**4. Conclusions**

In this work, we conducted local mode analysis, electron density analysis, and aromaticity delocalization index (AI) calculations (based upon vibrational frequencies) for a set of six neutral pancake-bonded systems, di-chalcodiazoyl dimers (**1**–**3**) and phenalenyl-based dimers (**4**–**6**), as to elucidate on the strength of pancake bond interactions within dimers, the ring strength of their monomers, the nature of the pancake bond interactions, the effect of substituents on the aromaticity of phenalenyl-based species, and the effect of dimerization on the aromaticity for phenalenyl-based species. The local stretching force constants, being suitable descriptors of bond strength and *π*-delocalization, are used to describe the pancake bond interactions of **1**–**6** and the degree of *π*-delocalization amongst these bonds and their corresponding dimer species. Directly from computed local stretching force constants we derived bond strength orders. We use measures of AI, and corresponding WS and ALT parameters, to determine what bond property, of the phenalenyl-based species investigated, predominately governs changes in aromaticity. From the results of our work we draw the following: [**1**] We find that dimer species **1** (1,2,3,5-dithiadiazolyl) and **2** (1,2,3,5-diselenadiazolyl) are significantly stabilized by their chalcogen⋯chalcogen contacts. Unlike **1** and **2**, which have *C*2*<sup>v</sup>* symmtery, the 1,2,3,5-ditelluradiazolyl (**3**) dimer is found to be stable in C2 symmetry as the singlet state is energetically favored over the triplet state, revealed from a negative ΔE*ST*. [**2**] In regard to the phenalenyl-based dimers, as the substituent size increased from **4** to **6** the stability of the system steadily declined as the steric repulsion between the substituent groups hindered the monomers of these dimers from changing into a orientation of lower energy. [**3**] As the radius of the chalcogen atoms di-chalcodiazoyl dimers **1**–**3** increase (Te < Se < S) the strength of the C⋯C contacts decreases. As the strength of the chalcogen⋯chalcogen interactions (i.e. contacts) decrease from **1** to **3** the overall ring strength decreases and the strength of the central (i.e. interdimer) C–C bond decreases [**4**]. For all phenalenyl-based dimers (**4**–**6**) we observed that the BSO *n* values of peripheral C⋯C are stronger that of their corresponding central C–C bonds. Revealing that pancake bonding interactions contribute largely to the stability of these species [**5**]. From energy density analysis *Hb*, following the Cremer-Kraka criteria, we observe the chalcogen⋯chalcogen pancake bonding interactions of the 1,2-dithia-3,5 diazolyl dimer (**1**) and 1,2-diselena-3,5-diazolyl dimer (**2**) are covalent in nature as they have negative (stabilizing) *Hb* values at their bond critical point *rb*. [**6**] Unlike the other 1,2-chalcogen-3,5-diazole dimers (**1** and **2**) the chalcogen⋯chalcogen contacts (i.e. Te⋯Te) of **3** are much weaker in strength and have a positive (destabilizing) energy density value *Hb* at the Te⋯Te bond critical point *rb* revealing that the Te⋯Te do not have a typical pancake bond nature as we observed **1** and **2**. [**7**] All pancake bonding interactions within the phenalenyl dimer (**4**), 2,5,8 trimethylphenalenyldimer (**5**), and the 2,5,8-tri-t-butylphenalenyl dimer (**6**) were observed to have postive (destabilizing) *Hb* values revealing that their pancake interactions are electrostatic in nature. [**8**] From BSO n(CC) values, the calculated AI, and related WS and ALT parameters we found that the dimerization of phenalenyl-based monomers leads to an increased aromaticity primarily due to CC bond strengthening. [**9**] From the same parameters mentioned above we observed that the substitution of the phenalenyl dimer, which is necessary for inhibiting

*σ*-dimerization, results in an overall reduction of system aromaticity predominantly through CC bond weakening.
