Asymmetrical benzo[ a ]-fused N 2 O 2 -boron-chelated BODIPYs as red to near-infrared absorbing chromophores: synthesis, characteristics and device applications for opto-electronics †

b Asymmetric benzo[ a ]-fused BODIPYs with benzo(thieno)[1,3,2]oxazaborinine units 1–4 were synthesized for the first time. The structural feature was elucidated by X-ray crystallography and NICS(0) calculation, indicating that the isoindole ring may possess azafulvene character. The photophysical properties of the dyes were investigated, demonstrating that they absorb far-red and NIR light with a l max value of 663–709 nm in THF. Replacement of benzo[1,3,2]oxazaborinine with the thieno-counterpart caused a red-shift in the absorption band. Interestingly, the effect of thiophene insertion on the photophysical properties is dependent on the position of the thiophene in the chromophore, as determined from cyclic voltammetry (CV) measurement and theoretical calculations. Further, N 2 O 2 -ligated sp 3 -boron induces chirality into the chromophore, facilitating optical resolution of 2 by chiral high-pressure liquid chromatography; helical dyes P - 2 and M - 2 were successfully characterized. As a potential device application, a single component device (ITO/dye 2 /Al) was fabricated to produce a photocurrent of 8.42 (cid:2) 10 (cid:3) 7 A cm (cid:3) 2 at a bias potential of 1 V under photoirradiation at 750 nm (128 m W cm (cid:3) 2 ). Compared to the dark current, the on/off current ratio at the same bias potential was determined to be 1.42 (cid:2) 10 2 . The result suggests that a sophisticated combination of asymmetrical BODIPY with a n-type acceptor would allow for NIR photodiodes with a p–n interface.


Introduction
Functional dyes with near-infrared (NIR) absorption bands have received much attention due to their tremendous utility in applications such as optoelectronics, 1,2 photovoltaics, 3,4 dye-sensitized solar cells, 5,6 hydrogen production, 7,8 chemosensing, 9 bioprobes, 3,10,11 photodynamic therapy, 12,13 photoimmuno therapy, 14 etc. Although numerous NIR dyes have been prepared to date, [15][16][17] they often suffer from low thermal and poor photo-stability. Accordingly, band-gap engineering is highly desired for the rational design of useful related materials. BODIPY (4,4-difluoro-4-bora-3a,4adiaza-s-indacene) [18][19][20] is among the most promising candidates due to its excellent photophysical properties and amenability to structural tuning. Structural modification at the periphery and at the boron center 21 enables tuning of the photophysical properties. 22 The traditional strategy is p-extension and fusion of aromatic rings such as benzene, [23][24][25] thiophene, [26][27][28] furan, 29,30 naphthalene, 31 phenanthrene, 32 and others 33 at the a-bond or b-bond of the pyrrole, which leads to a large red-shift in the absorption and fluorescence bands. Alternatively, p-extension by fusing and merging two and more BODIPY units have been studied. [34][35][36][37] Furthermore, it was reported that oligomerization of BODIPY chromophores caused J-aggregation to give redshift in the absorption band. 38 Given that electronic delocalization extended by intramolecular B,O-chelation leads to a dramatic red-shift in the absorption band, [39][40][41][42] structural combination of benzene-annulation at the a-bond and B,O-chelation allowed for the production of NIR BODIPYs. [43][44][45] Using that approach, we demonstrated that the related NIR dyes have great promise for use as NIR photosensitizers in optoelectronics devices. [46][47][48] Given that the BODIPY skeleton is amenable to tuning, the synthesis and characterization of asymmetrical benzo[a]-fused BODIPYs are worthwhile pursuits. [49][50][51][52][53][54][55][56][57][58][59][60] This is because the push-pull character of the asymmetrical p-conjugated structures would impact the photophysical properties. 34,52 However, asymmetrical BODIPYs featuring NIR absorbing bands have rarely been reported. To the best of our knowledge, styryl-containing asymmetric aza-BODIPYs are the only known such species to date 61 It is thus of interest to gain insight into the impact of cyclic fixation of asymmetric 3,5-diaryl (thienyl) BODIPY through B,O-chelation on the photophysical properties. In addition, the resultant tetrahedral geometry at boron can induce helicity 62 in the chromophore to create enantiomeric BODIPYs with an asymmetrical structure.
From the standpoint of device applications, p-conjugated systems hold great promise for developing organic semiconductors. 63,64 Notably, photodetectors that can convert incident NIR photons into an electrical signal have attracted considerable attention due to their many applications in image sensing, optical communication, night surveillance, etc. [65][66][67][68][69] To this end, the development of sensitive photodetectors capable of capturing very weak NIR signals with fast response is highly desired. p-Conjugated polymers with a narrow-bandgap, which serve as organic semiconductors, make it possible to achieve highperformance photodetectors with spectral responses. [70][71][72][73][74][75] On another front, given the advantage of small molecules, including their high purities and well-defined molecular structures, BOD-IPYs are among the most promising candidates. 47,48,76 However, asymmetrical derivatives that can be used in optoelectronic materials with photocurrent generation properties have not yet been reported.
In this work, asymmetrical benzo(thieno) [1,3,2]oxazaborininecontaining BODIPYs 1-4 ( Fig. 1) are synthesized and characterized. The related asymmetric BF 2 -dye 5 is also prepared as a control chromophore. X-ray crystallographic analysis of 2 reveals the packing structure, indicating that the enantiomer pair is present in a triclinic space group due to tetradentate N 2 O 2 -coordination to boron. The racemic mixture is successfully separated by using semi-preparative chiral HPLC. The photophysical properties of the dyes were investigated, demonstrating that they absorb far-red and NIR light with a l max value of 663-709 nm in THF. Replacement of benzo [1,3,2]oxazaborinine with the thieno-counterpart caused a red-shift in the absorption band. Interestingly, the effect of thiophene insertion on the photophysical properties is dependent on the position of the thiophene in the chromophore, as determined from cyclic voltammetry (CV) measurement and theoretical calculations. Thiophene insertion led to longerwavelength emission, with lower fluorescence quantum yields (f). Nevertheless, dye 2 is a NIR fluorophore that emits at 721 nm with F value of 0.21 when excited at 610 nm in THF. Our continuous interest is to fabricate organic photodetectors with asymmetrical benzo[a]-fused BODIPY. Towards this end, a 2-containing film was prepared by vacuum deposition, where the film absorbed NIR light at 735 nm with a long spectral width and a l onset value of 787 nm. As a preliminary result, an indium tin oxide (ITO)/2/Al device allowed for photocurrent detection under light irradiation at 750 nm (128 mW cm À2 ). We demonstrate for the first time that asymmetrical BODIPY-based NIR dyes can serve as organic photodetectors for application in optoelectronics.

