Table 3.4: Emission energies (λem in nm), Corresponding Oscillator Strength(fosc),
|µ|,|m|, cosθ,R,gCPL,Kf, and|µ|/|m|of(PT)3,(PT)4, and(PT)5. |µ|,|m|andR are in esu·cm, erg·G−1 and esu·cm·erg·G−1 units, respectively. Kf is in ns−1. All the
considered transitions are S1→S0.
System λem fosc |µ|/10−20 |m|/10−20 cosθ R/10−40 gCPL |m|/|µ| Kf (PT)3 388 0.705 762.72 4.55 0.000 -0.18 0.000 0.006 0.3120 (PT)4 389 0.438 602.18 6.41 -0.138 -533.93 -0.006 0.011 0.1926 (PT)5 393 0.318 516.08 4.61 -0.120 -284.74 -0.004 0.009 0.1372
by ADC(2).
M06-2X andωB97XD functionals match well with the benchmark ADC(2) results. Re- sults of PBE0 and B3LYP are very different than the ADC(2) results, even for the planar oligomers. For the UV spectra, while the pure functionals’ results are red-shifted, results of long-range corrected pure functionals are blue-shifted in comparison to the ADC(2) results. Analysis of excited states was also carried out by computing descriptors such as exciton sizes, charge transfer numbers, and number of NTOs participating in a particular transition. Here the results of CAM-B3LYP, M06-2X, and ωB97XD functionals match with each other. But these DFT descriptor results sometimes differ from the ADC(2) results regarding the state ordering and number of NTOs participating. Overall, it is found that functionals like CAM-B3LYP, M06-2X, and ωB97XD work better than the others for oligomers of pyridine-thiophene. Future work will focus on the exciton dynamics in these types of helical systems.
Chiroptical properties of polyaza[9]helicene and its
derivatives
Fusion Elongation
Chiroptical Properties
|m|=1.96
|m|=0.65 gCPL=0.000
|m|=2.01 gCPL=-0.055
|m|=2.33 gCPL=-0.037 gCPL=-0.052
In this chapter, the effect of lateral and helical extensions on the physical and chiroptical properties of azahelicenes is reported. Starting with the experimentally re- ported azahelicene 9Ha, three derivatives, two with laterally fused rings and the third with larger helical length, are designed. For the excited state properties such as UV-vis and CD spectra, performance of different DFT functionals is evaluated by comparing the results against the ADC(2) results. CPL properties are calculated at DFT level.
Among the three designed systems, pyrazine based 9HaP shows improved gCPL value compared to the parent 9Ha. On the other hand, quinoxaline based9HaQ is found to be the worst CPL emitter with the lowest dissymmetry factor. The helically extended derivative, 11Ha, shows good CPL results, but the gCPL remains smaller than for the parent system. The CPL results are analyzed in terms of electric and magnetic dipole transition moment vectors, and angles between these two vectors. A part of the content of this chapter is published in J. Phys. Chem. A,2022, 126, 8, 1412-1421.
4.1 Introduction
Efficiencies of CD and CPL active systems are evaluated in terms of dissymmetry factors, gCD and gCPL, corresponding to CD and CPL, respectively,27 as mentioned in Section 2.9. A good light emitter should also show high ΦFL, in addition to showing good CPL properties. Achieving good values for both gCPL and ΦFL simultaneously is difficult, and this has been an active area of research. For carbohelicenes, both the ΦFLand gCPL
values have been found to be small.7,59,108,109 The smaller value of ΦFL is attributed to the favored intersystem crossing pathways.
In many cases, substitutions by different groups53,54,57 and introduction of aro- matic rings to produce ring-fused carbo[n]helicenes7,58–62 have been shown to be good strategies to improve the ΦFL values and alter the chiroptical properties. Introduc- tion of quinoxaline (Qx) unit has been shown to result in a four-fold increase in ΦFL
in carbo[7]he-licene.58 In addition, it was also shown that introduction of alkyl chains in Qx-fused carbohelicenes produced a much larger ΦFL at 0.25.7 Similarly, fusion of maleimide unit with the carbo[5]helicene resulted in a larger ΦFL and good gCPL val- ues.59 Recently, two carbo[6]helicenic fragments fused with the perylene diimide unit showed excellent ΦFL and CPL properties.60
In addition to carbo[n]helicenes, hetero[n]helicenes having aromatic rings with het- eroatoms in the main skeleton have also been explored in recent times. Presence of atoms such as S, O, and N helps in tuning the electronic structures and properties.28,29,66–69,111
Yamamoto et al.69 showed that introduction of modified Qx onto tetrathia[9]helicene skeleton enhanced both the ΦFL and gCPL values. Helicenes consisting of one or more nitrogen atoms are known as azahelicenes. Otani and co-workers112 have synthesized orange-to-red CPL emittingQx-fused polyaza[7]helicenes (denoted as7HQ). Recently, Otani et al.11 reported the synthesis of polyaza[5]-[9]helicenes. While gCPL value was the highest for the polyaza[9]helicene (denoted as 9Ha in this work), ΦFL was found
the central unit of 9Ha (the resultant systems are denoted as 9HaP and 9HaQ, re- spectively) and the third one is with a larger helical length containing eleven rings, polyaza[11]helicene (denoted as11Ha). As mentioned in the previous paragraph, many Qx-based helicenes have been synthesized recently and chiroptical properties have been studied.7,58,69,112,241–244Similarly, many pyrazine-fused helicenes are also reported in the literature.241–244 In addition, extension of helical length has also been shown to improve the results in many cases.4,11,62,245–247 Keeping the above in mind, our aim is to explore the effects of introduction of these two aromatic acceptor rings and the enlargement of the helical size on the chiroptical properties such asgCD/gCPLandKf. For an electronic transition, these dissymmetry factors are calculated by using the Eq. 2.46, mentioned in Chapter 2. Typically, magnitudes of m are very small for π-π∗ transitions in most organic molecules.28,66 Therefore, there is a large number of studies in the literature to maximize the|m|.28,29,57,66,248 Keeping the above in mind, DFT based calculations are carried out to explore the absorption, CD and CPL properties of these three systems, and compare these results against the values for9Ha. In the following section, we pro- vide the computational details of the calculations. This is followed by presentation of results in the Results & discussion section. In the last section, we provide a conclusion of our studies.