Triplet excited state absorption

Two-photon excited fluorescence detection at the single-molecule level has been demonstrated for chromophores in cryogenic solids [M], room-temperature surfaces [61], membranes [62] and liquids [63, M and 65]- Although multiphoton excited fluorescence has been embraced with great enthusiasm as a technique for both ordinary confocal microscopy and single-molecule detection, it is not a panacea in particular, photochemical degradation in multiphoton excitation may be more severe than with ordinary linear excitation, probably due to absorption of more than the desired number of photons from the intense laser pulse (e.g. triplet excited state absorption) [61],... 

These polynuclear gold(I) phosphine complexes have been found to possess rich photochemistry. The phosphorescence of 49 is found to be quenched by a series of energy acceptors such as trans- and cis-stilbene, styrene, hept-l-ene, and cyclohexene [129], The transient absorption difference spectrum of a degassed acetonitrile solution of 49 and irans-stilbene displays absorption bands at about 365 and 390 nm, typical of the triplet excited state absorption spectrum of trans-stilbene, indicating the energy transfer nature of the quenching mechanism, Be-... 

Capellos and Suryanarayanan (Ref 28) described a ruby laser nanosecond flash photolysis system to study the chemical reactivity of electrically excited state of aromatic nitrocompds. The system was capable of recording absorption spectra of transient species with half-lives in the range of 20 nanoseconds (20 x lO sec) to 1 millisecond (1 O 3sec). Kinetic data pertaining to the lifetime of electronically excited states could be recorded by following the transient absorption as a function of time. Preliminary data on the spectroscopic and kinetic behavior of 1,4-dinitronaphthalene triplet excited state were obtained with this equipment... 

From the pulse radiolysis of DMSO solutions of naphthalene and the absorption of the naphthalene triplets (e = 2.26 x 104 M "1 cm 1) Hayon23 found that the yield of triplet excited states in irradiated DMSO is G = 0.57 and G = 0.36 for DMSO saturated with argon gas or N20 gas, respectively. 

The use of emission (fluorescence and phosphorescence) as welt as absorption spectroscopy. From these spectra the presence of as well as the energy and lifetime of singlet and triplet excited states can often be calculated. 

A-T ax are the spectral maximum of UV-Vis absorption, emission, and triplet state absorption, respectively, is the molar extinction coefficient, ta and Tt, Oa and r, s and Et, are the lifetime, quantum yield, and energy content of the singlet and triplet excited states of RF, respectively. 

Table 1. Lifetime of the first triplet excited state of 1-8 in air-equilibrated solutions determined by transient absorption at room temperatme [31] ...

Transient absorption spectra of the CeoN cluster (C6oN" ) -MePH system following laser excitation at 355 nm indicate that the photoinduced intermolecular electron-transfer from the triplet excited state of PH to the QqN cluster (CfioN) occurs as shown in Figure 15.9a. 

Anthraquinones are nearly perfect sensitizers for the one-electron oxidation of DNA. They absorb light in the near-UV spectral region (350 nm) where DNA is essentially transparent. This permits excitation of the quinone without the simultaneous absorption of light by DNA, which would confuse chemical and mechanistic analyses. Absorption of a photon by an anthraquinone molecule initially generates a singlet excited state however, intersystem crossing is rapid and a triplet state of the anthraquinone is normally formed within a few picoseconds of excitation, see Fig. 1 [11]. Application of the Weller equation indicates that both the singlet and the triplet excited states of anthraquinones are capable of the exothermic one-electron oxidation of any of the four DNA bases to form the anthraquinone radical anion (AQ ) and a base radical cation (B+ ). 

Table 9 Properties of T-T absorption bands and reaction rates of bithiophene derivatives in triplet excited state...

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

Nanosecond laser flash photolysis was applied to study excited-state 2-nitrothiophene in polar and non-polar solvents76 the transient absorption at 545 5 nm was assigned to its lowest triplet state. The rate constants of the interaction of this triplet excited state, with a number of substrates such as cyanide and hydroxide ions, have been determined77. Similarly, the transient absorption at 490 5 nm was assigned to the lowest triplet excited state of 5-nitro-2-furoic acid78, and that at 500 5 nm to that of 7V-(n-butyl)-5-nitro-2-furamide79. 

In contrast to the case of ZnCh-Ceo, no transient formation of Ceo" was detected at 1000 nm for any other dyad in Scheme 4b [65]. In each case, only the triplet-triplet absorption due to the chlorin or porphyrin moiety was observed due to the higher energy of the radical ion pair as compared to the triplet excited state as is expected from the redox potentials. Thus, the energy level of the radical ion pair in reference to the triplet energy of a component is an important factor in determining the lifetime of the radical ion pair. 

The transient absorption formed from laser flash photolysis of 15 was not quenched in oxygen-saturated solutions, presumably because the photoenolization to form 16 takes place from a singlet excited state of 15. Furthermore, Schworer and Wirz did not observe any transient absorption that can be attributed to the formation of the triplet excited state of 15 from direct irradiation. In contrast. Hurley and Testa used energy transfer to estimate that the triplet excited state of 15 is formed in 67% yield,whereas Takezaki et al. have measured the yields for forming the triplet excited state of 15 to be slightly higher or above 80%. They estimated the lifetime of the lowest triplet excited state of 15 to be 350 ps in ethanol. 

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