Probe molecules singlet excited state

Figure 3-23 illustrates the basic scheme involved in a typical pump-probe experiment. First, molecules are excited from So (singlet ground state) to Si (singlet excited state) by a pump laser of frequency Vo. Molecules excited to Si undergo nonradiative decay (intersystem crossing) to 7) (triplet state). Since the pump pulse width is much narrower than the lifetime of the 7) state (milli microseconds), excitation to the Si state by a pump laser... 

In all the examples quoted until now, molecules with singlet excited states were used as probes, with relatively short lifetimes, usually in the nanosecond time range. 

The electronic excited state is inherently unstable and can decay back to the ground state in various ways, some of which involve (re-)emission of a photon, which leads to luminescence phenomena (fluorescence, phosphorescence, and chemiluminescence) (22). Some biologic molecules are naturally fluorescent, and phosphorescence is a common property of many marine and other organisms. (Fluorescence is photon emission caused by an electronic transition to ground state from an excited singlet state and is usually quite rapid. Phosphorescence is a much longer-lived process that involves formally forbidden transitions from electronic triplet states of a molecule.) Fluorescence measurement techniques can be extremely sensitive, and the use of fluorescent probes or dyes is now widespread in biomolecular analysis. For example, the large increase in fluorescence... 

In the present report we describe new time-resolved measurements of solvation dynamics using the probe molecule coumarin 153 (hereafter referred to as C153) [8]. This well-known solvation probe has been thoroughly characterized in a number of past studies [2,9]. Its main attribute which makes it an excellent solvation probe is that upon electronic excitation its dipole moment changes from a ground state value of -6 D to a value of -IS D in 5, [10]. Furthermore, its S state is well separated fiom other excited singlet states and there is no indication of the occurrence of any excited-state reaction in most solvents [8,9]. 

Another example of intramolecular CT complex formation is provided by trans-4-dimethvlamino-4 -(1-oxobutvl)stilbene Solvent effects on the spectrum give a value of 22D for the excited state dipole moment. The effect of electric field on the fluorescence of 4-(9-anthry1)-N.N.-2.3,5,G-hexamethy1-aniline shows this compound forms an excited state whose dipole moment does not change with solvent . Chiral discrimination in exciplex formation between 1-dipyrenylamine and chiral amines is very weak . In the probe molecule PRODAN (6-propionyl)-2-(dimethylamino)—naphthalene the initially formed excited state converts to a lower CT state as directly evidenced by time-resolved spectra in n-butanol. Rate constants for intramolecular electron transfer have been measured in both singlet and triplet states of covalently porphyrin-amide-quinone molecules . Intramolecular excimer formation occurs during the lifetime of the excited state of bis-(naphthalene)hydrazides which are used as photochemical deactivators of metals in polyethylene . ... 

The earliest studies of excited-state proton reactions were concerned with the measurement of excited-state pK values both in the singlet and the triplet manifold. These studies are thoroughly reviewed in Refs. 13 and 18. Since the excited singlet state of most aromatic molecules lasts only a few nanoseconds, it was difficult to measure directly the dynamics of intermolecular proton transfer. Instead, as outlined in the introduction, steady-state measurements were used. Picosecond spectroscopy has made it possible to probe in real time the intermolecular transfer process. 

Excited singlet lifetimes, also called fluorescence lifetimes, of organic molecules are normally smaller than 10 ns. Notable exceptions are polycyclic aromatic hydrocarbons, such as pyrene and naphthalene. Due to their short lifetimes, fluorescent probes can only explore a small volume therefore, competition between the probe s decay to the ground state and dissociation from the supramolecular structure to the homogeneous phase occurs only infrequently. For example, in order to compete with the decay of the excited state, a probe with a 10 ns excited state lifetime must possess a dissociation rate constant from the supramolecular structure that is larger than 10 s. For this reason, fluorescent probes are normally assumed not to relocate during their lifetimes hence, explore a very limited volume. 

Case 2 Decay of excited states is much faster than the dynamic processes. This situation is most frequently encountered for excited singlet states, and the condition applies when [H] and kp are much smaller than Aq and k. The system can be viewed as having two populations of excited probe molecules which do not interchange during the excited state lifetime. Thus, no information on the dynamics of the probe can be obtained by directly following the decay of the excited states rather, each population of probe reports on their respective environments. 

In the case of DNA, no relevant quenching data for excited singlet probes have been reported. The fact that no extensive studies have been done with organic excited singlet states probably reflects the fact that nucleotides are efficient quenchers. However, inorganic probe molecules have been employed [146-151]. 

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