Intersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different spin multiplicity. [1]
When an electron in a molecule with a singlet ground state is excited (via absorption of radiation) to a higher energy level, either an excited singlet state or an excited triplet state will form. Singlet state is a molecular electronic state such that all electron spins are paired. That is, the spin of the excited electron is still paired with the ground state electron (a pair of electrons in the same energy level must have opposite spins, per the Pauli exclusion principle). In a triplet state the excited electron is no longer paired with the ground state electron; that is, they are parallel (same spin). Since excitation to a triplet state involves an additional "forbidden" spin transition, it is less probable that a triplet state will form when the molecule absorbs radiation.
When a singlet state nonradiatively passes to a triplet state, or conversely a triplet transitions to a singlet, that process is known as intersystem crossing. In essence, the spin of the excited electron is reversed. The probability of this process occurring is more favorable when the vibrational levels of the two excited states overlap, since little or no energy must be gained or lost in the transition. As the spin/orbital interactions in such molecules are substantial and a change in spin is thus more favourable, intersystem crossing is most common in heavy-atom molecules (e.g. those containing iodine or bromine). This process is called "spin-orbit coupling". Simply-stated, it involves coupling of the electron spin with the orbital angular momentum of non-circular orbits. In addition, the presence of paramagnetic species in solution enhances intersystem crossing. [2]
The radiative decay from an excited triplet state back to a singlet state is known as phosphorescence. Since a transition in spin multiplicity occurs, phosphorescence is a manifestation of intersystem crossing. The time scale of intersystem crossing is on the order of 10−8 to 10−3 s, one of the slowest forms of relaxation. [3]
Once a metal complex undergoes metal-to-ligand charge transfer, the system can undergo intersystem crossing, which, in conjunction with the tunability of MLCT excitation energies, produces a long-lived intermediate whose energy can be adjusted by altering the ligands used in the complex. Another species can then react with the long-lived excited state via oxidation or reduction, thereby initiating a redox pathway via tunable photoexcitation. Complexes containing high atomic number d6 metal centers, such as Ru(II) and Ir(III), are commonly used for such applications due to them favoring intersystem crossing as a result of their more intense spin-orbit coupling. [4]
Complexes that have access to d orbitals are able to access spin multiplicities besides the singlet and triplet states, as some complexes have orbitals of similar or degenerate energies so that it is energetically favorable for electrons to be unpaired. It is possible then for a single complex to undergo multiple intersystem crossings, which is the case in light-induced excited spin-state trapping (LIESST), where, at low temperatures, a low-spin complex can be irradiated and undergo two instances of intersystem crossing. For Fe(II) complexes, the first intersystem crossing occurs from the singlet to the triplet state, which is then followed by intersystem crossing between the triplet and the quintet state. At low temperatures, the low-spin state is favored, but the quintet state is unable to relax back to the low-spin ground state due to their differences in zero-point energy and metal-ligand bond length. The reverse process is also possible for cases such as [Fe(ptz)6](BF4)2, but the singlet state is not fully regenerated, as the energy needed to excite the quintet ground state to the necessary excited state to undergo intersystem crossing to the triplet state overlaps with multiple bands corresponding to excitations of the singlet state that lead back to the quintet state. [5]
Fluorescence microscopy relies upon fluorescent compounds, or fluorophores, in order to image biological systems. Since fluorescence and phosphorescence are competitive methods of relaxation, a fluorophore that undergoes intersystem crossing to the triplet excited state no longer fluoresces and instead remains in the triplet excited state, which has a relatively long lifetime, before phosphorescing and relaxing back to the singlet ground state so that it may continue to undergo repeated excitation and fluorescence. This process in which fluorophores temporarily do not fluoresce is called blinking. While in the triplet excited state, the fluorophore may undergo photobleaching, a process in which the fluorophore reacts with another species in the system, which can lead to the loss of the fluorescent characteristic of the fluorophore. [6]
In order to regulate these processes dependent upon the triplet state, the rate of intersystem crossing can be adjusted to either favor or disfavor formation of the triplet state. Fluorescent biomarkers, including both quantum dots and fluorescent proteins, are often optimized in order to maximize quantum yield and intensity of fluorescent signal, which in part is accomplished by decreasing the rate of intersystem crossing. Methods of adjusting the rate of intersystem crossing include the addition of Mn2+ to the system, which increases the rate of intersystem crossing for rhodamine and cyanine dyes. [7] The changing of the metal that is a part of the photosensitizer groups bound to CdTe quantum dots can also affect rate of intersystem crossing, as the use of a heavier metal can cause intersystem crossing to be favored due to the heavy atom effect. [8]
