Structural, optoelectronic and supplementary characterization data for “The Role of Alkyl Chain Length and Halide Counter Ion in Layered Dion-Jacobson Perovskites with Aromatic Spacers”, doi.org/10.1021/acs.jpclett.1c02937. Figure 2.zip: 2a and 2b (XRD patterns) in *.csv files. 2c and 2d raw and processed solid state NMR spectra of (PDEA)PbI4 and (PDEA)I2, comprising 207Pb, 207Pb->1H HETCOR. NMR data is given in the JCAMP-DX 6.0 format including acquisition and processing parameters. The TopSpin software is recommended as this was used to record the NMR data. 2e-h GIWAXS images in *.tiff format Figure 3.zip: 3a and 3c (UV-Vis absorption spectra) in *.csv format 3b and 3d (PL emission) in *.xlsx data Figure 4.zip: Transient absorption spectroscopy data in *.xlsx format Figure 5.zip: 5a-f (XRD patterns) in *.csv data Figure S2.zip: S2a-b (UV-Vis spectra) in *.csv data S2c-d (XRD patterns) in *.csv data Figure S3.zip: GIWAXS images in *.tiff format Figure S4.zip: S4 Raw and processed solid state NMR spectra of 1H->13C cross polarisation spectra. NMR data is given in the JCAMP-DX 6.0 format including acquisition and processing parameters. The TopSpin software is recommended as this was used to record the NMR data. Figure S5-S6.zip: Raw and processed solution NMR spectra of (PDEA)I2 and (PDEA)Br2 in deuterated DMSO, comprising 1H and 13C 1D and 2D HSQC/HMBC spectra. NMR data is given in the JCAMP-DX 6.0 format including acquisition and processing parameters. The TopSpin software is recommended as this was used to record the NMR data. Figure S7.zip: PL emission data in *.xlsx format Figure S8.zip: Transient absorption spectroscopy data in *.xlsx format Figure S9.zip: S9a-f optical absorption data in *.csv format Figure S10.zip: PL emission data in *.xlsx format Figure S11.zip: *.png image of contact angle measurements

This work was supported by the Max Planck Society and the Max Planck-EPFL center.