The electron-transfer rate from methylene-bridged molecular adsorbates to SnO2 semiconductor nanocrystalline film was systematically investigated for short bridge lengths. The synthesized sensitizers were the homologues of Re(CO)(3)Cl(dcbpy) [dcbpy = 4,4'-dicarboxy-2,2'-bipyridinel (ReCnA) with methylene units (CH2)(n) (n = 1-5) inserted between the bipyridine rings and the carboxylate anchoring groups. Use of femtosecond infrared spectroscopy provided the time resolution necessary to study the ultrafast electron transfer that occurs over these very short bridges. Electron injection is unambiguously observed by signals arising from both the injected electron and the CO stretching mode of the oxidized molecule. All dyes exhibited nonexponential electron injection. The kinetic traces for the different spacers can be superimposed on each other by scaling their time axes, which allows a quantitative comparison of these nonexponential injection rates. This comparison revealed an exponential decrease of injection rate with bridge length for 3 less than or equal to n less than or equal to 5 with a decay constant beta of 1.0 per CH2 unit. Deviation from the exponential dependence was observed for n = 1 or 2. The applicability of the time-scaling method also implies a rate distribution that is nominally independent of bridge length for all of the investigated samples.