The viability of organometallic polymers in bulk heterojunction organic solar cells has been investigated due to their donor capability. The efficiency of charge separation at the donor-acceptor interface can be improved through the use of heavy metals, as their increased spin-orbit coupling promotes the formation of the triplet MLCT excited state, which could improve exciton diffusion length and reduce the probability of recombination due to the extended lifespan of the spin-forbidden excited state. By improving the efficiency of charge separation step of the bulk heterojunction solar cell mechanism, the power conversion efficiency also improves. Improved charge separation efficiency has been shown to be a result of the formation of the triplet excited state in some conjugated platinum-acetylide polymers. However, as the size of the conjugated system increases, the increased conjugation reduces the impact of the heavy atom effect and instead makes the polymer more efficient due to the increased conjugation reducing the bandgap. [9]
In 1933, Aleksander Jabłoński published his conclusion that the extended lifetime of phosphorescence was due to a metastable excited state at an energy lower than the state first achieved upon excitation. Based upon this research, Gilbert Lewis and coworkers, during their investigation of organic molecule luminescence in the 1940s, concluded that this metastable energy state corresponded to the triplet electron configuration. The triplet state was confirmed by Lewis via application of a magnetic field to the excited phosphor, as only the metastable state would have a long enough lifetime to be analyzed and the phosphor would have only responded if it was paramagnetic due to it having at least one unpaired electron. Their proposed pathway of phosphorescence included the forbidden spin transition occurring when the potential energy curves of the singlet excited state and the triplet excited state crossed, from which the term intersystem crossing arose. [10]
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, than the absorbed radiation. A perceptible example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the electromagnetic spectrum, while the emitted light is in the visible region; this gives the fluorescent substance a distinct color that can only be seen when exposed to UV light. Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.
Photoluminescence is light emission from any form of matter after the absorption of photons. It is one of many forms of luminescence and is initiated by photoexcitation, hence the prefix photo-. Following excitation, various relaxation processes typically occur in which other photons are re-radiated. Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors up to milliseconds for phosphoresence processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours.
Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed.
Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet, visible light (400–750 nm) or infrared radiation (750–2500 nm).
Scintillation is a flash of light produced in a transparent material by the passage of a particle. See scintillator and scintillation counter for practical applications.
CIDNP, often pronounced like "kidnip", is a nuclear magnetic resonance (NMR) technique that is used to study chemical reactions that involve radicals. It detects the non-Boltzmann (non-thermal) nuclear spin state distribution produced in these reactions as enhanced absorption or emission signals.
Internal conversion is a transition from a higher to a lower electronic state in a molecule or atom. It is sometimes called "radiationless de-excitation", because no photons are emitted. It differs from intersystem crossing in that, while both are radiationless methods of de-excitation, the molecular spin state for internal conversion remains the same, whereas it changes for intersystem crossing. The energy of the electronically excited state is given off to vibrational modes of the molecule. The excitation energy is transformed into heat.
The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.
Phosphorescent organic light-emitting diodes (PHOLED) are a type of organic light-emitting diode (OLED) that use the principle of phosphorescence to obtain higher internal efficiencies than fluorescent OLEDs. This technology is currently under development by many industrial and academic research groups.
Photosensitizers produce a physicochemical change in a neighboring molecule by either donating an electron to the substrate or by abstracting a hydrogen atom from the substrate. At the end of this process, the photosensitizer eventually returns to its ground state, where it remains chemically intact until the photosensitizer absorbs more light. This means that the photosensitizer remains unchanged before and after the energetic exchange, much like heterogeneous photocatalysis. One branch of chemistry which frequently utilizes photosensitizers is polymer chemistry, using photosensitizers in reactions such as photopolymerization, photocrosslinking, and photodegradation. Photosensitizers are also used to generate prolonged excited electronic states in organic molecules with uses in photocatalysis, photon upconversion and photodynamic therapy. Generally, photosensitizers absorb electromagnetic radiation consisting of infrared radiation, visible light radiation, and ultraviolet radiation and transfer absorbed energy into neighboring molecules. This absorption of light is made possible by photosensitizers' large de-localized π-systems, which lowers the energy of HOMO and LUMO orbitals to promote photoexcitation. While many photosensitizers are organic or organometallic compounds, there are also examples of using semiconductor quantum dots as photosensitizers.
