohexyluracil.

The photophysical relaxation mechanisms of 1-cyclohexyluracil, in vacuum and water, were investigated by employing the Multi-State CASPT2 (MS-CASPT2, Multi-State Complete Active-Space Second-Order Perturbation Theory) quantum chemical method and Dunning’s cc-pVDZ basis sets. In both environments, our results suggest that the primary photophysical event is the population of the bright state. Afterwards, two likely deactivation pathways can take place, which is sustained by linear interpolation in internal coordinates defined via Z-Matrix scans connecting the most important characteristic points. The first one (Route 1) is the same relaxation mechanism observed for uracil, its canonical analogue, i.e., internal conversion to the ground state through an ethylenic-like conical intersection. The other route (Route 2) is the direct population transfer from the bright state to the triplet state via an intersystem crossing process involving the (/) singlet-triplet crossing point. As the spin-orbit coupling is not too large in either environment, we propose that most of the electronic population initially on the state returns to the ground following the same ultrafast deactivation mechanism observed in uracil (Route 1), while a smaller percentage goes to the triplet manifold. The presence of a minimum on the potential energy hypersurface in water can help to understand why experimentally it is noticed suppression of the triplet states population in polar protic solvent.

A bis-pyrene chalcone based fluorescent material for ratiometric sensing of hydrazine: An acid/base molecular switch and solid-state emitter

In this work, we have designed and synthesized a new fluorescent molecular probe, DPY comprising of pyrene-diacetylpyridine conjugate, which was found to be sensitive to hydrazine as well as protonation. DPY is characterised by a strong emission both in solution (λem = 530 nm) as well as in solid state (λem = 610 nm), attributed to intramolecular charge-transfer. The probe responds to hydrazine with a ratiometric fluorescence emission change from yellow to blue, due to chalcone cyclisation reaction of α, β-unsaturated carbonyl group resulting in the pyrazoline compound, DPY-Hy, imparting a strong greenish-blue emission in solution. Further, the strong fluorescence emission of DPY in powder and thin film was quenched upon exposure to TFA, and revived upon exposure to TEA. For developing on-site detection protocol, when DPY was drop-casted on nonfluorescent silica plate a vivid naked-eye colour change from orange-red to dark blue was realized. Interestingly, in the aggregated state, DPY exhibited a broad range emission from green to orange in a mixed solvent system of THF:H2O. A plausible explanation of the photophysical events is substantiated with theoretical calculations.

Kinetics of photon upconversion by triplet-triplet annihilation: a comprehensive tutorial.

This perspective article provides a comprehensive but organized tutorial introduction of the kinetics related to photon upconversion (UC) by triplet-triplet annihilation (TTA) (TTA-UC). The field of TTA-UC is multi-disciplinary and rapidly growing with the involvement of researchers from diverse backgrounds. TTA-UC consists of a series of tangled photophysical processes, so a solid understanding of the kinetic features and consequences is important to develop and evaluate materials for TTA-UC. This tutorial starts with an introduction of the standard model of TTA-UC along with the assumptions used in the model. The essential concept of the spin statistics for TTA and how this concept is related to the singlet branching ratio, which directly affects the efficiency of UC, are then explained through step-by-step analyses. Using these foundations, solutions for the steady-state behaviors are derived, featuring the universal curve that describes the excitation intensity dependence of the UC quantum yield for any sample type. Various useful functions for analyzing experimental data are also introduced and summarized. The transient behaviors of TTA-UC are then discussed along with their equations, and the usefulness for analyzing transient experimental data is explained using examples. In this article, self-consistent derivations and relevant references are provided for an easy understanding of the advanced discussion and analyses.

Self-Assembly Directed Organization of Fullerene-Bisporphyrins into Supramolecular Donut Structures for Excited State Charge Stabilization

