Subpicosecond flash photolysis of chromium(III) coordination complexes

Abstract

87assume the dimers have no internal energy [Eint(2) = 01, then the internal energy of the remaining monomers can be. calculated from eq 8 as 0.85 X J/molecule (427 cm-I). On the other hand, the maximum value for Ein,(2) is Do = 196 cm-I; otherwise, the dimers would dissociate. With this value, Eint(l) = 0.68 X J/molecule (340 cm-I). In both these calculations, E,(l) has been taken to be equal to AHvap10 These values for the internal energy are within experimental error of that previously reported, (0.97 f 0.34) X J/molecule (488 cm-I), and the value calculated from the isenthalpic expansion equation, 1.15 X J/molecule (578 cm-I). Since the "negative" signal observed in these experiments decreases by 90% upon heating the nozzle to 370 K, the cluster percentage clearly decreases at higher temperatures. At 400 torr, the percentage dimer limits are between 1% and 20% at 370 K instead of 9. 9%-39% as calculated for a 290 K, 400 torr source. This makes it unlikely that the difference between the measured value of Eint and that calculated from the isenthalpic expansion equation, which became significant at elevated temperatures,I0 could be accounted for by the presence of dimers.In conclusion, a sensitive laser technique has shown that dimers are present in neat beams of SF, under conditions corresponding to nozzle pressures ranging from 200 to 1000 torr and nozzle temperatures of 290 to 370 K. The percent of dimer found, however, is relatively small and does not appear to significantly affect the results of previous work.Acknowledgment. The authors appreciate many helpful discussions of the present work with Professor R. B. Bernstein.Flash photolysis and transient spectroscopy of six Cr(II1) compounds in liquid solution at room temperature have achieved a time resolution of 1 ps, about an order of magnitude better than in previous studies. Measurements were carried out in water and in five different organic solvent with no observable differences in the results. Excitation was at 314, 585, and 628 nm; transient spectra were monitored from 340 to 750 nm. In contrast to previous results (all using excitation at 530 nm), transient absorption appeared in less than 1 ps in all cases. For one particular set of excitation conditions, there was evidence for a transient with a lifetime of 1-3 ps. This could be the initially populated quartet or another species along the decay coordinate; or it may be artifactual. Except for that phenomenon, all transients may be explained by the known absorption of the lowest doublet in each complex. IntroductionThe photochemistry and photophysics of Cr(II1) coordination compounds have been the focus of considerable study.1-2 The electronic ground state is a quartet. Ligand field theory explains that the energy gap to the lowest excited doublet is approximately constant, while the energy of the lowest excited quartet state is variable, determined by the ligand field strength. Photochemical and photophysical behavior depend critically upon the relative positioning of the lowest state of each man...

Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices

“…These observations suggest that the higher energy irradiation populates quartet Franck-Condon states at energies above the quartet/doublet crossing point such that quartet to doublet ISC effectively competes with vibrational cooling. This interpretation was supported by ps laser studies on several Cr(III) complexes that demonstrated a rise time for 2 E g formation shorter than the time response of the instrument (1-2 ps) [59,60]. More recent studies on Cr(acac) 3 indicate that k ISC > 1 × 10 13 s −1 , thus ISC from such Franck-Condon states is at least competitive with VR to the lowest quartet [54].…”

Photochemistry and Photophysics of Chromium(III) Complexes

“…Fluorescence or delayed fluorescence has been searched for in room-temperature solution studies by many workers, and only one report remains viable. Consequently, little is known about the energy of the thermalized quartet excited state in solution and the reported rise time for doublet phosphorescence, <1 ps, implies that this state may have only a fleeting existence . Regarding its chemical reactivity, however, doublet quenching experiments have clearly shown that photochemistry occurs from the quartet state as there is often an unquenchable component of reaction.…”

Photochemistry and Photophysics of Coordination Compounds

“…In the present work ) and in our earlier communication (Serpone 1984) we employed a solid-state Nd:Yag laser svstem (^30 ps pulses; 2-3 mj/pulse) ; photon flux per pulse ^ 10 16 -10 lS 7 photons/cm 2 , some 5-6 orders of magnitude greater than that used by Rojas (1986Rojas ( 1987 3,4,7, phen) revealed that excitation (355 nm) of all these complexes results in a fast emission component below 650 nm that decays in ^ 50 ps (Fig. 2a), and, with the exception of NN = 5-Phphen, 4,4'-Ph^bpy, and Cr(terpy) 2 3 + , a slow emission rise at ^ 700 nm.…”

“…1980,35b,554 (9) Strand, D. unpublished results (10) Schonherr, T., Linder, R.; Eyring, G. Z. Naturforsch.1983,38a,736 (11) are a few ps or less (Kirk 1976;Rojas 1986Rojas 1987Pyke 1978 ;LeSage 1983 ;Nicollin 1980). Recently, we reported (Serpone 1984) that 355-nm excitation of Cr(bpy) 3 and Cr(4,7-Me 2 phen) 3 3+ in 1M HCl aqueous media results in a grow-in of the luminescence from their 2 Tj_/ 2 E states with x(rise) ^ 10-20 ns, attributed to intersystem crossing from the quartet to the doublet manifolds.…”