In junior-high and high-school science you learn that the red pigment hemoglobin carries oxygen in blood and that the green pigment chlorophyll drives photosynthesis in plants. Both pigments share a large ring-shaped molecular framework called a porphyrin. Dr. Fischer used step-by-step chemical experiments to reveal the once-mysterious structure of this framework. He then synthesized haemin, a derivative isolated from blood, and verified that his laboratory product behaved exactly like the natural compound. These achievements made it possible to describe biological reactions with precise chemical formulas and strongly influenced drug development and analytical chemistry. By linking a pigment’s structure to its color, Fischer also built a bridge between organic chemistry and biology.

Fischer’s contribution is epitomized by his quantitative structural elucidation and total synthesis of porphyrin pigments. By oxidizing, reducing, and hydrolyzing haemin he isolated individual pyrrole fragments and used their patterns to deduce the substitution map of the tetrapyrrole ring. His pinpointing of the positions of vinyl and methyl groups on the protoporphyrin IX backbone was especially groundbreaking. He then designed a stepwise condensation of pyrrole monomers, introducing Fe(III) at the final stage to afford a laboratory compound that was spectroscopically identical to natural haemin. For chlorophyll he prepared metal-substituted analogues, replacing the central magnesium with zinc or copper, and showed how the metal center modulates light-absorption behavior. These studies opened the dawning field of bioinorganic and coordination chemistry, providing new analytical tools for hematology and photosynthesis research. Moreover, they spurred the development of spectroscopic methods that exploit the intense UV–visible bands of porphyrins, underpinning modern pulse oximetry and photodynamic therapy. Fischer’s systematic strategy is regarded as an early example of "target-oriented synthesis" and remains a textbook model for complex natural-product synthesis.

In elucidating the structure of haem-derived haemin, Fischer analyzed deferrated porphyrins produced via the Kühn–Wittmann reduction and assigned Haber degradation products to specific ring positions. Combining Ostwald acid-number titration with post-methylation GC analysis, he confirmed that vinyl groups occupy C2 and C7 and methyl groups C3, C8, C13, and C17 of protoporphyrin IX. In the total synthesis he assembled α,β-substituted pyrroles through Knorr condensations and adopted stepwise condensation of a linear tetrapyrrole precursor to circumvent late-stage macrocyclization. Oxidative cyclization was optimized with FeCl3/DMF, and pyridine coordination was used to stabilize the high-spin Fe(III) center. Identity with the natural compound was proven by congruent Soret (λmax = 398 nm) and Q-band positions. In chlorophyll studies he hydrolyzed the phytol ester, demetallated and remetallated the macrocycle, and documented absorption shifts across the metallochlorophyll series. These results systematized the electronic structure and coordination chemistry of metalloporphyrins, informing later design of picket-fence porphyrins and electron-transfer models. Fischer’s experimental rigor and stepwise logic remain cited protocols in contemporary metalloporphyrin functionalization research.