Complete Mechanism of Hemithioindigo Motor Rotation

Abstract

Hemithioindigo-based molecular motors are powered by nondamaging visible light and provide very fast directional rotations at ambient conditions. Their ground state energy profile has been probed in detail, but the crucial excited state processes are completely unknown so far. In addition, very fast processes in the ground state are also still elusive to date and thus knowledge of the whole operational mechanism remains to a large extent in the dark. In this work we elucidate the complete lightdriven rotation mechanism by a combination of multiscale broadband transient absorption measurements covering a time scale from fs to ms in conjunction with a high level theoretical description of the excited state. In addition to a full description of the excited state dynamics in the various time regimes, we also provide the first experimental evidence for the elusive fourth intermediate ground state of the original HTI motor. The fate of this intermediate also is followed directly proving complete unidirectionality for both 180°rotation steps. At the same time, we uncover the hitherto unknown involvement of an unproductive triplet state pathway, which slightly diminishes the quantum yield of the E to Z photoisomerization. A rate model analysis shows that increasing the speed of motor rotation is most effectively done by increasing the photoisomerization quantum yields instead of barrier reduction for the thermal ratcheting steps. Our findings are of crucial importance for improved future designs of any light-driven molecular motor in general to yield better efficiencies and applicability.

Übertragung unidirektionaler molekularer Motorrotation auf eine räumlich getrennte Biarylachse

“…Dadurch konnte eindeutig gezeigt werden, dass die ersten 180° der Rotation um die Doppelbindung von C ‐ 1 nach A ‐ 1 vollständig direktional ablaufen (mindestens 95 %, wenn angenommen wird, dass 5 % verbleibendes D ‐ 1 durch NMR‐Spektroskopie detektiert werden kann). Ähnlich zum originalen HTI‐Motor kann durch Bestrahlung von reinem A ‐ 1 auch bei −80 °C wegen des sehr schnellen thermischen Zerfalls zu C ‐ 1 kein B ‐ 1 ‐Isomer beobachtet werden ,. Unter der Annahme, dass 1.5 min Halbwertszeit noch durch NMR‐Spektroskopie detektierbar sind, kann die entsprechende Energiebarriere folglich nicht größer als 13 kcal mol −1 sein.…”

Toward Fast and Efficient Visible‐Light‐Driven Molecular Motors: A Minimal Design

“…From the point of view of applications, adding to these challenges is the fact that most overcrowded‐alkene motors are driven by energetic UV light, which is more damaging to the motors and their environment than visible light. Thus, a key goal in the development of light‐driven molecular motors is to make them responsive to visible light, so as to facilitate their usage in biological systems and other soft materials …”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Among the various types of synthetic molecular motors available today,t hose that achieve 3608 unidirectional rotary motion through the absorption of UV or visible light are commonlyk nowna sl ight-driven rotary molecularm otors. [17][18][19][20][21][22][23][24][25] Typically, the clockwise (CW) or counterclockwise (CCW)-as determined by the molecular chirality-rotary motion results from consecutive E!Z and Z!E photoisomerizations around ac arbon-or carbon-nitrogen double bond that connects two distinct molecular halves. Furthermore, for mostm otors of this type (but not all [21] )t he reaction cycles also include slower thermalconformational relaxation steps.…”

“…[34,35,46,47] From the point of view of applications, addingt ot hese challenges is the fact that most overcrowdedalkenem otorsa re driven by energetic UV light, which is more damaging to the motors and their environmentt han visible light. Thus, ak ey goali nt he development of light-driven molecular motors is to make them responsive to visible light, [22,25,[48][49][50][51][52] so as to facilitate their usage in biological systems and other soft materials. [53] Against this background,c onsiderable efforts have been devoted both to finding ways to accelerate the thermals teps of overcrowded-alkene motors [26-30, 33, 36-38, 54-57] and to put forth alternative light-driven motor designs that require no thermal steps to complete af ull 3608 rotation.…”

Reversible Photoswitching of Isolated Ionic Hemiindigos with Visible Light

“…[27] Although HI molecules were first synthesized over a century ago [24] and their basic photoswitching characteristics have been known for at least two decades, [25] interest has recently grown due to the development of new derivatives that respond to red light, have high thermal stability of their switching states, are simple to synthesize and function-alize, and exhibit high photostability. [17] Recent contributions have reported asymmetric HI systems that exhibit photoswitching of chiroptical properties, [26] and HTI chromophores that can be deployed as controllable molecular motors, [23,[28][29][30][31][32] receptors [33][34] or tweezers. [19,35] Other applications include light controlled peptide structure [36][37][38] and supramolecular chemistry.…”