Intersystem Crossing and Internal Conversion of Acridine
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Intersystem Crossing and Internal Conversion Quantum Yields of Acridine in Polar and Nonpolar Solvents Arlette Kellmann Laboratoire de Photophysique Mol6culaire du CNRS, Universit6 de Paris Sud, 9 1405 Orsay, France (Received December 6, 1976) Publication costs assisted by CNRS
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The present work reports measurements of S T intersystem crossing quantum yields of acridine in polar and nonpolar solvents, using the third harmonic of a Nd-glass laser. The intersystem crossing yield was determined by comparing the triplet formation of acridine with that of anthracene in ethanol, used as a reference. The quantum yields of acridine in benzene, tert-butyl alcohol, and water were found to be 0.73, 0.61, and 0.39, respectively. These data combined with the fluorescence yields show the existence of internal conversion from the first excited singlet state of acridine in the three solvents. The results indicate a strong solvent effect for the rate constants of the radiationless transitions. No significant deuterium effect was observed on the quantum yields.
Introduction The radiative and radiationless processes in aromatic molecules have been extensively studied during the last years. The nitrogen-heterocyclic compounds are of particular interest owing to the presence of n,a* states.' Aza and diaza analogues of benzene and naphthalene have been the subject of a number of experimental and theoretical However, little quantitative information is available concerning the nonradiative transitions (intersystem crossing and internal conversion) in acridine, the aza analogue of anthracene. In this paper we report an experimental determination by use of a laser of the singlet triplet intersystem crossing quantum yields (aisc) for acridine in polar and nonpolar solvents (water, tert-butyl alcohol, benzene). These systems were chosen for study, as their photochemistry5-'' and emission properties'&18are fairly well known. Acridine (Ac) irradiated in hydrogen-donating solvents abstracts, via an excited state, a hydrogen from the solvent, giving the acridanyl radical (AcH-). This radical leads to acridan (AcH,), substituted acridan, and to diacridan (AczHz). The nature of the excited state involved in the photochemical reaction has been d i s c u ~ s e d , l and ~ - ~the ~ more recent studies suggest that a n,a* singlet state is the reactive state."'15 In contrast to anthracene, acridine does not show fluorescence in hydrocarbons at room temperature, but fluoresces slightly in alcohols and strongly in ~ater;'~t''there is no doubt that the fluorescent state is (r,x)*in nature.18 The solvent dependence of the fluorescence of several polycyclic monoazines has been interpreted as involving the interchange of the lowest n,a* singlet state in a hydrocarbon solvent to a r,a* singlet in a hydroxylic solvent, due to hydrogen bonding.lg The s ectroscopic work of Coppens and co-workers on acridineZ P suggests that the n,n* singlet state is slightly higher than the r,a*state in polar solvents, in agreement with some other theoretical and experimental s t ~ d i e s ; ' ~ however, ~ ' ~ ~ ' ~Whitten ~ ~ ~ and Lee'' proposed a lower n,r* state irrespective of the solvent polarity. Indeed, even in the absence of precise information about the relative position of T,T* and n,r* singlet states, it can be assumed that these two states, being very close, are strongly mixed by vibronic coupling irres ective of the order of the levels. It has been emphasizedBa that vibronic coupling may be important in the deactivation via internal conversion of the first excited singlet state of nitrogenheterocyclic compounds. Indeed our present results on acridine lead to the conclusion of the existence of an
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internal conversion from the lowest excited singlet state to the ground state.
Experimental Section Materials. Acridine (Eastman Kodak) was recrystallized three times from an ethanol-water mixture after refluxing with active carbon. Acridine-dg(Merck Sharp and Dohme) was purified by vacuum sublimation. Anthracene was an Aldrich Gold Label product. Benzene, ethanol, and tert-butyl alcohol (Merck Uvasol) were used without further purification. The aqueous solutions were prepared from twice distilled deionized water. The solutions were made basic using 0.01 M NaOH. Samples for flash and laser photolysis were degassed by several cycles of the freeze-pump-thaw procedure; after each pumping, argon was introduced above the solution. Apparatus and Method. The conventional flash apparatus used has been previously described in The excitation light source of the laser flash spectroscopy setup is a Nd3+doped glass laser (C.G.E., Model VD 231) emitting at 1058 nm. The third harmonic (353 nm), generated by means of KDP crystals, was used in the present work at energy of a few mJ (1-20 mJ); the pulse half-width was 30 ns. The laser beam was projected on one side of a 10-mm square silica cell with polished sides containing the sample. A frosted plate of silica was placed close to the cell, to homogenize the laser beam. The analyzing light, a xenon flash (with a maximum remaining constant for more than 20 ps after the laser pulse), crossed the optical cell at 90° to the laser beam, through a 2-mm wide section of the irradiated part of the solution, close to the laser entrance window; the monitoring light was then focused on the slit of a monochromator (Jarrell-Ash f = 0.25 m, band width 2 nm). The transient signal monitored by a photomultiplier tube (Radiotechnique 150 UVP) was displayed on a dual-beam oscilloscope (Fairchild Model 777). Relative values of the laser energy were obtained by measuring a small fraction of the UV laser beam deflected on a calibrated photodiode (ITT 4000 S) and its integrated photocurrent was displayed on the oscilloscope (from these measurements a linear correction of the transient optical densities was applied for variations in the laser energy). The oscillograms were recorded photographically; Figure 1 shows an example of oscilloscope traces of triplets of acridine and anthracene, and of the relative values of the energy laser. The Journal of Physlcal Chemistry, Voi. 8 1 , No. 12, 1977
Arlene Kelhnann
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TABLE I: Intersystem Crossing and Internal Conversion Quantum Yields Molecule
Solvent
Benzene
A crid in e Acridine-d, Acridine Acridine Anthracene
Reference 17.
Benzene tert-BuOH Water, 0.01 M NaOH EtOH
Reference 34.
e
q , M - ' em-'
et
011
27 000 26 000 26 000 19 000 60 000
