Perylene, excitation transfer

Two typical dye molecules. The europium complex (a) transfers absorbed light to excited-state levels of the complexed Eu , from which lasing occurs. The perylene molecule (b) converts incident radiation into a triplet state, which decays slowly and so allows lasing to occur. 

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

Chemiluminescence also occurs during electrolysis of mixtures of DPACI2 99 and rubrene or perylene In the case of rubrene the chemiluminescence matches the fluorescence of the latter at the reduction potential of rubrene radical anion formation ( — 1.4 V) at —1.9 V, the reduction potential of DPA radical anion, a mixed emission is observed consisting of rubrene and DPA fluorescence. Similar results were obtained with the dibromide 100 and DPA and/or rubrene. An energy-transfer mechanism from excited DPA to rubrene could not be detected under the reaction conditions (see also 154>). There seems to be no explanation yet as to why, in mixtures of halides like DPACI2 and aromatic hydrocarbons, electrogenerated chemiluminescence always stems from that hydrocarbon which is most easily reduced. A great number of aryl and alkyl halides is reported to exhibit this type of rather efficient chemiluminescence 155>. 

The main features of the chemiluminescence mechanism are exemplarily illustrated in Scheme 11 for the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of imidazole (IMI-H) as base catalyst and the chemiluminescent activators (ACT) anthracene, 9,10-diphenylanthracene, 2,5-diphenyloxazole, perylene and rubrene. In this mechanism, the replacement of the phenolic substituents in TCPO by IMI-H constitutes the slow step, whereas the nucleophilic attack of hydrogen peroxide on the intermediary l,l -oxalyl diimidazole (ODI) is fast. This rate difference is manifested by a two-exponential behavior of the chemiluminescence kinetics. The observed dependence of the chemiexcitation yield on the electrochemical characteristics of the activator has been rationalized in terms of the intermolecular CIEEL mechanism (Scheme 12), in which the free-energy balance for the electron back-transfer (BET) determines whether the singlet-excited activator, the species responsible for the light emission, is formed ... 

The peroxyoxalate system is the only intermolecular chemiluminescent reaction presumably involving the (71EEL sequence (Scheme 44), which shows high singlet excitation yields (4>s), as confirmed independently by several authors Moreover, Stevani and coworkers reported a correlation between the singlet quantum yields, extrapolated to infinite activator concentrations (4> ), and the free energy involved in back electron-transfer (AG bet), as well as between the catalytic electron-transfer/deactivation rate constants ratio, ln( cAx( i3), and E j2° (see Section V). A linear correlation of ln( cAx( i3) and E /2° was obtained for the peroxyoxalate reaction with TCPO and H2O2 catalyzed by imidazole and for the imidazole-catalyzed reaction of 57, both in the presence of five activators commonly used in CIEEL studies (anthracene, DPA, PPO, perylene and rubrene). A further confirmation of the validity of the CIEEL mechanism in the excitation step of... 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

For the excited singlet state of rhodamine as product state the free energy plot of the reverse electron transfer from the reduced dye to the hole (dashed curve in Fig. 31) is a mirror image to the free energy plot of the forward reaction relative to AG° =0. We immediately see from Figs. 31 and 32 that this reverse reaction is very fast at phenanthrene and slower at chrysene. It is still slower at anthracene and extremely slow at perylene. At phenanthrene this reverse reaction can compete with the dissociation of the hole from the reduced dye as is borne out by the recombination controlled current in this system (Fig. 27). 

Therefore, we have developed a pump/pump-probe experiment to obtain more informations on the structures of these geminate ion pairs. It allows the investigation of the excited states dynamics of the transient species at different time delays after photo-triggering the charge transfer, by monitoring the ground state recovery (GSR) of those transient species (Fig. lb). In the present study, we have used perylene (Pe) as fluorescer (electron donor) and either trans-l,2-dicyanoethylene (DCE) or 1,4-dicyanobenzene (DCB) as quencher (electron acceptor) in acetonitrile (ACN). 

If the reduction potential of the BX compound is too negative compared to the A compound, the ability of AT to transfer an electron to BX may be enhanced by exciting AT photochemically. Thus pyrene anion radical reacts very slowly with m-chlorotoluene, but shining green light on the red anion-radical makes the reaction proceed very fast (Fig. 5).32,33 Another possiblity is to use the dianion thus perylene anion-radical does not react with 1,4-dichlorobenzene, whereas it is rapidly reduced by perylene dianion.25 (Fig. 6). 