Experimental section
General NMR spectra were measured on a Bruker Avance 500 spectrometer ( 1 H: 500 MHz, 13 19 F NMR, respectively. Mass spectrometry data were taken by using a JEOL JMS-700 fast atom bombardment (FAB) spectrometer where m-nitrobenzylalcohol was used as a matrix or a Bruker micrOTOF II-SDT1 spectrometer with atmospheric pressure chemical ionization (APCI) method. FT-IR spectra were recorded on a JASCO FT/IR-4600 equipped with a diamond ATR crystal (JASCO PRO 450S). Melting points for 1, 3, 4, 5, 12 and 13 were recorded on Seiko Instruments Inc EXSTAR6000. Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) for dye 2 were taken by using TGA/DSC 1 (Mettler Toledo). The absorption and fluorescence were measured using Shimadzu UV-3600 UV/vis/NIR spectroscopy and a JASCO FP-6500 spectrofluorometer, respectively. Fluorescence lifetimes were determined by Hamamatsu photonics Quantaurus-Tau C11367-21. Elemental analyses were performed on an Exeter Analytical, Inc. CE-440F Elemental Analyzer. Ionization potential data were measured by a Riken Keiki photoelectron spectroscopy instrument, AC-3. Atomic Force Microscope (AFM) was used AFM-5400 (Hitachi High-Technologies Corporations) in Dynamic Force mode using silicon probes with resonant frequency of 70 kHz and a force constant of 2 N m À1 .

Determination of fluorescence quantum yield
The fluorescence quantum yield (F exp ) was calculated from the eqn (1). 83 where F(l) and F R (l) describe the measured fluorescence intensities of the dye and the reference, respectively, and A and A R describe the corresponding absorbances at the excitation wavelength. The reference used was zinc phthalocyanine (F R = 0.23 in THF) 84

Device fabrication
The dye 2 film with a thickness of 90 nm were formed on the surface of the indium tin oxide (ITO) glass substrate using a resistance heating type vacuum deposition method. Subsequently, the devices were completed by evaporating a 100 nm thick aluminum film as an electrode. Current-voltage (J-V) measurements of the dye-loaded devices were performed using a Keithley 4200-SCS source meter under irradiation at 750 nm (128 mW cm À2 ) by means of a PVL-3300 (Asahi Spectra Co., Ltd) for the device with 2.

