*From*: ilfir.,at,.bgumail.bgu.ac.il*Subject*: summary: fluorescence lifetime calculation*Date*: Tue, 17 Sep 2002 19:19:12 +0200

Hello, It is summary of replies on my question that was sent one week ago: Initial question was: "I am interested in prediction of fluorescence lifetime of organic molecules. Is there any reasonable approach to calculate this value or make approximate estimation. I suppose I may use difference of energies between ground and exited singlet state. Please send on my mail as I am not subscriber. Thank you." I took several answers (thank you very much!!!) those are very promising (!): ******************************************************* 1. "Najeeb Said" <najeeb.said.,at,.uk.amershambiosciences.com>: Although some semi-empirical methods allow you to predict wavelength of a photon emitted from the excited state (http://www.accelrys.com/cerius2/zindo.html), these methods will not tell you if an efficient non-radiative relaxation pathway to the ground state is present e.g. cis/trans isomerisation or electron transfer. Ab-initio calculations on the excited state are required to determine this and these are computationally expensive. As an example see (Ultrafast Radiationless Deactivation of Organic Dyes: Evidence for a Two-State Two-Mode Pathway in Polymethine Cyanines; Sanchez-Galvez, A et al; J. Am. Chem. Soc., 122 (12), 2911 -2924, 2000). The calculation of fluorescence lifetime is an even more challenging area. As I mentioned earlier, depopulation of the excited state can occur by via fluorescence, internal conversion, electron transfer and other processes. These competing processes can be described as a series of rate constants i.e. K(depop) = K(fluorescence) + K(non-fluorescence) Where K(non-fluorescence) = K(internal conversion) + K(electron transfer) + K(other pathways). To determine the fluorescent lifetime (1/ Kfluorescence) of a compound, you would need to follow the trajectory of its excited state along a reaction co-ordinate (time). When you consider that a typical organic dye has a fluorescent lifetime in the nanosecond region and a typical timestep in these calculations is in the fentosecond scale (Jolibois, F et al. J. Am. Chem. Soc.; 2000; 122; 5801-5810), the calculation of fluorescent lifetimes of organic dyes is beyond our capabilities. The area of fluorescence modelling is a fascinating, but challenging area. Can you please send a summary of your answers to this list. Regards, Najeeb Said ================= Amersham Biosciences The Maynard Centre Cardiff, UK ******************************************************* 2. Sergei Tretiak <serg.,at,.markov.chem.rochester.edu> Hi Ifir, You could calculate the radiative lifetime "tau" with formulae k_rad=4/3 * E^3 d^2/c^3 where k is the radiative decay rate (1/tau), c is the speed of light, E is the difference between ground and exited singlet state energies and d is the transition dipole moment associated with this transition. Practical expression is tau[ns]=6000/d^2[Debye]/E^3[eV] The transition dipole is well defined, it is dipole moment associated with optical transition between states, i.e. d_ge=<\psi_e| D | \psi_g>, where \psi_e and \psi_g are excited and ground state electronic wavefunctions and D is the dipole moment operator (note that you have x, y and z components of d and their vector's sum). It does not associated with ground or excited state dipoles given by d_gg=<\psi_g| D | \psi_g > and d_ee=<\psi_e| D | \psi_e >, but closely related to the oscillator strength f for a given optical transition f=2/3 * d^2 * E_e, where E_e is the transition frequency. Note that this way you are going to calculate the radiative decay rate only, internal conversion (e.g. to triplet), energy and electron transfer channels will not be accounted for. The total depopulation rate would be the sum of all these rates. Best, Sergei .-----------------------------------------------------------------------. |\ / \ | Sergei Tretiak | / \ /| |.\/...\|.......................................................|/...\/.| | Theoretical Division | Voice: (505) 667-8351 | | Mail stop B262 | Fax: (505) 665-4063 | | Los Alamos National Lab | E-mail: serg.,at,.markov.chem.rochester.edu | | Los Alamos, NM 87545 | serg.,at,.wigner.chem.rochester.edu | | Homepage: http://markov.chem.rochester.edu/serg/welcome.html | |_______________________________________________________________________| ******************************************************* 3. Qadir <Qadir.,at,.cermm.concordia.ca> (Centre de Recherche en de Modélisation Moléculaire) Well, the problem is really complicated, and one can not really get anything from considering just energies between ground and exited states. At least you have to consider transition moments and the energy gap (i.e. one needs to calculate the oscillator strength, which is something like f=1/3*TD^2*deltaE). Using this may be you can make some correlations between fluorescence lifetime and calculated properties. I am sure there are some rigorous way to calculate fluorescence lifetime but I don't think it is too easy... ******************************************************* Ok, now I see there is practical approach to the problem. It should be tried! If anyone will have good experience with with expression please send message to this board and to me. Best regards, Ilfir mailto:ilfir.,at,.bgumail.bgu.ac.il http://ilfir.cjb.net