Axially Chiral Organic Semiconductors for Visible‐Blind UV‐Selective Circularly Polarized Light Detection

Abstract Technologies that detect circularly polarized light (CPL), particularly in the UV region, have significant potential for various applications, including bioimaging and optical communication. However, a major challenge in directly sensing CPL arises from the conflicting requirements of planar structures for efficient charge transport and distorted structures for effective interaction with CPL. Here, a novel design of an axially chiral n‐type organic semiconductor is presented to surmount the challenge, in which a binaphthyl group results in a high dissymmetry factor at the molecular level, while maintaining excellent electron‐transporting characteristics through the naphthalene diimide group. Experimental and computational methods reveal different stacking behaviors in homochiral and heterochiral assemblies, yielding different structures: Nanowires and nanoparticles, respectively. Especially, the homochiral assemblies exhibit effective π–π stacking between naphthalene diimides despite axial chirality. Thus, phototransistors fabricated using enantiomers exhibit a high maximum electron mobility of 0.22 cm2 V−1 s−1 and a detectivity of 3.9 × 1012 Jones, alongside the CPL distinguishing ability with a dissymmetry factor of responsivity of 0.05. Furthermore, the material possesses a wide bandgap, contributing to its excellent visible‐blind UV‐selective detection. These findings highlight the new strategy for compact CPL detectors, coupled with the demonstration of less‐explored n‐type and UV region phototransistors.


Experimental Procedures
OFET fabrication.OFETs based on thin films of (S)-1 and (R)-1 were fabricated using heavily n-doped (100) silicon wafers with 300 nm thick SiO2 (Ci = 11.5 nF cm -2 ).Wafers were cleaned with a piranha solution for 30 min and underwent UV-ozone treatment.The wafer surface was then treated with n-octadecyltirmethoxysilane (OTS) to form a selfassembled monolayer.3 mM OTS solution in trichloroethylene was spin-coated at 500 rpm for 5 s, at 1500 rpm for 30 s, and at 500 rpm for 5 s.Then, the wafer was exposed to ammonia vapor overnight in a glass desiccator.Synthesized (R)-1 and (S)-1 was deposited (40 nm thickness) on the wafer by thermal evaporator under different substrate temperatures.Thermal evaporation was conducted at a crucible temperature of 230−260 o C in a rate of 0.1 Å s -1 under ultrahigh vacuum condition (< 5 × 10 -6 torr).Gold electrodes (40 nm thickness) were thermally evaporated using shadow masks to form source and drain electrodes with a channel length (L) of 50 μm and a channel width (W) of 1000 μm.
Optoelectronic measurements.The electrical performance of OFETs was measured in N2 environment using Kiethley 4200-SCS semiconductor parametric analyzer.Photoresponse characteristics were measured inside a vacuum chamber and monochromic 385nm LED (Thorlabs, M385L3) was used.For testing the spectral response, monochromic light was generated using a 300 W Xenon lamp and Oriel Cornerstone 130 monochromator with dual gatings.The circularly polarized light was generated using a linear polarizer and a quarterwave plate (Thorlabs, AQWP05M-340).The generated light was calibrated by placing the conventional Si photodetector in the same position with the devices.