Synthesis and structural analysis
The path for synthesis of the targeted dyes is summarized in Scheme 1, where the preparation of the control dye 5 is also included. The key synthons, 1-formyl-3-(2-methoxyphenyl or 2-methoxythionyl)isoindoles 8a and 8b were prepared in 74% and 64% yield, respectively, by the Suzuki-cross coupling reaction of 6 77 with 7a and 7b (Scheme S1, ESI †). Another synthon, 2-(2-methoxythiony)-4-phenyl-1H-pyrrole 9b, was prepared from 10 through a two-step process 48 (Scheme S1, ESI †). The subsequent nucleophilic addition reaction of 8 and 9 in dry CH 2 Cl 2 , followed by boron chelation, demethylation with BBr 3 , and N 2 O 2 boron coordination in the presence of diisopropylethylamine (DIPEA) afforded targets 1-4. As a control, BF 2 dye 5 was also prepared in a similar manner, except for demethylation and N 2 O 2 boron coordination. The dyes were characterized by 1 H and 13 C NMR spectroscopy, mass spectrometry and elemental analysis.
A suitable single crystal of 3-thieno-5-benzo-N 2 O 2 -type BOD-IPY 2 was obtained by the (solvent) vapor diffusion technique using CH 2 Cl 2 /hexane (1 : 1 v/v), where the crystals had triclinic crystal symmetry. The structure was analyzed by X-ray crystallography (CCDC no. 2024003 †); the Oak Ridge thermal ellipsoid plot (ORTEP) diagram is presented in Fig. 2. Boron adopted a tetrahedral geometry in which it binds to the two nitrogen atoms of the isoindole and pyrrole rings, as well as two oxygens linked by thiophene and benzene rings. The N1-B1-O1 and N2-B1-O2 bite angles are 107.7(1)1 and 106.4(1)1, respectively. Although the angles are somewhat smaller than that of prototype N 2 O 2 -boron chelated BODIPY reported by Burgess et al., 87 they are close to the ideal tetrahedral angle of 109.51. The 3-thiophene ring is tilted by 29.61 relative to the pyrrole plane, where this angle is larger than the dihedral angle between the 5-benzene ring and the isoindole plane (Fig. S1, ESI †). Interestingly, the C15-C16 (1.385 (2)) and C23-N2 (1.350(2)) bond distances (Å) are shorter than those of C15-C14 (1.405 (3)) and C5-N1 (1.365 (2)), respectively. NICS(0) calculation 88 based on GIAO CAM B3LYP/6-31G(d,p) was conducted to understand the structural features. The NICS(0) value for the pyrrole unit of the isoindole ring was calculated to be À6.14, which is more positive than that of the pyrrole ring (À7.44), as shown in Fig. 3a. This suggests that the isoindole ring may possess azafulvene character, which means that dye 2 would primarily adopt resonance structure 2B in the resonance equilibration between 2A and 2B (Fig. 3b). Another interesting feature arises from the packing structure shown in Fig. 2b; the two dyes are present as a pair of enantiomers, where a more detailed representation is presented in Fig. S2 (ESI †). This is ascribable to the chiral peripherical environment at the sp 3 -boron Scheme 1 Synthetic routes for of dyes 1-5. Reagent and reaction conditions; (i) POCl 3 , dry CH 2 Cl 2 , room temperature, 2 h. (ii) NEt 3 , BF 3 ÁOEt 2 , dry toluene, 100 1C, 2 h for 13a, 13b and 5. overnight for 13c and 13d (iii) BBr 3 , dry CH 2 Cl 2 , icy condition, 2 h and then DIPEA, BF 3 ÁOEt 2 , dry toluene, reflux, 2 h. (iv) BBr 3 , dry CH 2 Cl 2 , reflux, overnight and then B(OMe) 3 , dry THF, reflux, overnight.

Paper Materials Advances
Open center, 89 where a dimer is formed through intermolecular p-p stacking interaction between the isoindole and pyrrole units, with a distance of 3.624 Å. 90 Further, it was found that CH-p interactions were operative between isoindole-H (H20) and the phenyl ring at the 1 position of an adjacent dye with the same enantiomer (CHÁ Á Ábenzene ring = 2.798 Å).

