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Light, Life, and Measurement
Published in Thomas M. Nordlund, Peter M. Hoffmann, Quantitative Understanding of Biosystems, 2019
Thomas M. Nordlund, Peter M. Hoffmann
The rate kisc in Figure 9.15 is the “intersystem crossing” rate. The ground state of organic molecules typically has total electronic spin equal to zero. The set of spin 0 electronic states is called the singlet system or singlet manifold of states. We noted before that since photons carry one unit of angular momentum, photon absorption normally requires a change of one unit of electronic orbital angular momentum during the excitation process. ΔL = ±1 is the first selection rule for transitions involving photon absorption or emission. The “intersystem crossing” from singlet states to triplet (S = 1) states is to a triplet state of equal energy, if it is available. If not, crossing can still occur, but some thermal, vibrational energy will have to be found to simultaneously make up any energy difference, making the transition less likely (slower).
Review of Nanoscale Spectroscopy in Medicine
Published in Sarhan M. Musa, Nanoscale Spectroscopy with Applications, 2018
Chintha C. Handapangoda, Saeid Nahavandi, Malin Premaratne
Radiative relaxation includes fluorescence (Vo-Dinh and Cullum 2003) and phosphorescence (Apreleva et al. 2006). Nonradiative relaxation involves many small collisional relaxations and tiny temperature rises of surrounding species. The ground state of most molecules involves paired electrons with a total spin equaling to zero. These states are called singlet states, and they are labeled S0, S1, S2, and so on, in the order of increasing energy (Prasad 2003). A state with a net spin value (i.e., unpaired electrons) is called a triplet state and is denoted by T. Figure 11.4 shows the typical spin arrangement in molecular orbitals of the ground state and excited singlet and triplet states (Vo-Dinh and Cullum 2003). As depicted in Figure 11.4, the excitation of an electron from a paired electron pair of a molecule whose ground state is S0 can produce either a state where the two electrons are still paired (a singlet state) or a state where the two electrons are unpaired (a triplet state) (Prasad 2003).
Environmental Photochemistry
Published in Richard A. Larson, Eric J. Weber, Reaction Mechanisms in Environmental Organic Chemistry, 2018
Richard A. Larson, Eric J. Weber
The triplet state, having spin-unpaired electrons, differs chemically in its chemical properties from the singlet state. Triplet states are far more likely to take part in chemical reactions than singlets are. Their lifetimes are usually many orders of magnitude greater, giving them a much higher probability of encountering another species with which it can react. Photochemical reactions of triplet species take many forms, some of which will be discussed under the reactions of individual compounds later in the chapter.
The role of the bulky blocking unit of the fluorescent emitter in efficient green hyper-fluorescent organic light-emitting diodes
Published in Journal of Information Display, 2021
Fluorescent organic light-emitting diodes (OLEDs) have been used for several decades because of their good stability [1–3]. However, they have limited internal quantum efficiency because only singlet excitons can be used for light emission [4]. To overcome such limitation by utilizing all generated excitons, thermally activated delayed fluorescence (TADF) emitters emerged to harvest the triplet excitons using the small singlet–triplet energy gap [5–8]. The small energy gap between the singlet state and the triplet state in the TADF materials can induce reverse intersystem crossing from the triplet state to the singlet state. Therefore, the TADF materials can achieve 100% internal quantum efficiency. Although the TADF materials have the merit of quantum efficiency, they do not have good color purity and good emission ability due to the torsion between their donor and their acceptor [9].
Performance of density functional theory and orbital-optimised second-order perturbation theory methods for geometries and singlet–triplet state splittings of aryl-carbenes
Published in Molecular Physics, 2020
Reza Ghafarian Shirazi, Dimitrios A. Pantazis, Frank Neese
Carbenes are highly reactive organic molecules where a neutral carbon atom has two electrons less than an octet structure [1,2]. In the language of valence bond theory, carbene centres adopt sp2 hybridisation and hence are bent. Their electronic structure and spin state multiplicity can be described in a simplified manner by assuming two electrons to be distributed in two nearly degenerate p and sp2 orbitals (Figure 1). Formation of parallel spins in this configuration results in a high-spin triplet state (S = 1), while pairing of the orbitals in the sp2 orbital leads to a singlet state (S = 0). The triplet and singlet carbenes are significantly different species both structurally and electronically. Most carbenes adopt a triplet ground state, but singlet ground state carbenes can also form, or spin-state equilibria established, through interaction with proper substituents and steric restrictions [3–6]. The significant difference in electron distribution between the two spin states results in distinctly different chemical properties and reactivity. For instance, the often higher-lying singlet state participates in reactions rather than the lower-lying triplet state. Therefore, knowledge of accurate singlet–triplet energy gaps are of high importance in carbene chemistry.
Analysis of time-resolved EPR spectra observed in the photolysis of disulphonated anthraquinones included in cyclodextrins – evidence of the S-T_ mixing of radical pair mechanism
Published in Molecular Physics, 2019
Figure 2(a) shows the image of the induction of spin polarisation in the triplet precursor case and predictable CIDEP spectrum by a typical mixing shown in Figure 1. Here, an RP is assumed to be composed of a main radical (R1) having one nuclear spin (I = 1/2) and an HFC constant of a1, and another counter radical (R2) with many small HFC constants . Here, the sign of the HFC constant (a1) of R1 is assumed to be positive. To complete the mixing, we have to handle the strongly interacting RP by the exchange interaction J (2|J| ∼ gµBB). Here, the population of every triplet sub-state is assumed to be equivalent; in other words, the initial spin polarisation conveyed from the excited triplet state is quenched before the interaction. There are two possible conversions; electron spin (βe) of R2 is inverted along with simultaneous inversion of the nuclear spin (αN) of R1 having a large HFC constant of ‘a1’; that is, the flip-flop of the electron spin of R2 and the nuclear spin of R1 takes place as shown in the figure. Another conversion is simultaneous flip-flop of the electron spin and the nuclear spin only in R1. Accordingly, population change of R1 and R2 is easily obtained as shown in the figure. Therefore, we can simulate the EPR spectrum of mixing as shown in the bottom part of Figure 2(a). The spectrum is composed of one lowest emissive hyperfine line of R1 and an emissive spectrum of R2 at g = 2.00, but the highest hyperfine line of R1 does not appear. In the case of negative HFC constant, only the highest one of R1 appears along with that of R2 (not shown here). This means that the hyperfine structure induced by mixing is sensitive to the sign of the HFC constants; in other words, we can get one of the exclusive information (sign of the HFC constant) to determine the structure of intermediate radical species.