The nature of the dark energy is still a mystery and several models have been proposed to explain it. Here we consider a phenomenological model for dark energy decay into photons and particles as proposed by Lima (J. Lima, Phys. Rev. D 54, 2571 (1996)). He studied the thermodynamic aspects of decaying dark energy models in particular in the case of a continuous photon creation and/or disruption. Following his approach, we derive a temperature redshift relation for the CMB which depends on the effective equation of state $w_{eff}$ and on the adiabatic index $gamma$. Comparing our relation with the data on the CMB temperature as a function of the redshift obtained from Sunyaev-Zeldovich observations and at higher redshift from quasar absorption line spectra, we find $w_{eff}=-0.97 pm 0.034$, adopting for the adiabatic index $gamma=4/3$, in good agreement with current estimates and still compatible with $w_{eff}=-1$, implying that the dark energy content being constant in time.
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