Photophysical properties
The photophysical properties of asymmetric N 2 O 2 -boron chelated dyes 1-4 and control dye 5 are shown in Fig. 4 [1,3,2]oxazaborinine unit with the corresponding thiophene counterpart further caused a red-shift in the absorption band, which depended on the position of thiophene in the chromophore; 2 with the thiophene ring at the 3 position absorbed far-red light with l max at 699 nm (e max = 6.03 Â 10 4 M À1 cm À1 ), which is 30 nm longer than that of 5-thiophene derivative 3, for which the e max value increased by 1.25 times. 3,5-Dithiophene-containing dye 4 showed the longest absorption band at 709 nm among the dyes tested herein, with longer wavelength absorption edges beyond 753 nm. This indicates that increasing the number of thiophene rings in the chromophore is beneficial for producing NIR dyes, although the e max value was lower than that of dye 2. Fig. 4b displays the fluorescence spectra of the dyes when excited at 610 nm, showing a typical mirror-image relationship between the absorption and fluorescence spectra. Dye 1 emitted far-red light at 682 nm when excited at 610 nm in THF, with a fluorescence quantum yield (F) of 0.48. The lower F value compared to that of 5 is possibly due to increased nonradiative deactivation as the result of a smaller energy gap. 16 It was also found that thiophene-based B,O chelation caused a red-shift in the emission spectra (Table 1). Consequently, 3,5-bis(thiophene)-incorporated dye 4 emitted NIR light at 741 nm, with a F value of 10% in THF. In this way, as l F was redshifted, F value was lowered, which is consistent with the trend of fluorescent lifetime (t) ( Table 1 and Fig. S3, ESI †). However, control dye 5 showed a relatively small value of t (5.78 ns), which could be interrupted by the presence of free rotatable phenyl units at the 3,5 positions because of acceleration of non-radiative deactivation. Evaluation of the solvent dependency of the photophysical properties allowed us to find that the absorption and fluorescence spectra of the dyes were almost insensitive to the solvent polarity (Fig. S4, ESI †),  indicating that these dyes undergo typical p-p* transition, as inferred from the small Stokes shift.

Electrochemistry
To better understand the photophysical character, the electrochemical properties of the dyes were investigated using cyclic voltammetry (CV), as shown in Fig. 5. The formal potential of was 0.197 V versus Ag/Ag + . Reversible oxidation and reduction waves were observed for the N 2 O 2 -dyes (1-4) enabling us to determine the half-wave potentials ((E ox 1/2 ) and (E red 1/2 )) as summarized in Table 2. On another front, control 5 exhibited an irreversible reduction wave, from which the corresponding onset potentials were determined. The electrical gap of the dyes was inconsistent with the absorption bands observed in Fig. 4a, possibly due to some involvement of transitions other than the HOMO -LUMO transition in the absorption band. The HOMO and LUMO energy levels were estimated on the basis of those potential values (Table 2). When compared to BF 2 dye 5, intramolecular B,O cyclization made the HOMO more negative than the LUMO, resulting in a narrower band gap and consequent red-shift in the absorption and fluorescence bands. It was also found that thiopheneinsertion in the chromophore had a stronger impact on the HOMO level than on the LUMO. Notably, the HOMO level of 3-thiophene analogue 2 was higher-lying than that of 5-thiophene stereoisomer 3. Moreover, the HOMO level became more destabilized as the number of thiophene units in the chromophore increased. In this way, the HOMO levels were energetically higher in the following order: 4 4 2 4 3 4 1, resulting in a smaller HOMO-LUMO bandgap. The trend observed here was rationalized by theoretical calculation (vide infra).

