Do you know the quantum yield of the monomer? This could be used to calculate the QY of the aggregate by comparing fluorescence intensities at equal monomer concentrations. If you don't know the quantum yield of the compound, then you can determine it by comparing it to some other compound for which the QY is known, by measuring the fluorescence intensity of the 2 substances at the same wavelength and concentration in the same medium.

Thanks Adam, but when dual fluorescence takes place what is the meaning of the fluorescence yield? Is it wrong if I sum the quantum yields at the two states (fluoresence yield  at state A and state B?

I suppose that the quantum yield of the mixture would be the weighted average of the quantum yields of the monomer and the aggregate, weighted by their mole fractions.

We also faced with a problem of J-aggregates quantum yield measurement. It is much more difficult comparing with monomer solution due to aggregation conditions including a dye concentration could strongly effect on its optical properties including QY. So, my own opinion that only absolute quantum yield measurement using integral spheres should be used in case of J-aggregates but not a relative one. One of the very important things you should to take into account is a blue edge of fluorescent band self-quenching due to small Stokes shift. If it is interesting I could give some references.

Thanks Alexander for your answer. Yes please send some interested publications to me 

Sorry for the long delay.

The methods (2 measurements or 3 measurements)  of QY measurements using integrating sphere: Faulkner, D. O.; McDowell, J. J.; Price, A. J.; Perovic, D. D.; Kherani, N. P.; Ozin, G. A. Measurement of Absolute Photoluminescence Quantum Yields Using Integrating Spheres – Which Way to Go? Las. Photonics Rev. 2012, 6, 802–806.

Self-absorption correction:  Ahn, T.-S.; Al-Kaysi, R. O.; Müller, A. M.; Wentz, K. M.; Bardeen, Ch. J. Self-Absorption Correction for Solid-State Photoluminescence Quantum Yields Obtained from Integrating Sphere Measurements. Rev. Sci. Instr. 2007, 78, 086105.

Our attempts to measure QY for J-aggregates:  Guralchuk, G. Ya.; Katrunov, I. K.; Grynyov, R. S.; Sorokin, A. V.; Yefimova, S. L.; Borovoy, I. A.; Malyukin, Yu. V. Anomalous surfactant-induced enhancement of luminescence quantum yield of cyanine dye J-aggregates. J. Phys. Chem. C 2008, 112, 14762-14768.

Other QY measurement for J-aggregates:

1) Obara, Yu.; Saitoh, K.; Oda, M. and Tani T. Room-Temperature Fluorescence Lifetime of Pseudoisocyanine (PIC) J Excitons with Various Aggregate Morphologies in Relation to Microcavity Polariton Formation Int. J. Mol. Sci. 2012, 13, 5851-5865;

2) Ermolaeva, G. M.; Maslov, V. G.; Orlova, A. O.; Panfutova, A. S.; Rosanov, N. N.; Fainberg, B. D.; Shakhverdov, T. A.; Shilov, V. B. Dynamics of Optical Response of Solutions of Pseudoisocyanine J Aggregates upon Pico- and Subnanosecond Excitation Opt. Spectrosc. 2011, 110, 871–879;

3)  Horng, M.-L.; Quitevis, E. L. Excited-State Dynamics of Polymer-Bound J-Aggregates J. Phys. Chem. 1993, 97, 12408-12415.

If you have any questions don't hesitate to ask me.

Alexander Sorokin