OFFICE OF NAVAL RESEARCH. Contract Nonr-4756(07) Task No. NR Determination of the Rate Constants

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1 OFFICE OF NAVAL RESEARCH a Contract Nonr-4756(07) Task No. NR CD O I - J > HI l! Determination of the Rate Constants of the Intersyntem Crossing Processes to the Individual Multlplets of the Lowest Triplet State M. A. El-Sayed and L. Hall Department of Chemistry L^ * ' ^-^ 1 Esrae: i tannakj Reproduction in whole or in pa:, t Is permitted for any purpose of the United Spates Government DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED October, 1968 Reproduced by ;he CLEARINGHOUSE for Federal Scientific & Technical Information Springfield V.j. 2215! rail QBKixutu: ",(..";,i<.

2 DETERMINATION OF THE RATE CONSTANTS OF THE INTERSYSTEM CROSSING PROCESSES TO THE INDIVIDUAL MULTIPLETS OF THE LOWEST TRIPLET STATE M. A. El-Sayed and L. Hall Department of Chemistry University of California Los Angeles, California -.. The decay of Che phosphorescence emission from the three individual sublevels of the lowest triplet state has recently been reported for the first time for pyrazine at K. This is observed when the spin-lattice relaxation processes between the three sublevels are slower than the radiative processes from the individual sublevels. * " * Furthermore, the three observed emission lifetimes should be different so that they may be resolved in time. There are a good number of molecules for which these conditions are satisfied. (3) It thus appears that one can study the radiative and nonradiative properties of the individual sublevels of the lowest triplet state of phosphorescent molecules. A number of properties are now being examined in our laboratories. We report here on a new method that can be used to determine the rate constants of the intersystem crossing processes leading to the population of the three different sublevels of the lowest triplet states of phosphorescent molecules. A knowledge of these constants (or their ratios) is vital in determining the relative importance of the different routes of the intersystem crossing processes in molecules. T Alfred P. Sloan Fellow. * Part of PhJ). dissertation at UCLA. Contribution No from UCLA.

3 BMaMv A molecule excited to a singlet state loses its energy by internal conversion processes and finally reaches the lowest singlet state, v 2. Intersystem crossing process proceeds from S. to the different triplet sublevels T.., T- and T_ of the lowest tripled statev either directly or indirectly via other triplet states located between S, and the lowest triplet state. The pumping rates of the T., T and T sublevels from S. have the rate constants k, k and k., respectively. Each one of these rate constants may or may not be a composite of other rate constants, depending on the exact mechanism involved in the S.-»*T radiationless trancitions. 3. The system is at temperatures for which the S.L.R. processes between T., T and T are much slower than the radiative &nd nonradiative processes from the three sublevels to the ground state. Simple Relationships: The application of the steady state approximation to the formation and disappearance of any of the multiplets, e.g., T,, of the above system gives the following relation: K (IS)1 [S 1 ] ' K i [T i ] (1) where K,,. is the rate constant for the intersysf am crossing process from ( LS)i S, to the T. sublevel, K. ("K,, N + K,., t, v.^ls the observed rate 1 i ' 1 v (rad)i (noaradiative)l^ constant of the decay of sublevel i, and [f-j, [T.] represent the steady state concentrations of the lowest singlet stat» and sublevel (i) of the lowest triplet state, respectively. The observed steady state intensity, I., of the emission from sublevel T. is given by ^ = VadH^J = V T i ]W i = K (IS)i [S l ]Cp i (2)

4 -3- K (rad)i where cp. is the pfiosphorescence quantum yield of su'olevel i. 1 K 1 Applying equation (2) to the emissjon from any two of th? sublevels, e.g., T. and T-, one obtains: ll K (IS)1 i h K (IS)2 ^ x r <3) ^ If one determines cp., cp. and cp. at these temperatures, two independent equations 1 ' Xl ' x, can be obtained relating r - and = to ratios involving K/ TC v J. J.f the total 12 3 l**ll K-- ( K/jgN, + K/jg\ 2 + K/jg^) is determined in the usual manner, one can solve for K, IS., X / Is \2 and K (is)3 individuall y' Applications ^o Pvrazine-d,: Since cp,, cp» and cp. are not yet known for any molecule, wa selected a molecule with very high phosphorescent quantum yield, pyrazinu-d.. The change of the phosphorescence lifetime upon deuteration is found to be absent for the short-lived emission and only small for the medium- and long-lived emissions. These results strongly suggest that the observed lifetime is near the radiative one for a molecule having as high a triplet-state energy as pyrazine. These facts justify making the assumption that cp = co = cp = 1. The initial intnn^ily of the three decay components is determined from the decay curve. From polarization and theoretical considerations the short-, medium-, and long-lived emissions are assigned to radiation from the T, T and T sublevels y z x 3 + of foe B. state, respe' lively. Using equation (3), one determines the v ratios IS,_. and K TC/. to be 2% and 47,, respectively. This means "^IS, v K IS(T ) (T y ) y 7 that the most probable intersystem crossing route in pyrazine is the one(s) that populates the T sublevel of the B~' state. Such a conclusion is in agreement with predictions made previously^ ' which proposed that the most ^The N...N and the out-of-plane axes are designated as the z and x axes, respectively.