Theoretical analysis
Time-dependent density functional theory (TD-DFT) and DFT analyses at the CAM-B3LYP/6-31G(d,p) level were conducted using Gaussian 16 software 86 to understand the photophysical properties of the dyes. Tables S1-S5 (ESI †) summarize selected transitions from the theoretical data for the N 2 O 2 -type dyes (1-4) and 5. Based on the oscillator strength (f), the major absorption bands were from S 0 -S 1 transitions, corresponding to the HOMO -LUMO transition. Initially, the HOMO/LUMO energy    gap of dye 1 was smaller than that of 5 by 0.20 eV. This observation supports the deduction that intramolecular B,Ochelation causes a red-shift in the absorption band. The electron-density distributions of the HOMO and LUMO levels are also presented in Tables S1-S5 (ESI †), which indicate that the p-p* transitions are primarily derived from both orbitals spread over the BODIPY backbone involving substituents at the 3 and/or 5 positions, as supported by the small Stokes shifts of the dyes (vide supra; Table 1). However, comparison of the surface plot of the HOMO with that of the LUMO suggests a decrease in the electrondensity distributions of the 3-benzo(thieno) [1,3,2]oxazaborinine unit based on the energy of the HOMO -LUMO transition. In order to visualize the electron-density redistribution during the HOMO -LUMO transition, electron density difference plots (EDD) were employed (Fig. 6), 91 indicative of partial charge-transfer character with a transition from the benzo(thieno) [1,3,2]oxazaborinine at the 3-position to the BODIPY core. Given the electron-donor character of thiophene, the electron density depletion of the 3-thiophene ring during the transition is larger than that of the 5-thiophene ring, indicating that introduction of the thiophene donor at the 3 position in dye 2 is more effective than that at the 5 position in 3. Accordingly, 3-thiophene dye 2 has a higher-lying HOMO than stereoisomer 3, which is consistent with the CV data. Such insight strongly supports the red-shift in the absorption and fluorescence bands of 2 compared to those of 3. Finally, the 3,5-dithiophene-containing dye 4 had the smallest calculated energy gap for the S 0 -S 1 transition, which reproduces the experimental result. Taken together, incorporation of thieno [1,3,2]oxazaborinine into the dye skeleton would not only contribute to expansion of the p-p* transition of the chromophore, but also tune the HOMO energy level. The broad absorption band ranging from 300 to 450 nm may include plural transitions of HOMOÀ1 -LUMO, HOMO -LUMO+1, and HOMO -LUMO+2.

Optical resolution
Another interesting feature of the N 2 O 2 -boron-chelated BODI-PYs is the helicity with respect to the central boron atom (vide supra; Fig. 2). 89 Optical resolution of dye 2 was conducted by preparative chiral HPLC (Daicel CHIRALPACK-IE) to successfully obtain the corresponding enantiomers (Fraction A and B; Fig. S11, ESI †). The normalized ECD spectra of each fraction, as well as the UV/vis absorption spectra in THF, are shown in Fig. 7. For Fraction A, a positive significant CD signal was observed in the far-red region with l max of 699 nm, assignable to the S 0 -S 1 transition, whereas a relatively low-intensity negative CD signal was detectable in the shorter wavelength region (300-480 nm). In contrast, a mirror image CD spectrum was observed for Fraction B, indicating an enantiomeric counterpart of Fraction A. In order to determine the absolute configuration of each fraction, the CD spectrum of the P-isomer of 2 was calculated and compared with the experimental CD spectrum (Fig. S11, ESI †). Thus, Fraction A was assigned as the P-isomer. Accordingly, Fraction B is likely to be the M-isomer. Taken together, the CD intensity was derived from the BODIPY chromophore with a helical conformation of the peripherical boron center. The anisotropic factor g was determined to be 2.26 Â 10 À3 and 2.24 Â 10 À3 for the P-isomer  and M-isomer, respectively. Chiral BODIPYs with a significant absorption band in the far-red region deserve further investigation in terms of chiroptical applications. 92

