Crystal phosphorescence excitation

Let us note that there is a small gap of about 30 cm-i between the O —0" band observed in phosphorescence excitation and phosphorescence emission (Fig. 20). Such an effect is typical for crystals where emission occurs generally from traps that lie 20—50 cm i below the host level. The phosphorescence spectrum is dominated by the strong 0 - 0" band, an indication that there is little geometry change between So and T. ... 

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

The excitation levels of semiconductors (semi-insulators) do not extend to the vacuum level of zero energy. However, due to the crystal or crystal like structure of the material, electrons excited into one of the excitation bands may travel within the band throughout the crystal. These excited electrons may be de-excited at dislocations in the crystal, at edges of the crystal or by fluorescence and phosphorescence. 

The emission spectra from powders and crystal suspensions of the aromatic amino acids, however, are quite different from those shown in Figs. 3 and 4. The fluorescence and phosphorescence maxima of the powders are red shifted in each case by up to 50 nm and the phosphorescence lifetimes at 77 K are found to be 1.5 sec for tryptophan, 0.4 sec for t5Tosine and 0.5 sec for phenylalanine The phosphorescence lifetimes decrease markedly as the sample temperature increases In the powders the phosphorescence to fluorescence ratios are considerably smaller than in frozen aqueous solutions. The crystal suspensions at room temperature show even greater red shifts in the phosphorescence maxima, with no fine structure observed and with lifetimes on the order of 0.2 sec for each aromatic amino acid >. Although these results along with the phosphorescence excitation spectra bear little resemblence to those expected for the aromatic amino acids, Bogach et al. point out the similarities to phosphorescence and excitation spectra of photoproducts formed in solutions of the aromatic amino acids at low temperatures... 

Free Bp is fluorescent in solution although its quantum yield is low. However, cocrystal 1 does not fluoresce or phosphoresce because the two phenyls in Bp are not co-planar. Compared to 1, cocrystals 2-6 emit strong phosphorescence, and their phosphorescence excitation and emission characteristics are listed in Table 5. On one hand, the introduction of haloperfluorobenzenes into the crystal dilutes the concentration of phosphors (Nap, Phe, and Pyr), thus preventing luminescence from concentration- or aggregation-quenching. On the other hand, the strong spin-orbital coupling of heavy atom iodine/bromine makes the forbidden Si-Ti and Ti-Sq transitions possible. 

The water lattice may be an important element in forming the ordered thymine structure necessary for dimerization, as pointed out by Beukers and Berends.37 Thymine can crystallize from solution as a monohydrate (a real hydrate)38 in whose crystal lattice one thymine is directly above another. The influence of humidity upon dimer yield in dry films may be connected with monohydrate formation, and monohydrate formation in frozen solutions may be the reason for the almost theoretically maximum quantum yields for dimer formation.31 The possible existence of aggregates in frozen aqueous solutions is supported by a tenfold increase in purine phosphorescence at 44°K produced by the presence of 1% ethanol and by a blue shift of excitation and emission spectra.39... 

Spectroscopic evidence has also been adduced for the ability of the anthracene T2 to transfer energy to other substances.239 Selective excitation of guest anthracene in a host dibenzofuran crystal also containing napthalene-d8 as a guest resulted in naphthalene phosphorescence. As shown in Figure 8, the path of the energy is S0 -> Sj excitation of anthracene, intersystem crossing... 

The experiment which unequivocally demonstrated that the metastable phosphorescent state of organic compounds is an excited triplet was performed by Lewis and Calvin in 1945.3 They actually measured the paramagnetic susceptibility of fluorescein dissolved in boric acid crystals under intense illumination. 

Hutchison reported the first ESR spectrum of a metastable phosphorescent state by study of naphthalene oriented in durene crystals.4 Since then, similar spectra have been recorded for several other polynuclear aromatics both oriented in host crystals and randomly suspended in glassy matrices. D values for all these ir,n excited states are quite low, indicating little interaction between the unpaired electrons. Interestingly, D for the quinolines equals 0.10cm"1 just as in naphthalene,197 indicating that the presence of a heteroatom does not necessarily change the ir,w nature of the lowest triplet state very much. A similar conclusion has been reached from a comparison of the ESR spectra of fluorene, carbazole, dibenzofuran, and dibenzothiophene.198... 

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

The H(D) atom abstraction rate constants in durene crystals by the impurity molecules quinoline, isoquinoline, quinoxaline, and quinozaline in their excited triplet state were measured by Hoshi et al. [1990] using the phosphorescence method described above. The transfer occurs in the fragment CH N formed by a methyl group of durene and a nitrogen atom of the impurity molecule. In the interval 300-100 K the activation energy drops from 3.5 kcal/mol to 1.6 kcal/mol. Deuteration reduces the... 

The optical absorption, measurable via the excitation spectra of secondary emissions (delayed fluorescence,143 phosphorescence,142 EPR triplet spectra,166 etc.). The absorption is the response of the crystal to an excitation spatially homogeneous at the microscopic scale ... 

Fig. 18. Schematic of apparatus used to measure fluorescence kinetics with a streak camera. The Nd glass laser emits a train of one hundred 1.06 pm pulses separated by 6 ns. A single pulse in the earlier portion of the train is selected by a Pockels cell and crossed polarizers (Pi and P2). The high voltage pulse ( 5 ns) at the Pockels cell is supplied by a laser triggered spark gap and a charged line. The single pulse ( 8 ps, 109 W) can be amplified. The second harmonic is generated from a phase matched KDP crystal. Beam splitters provide two side beams beam (1) triggers the streak camera beam (2) arriving at the streak camera at an earlier time acts as a calibrating pulse. The main 0.53 pm beam excites the sample for fluorescence measurement. The fluorescence collected with f/1.25 optics is focused into the 30 pm slit of the streak camera. The streak produced at the phosphorescent screen is recorded by an optical multichannel analyzer. (After ref. 67.)...

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

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