Role of Hydroxyl Groups in the Photophysics, Photostability, and (Opto)electronic Properties of the Fungi-Derived Pigment Xylindein

Abstract

Organic semiconductors have attracted increasing attention due to their low cost, solution processability, and tunable properties. Of special interest are molecules with enhanced environmental stability. We have recently reported on the (opto)electronic properties of a remarkably stable, naturally derived pigment xylindein. Here, we establish that one particular aspect of xylindein's molecular structure, namely the presence of hydroxyl (OH) groups, is critical for enabling its enhanced stability and relatively high electron mobility. In particular, we synthesized a methylated derivative of xylindein, dimethylxylindein, where the OH groups are replaced with OCH 3 groups, and compared photophysics and the (opto)electronic properties of dimethylxylindein and xylindein. We reveal the presence of a long-lived excited state in dimethylxylindein, in contrast to xylindein, which has an efficient fast nonradiative pathway to the ground state. This results in significantly reduced photostability of dimethylxylindein as compared to xylindein. The effective electron mobility, obtained from space-charge-limited currents, in amorphous xylindein films was found to be 4 orders of magnitude higher than that in amorphous and crystalline dimethylxylindein films. In contrast, the photosensitivity of dimethylxylindein is about 2 orders of magnitude higher than that of xylindein. The mechanism of charge transport in all films was thermally activated hopping, with the xylindein films characterized by considerably shallower charge traps than dimethylxylindein films, attributed to hydrogen bonding via hydroxyl groups promoting an efficient conductive network in xylindein.

Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy

“…For example, dark and ac (photo)currents in thin films (see Figures 2a and b) show lower activation energy in xylindein than dimethylxylindein, partly attributed to the enhanced intra‐ and intermolecular H‐bonding by the hydroxy groups of xylindein (with electron mobility up to 0.4 cm 2 /Vs) [20b,31] . The photosensitivity, however, is notably higher for dimethylxylindein [31] .…”

“…Stabilization of π‐systems without hindering carrier injection can occur by introducing heteroatoms into reactive sites of conjugated structures [30] . Triplet states can also affect photostability through the generation of reactive oxygen species (ROS), [5b,31] while UV radiation can generate radical species with cascading effects. Strongly fluorescent molecules are often less photostable than weakly fluorescent molecules because the latter can quickly deactivate the excited state through nonradiative processes.…”

“…An intriguing area of research thus involves the relationship between ES(I)PT and charge transfer (CT) in devices. Notably, featuring an ultrafast ESIPT does not guarantee chromophore stability as exemplified by the photostability of several molecules under simulated solar irradiation (Figure 2d) [31] . Xylindein and alizarin in DCM solution display superior photostability, whereas indigo and dimethylxylindein become nearly photobleached within ∼3 h. In addition to triplet formation in dimethylxylindein, [19,31] both dimethylxylindein and deprotonated xylindein [15b,31] exhibit reduced photostability due to the weakened H‐bonding.…”

“…Moreover, small‐scale molecular rearrangements within the excimer can rapidly reach a more relaxed structure by decreasing the intermolecular distance to improve the electronic and vibrational coupling, [23b] while electronic delocalization splits the excited‐state energy relative to monomer (Figure 3f) [33] . Given that such distance/coupling is regulated by the tail extensions of xylindein, a targeted design principle could involve extended or branched tail sidechains to hinder excimer formation while still maintaining a certain degree of π‐π stacking (see Figure 3g insets), thus improving the photoconductivity of xylindein thin films [15b,31] …”

“…Upon replacing the two hydroxy groups of xylindein with methoxy groups, dimethylxylindein without ESIPT provides deep insights into an altered H‐bonding network [19,31] . In contrast to xylindein, [15b] dimethylxylindein in DCM exhibits an incomplete decay of blue ESA and recovery of GSB (Figure 4a) due to triplet formation [18] .…”