33 L a s e r Studies of Radiationless D e c a y M e c h a n i s m s
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 19, 2018 | https://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch033
in
2+/3+
Os
P o l y p y r i d i n e Complexes
THOMAS L. NETZEL and MICHAEL A. BERGKAMP Brookhaven National Laboratory, Chemistry Department, Upton, NY 11973 The lowest energy excited states in Os(II) polypyridine complexes are of a metal-to-ligand charge transfer (MLCT) type and live for 10-40 µs at 4.2 K.1 The long wavelength absorptions in the visible region of the spectrum in Os(III) polypyridine complexes arise from ligand-to-metal charge transfer (LMCT) transitions and do not produce detectable luminescence. This suggests that these LMCT states are very short lived. We report here the results of picosecond absorption studies on the lifetimes the LMCT states in OsL33+ complexes [L = 2,2'-bipyridine(bpy) or 1,10-phenanthroline(phen)] as functions of temperature and isotopic substitution. The LMCT lifetimes at low temperature are then contrasted with the low temperature lifetimes of the MLCT states of OsL 2+ complexes and both are examined from the perspective of a coarse-grained radiationless decay theory developed by Englman and Jortner. In particular we seek to understand which molecular factors are responsible for the experimentally observed lifetimes. Results For picosecond kinetic measurements of change-in-absorbance ( AA.) spectra, the samples were degassed in 2 mm path length cells and held at constant temperature in a flowing-helium cryostat. The samples were excited at 527 nm with 6 ps laser pulses and the M. spectra of the excited states were measured with 8 ps white probe pulses. The laser system has been described elsewhere. The observed ΔΑ. signals are consistent with the known MLCT spectra of ground state OsL^ * complexes and support our assignment of the observed optical transients in 0sL3^" complexes to the production of LMCT states. Table I lists the excited state lifetimes for 0s(phen)3^" at three temperatures. The striking result is the lack of any significant change in the LMCT state*s lifetime on going from 295 to 10 K. Table I also presents data on the effects of deuteration on the charge transfer state lifetimes of Os(bpy)3^"/^ complexes. The lifetimes are lengthened, but only by factors of 2 and 2.5 respectively, for Os(bpy)3 and 0s(bpy)3^". 3
2
2
3+
0097-6156/83/0211-0515 $06.00/0 © 1983 American Chemical Society Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
516
INORGANIC C H E M I S T R Y : TOWARD T H E
21 ST
CENTURY
3
Table 1. Charge Transfer E x c i t e d State L i f e t i m e s .
Os(phen)
Lifetime (ps)
Temperature (K)
Compound
3^">Jl
295
< 9
80
20 ± 3
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 19, 2018 | https://pubs.acs.org Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch033
10 Os(bpy)
a f 3
^
Os(d -bpy) Zi
3
8
120
4
Os(phen) 3^*" Os(bpy) -»i Os(d -bpy)
62 ± 4£
10
3^*1
8
19 ± 2
5
2 f 3
»i
± 10
32
10
1.05
10
2.5
]ssl ± 0.04 ± 0.2
psi psl
JL The f o l l o w i n g abbreviations are used i n t h i s t a b l e : phen = 1,10-phenanthroline and bpy = 2 , 2 ' - b i p y r i d i n e . J> In H2O with 9 M H2S01+. £ The l i f e t i m e i n H 0 with 9 M D2S0Î* i s 64 ± 4 ps. 1 In D2O with 9 M D2S01+. £ From r e f . 1. JL Determined i n t h i s work by measurement of emission decay. 2
3 +
While the l i f e t i m e s of the OsL^ and O s L 3 charge t r a n s f e r states d i f f e r by a f a c t o r of 10 -10 , the energy gaps between t h e i r ground and charge t r a n s f e r states are s i m i l a r , 14.5 χ 10 and 16.0 χ 10 cm" , r e s p e c t i v e l y . However, O s L ^ " complexes have IR bands that OsL^ complexes don't have. Our observation of these absorptions agrees with a recent report by Kober an