Computational analysis (Molecular Dynamics Simulations
).Three systems were simulated, named Rsys, Ssys and Racesys.Each system consisted of 24 molecules of (R)-1, 24 molecules of (S)-1 and 12 molecules of both (R)-1 and (S)-1, with 2400 molecules of MCH solvent.Initial box size of all systems was 9.5×9.5×9.5 nm.During the process of constructing the initial configuration of Rsys, a 2×3×4 array of 24 molecules of (R)-1 was placed in parallel.The COG distances between the parallel NDIs of each molecule along the x, y, and z axes were set to 2.0 nm, 2.5 nm, and 1.5 nm.S1] The initial configuration of Ssys consisted of 4×3×2 array of 24 molecules of (S)-1, with distances of 1.5 nm, 2.0 nm, and 2.5 nm along the axes.For Racesys, a 4×3×2 array of S5 placement of same molecule type, (R)-1 and (S)-1 were alternately placed.A solvent was randomly placed in the void to prevent atom overlapping.
All the systems and simulations were performed under 1 bar and 363 K, which is below the boiling point of MCH solvent.The simulation process started with energy minimization with steepest descent method.Subsequently, a 1 ns NPT run was conducted with Velocity Rescaling (v-rescale) thermostat and Berendsen barostat to rapidly approach density values near equilibrium.A 5 ns NPT run was carried out with Nose-Hoover thermostat and Parrinello-Rahman barostat was followed by a 150 ns NVT run with Nose-Hoover thermostat to achieve an equilibrium state.Additionally, 20ns production run under NVT ensemble was performed and used in analysis process.
S5] In the simulations, cutoff distance of 1.0 nm for an electrostatic and Van der Waals potential was set, and periodic boundary conditions were applied.For handling long-range electrostatic potentials, Particle Mesh Ewald method [S6] was applied.During the production run, simulation time step was 1fs and trajectory data was saved at intervals of 1 ps.
S9] The structure optimization at the gamma point with 520 eV of a plane wave energy cutoff.
The Perdew-Burke-Ernzerhof (PBE) functional [S10] and van der Waals dispersion correction were applied to calculate energy of stable structures.The calculations were continued until the energy difference of the total and the band structure energy became smaller than 10 -6 eV, with a maximum force criterion for convergence set to 0.05 eV Å -1 .Mixing parameters from Kerker mixing scheme [S11] of A, and B were set to 0.3 and 0.5.
For the initial configuration, systems were constructed with 2 molecules of either (R)-1 and (S)-1 of the same type of different types.These two molecules were positioned at a minimum distance which is about 3.85 ± 0.2 Å from one of the inner atoms constituting the COG of within the NDIs, which is shorter than the stacking criteria distance, in order to mimic the stacking structure without atomic overlap.To focus on a single pure stacking interaction between NDIs, a vacuum was set along all axes more than 10 Å .Additionally, calculations were performed for systems consisting of a single molecule of (R)-1 and (S)-1 under the same conditions above to determine the energy difference between those two molecule systems, which represents the formation energy.
For heterochiral stacking formation energy, For homochiral stacking formation energy, Cyclic Voltammetry.Cyclic voltammetric measurements were carried out using a threeelectrode cell consisting of a polished 2 mm glassy carbon as the working electrode, Pt as the counter electrode, and Ag/AgNO3 as the reference electrode.Solutions in chloroform were prepared with concentrations of 1.0 × 10 −3 M for (R)-1 and (S)-1, and 0.10 M for the supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAPF6).To determine the LUMO levels relative to the vacuum level, the redox data were standardized to the ferrocene/ferrocenium couple with a calculated absolute energy of −4.8 eV, using the following equation;

Estimation of Optoelectrical Properties
To explore the photosensitivity of organic phototransistors, the transfer characteristics were analyzed under light irradiation to determine the photoresponsivity (R) and the ratio of photocurrent to dark current (P).The values for R and P were determined using the following equations: where Iph is the photocurrent, Pinc the incident illumination power on the channel of the device, Ilight the drain current under illumination, and Idark the drain current in the dark, respectively.
Furthermore, the external quantum efficiency (EQE) (η) of OPTs was calculated.This quantity is expressed as the ratio of the number of photogenerated carriers that effectively increase the drain current to the number of photons that are incident on the area of the OPT channel.This can be determined using the following equation.
where h is the plank constant, c the speed of light, e the fundamental unit of charge, Pint the incident power density, A the area of the transistor channel, and λpeak the peak wavelength of the incident light, respectively.
The concept of detectivity typically refers to the minimum detectable signal, which allows the comparison of photodetector devices with varying configurations and sizes.In this study, D * was determined by applying equations ( S7) and (S8).
In these equations, A is the photodetector active area, NEP the noise equivalent power, and   2 the measured noise current, and Δf the bandwidth.If the major limit to detectivity is shot noise from the drain current under dark conditions, D * can be simplified as equation (S9).
The rise time is defined to be the time required for the current to increase from 10% to 90% of the peak value after light illumination, whereas the fall time is estimated to be the time required for the current to decrease from 90% to 10% of the maximum value.
The resulting precipitate was filtered and dissolved in dichloromethane.The compound was washed with brine and dried over MgSO4. 2 was used directly for next reaction without further purification.Only a small amount was purified by silica gel column chromatography for characterization purpose.

Figure S14 .
Figure S14.CD spectra of (S)-1 films deposited at substrate temperature of RT, 125 o C, 160 o C and 180 o C.

Table S4 .
Comparison of this work and other recently published chiral organic small molecule OPTs