Supramolecular structures formed from intermolecular association of repeated units of redox- and photo-active molecular monomers, in solution or in bulk, are of greater significance due to their importance in biology performing various functions making life on earth possible. One such supramolecular organization is ‘photosynthetic antenna-reaction centre’ complex found in green plants and bacteria which is involved in converting sunlight into chemical energy. The core antenna complex in bacteria form concentric rings, LH1 and LH2. LH1 consists of 16 ab protomers with a 68 Å hole in the middle while the LH2 consists of 9 ab protomers. These work in synergy in terms of light capture, transport and generating charge separated states.Several covalent and supramolecular structures containing fullerenes have been described in literature, however, only a few examples where the components are either redox or photochemically amphoteric. This property becomes especially important in light energy harvesting where the presence of an electron donor-acceptor pair is a basic necessity with at least one of them being photo-active to promote photoinduced charge separation leading to the formation of radical ion-pairs. Here, we describe the synthesis, by tether-directed functionalization, of a [60]fullerene e -bisadduct carrying two Zn-porphyrins (see figure inset), its supramolecular organization, and photophysical events. Remarkably, the supramolecular assembly of the present bisporphyrin-C60 forms donuts-shape aggregates (see figure for AFM image), primarily via pi-pi type charge transfer interactions, as demonstrated by means of optical absorption and emission, variable temperature 1H NMR and AFM and supported by theoretical calculations. More importantly, upon photoexcitation, the supramolecular assembly generates long-lived charge separated states of ~1 ms lifetime due to electron/hole delocalization within the supramolecular structure. The unprecedented results of the present study demonstrate the success of supramolecular organization of donor-acceptor pairs in biomimetic light energy harvesting. Figure 1

Ethylene Diamine Functionalized Citrate-Capped Gold Nanoparticles for Metal-Enhanced Bioluminescence

Metal-enhanced bioluminescence (MEB) is a complex photophysical event which manifests itself in manifold luminescence enhancement and depends on many parameters. The parameters include the distance between the luminescent source and the nanomaterial surface, the size and shape of nanoparticles, localized surface plasmon resonance (LSPR) peaks of the nanomaterial and the dielectric constant of the surrounding medium. Studying distance-dependent MEB in the presence of linkers of specified length may provide new insights into luminescence enhancement. In this regard, the present investigation is aimed to understand the role of ethylene diamine as a potential linker in model systems for distance-dependent metal-enhanced bioluminescence (MEB) with gold nanoparticles (AuNPs). Four different types of AuNPs (AuNP1, AuNP2, AuNP3, AuNP4) were synthesized by varying the trisodium citrate (TC) and silver nitrate (AgNO3) concentrations with maximum absorbance values of 0.99, 1.24, 1.21 and 1.38 respectively and the corresponding LSPR peaks (λmax) of 520 nm, 535 nm, 525 nm, and 525 nm. Luminescence enhancement up to 1.44-fold was observed when ethylene diamine (ED) was used as a linker in the presence of 1-N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC), ATP and AuNP1. The sample consisting of FMN, EDC and AuNP1 showed 1.3-fold luminescence enhancement. It was noted that AuNPs synthesized using AgNO3 as an additional component did not enhance luminescence in all the investigations. The suggested technique of using linkers of predetermined lengths may also prove fairly effective for studying other parameters which can influence MEB and cause sensitivity enhancement of luminescence-based biosensors

Distributions of Potential and Contact-Induced Charges in Conventional Organic Photovoltaics.

The interfaces of dissimilar materials play central roles in photophysical events in organic photovoltaics (OPVs). Depth profiles of electrostatic potential and contact-induced charges determine the energy-level lineup of the frontier orbitals at electrode/organic and organic heterointerfaces. They are critical for the elementary processes in an OPV cell, such as generation and diffusion of free carriers. A simple electrostatic model describes the energetics in organic heterojunctions supported by an electrode, and experiments via photoelectron spectroscopy and the Kelvin probe method validate the potential distribution in the stacking direction of the device. A comparative study has clarified the significance of Fermi-level pinning and resulting electrostatic fields in determining the energy-level alignment. In this review, we discuss how parameters of device constituents affect the distributions of potential and the dark charges in conventional OPVs comprising metallophthalocyanine and C60 as donor and acceptor, respectively. The results of previous studies, together with additional numerical simulations, suggest that a number of the factors influence the depth profiles of the dark charge and potential, such as the work function of bottom materials, layer thickness, structural inhomogeneity at interfaces, top electrode, and stacking sequence.

Early photophysical events of a ruthenium(ii) molecular dyad capable of performing photochemical water oxidation and of its model compounds.

