Molecular aggregates 2004. 4. 8. Myounghee Lee

Nanoparticles stability of silole in acetone/water mixture Blue-shift of PL peak & Decrease of PL intensity with time

J-Aggregates ? Red-shift of absorption spectra of TDBC in water at RT Linear aggregate Exciton model Mirjam van Burgel, Douwe A. Wiersma, and Koos duppen, J. Chem. Phys. 2001, 102(1), 20.

J-Aggregates ? J-aggregates (or Scheibe aggregates) named after one of its discorverers E. E. Jelly & G. Scheibe in the late 1930s[1,2] pseudo isocyanine (PIC) Molecular aggregates formed by cyanine dye Strongly coupled transition dipoles, Frenkel excitons Geometrical arrangement, coupling energy Splitting of excitation energy one Molecule (M) upper state allowed (H) : H-aggregate, parallel alignment lower state allowed (J) : J-aggregate, oblique alignment For more complex alignment, splitting of the excitonic band into the several bands [3] Excitonic energy states of monomer and dimers [1] Scheibe, G. Angew. Chem. 1937, 50, 51. [2] Jellly, E. E. Nature 1936, 138, 1009: Nature 1937, 139, 631. [3] S. Kirstein, R. Steitz, R. Garbella and H. Möhwald J. Chem. Phys. 103(2) (1995) 815.

An example of H- and J-aggregates of a cyanine dye, DiSC3+(5) 1st splitting - stacking interaction bet. Cyanine dimers 2nd splitting - end-to-end interactions bet. dimers along the length of the H- and J-aggregates Arindam Chowdhure, Sebastian Wachsmann-Hogiu, Prakriti R. Bangal, Izzat Raheem, and Linda A. Peteanu, J. Phys. Chem. B 2001, 105, 12196.

Coupling energy and red-shift of J-aggregates Quantum mechanical Hamiltonian of an unperturbed J-aggregate, Resonant Coulomb-coupling energies of the system of extended dipoles arranged on the hexagonal lattice, Red-shift of the J-band in the observed spectra of the aggregate, Bn+ and Bn - the creation and annihilation operators of the excited state of the molecule at the site n, - the excitation energy of the corresponding monomers, Dn - the energy shift due to the Electrostatic interaction of the monomers with the lattice, Jn,m - the resonant coupling Energy between the chromophores at the site n and m, l – extended dipole length (double charges of magnitude Q and moment, ) S. Engelhard and F. H. M. Faisal, J. Chem. Phys. 1999, 110, 3596.

Radiative Dynamics in solution and in molecular assemblies of an H-aggregate-forming stilbazolium amphiphile Q. Song, P. W. Bohn, and G. J. Blanchard J. Phys. Chem. B 1997, 101, 8865

Kinetic diagram of the optical transition for a cyanine molecule Hemicyanine - H-aggregate at air-water interface The relaxation diagram of ground and excited electronic potential surface Samples used in this paper - I. Monomer in solvents II. H-aggregates in Langmuir-Blodgett (LB) films on SiO2 exciton hopping intramolecular coupling for excited state intermolecular coupling (energy transfer) * Monolayer structure : aggregate islands (99.5%)with monomer (0.5%) at grain boundaries and in the interstices

Frenkel Exciton Exciton hopping Energy transfer

Kinetic diagram of the optical transition for a cyanine molecule Decay lifetime Hemicyanine - H-aggregate at air-water interface Size of aggregate e h Samples used in this paper - I. Monomer in solvents II. H-aggregates in Langmuir-Blodgett (LB) films on SiO2 exciton hopping intramolecular coupling for excited state intermolecular coupling (energy transfer) Monolayer structure : aggregate islands (99.5%)with monomer (0.5%) at grain boundaries and in the interstices

Kinetic response in monomer Time-dependent population changes for the energy level, N0, N1, N2 – populations of the ground state, initially populated level, and the TICT state The solution for this set of coupled linear differential equations has the form, N10 – initial populations of the directly populated excited satate

Kinetic response in monomer The signal observed is, A single-exponential decay function is not sufficient to describe the fluorescence behavior when reversible coupling exists between an emissive state and other excited electronic states. The solution for this set of coupled linear differential equations has the form, Using Taylor series expanding Further simplification,

Lifetimes in n-Alkanol solutions Excited at 480 nm and monitored at 660(+,-30) nm

Lifetimes in LB film

Kinetic diagram of the optical transition for molecules in various states The relaxation diagram in mixed state of monomer and aggregates, Time-dependent population changes for LB film, A* : excited aggregate M* : excited monomer Kxt : rate of exciton transfer

Modeling of aggregate decay dynamics Decreasing the nonradiative rate relative to the rate for exciton transport - introduce rise time Decreasing the rate of exciton transport - produces weaker fluorescence Fitting parameters

Monte Carlo simulation Random walk model for exciton hopping The distribution of the distance of the final positions from the origin - runs ranging from 50 to 500 hops Mean distance traveled for exciton diffusion as a Function of the number of discrete steps taken - the relation can be used to estimate the aggregate size

Monte Carlo simulation

The size of an H-aggregate Fitting parameters from the measured value, average lifetime of exciton, exciton hopping time for organic monolayers, typically very fast, ~0.1 ps average distance traveled, 45 steps = 45 molecules 26.5 angstrom

Monte Carlo simulation Blue area ? 1. randomly select a location within the rectangle 2. if it is within the blue area, record this instance a hit 3. generate a new location and repeat 10,000 times

LB film