Fig. 12. Perrin quenching radii, R, [33J vs. variations of the free energy, - AG°, of electron transfer from the excited donor molecule to the acceptor molecule for donor-acceptor pairs in vitreous /nms-l,5-decalindiol. 1, Rubrene + A/ AT-diethylamline (DEA) 2, rubrene + N,N,-Ar,Ar-tetramethyl-p-phenylenediamine (TMPD) 3, rubrene + tetrakis(dimethylaminoethy-lene) 4, tetracene + DEA 5, tetracene + TMPD 6, 9,10-dinaphthylanthracene + DEA 7, 9,10-dinaphthylanthracene + TMPD 8, perylene + DEA 9, perylene + TMPD 10, 9-methylanthracene + TMPD 11, 9,10-diphenylanthracene + TMPD 12, coronene + TMPD 13, benzo[ Ai jperylene + TMPD 14, fluoranthene + DEA 15, acridine + DEA.

In several cases, dependent on the donor, the electron transfer triplet energy transfer from the triplet state of the fullerenes to the donor was observed. For example, excitation of C6o/perylene (Pe) mixtures leads to 3Pe and C6o in a fast reaction ((1.4 0.1) X 109 M 1 s-1). The electron transfer from Pe to 3C o occurs with a rate one-third of triplet energy transfer [127]. Ito et al. investigated the photoexcitation of mixed system of C6o and (3-carotene [141], They observed triplet energy transfer from 3C o to (3-carotene in polar as well as in nonpolar solvents besides electron transfer from (3-carotene to 3C o However, the electron transfer rate constant increases with solvent polarity while the energy transfer is only less effected by the change of solvent polarity (Table 5). 

Further work used a similar system to inhibit the formation of a second ion pair completely, using the electric field of an initial ion pair. In compound 14, Zn3PN and 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide (pyr-PMI) are the electron donors, while NI and PI are once again electron acceptors.11701 Photoinduced electron transfer from Zn3PN to PI with 416 nm laser pulses occurs with t = 27 ps however, if a 645 nm laser pulse is used to excite pyr-PMI first, this event is completely inhibited. 

Figure 5.4, one can easily understand why the interfacial electron transfer should take place in the 10-100 fsec range because this ET process should be faster than the photo-luminescence of the dye molecules and energy transfer between the molecules. Recently Zimmermann et al. [58] have employed the 20 fsec laser pulses to study the ET dynamics in the DTB-Pe/TiC>2 system and for comparison, they have also studied the excited-state dynamics of free perylene in toluene solution. Limited by the 20 fsec pulse-duration, from the uncertainty principle, they can only observe the vibrational coherences (i.e., vibrational wave packets) of low-frequency modes (see Figure 5.5). Six significant modes, 275, 360, 420, 460, 500 and 625 cm-1, have been resolved from the Fourier transform spectra of ultrashort pulse measurements. The Fourier transform spectrum has also been compared with the Raman spectrum. A good agreement can be seen (Figure 5.5). For detail of the analysis of the quantum beat, refer to Figures 5.5-5.7 of Zimmermann et al. s paper [58], These modes should play an important role not only in ET dynamics or excited-state dynamics, but also in absorption spectra. Therefore, the steady state absorption spectra of DTB-Pe, both in...

I. Introduction Excited-State Intramolecular H-Atom Transfer in Hypericin-Like Perylene Quinones... 

I. INTRODUCTION EXCITED-STATE INTRAMOLECULAR H-ATOM TRANSFER IN HYPERICINLIKE PERYLENE QUINONES... 

As noted above, a possible objection to our assignment of the excited-state reaction to H-atom transfer in these perylene quinone systems is the observation of mirror image symmetry between the absorption and the emission spectra, which indicates minimal structural changes between the absorbing and the emitting species. Our first attempt to explain this symmetry was to suggest that the... 

If subsequent experiments do indeed demonstrate that excited-state H-atom transfer does not occur in hypomycin B, then one may draw the conclusion that multiple transfers (either concerted or stepwise) must occur in these pery-lene quinones and that by frustrating the process in one half of the molecule, the process in the other half is impeded. At this point, such reasoning is speculative and contrary to the growing body of evidence provided by theory and experiment. As indicated above, quantum mechanical calculations indicate that the double-H-atom transfer in hypericin [67] and in the perylene quinone nucleus [75] of hypocrellin is energetically unfavorable compared to the single-transfer event. Experiments for hypericin in which one half of the molecule cannot participate in H-atom transfer owing to protonation of the carbonyl group (or even perhaps complexation with a metal ion) [76] also indicate that the transfer process can still occur. 

As we have discussed in depth elsewhere, despite the similarities in the structures of hypericin and hypocrellin, which are centered about the perylene quinone nucleus, their excited-state photophysics exhibit rich and varied behavior. The H-atom transfer is characterized by a wide range of time constants, which in certain cases exhibit deuterium isotope effects and solvent dependence. Of particular interest is that the shortest time constant we have observed for the H-atom transfer is 10 ps. This is exceptionally long for such a process, 100 fs being expected when the solute H atom does not hydrogen bond to the solvent [62]. That the transfer time is so long in the perylene quinones has been attributed to the identification of the reaction coordinate with skeletal motions of the molecule [48, 50]. 

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