R.P.H. Rettschnick

Fluorescence from the second excited singlets of pyrene and 3,4-benzpyrene

Abstract The fluorescence from the second excited singlet state of pyrene and 3,4-benzpyrene in the vapour phase has been observed. The effect of inert gas, temperature and excitation frequency has been investigated. The estimated rate constants for the internal conversion and the reverse process for pyrene are 2 × 10 12 s −1 and 10 7 s −1 respectively.

Vibronic coupling and radiative transitions

Abstract A dynamical treatment of vibration-induced radiative transitions is given by considering the relevant electronic wavefunction to be dependent on both the position and the velocity of the nuclei along some specific (antisymmetrical) vibrational co-ordinate.

Studies on Triplet Radiative Lifetimes, Phosphorescence, and Delayed Fluorescence Yields of Aromatic Hydrocarbons in Liquid Solutions

The phosphorescence of phenanthrene, pyrene, and naphthalene in fluid solutions has been studied. The radiative lifetimes are calculated from the quantum yields and actual lifetimes and vary only slightly or not at all with temperature within the experimental error. For phenanthrene, pyrene, naphthalene, and anthracene the radiative lifetimes at 77°K in ethanol are 25, 30, 75, and 180 sec, respectively. The temperature dependence and the absolute value of the triplet–triplet annihilation efficiency vary from molecule to molecule. The results are discussed in terms of a re‐encounter association of the annihilation partners to form an excimer.

Monomer and excimer phosphoresence of phenanthrene and naphthalene in solution

Abstract The phosphoresence spectra of phenanthrene and naphthalene have been measured in liquid ethanol solutions. Both spectra are superpositions of a structured and a broad structureless component. The structured component is ascribed to the triplet monomer, the structureless one to the triplet excimer phosphorescence emission. Decay constants for the two emissions have been measured as a function of the temperature.

Determination of the barrier to intramolecular exciplex formation in a jet-cooled, bichromophoric molecule

The emissive properties of a bichromophor molecule (1) are reported. This contains an anilino group as an electron donor (D) and a 1-cyanonaphthalene group as an electron acceptor (A) interconnected by a saturated hydrocarbon bridge of limited flexibility, which holds D and A far apart in the electronic ground state. The emission spectrum of 1, both in solution and in the gas phase, indicates that quantitative formation of an intramolecular exciplex between D and A occurs. This exciplex formation was studied as a function of excitation energy in molecules of 1 isolated in a supersonic free jet. A barrier of 1700 ± 200 cm−1 was found between the Franck-Condon excited conformation and the conformation of the exciplex. Although this value is significantly higher than that reported earlier for exciplex formation between chromophores connected by a simple polymethylene chain (≈ 900 cm−1) it is much lower than the barrier predicted for folding the bridge in 1 sufficiently to bring D and A in close contact. A tentative explanation of this discrepancy is given.

Time resolved mode‐to‐mode vibrational energy flow in the isolated s‐tetrazine argon complex

This paper reports the first direct observation of intramolecular vibrational redistribution (IVR) where two consecutive steps have been distinguished. The initially excited vibronic state ∼(6b2) of the van der Waals complex of s‐tetrazine and argon changes into the intermediate state ∼(16a116b1) from which subsequently the final state ∼(16a2) originates. The temporal intensity profiles of selectively detected emission from each of these states of the complex have been measured on a picosecond time scale.

Long-range, photoinduced charge separation in solvent-free donor—bridge—acceptor molecules

Abstract Charge separation in isolated supermolecules, consisting of an aniline or 4-methoxy aniline donor and a cyanonaphtalene acceptor connected by a semirigid bridge, is described in terms of irreversible sequential intramolecular relaxation involving electron transfer from an electronically excited molecular state and electrostatically driven folding. Sequential folding of the charge transfer state extended the energy range of this state, which is accessible by radiationless electron transfer. The analysis of the intramolecular electron transfer dynamics led to agreement between the distance scale for charge separation inferred from energetic and structural data and accounted for the excess energy dependence of the nuclear Franck—Condon vibrational overlap factors.

A dramatic effect of the donor ionization potential on the apparent barrier to intramolecular exciplex formation in jet-cooled, bichromophoric molecules

Abstract The fluorescence of three nearly identical donor-bridge-acceptor molecules was studied under jet-cooled conditions. These compounds contain a 1-cyanonaphthalene group as an electron acceptor (A) and an anilino derivative as an electron donor (D). Their only difference is in the substituent on the para position of the anilinogroup and therefore in ionization potential (IP). D and A are interconnected by a saturated hydrocarbon bridge of limited flexibility, which holds D and A far apart in the electronic ground state. Nevertheless, an intramolecular exciplex is formed upon excitation of the cyanonaphthalene group with sufficient excess vibrational energy. The barrier to this exciplex formation appears to be much lower than the barrier predicted for folding the bridge to bring D and A in close contact and moreover, the molecule with the lowest IP has by far the lowest barrier. This is taken as conclusive evidence that long-range electron transfer is the first step in the exciplex formation and determines the height of the barrier. The subsequent folding of the bridge would then be a result of the Coulombic attraction forces between D and A.

Intermediate strong coupling and vibrational redistribution effects in the S2 emission of pyrene

Abstract Upon excitation of pyrene in the third excited singlet state normal (τ = 75 nsec) and hot band (τ = 15 nsec) fluorescence of the second excited singlet has been observed. Both emissions exhibit intermediate strong coupling characteristics while the lifetime of the latter is also affected by a vibrational redistribution process.

The role of the triplet state in depopulation of the electronically excited proton-transferring system [2,2′-bipyridyl]-3,3′-diol

Abstract Electronic excitation of [2,2′-bipyridyl]-3,3′-diol (BP(OH) 2 ) generates the diketo-tautomeric form of the system. The long-lived transient absorption (TA) of BP(OH) 2 was assigned as the T-T absorption of the diketo-tautomer. The effect of the stimulated emission pumping on TA excludes the essential contribution of the absorption from the ground-state diketo form. The rate constants of T-T annihilation and of oxygen quenching are k TTA = 2.4 × 10 10 M −1 s −1 and k q O 2 = (2–3) × 10 9 M −1 s −1 . The contribution of the second-order process to the overall decay of the triplet BP (OH) 2 , and the origin of the delayed fluorescence of the diketo form, are discussed.