Hiroshi Kokubun

Fluorescence yields of azulene derivatives

Fluorescence yields of a number of azulene derivatives were determined in cyclohexane at room temperature. It was found that the yield decreases with the decrease of the energy gap, ΔE, between the first and the second excited states and that the rate constant of radiationless transition increases exponentially with decreasing ΔE.

Nonradiative relaxation processes of the higher excited triplet states of anthracenes studied by a double excitation method

Abstract Intersystem crossing from higher excited triplet states, T n ( n ⩾ 2), to the lowest excited singlet state, S 1 , has been studied by a newly devised double excitation method. The quantum yields of T n ⇌ S 1 intersystem crossing of anthracene, 9-methylanthracene, 9-phenylanthracene, 9,10-dichloroanthracene and 9,10-dibromoanthracene are determined to be 2.6 × 10 −5 , 3.6 × 10 −4 , 4.7 × 10 −4 , 1.5 × 10 −2 and 2.7 × 10 −1 , respectively. The large increases in quantum yields of meso-derivatives are interpreted in terms of level inversion of T 2 and S 1 with substitution at the meso-position. Internal heavy atom effects are observed in T 2 → S 1 intersystem crossing. In comparison with T n ← S 1 intersystem crossing the rate of T 2 → T 1 internal conversion is estimated to be 10 11 s −1 for all compounds studied.

Quenching of excited 2,5-diphenyloxazole by CCl4

Abstract The interactions between the ground and excited states of 2,5-diphenyloxazole and CCl4 were investigated in cyclohexane at 23°C. Distinct complex formation was not observed either in the ground or in the excited states. The quantum yields of the fluorescene and intersystem crossing were determined to be 0.85 and 0.12 respectively. The fluorescence is quenched and the photobleaching is extremely enhanced by the addition of CCl4. The excited singlet and triplet states are deactivated by CCl4 with rate constants kq = 2.0 × 109 M−1 s−1 and kqt = 7.8 × 103 M−1 s−1 respectively. Using the emmision-absorption flash technique it was found that the fluorescence quenching by CCl4 is caused by the photochemical reaction (kr), intersystem crossing k′ST and physical quenching kd with the relative rates kr:k′ST:kd = 1:0.4:21. The deactivation of the triplet by CCl4 is the result of physical quenching.

Fluorescence of the antharacenes following Tn → T1 excitation studied by a double excitation method

Abstract The fluorescence from the lowest excited singlet state following excitation of the lowest triplet state was observed for anthracene, 9-methylanthracene, and 9-phenylanthracene in ethanol by a newly devised double excitation method which is essentially the combination of flash and laser photolysis. The quantum yield of intersystem crossing from the excited triplet state, Tn(n ⩾ 2), to the lowest excited singlet state was markedly increased by methyl- and phenyl-substitution at the meso-position.

Temperature effect on inverse intersystem crossing of anthracenes

Abstract The temperature and heavy-atom effects on the quantum yield Φ TS of the inverse (T n → S 1 ) intersystem crossing of several anthracenes are studied by the stepwise two-photon excitation technique. The first effect is explained in terms of the relative positions of the S 1 and T 2 levels and the second effect in terms of the spin—orbit coupling between S 1 and T 2 . It was confirmed that the T n → S 1 inverse intersystem crossing occurs from T 2 to S 1 after rapid T n → T 2 internal conversion.

The Electron Transfer Reaction between Triplet Methylene Blue and Aromatic Compounds

The electron transfer reaction from aromatic compounds to méthylène blue in the triplet state has been studied in acetonitrile by flash photolysis. The relationship between the rate constant of the reaction in the triplet state and the free enthalpy change involved was investigated. For a positive free enthalpy change, the electron transfer reaction does not occur, in contrast to what has been observed in the excited singlet state.

The hydrogen bonding effect of pyridine on the deactivation of the excited states of acridone

Abstract The interaction between the ground and excited acridone and pyridine has been investigated by an emission-absorption flash and a laser technique. The hydrogen bonding equilibrium was observed in the ground state (K = 4.5 M−1 at 22°C). The triplet acridone interacts with pyridine (rate constant, 8 × 108 M−1 s−1) to produce the hydrogen-bonded triplet which decays with the rate constant of 3.3 × 106 s−1 in benzene, yielding the radical which is stable even in the presence of oxygen. The fluorescence yield and the life- time increase considerably by the hydrogen bond formation with pyridine. In pyridine where all of acridone seems to be hydrogen-bonded, the radical formation from the singlet excited state was not observed. Since ΦST + Φf ⋍1 in benzene and 0.37 in pyridine, it was concluded that the deactivation other than fluorescence and intersystem crossing is enhanced by the hydrogen bond formation.

The photoenolization mechanism of 2-methylbenzo-phenone

Abstract The photoenolization of 2-methylbenzophenone was investigated by conventional and laser flash photolysis. Two kinds of transient absorption were distinctly observed in the ethanol and the dioxane solutions. The short lived transient was assigned to the cis-dienol and the long lived transient to the trans-dienol. The Stern—Volmer plots for the yields of these dienols gave different quenching constants when naphthalene was used as a triplet quencher. It was concluded that the cis-dienol is produced from the n-Π* triplet state of 2-methylbenzophenone with s-cis conformation and the trans-dienol from the triplet state with s-trans conformation.

Protolytic reactions of acridine in the triplet state

Abstract The triplet—triplet (T—T) absorption spectrum of the acridinium ion (λmax=490 nm) was measured in acidic aqueous solution at pH 1.0 by means of the triplet energy transfer from the disodium salt of naphthalene disulphonic acid to the acridinium ion. The triplet acridinium ion decays by a first order process with a rate constant of 1.5 × 103 s−1 at a pH below about 3.8. The pKa of acridine in the triplet state was determined to be 5.6 at 23°C from the measurements of T—T absorption spectra of both the acridine molecule and the acridinium ion. The rate constants for protonation and deprotonation in the triplet state were determined to be (3.4 – 3.8) × 1010 M−1 s−1 and (8.5 – 9.5) × 104 s−1 respectively.

Determination of the S*—T transition probabilities of some acridine dyes and of thiopyronine by the method of sensitized delayed fluorescence

Abstract Parkers method of delayed fluorescence as a means of determining the S * —T transition probability Φ ST was extended to systems in which reversible triplet energy transfer occurs between donor and acceptor. The Φ ST values of proflavine, acriflavine and acridine orange in ethanol were determined by this extended method using anthracene or 9-methylanthracene as an acceptor and eosine as a reference donor. The Φ ST values thus obtained are 0.45 ± 0.05 (proflavine), 0.53 ± 0.07 (acriflavine) and 0.30 ± 0.05 (acridine orange). The Φ ST values of eosine and thiopyronine were also determined by the method in Parkers original form using perylene as an acceptor and erythrosine as a reference donor. The Φ ST values obtained, 0.44 ± 0.01 (eosine) and 0.24 (thiopyronine), agree satisfactorily with those obtained by a chemical method, 0.43 ± 0.04 (eosine) and 0.28 ± 0.03 (thiopyronine).