J. Phys. Chem. A 2010, 114, 8961–8968
8961
Vibrational Energy Flow Controls Internal Conversion in a Transition Metal Complex Gordon J. Hedley, Arvydas Ruseckas, and Ifor D. W. Samuel* Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, UniVersity of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, United Kingdom ReceiVed: February 4, 2010; ReVised Manuscript ReceiVed: July 20, 2010
Internal conversion (IC) between excited electronic states is a fundamental photophysical process that is important for understanding protection from UV radiation, energy transfer pathways and electron injection in artificial photosynthetic systems and organic solar cells. We have studied IC between three singlet MLCT states in an iridium complex using femtosecond fluorescence spectroscopy. Very fast IC with a time constant of 0.6 eV of vibrational energy stored in the complex that has to be dissipated by intramolecular vibrational redistribution before further IC to the lower energy states can occur. These results show that the ability to dissipate vibrational energy can control the relaxation process in this class of materials. 3
1. Introduction Internal conversion (IC) between excited electronic states is a way of dissipating excess electronic energy, and is important in the protection of biological molecules from harmful UV radiation.1,2 Energy and charge transfer processes in artificial photosynthesis3-5 and solar cells6 can compete with IC, mediating more efficient energy conversion from higher energy excited states.7 Transition metal complexes play important roles in biochemical reactions, such as those involving myoglobin8 and hemeglobin.9 They also show attractive optical and electronic properties for a wide range of applications including dye sensitized solar cells,10 artificial light harvesting complexes,3 and organic phosphorescent devices.11 Excited states in transition metal complexes are found12,13 to be a mixture of metal-ligand charge transfer (MLCT) and ligand centered (LC) π-π* states, the nature of the mixture is defined by the coordinating metal and by the structure of the ligands. Previous ultrafast photophysical studies have been reported on a number of different transition metal complexes. From fast to slow, this has included a very fast decay of fluorescence from singlet MLCT states in ruthenium14,15 and iron16 complexes, with time constants of 40 fs or less, and this was attributed to intersystem crossing (ISC) from the singlet to the triplet manifolds. Slightly slower ISC has been observed in iridium17 and rhenium18 complexes on the 70-150 fs time scale. When studying ISC in rhenium, Chergui and co-workers observed a correlation between the ISC rate and the vibrational periods of the metal-ligand modes, implying that the rate could be controlled by low-frequency skeletal vibrations.18 In ultrafast transient absorption studies by McCusker and co-workers excited state localization in a ruthenium complex was found to occur in 60 fs and was attributed to nondiffusive solvation dynamics,19 and evolution of the primary singlet MLCT state to the lowest excited triplet (MLCT) state was found to occur on a time scale of 100 fs.20 Recent work using both ultrafast luminescence16 and X-ray spectroscopy21 have looked at the ultrafast dynamics of an iron complex and found a 120 fs time constant for the population to leave the * To whom correspondence should be addressed. Phone: +44 1334 463 114. Fax: +44 1334 463 104. E-mail:
[email protected].
MLCT state for a quintet state. Finally, dynamics within the lowest 3MLCT state in an iridium complex have been found22 to equilibrate between three electronic substates with a time constant of 230 fs. Ultrafast photophysics experiments to date have thus explored a number of areas of the nature and rates of fast internal dynamics in transition metal complexes; however, a detailed study of IC between singlet MLCT states has not been explored. In this article we have observed the ultrafast dynamics of an iridium complex. Due to the presence of a number of closely spaced states we were able to monitor the relaxation of several consecutive MLCT states and observe the IC between thems observations that have not previously been accessible in transition metal complexes. We have found that the rate of IC varies, with an initial fast IC from a higher energy excited singlet state (decay time