The early photophysical events occurring in the dinuclear metal complex [(ttb-terpy)(I)Ru(μ-dntpz)Ru(bpy)2]3+ (2; ttb-terpy = 4,4',4''-tri-tert-butyl-terpy; bpy = 2,2'-bipyridine; dntpz = 2,5-di-(1,8-dinaphthyrid-2-yl)pyrazine) - a species containing the chromophoric {(bpy)2Ru(μ-dntpz)}2+ subunit and the catalytic {(I)(ttb-terpy)Ru(μ-dntpz)}+ unit, already reported to be able to perform photocatalytic water oxidation - have been studied by ultrafast pump-probe spectroscopy in acetonitrile solution. The model species [Ru(bpy)2(dntpz)]2+ (1), [(bpy)2Ru(μ-dntpz)Ru(bpy)2]4+ (3), and [(ttb-terpy)(I)Ru((μ-dntpz)Ru[(ttb-terpy)(I)]2+ (4) have also been studied. For completeness, the absorption spectra, redox behavior of 1-4 and the spectroelectrochemistry of the dinuclear species 2-4 have been investigated. The usual 3MLCT (metal-to-ligand charge transfer) decay, characterized by relatively long lifetimes on the ns timescale, takes place in 1 and 3, whose lowest-energy level involves a {(bpy)2Ru(dntpz)}2+ unit, whereas for 2 and 4, whose lowest-energy excited state involves a 3MLCT centered on the {(I)(ttb-terpy)Ru(μ-dntpz)}+ subunit, the excited-state lifetimes are on the ps timescale, possibly involving population of a low-lying 3MC (metal-centered) level. Compound 2 also exhibits a fast process, with a time constant of 170 fs, which is attributed to intercomponent energy transfer from the MLCT state centered in the {(bpy)2Ru(μ-dntpz)}2+ unit to the MLCT state involving the {(I)(ttb-terpy)Ru(μ-dntpz)}+ unit. Both the intercomponent energy transfer and the MLCT-to-MC activation process take place from non-equilibrated MLCT states.

Complete reconstruction of bound and unbound electronic wavefunctions in two-photon double ionization

To be complete, the characterization of the photoionization process of atoms and molecules requires the extraction of all quantum-mechanical phases and amplitudes. So far, complete experiments have accessed only the ionization process of neutral atoms and molecules. Here we report the quantum-mechanically complete characterization of the single and double ionization of neon to yield doubly charged ions. The first ionization step by intense, polarized extreme ultraviolet light from a free-electron laser leaves the ion in a polarized state (that is, one in which the angular momentum of the ion is aligned in space). By controlling the polarization of the light, we determine the bound and continuum components of the system in the first and second ionization steps leading to the formation of doubly charged neon ions. We test the validity of our approach by characterizing the influence of autoionizing ionic states on the two-photon double-ionization mechanism. Our results are important for understanding the physics of the interaction of extreme ultraviolet radiation with ions. Photoionization is one of the most important photophysical events. This process can now be characterized in a quantum-mechanically complete manner by use of polarization-controlled extreme-ultraviolet light derived from a free-electron laser

Observation of Near-Threshold Resonances in the Flavin Chromophore Anions Alloxazine and Lumichrome.

Lumichrome (LC) is the chromophore of the flavin family of photoactive biomolecules, where key biochemical activity involves interplay between redox and photophysical events. Questions remain about the relationship between the redox status of the ground and excited states and demand an improved understanding of the intrinsic photochemistry. Using anion photodissociation spectroscopy, we have measured the intrinsic electronic spectroscopy (564-220 nm) and accompanying photodegradation pathways of the deprotonated anionic form of LC. Experiments were also performed on alloxazine (AL), which is equivalent to LC minus two methyl groups. We observe a resonance state close to 3.8 eV for both anions for the first time, which we tentatively assign to dipole-bound excited states. For AL this state is sufficiently long-lived to facilitate dissociative electron attachment. Our results suggest that the presence of methyl group rotors at key positions along the molecular dipole may reduce the lifetime of the resonance state and hence provide a structural barrier to valence electron capture, and ensuing molecular dissociation.

Photolytic Properties of Antivitamins B12.

Antivitamins B12 represent an important class of vitamin B12 analogues that have gained recent interest in several research areas. In particular, 4-ethylphenylcobalamin (EtPhCbl) and phenylethynylcobalamin (PhEtyCbl) exemplify two such antivitamins B12 which have been characterized structurally and chemically. From a spectroscopic point of view, EtPhCbl is photolabile with a very low quantum yield of photoproducts, while PhEtyCbl is incredibly photostable. Herein, DFT and TD-DFT computations are provided to explore the photolytic properties of these compounds to shed light on the electronic properties that are indicative of these differences. Potential energy surfaces (PESs) were constructed to investigate the mechanisms of photodissociation leading to radical pair (RP) formation and the mechanisms of deactivation to the ground state. The S1 PESs for each antimetabolite contain two energy minima, one being the metal-to-ligand charge transfer (MLCT) and another the ligand-field (LF) state. There are two possible pathways for photodissociation that can be identified for EtPhCbl but only one (path B) is energetically feasible and involves the lengthening of the Co-NIm bond through the MLCT region followed by the lengthening of the Co-C bond through the LF region. For PhEtyCbl, there is not an energetically favorable path for photolysis; rather, internal conversion (IC) is the significantly preferred photophysical event.