Karl H. Grellmann

Dual phosphorescence from 2-(2′-hydroxyphenyl)benzoxazole due to keto-enol tautomerism in the metastable triplet state.

Abstract The photoexcitation of liquid solutions of 2-(2′-hydroxyphenyl)benzoxazole (HBO) in alkanes leads to a dual phosphorescence which can be assigned to the keto and enol forms of HBO. The keto-enol tautomerism in the metastable triplet state of HBO is practically independent of temperature. The keto and enol bands of the phosphorescence from HBO decay monoexponentially with the same rate constant. The experimental data on the phosphorescence from HBO are consistent with those on the transient absorption of HBO. At high triplet concentration and low solvent viscosity, a delayed fluorescence from HBO due to triplet-triplet annihilation is also observed.

Intramolecular proton transfer and tunnel effects in the metastable triplet states of 2-(2'-hydroxyphenyl) benzoxazole studied by micro- and nanosecond laser flash photolysis

Abstract The enol-keto tautomerism of 2-(2′-hydrophenyl)benzoxazole (HBO) in its metastable triplet states has been studied in fluid, non-polar solution. After laser flash excitation the keto triplet state is populated with high efficiency. The establishment of a ketoenol equilibrium was observed between 55 and 220 K. At low temperatures the rate of this reaction is determined by tunnel effects. The barrier height of the proton transfer is in the triplet state much larger than in the singlet state.

TRIPLET-STATE FORMATION AND CIS-]TRANS ISOMERIZATION IN THE EXCITED SINGLET-STATE OF THE KETO TAUTOMER OF 2-(2'-HYDROXYPHENYL)BENZOTHIAZOLE

Abstract Flash-excitation of degassed solutions of 2-(2′-hydroxyphenyl)benzothiazole (HBT) in an inert solvent leads to the metastable triplet state of HBT and to a cis → trans isomerization of the HBT keto tautomer. Both processes proceed from the first excited singlet state of the HBT keto tautomer, 1 K*. The trans-keto tautomer is not formed below 150 K, whereas the triplet yield increases at lower temperatures, like the fluorescence yield. Besides fluorescence, intersystem crossing and cis → trans isomerization, an additional deactivation channel of 1 K* is proposed in order to explain the different temperature dependencies of the quantum yields of fluorescence and cis → trans isomerization. It is suggested that in the singlet ground state the keto-trans isomer decays by a second-order reaction to the enol form, 1 E, by mutual hydrogen exchange: 2 1 K tr → 2 1 E tr → 2 1 E cis .

Flash-photolysis of 2-(2'-hydroxyphenyl)-3-H-indole. Ground-state keto-enol tautomerization by mutual hydrogen exchange and by proton catalysis

Abstract Efficient excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxy-5′-methylphenyl)-3,3-dimethyl-3H-indole (HBC) leads to the formation of an unstable trans-keto tautomer 1 K tr whose transient absorption can be observed in flash experiments. The assignment of this transient absorption to the trans-keto tautomer is corroborated by the observation that a similar transient is not formed from BHBC, which has been synthesized as a bridged, rotationally blocked counterpart of HBC. In hydrocarbon solvents, two 1 K tr molecules revert by a mutual hydrogen-exchange reaction to the stable enol form of HBC. This second-order reaction is diffusion-controlled and its rate constant is proportional to T /η over a wide range of the solvent viscosity η. In the presence of alcohols (ROH) a first-order, pseudo-monomolecular proton-catalysed re-enolization reaction competes with the second-order reaction. With methanol and ethanol as proton donors it is shown that two donor molecules are involved in this process. The rate of the proton-catalysed reaction increases with decreasing temperatures. A pre-equilibrium between a nonreactive 1 K tr and a reactive 1 K tr ::: (ROH) 2 complex is the reason for this unusual temperature dependence of the first-order 1 K tr decay rate.

Photoisomerizations of the photochromic anil salicylidene-1-naphthylamine in 3-methylpentane

Abstract The results of a nanosecond flash-photolysis study with solutions of salicylidene-1-naphthyl-amine (SN) in 3-methylpentane (3MP) at ambient and at low temperatures are reported. Besides a newly discovered triplet state, two photochromic transients (PCTs) are formed after excited state intramolecular proton-transfer (ESIPT) has taken place. Above 140 K the formation of a quinoid trans-keto tautomer (1Ktr) prevails. At 100 K a second PCT is observed in steady-state experiments which we tentatively assign to a twisted conformer of the keto form of SN. The transient 1Ktr re-enolizes in dry 3MP by second-order double proton-transfer; in the presence of 10−4 M methanol the re-enolization of 1Ktr is proton catalyzed and the lifetime of 1Ktr is shortened by more than two orders of magnitude. The properties of the flexible SN are compared with two other rotationally hindered ‘ESIPT molecules’.

Experimental studies of hydrogen tunnelling. Utilization of large isotope effects in a mechanistic study of a hydrogen shift reaction

Abstract The kinetics of a hydrogen shift reaction was investigated. The process is governed by large tunnel contributions and takes place during the rearrangement of a light-generated, transient hexahydrocarbazole into a stable hexahydrocarbazole. Large isotope effects were observed when the migrating hydrogen was replaced by deuterium. By appropriate labelling we could show that a [1,4] hydrogen shift takes place and that two combined [1,2] hydrogen shifts can be ruled out as a (theoretically possible) reaction route.

Tunnel effects on inter- and intramolecular hydrogen transfer reactions of transient dihydro- and hexahydrocarbazoles

Abstract We examine inter- and intramolecular hydrogen-transfer processes in two related metastable dihydrocarbazoles in nonpolar solvents of different viscosity and compare them with similar transfer processes in transient hexahydrocarbazoles. N-ethyldiphenylamine (A′) and N-ethyl-2,6-dimethyldiphenylamine (A) photocyclize in their triplet states, yielding the triplet states of the zwitterionic dihydrocarbazoles 3 Z′* and 3 Z*, respectively, which subsequently relax to their metastable singlet ground states 1 Z′ and 1 Z′. In spite of their similarity, the two transients 1 Z′ and 1 Z stabilize by completely different pathways: the unsubstituted transient 1 Z′ is converted into N-ethylcarbazole (C) and an N-ethyltetrahydrocarbazole (THC) by a bimolecular disproportionation reaction. The methylsubstituted intermediate 1 Z is converted into a stable dihydrocarbazole (D) by a sigmatropic, intramolecular [1,8]-H-shift and by an intermolecular, mutual hydrogen-exchange reaction within the encounter complex 1 (ZZ) which yields two molecules of D. The rates of the intra- and of the intermolecular transfer reaction of 1 Z are governed by tunnel effects. The rate of the intramolecular tunnel process does not depend on solvent friction and becomes temperature independent at low temperatures. The rate of the intermolecular, reaction-controlled exchange reaction 1 (ZZ) → 2 1 D becomes also temperature independent if the solvent is fluid enough. In more viscous solvents the reaction becomes diffusion controlled and, therefore, strongly temperature dependent. The intermolecular disproportionation reaction 2 1 Z′ → C + THC is also reaction controlled but no tunnel effects are observed.