5 -4- probable tntersystem cross Iris process in pyrazlne is: Un.TT* S.O. S^yTT*,. 3 n,tt*. v I» ^ B, ' (T)^ B' (T). 3u lu 3u _. l-tl.tt* S.O. 3_TT.TT* 1,,,,,,. t.,, The B. wt^^-fc». B ' is the only route allowed oy spin-orbit selection rules. The other two levels, T and T, are thus populated by processes involving spin orbit-vibronic or spin-vibronic type interaction, From the determined ratio, it is clear that these processes are times less probable than the first-order-allowed processes in pyrazine. The application of this method to other systems is now in progress, and a complete account of the work will be published later. Acknowledgements: We wish to acknowledge the financial support of the Office of Naval Research, Contract No. Nonr-4756(07).

6 REFERENCES 1. a. L. Hall, A. Armstrong, W. Moomaw, and M. A. El-Sayed, J. Chem. Phvs.. 48, 1395 (1968). b. M. A. El-Sayed, W. R. Moomaw, and D. Tintl, J. Chem. Phvs.. in press. c. M. A. El-Sayed, W. R. Moomaw, L. Hall, and A. Armstrong, In International Symposium on Excltcns. Magnons and Phonons In Molecular Crystals. A. B. Zahlan, Editor, Cambridge University Press (1968). 2. M. S. de Groot, I. A. Hesselmann, and J. van der Waals, Mol. Phvs (1967). 3. L. Hall and M. A. El-Sayed, work in progress. 4. a. For rev ew, see: S. K. Lower and M. A. El-Sayed, Chem. Rev.. 66, 199 (1966). b. D. Scott and 0. Maltenkieks, J. Phys. Chem.. 72., 3354 (1968). 5. G. W. Robinson and P. Frosch, J. Chem. Phvs.. 37, 1962 (1062); 38, 1187 (1963). 6. M. A. El-Sayed, "The Intersystem Crossing to and the Phosphorescence from the Individual Sublevels of the Lowest Triplet State in Pyrazine at K", in International Conference on Molecular Luminescence (Loyola University, August, 1968), E. Llm, Editor; W. Benjamin, Inc., in press. 7. M. A. El-Sayed, J. Chem. Phys.. 36, 573 (1962); ^8, 2834 (1963).

7 I WUÜMMßHiät St'ttinly Cljütifirnlion DOCUMENT CUKTROL DATA R&D Svt'iinty i In* >iln ution ol title, Irndy t«/ Mbsfr<ii t»ml inti^xirij*,h\not»lii>n nm*t hv nitfrvd wht n!>>< >>\ttjll ft puff i^ ftoggolgtfl?nig<n*rino»ctivi ry (Corporal* nnthor; Unlver.ilty of California, Los Angeles M. A. El-Sayed, Principal Investigator Lot Angelas, California :o. Rrro«i srcority CLASsinr A TION Unclassified }h. cwoup none 1 HCPOWT Tl TLE DETERMINATIOM OF THE RATE CONSTANTS OF THE INTERSYSTEM CROSSING PROCESSES TO THE INDIVIDUAL MÜLTIPLETS OF THE LOWEST TRIPLET STATE " 4 OeseniPTive NOTES (Typ» ol trport and Irrluiltt' ii«(rm Technical report 9 4u THOniSi (Flttt name, middle inltltl, /«>( numr) M. A. El-Sayed and L. Hall «MEPOHT DATE October, 1968 issrwuim H ' 6. PROJKCTNO ARPA Order No. 306 Task No. NR ». TO T AWNO OF PAGES 7h. NO OF REF5 1«. ORIGINATOR'S «tport NUMBERfS) c. <»K OTHER REPORT NOISI fanv nthrr numbers that may be mtaifned tbis repcrt) 10 DISTRIBUTION STATEMENT Distribution of this document is un united. II. SUPRLF.MFNTARV NOTES II SPONSORING Wl LI T AHY ACTIVITY I J AB» TRAC ' Office of Naval Research Physics Branch Washington,D. C. Equations are derived which can be used to determine the rate constants for the intersystem crossing processes involved in populating the three zero field spin levels of the lowest triplet states of phosphorescing molecules JS». i r, ( DD FORM I NO V 6S S/N « I 1473 (PAGF - n Unclassified Socuntv Classification

8 Unclassified Security CUYiUication KEY WORD» HOLE WT not. E Intersystem crossing radlatlonless transitions pyrazlne DD :T.M73 «BACK, (PAGE 2) Unclassified Security Classification