Quenching refers to any process which decreases the fluorescence intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching. As a consequence, quenching is often heavily dependent on pressure and temperature. Molecular oxygen, iodide ions and acrylamide are common chemical quenchers. The chloride ion is a well known quencher for quinine fluorescence. Quenching poses a problem for non-instant spectroscopic methods, such as laser-induced fluorescence.
Mostafa A. El-Sayed is an Egyptian-American physical chemist, a leading nanoscience researcher, a member of the National Academy of Sciences and a US National Medal of Science laureate. He was the editor-in-chief of the Journal of Physical Chemistry during a critical period of growth. He is also known for the spectroscopy rule named after him, the El-Sayed rule.
In molecular spectroscopy, a Jablonski diagram is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity. Nonradiative transitions are indicated by squiggly arrows and radiative transitions by straight arrows. The vibrational ground states of each electronic state are indicated with thick lines, the higher vibrational states with thinner lines. The diagram is named after the Polish physicist Aleksander Jabłoński.
In spectroscopy and quantum chemistry, the multiplicity of an energy level is defined as 2S+1, where S is the total spin angular momentum. States with multiplicity 1, 2, 3, 4, 5 are respectively called singlets, doublets, triplets, quartets and quintets.
The di-pi-methane rearrangement is a photochemical reaction of a molecular entity that contains two π-systems separated by a saturated carbon atom, to form an ene- substituted cyclopropane. The rearrangement reaction formally amounts to a 1,2 shift of one ene group or the aryl group and bond formation between the lateral carbons of the non-migrating moiety.
Fluorescence is used in the life sciences generally as a non-destructive way of tracking or analysing biological molecules. Some proteins or small molecules in cells are naturally fluorescent, which is called intrinsic fluorescence or autofluorescence. Alternatively, specific or general proteins, nucleic acids, lipids or small molecules can be "labelled" with an extrinsic fluorophore, a fluorescent dye which can be a small molecule, protein or quantum dot. Several techniques exist to exploit additional properties of fluorophores, such as fluorescence resonance energy transfer, where the energy is passed non-radiatively to a particular neighbouring dye, allowing proximity or protein activation to be detected; another is the change in properties, such as intensity, of certain dyes depending on their environment allowing their use in structural studies.
9-Fluorenylidene is an aryl carbene derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state. For this reason, fluorenylidene has been studied extensively in organic chemistry.
Photoredox catalysis is a branch of photochemistry that uses single-electron transfer. Photoredox catalysts are generally drawn from three classes of materials: transition-metal complexes, organic dyes, and semiconductors. While organic photoredox catalysts were dominant throughout the 1990s and early 2000s, soluble transition-metal complexes are more commonly used today.
Triplet-triplet annihilation (TTA) is an energy transfer mechanism between two molecules in their triplet state, and is related to the Dexter energy transfer mechanism. If triplet-triplet annihilation occurs between two molecules in their excited states one molecule transfers its excited state energy to the second molecule, resulting in one molecule returning to its ground state and the second molecule being promoted to a higher excited singlet, triplet or quintet state. Triplet-triplet annihilation was first discovered in the 1960s to explain the observation of delayed fluorescence in anthracene derivatives.
Thermally activated delayed fluorescence (TADF) is a process through which a molecular species in a non-emitting excited state can incorporate surrounding thermal energy to change states and only then undergo light emission. The TADF process involves an excited molecular species in a triplet state, which commonly has a forbidden transition to the ground state termed phosphorescence. By absorbing nearby thermal energy the triplet state can undergo reverse intersystem crossing (RISC) converting it to a singlet state, which can then de-excite to the ground state and emit light in a process termed fluorescence. Along with fluorescent and phosphorescent compounds, TADF compounds are one of the three main light-emitting materials used in organic light-emitting diodes (OLEDs).