  Aims. We aim to detect molecules in the atmosphere of the young forming
companion PDS70 b by searching for atmospheric absorption features typical of
substellar objects. Methods. We obtained medium-resolution (R$\approx$5075)
spectra of the PDS70 planetary system with the SINFONI integral field
spectrograph at the Very Large Telescope. We applied molecular mapping, based
on cross-correlation with synthetic spectra, to identify signatures of
molecular species in the atmosphere of the planet. Results. Although the planet
emission is clearly detected when resampling the data to lower resolution, no
molecular species could be identified with the cross-correlation technique. We
estimated upper limits on the abundances of H$_2$O, CO and CH$_4$
($\log(X_\mathrm{mol}) < -4.0$, $-4.1$ and $-4.9$, respectively) assuming a
clear atmosphere, and we explored the impact of clouds, which increase the
upper limits by a factor up to 0.7 dex. Assuming that the observations directly
probe the planet's atmosphere, we found a lack of molecular species compared to
other directly imaged companions or field objects. Under the assumption that
the planet atmosphere presents similar characteristics to other directly imaged
planets, we conclude that a dusty environment surrounds the planet, effectively
obscuring any feature generated in its atmosphere. We quantify the extinction
necessary to impede the detection ($A_V\approx16-17$ mag), pointing to the
possibility of higher optical thickness than previously estimated from other
studies. Finally, the non-detection of molecular species conflicts with
atmospheric models previously proposed to describe the forming planet.
Conclusions. To unveil how giant planets form, a comprehensive approach that
includes constraints from multiple techniques needs to be undertaken. Molecular
mapping emerges as an alternative to more classical techniques like SED
fitting.
