Intra- and intermolecular quenching of carbazole photoluminescence by imidazolidine radicals

Abstract

Mechanisms of carbazole photoluminescence quenching by the free and chemically bound nitroxyl radicals in the model bound system "carbazole (CBZ)-imidazolidine nitroxyl radical R • " were investigated and the photophysical properties of the system were studied and com pared with those of free CBZ and R • in solution. The quantum yield and lifetime of fluores cence from the local singlet excited state of the carbazole moiety in the bound CBZ-R • system is three orders of magnitude lower than in free CBZ. The lifetime of the local triplet excited state of the carbazole moiety in the bound system is shorter than 50 ns. The rate constants for intermolecular quenching of the singlet and triplet excited states of free CBZ by R • in acetoni trile were found to be (1.4±0.1)•10 10 and (1.5±0.2)•10 9 L mol -1 s -1 , respectively. The most plausible mechanisms of both free and covalently bound carbazole luminescence quenching by nitroxyl radicals are exchange energy transfer and acceleration of internal conversion due to electron exchange.Keywords: spin labeled luminophores, laser flash photolysis, fluorescence quenching.Spin catalysis of chemical reactions by paramagnetic species occurs in triads of paramagnetic species where spin conversion in a pair of species is initiated by the third electron spin. 1,2 This phenomenon was experimentally confirmed for a number of photochemical, biochemical, and radiation chemical systems. 3-5 The third spin plays the role of the source of external magnetic field in which a spin correlated radical pair created in some (e.g., radia tion chemical) way is placed. Systematic research on spin catalysis requires some model compounds comprising a paramagnetic fragment bound to a partner from the rad ical pair.In this connection, spin labeled luminophores are of particular interest. Theoretical calculations predict a num ber of interesting effects in radiation processes involving them. 6 However, in most cases the paramagnetic center acts as an efficient quencher of the luminescence of such compounds. 4,7-9 It should be noted that it was proposed to use the reverse effect, viz., the rise of the luminescence of fluorophore-nitroxide radical dyads due to reduction of the nitroxide group, for the design of sensors for the superoxide radical, antioxidants, hydrogen peroxide in bi ological systems, 10 and for alkyl radicals in polymers. 11The accelerated internal conversion proceeding by the electron exchange mechanism is the main quenching mechanism of the fluorescence of chemically bound sys tems "luminophore-nitroxide radical". 7,8 However, some other mechanisms of quenching in these systems were pro posed, e.g., acceleration of intersystem crossing (ISC) from the excited singlet to triplet state. 7 These assumptions are based on the fact that quenching of singlet states of lumi nophores by free (chemically unbound) nitroxide radicals (this process has been studied in more details) can follow all known channels including energy transfer, 12,13 elec tron transfer, 14 ISC, 15,16 and internal convers...

Mechanistic Aspects of Photoinduced Rearrangement of 2,3-Diarylcyclopentenone Bearing Benzene and Oxazole Moieties

“…The appropriate first-order rate constant of quenching determined as the reciprocal characteristic time τ 1 is equal to (4.0 ± 1.9) × 10 6 s –1 , which gives the rate constant of the triplet state quenching equal to k 5 = (1.7 ± 0.8) × 10 9 M –1 s –1 . This value is an order of magnitude less that the rate constant of diffusion-controlled reaction in acetonitrile at room temperature ( k D = 1.9 × 10 10 M –1 s –1 ) . The rate constants of ca.…”

“…The value of the O 2 concentration in air-saturated acetonitrile is 2.4 × 10 –3 M . The residual oxygen content of the solution after bubbling with high purity argon typically does not exceed 1% (note that the estimation performed in the original paper should be increased by an order of magnitude).…”

Kinetics and mechanism of photochromic transformations of a 2,3-diarylcyclopentenone

“…Assuming that this life time corresponds to the quenching of 1A triplet state by oxygen, one can calculate the rate constant of quenching to be (1.9 ± 0.2) × 10 9 L mol -1 s -1 . This value is one order of magnitude lower than the rate constant of diffusion controlled reaction in acetonitrile at room temperature (k D = 1.9 × 10 10 L mol -1 s -1 [25]). The rate constants of ca.…”

“…The probability of singlet oxygen formation in this process is between 0.25 and 1 [25,28]. The lifetime of ( 1 Δ g ) in acetonitrile was reported to be sensitizer dependent.…”

“…One of them is quenching by the residual oxygen. Taking into account the rate constant of quenching of triplet 1A, 1.9 × 10 9 L mol -1 s -1 (see above), and the fact that the residual oxygen content of the solution after bubbling with argon does not exceed 1% [25], one can esti mate the effective pseudo first order rate constant of this reaction by the value of 5 × 10 4 s -1 . Therefore, quenching by the residual oxygen could be responsi ble for ca.…”