Fabrication of photodetector
To determine if asymmetrical BODIPYs could serve as NIR photodetector materials, the 3-thiophene-5-benzene-containing dye 2 was selected as a plausible sensitizer for targeted device application. This is because 2 absorbs more strongly far-red and near infrared light, as shown in Fig. 4a (l max = 699 nm, e max = 6.03 Â 10 4 M À1 cm À1 ). Prior to device fabrication, the thermal properties of 2 were investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) (Fig. S12, ESI †); the observed melting point of 2 was 286 1C. Based on the thermal analysis, resistance-heating type vacuum deposition was employed to prepare a dye film on a quartz plate. The absorption spectrum of the film was measured, which showed a nearinfrared absorption band at 735 nm and a l onset value of 787 nm (Fig. 8a). Compared to that in THF, a bathochromic shift of 36 nm was detected. An amorphous film was successfully formed on an ITO glass substrate in a similar method, and a flat film with a root mean square (RMS) thickness of 0.54 nm was obtained, as inferred from the atomic force microscopy (AFM) image of the 1 Â 1 mm area (Fig. 8b). The ionization energy of 5.30 eV for the film was determined by photoelectron spectroscopy 93 under ambient conditions (Fig. S13, ESI †), where the energy was transferred to the HOMO energy level as À5.30 eV. The value is lower than that of the ITO electrode (À5.0 eV). On another front, the LUMO energy level determined from the l onset value of the film was À3.72 eV, which is higher than the work function of Al (À4.3 eV). Taken together, photo-generated electron and hole injection would be energetically favorable enough to provide photodetector functionality. The current-voltage ( J-V) measurements for the dye-loaded device under photoirradiation at 750 nm (128 mW cm À2 ) are shown in Fig. 9a. The 2-loaded device produced a photocurrent of 8.42 Â 10 À7 A cm À2 at a bias potential of 1 V. Compared to the dark current, the on/off current ratio at the same bias potential was determined to be 1.42 Â 10 2 .
In this way, an increase in the forward injection of current under NIR irradiation was clearly detected. Fig. 9b shows the wavelengthdependent photocurrent densities of the 2-loaded device (ITO/ 2/Al); the spectral profile (with a maximum photocurrent at 735 nm) was almost the same as the absorption band of 2 in the film. Thus, near-infrared photons absorbed by 2 were converted to a photocurrent in the single-component device. Fig. 10 presents the light-associated cycle for photocurrent production and non-production, where the device showed a quick response to irradiation at 750 nm (128 mW cm À2 ). On the other hand, the photocurrent quickly fell to zero when the 750 nm irradiation was discontinued. This result demonstrates the potential of  where R (A W À1 ) is the responsivity, which is defined as the ratio of photocurrent ( J ph ) to incident-light intensity (L light ), and q and J d the absolute value of electron charge (1.6 Â 10 À19 Coulomb) and dark current, respectively. Given that J ph and J d were measured to be 2.89 Â 10 2 A cm À2 and 4.31 Â 10 À11 A cm À2 at 0.1 V bias potential, respectively. As a result, ITO/2/Al device showed a photodetectivity of 6.08 Â 10 11 Jones at 750 nm.

Conclusion
In summary, this work for the first time presents the synthesis and characterization of a new family of B-O constrained asymmetric BODIPYs. The structural feature was elucidated by X-ray crystallography and NICS(0) calculation, indicating that the isoindole ring may possess azafulvene character. It was found that the structural rigidity imparted upon coordination to boron caused a significant red-shift in the absorbance and fluorescence properties. Based on theoretical calculations, the longest absorption band of the dyes is derived primarily from the p-p* transition within the BODIPY core, which is significantly influenced by thiophene insertion, accompanied by destabilization of the HOMO energy level. As a result, 3-benzo-5-thiophene-containing analogue 2 absorbed far-red and NIR light at l max = 699 nm in THF. The electrochemical measurement and theoretical approach indicate that incorporation of thieno [1,3,2]oxazaborinine into the dye skeleton would not only contribute to the expansion of the p-p* transition of the chromophore, but also tune the HOMO energy level. In the latter case, the EDD plot suggests that the chromophore may possess partial charge-transfer character, with a transition from the constrained thiophene ring at the 3 position to the BODIPY core, which is ascribable to an increase in the HOMO level. This deduction is supported by the fact that the absorption band of 3-thiophene 2 is red-shifted relative to that of 5-thiophene 3. Further, as the number of thiophene units increased, the absorption band was more strongly red-shifted to the NIR region for 3,5-dithiophene 4 (l max = 709 nm, l onset = 753 nm). Another interesting feature is that N 2 O 2 -type BODIPYs possess helicity with respect to the central boron atom. Optical resolution of 2 was conducted by preparative chiral HPLC to give both P and M-enantiomers, with an intense absorption band in the far-red region. Our ongoing program exploring NIR photodetectors in optoelectronics motivated us to fabricate a singlecomponent device (ITO/2/Al). When illuminated by 750 nm NIR light (128 mW cm À2 ) the device produced a photocurrent of 8.42 Â 10 À7 A cm À2 at a bias potential of 1 V. Compared to the dark current, the on/off current ratio at the same bias potential was determined to be 1.42 Â 10 2 . The increase in the forward injection of current under NIR irradiation was ascribed to direct dissociation of the photoexcited electron-hole pairs. The results suggest that a sophisticated combination of dye 2 with an n-type acceptor would allow for NIR photodiodes with a p-n interface 69 and deserve further investigation.

Conflicts of interest
There are no conflicts to declare.