Ultrafast spectral hole burning reveals the distinct chromophores in eumelanin and their common photoresponse

Abstract

Seeing the colors in black: ultrafast transient hole burning spectroscopy reveals the absorption properties of discrete chromophores and their interactions in the skin pigment eumelanin.

Probing the heterogeneous structure of eumelanin using ultrafast vibrational fingerprinting

“…Each subset of selected chromophores has the same transient vibrational spectrum, or fingerprint, in the measurement window spanning 1300–1900 cm −1 and identical decay kinetics. This suggests that each subset of electronic chromophores contains the same distribution of functional groups as any other set, a property that we propose underlies the universal photoresponse observed in eumelanin 11 .…”

“…This causes bleaching of virtually all double-bond vibrational bands in the electronic ground state owing to frequency shifts upon excitation 24 . In a heterogeneous sample like eumelanin, subsets of absorbers can be selectively photoexcited by tuning the excitation wavelength 11 . In the language of hole burning spectroscopy, photoexcitation excites an isochromat of molecules that have a common ability to absorb the pump wavelength 25 .…”

“…The chemical disorder model proposed by Tran et al 10 postulates that the broad and featureless absorption spectrum of eumelanin is a superposition of absorption bands owing to chemically heterogeneous subunits. Transient electronic spectral hole burning observed in femtosecond laser experiments on eumelanins 11 , 12 (Fig. 1b ) indicates the presence of distinct chromophores that differ in their absorption spectra.…”

“…Investigators have augmented the chemical disorder model by pointing out that electronic conjugation length 16 , redox state variations 16 – 20 , and excitonic delocalization 14 , 15 could all affect the spectral and dynamical properties of eumelanin chromophores, but to an unknown degree. Although it is currently difficult to disentangle these effects, the observation of spectral hole burning 11 in electronic transient absorption experiments (e.g., Fig. 1b ) suggests that different subsets of chromophores in eumelanin can be interrogated spectroscopically.…”

“…Here, we combine the recently demonstrated ability to selectively excite eumelanin chromophores based on their transition energies 11 with time-resolved vibrational spectroscopy, a technique that has not been used previously to study any form of melanin. By tuning an excitation pulse from the UV through the visible, subsets of electronic chromophores are selected, and a mid-IR probe pulse detects transient changes in vibrational modes in an approach we term ultrafast vibrational fingerprinting (UVF).…”