Visible absorbing symmetrical squaraine and croconine dye derivatives: A comparative computational study

Abstract

Visible absorbing C-N bonding squaraines (SQ1-SQ5) and croconines (CR1-CR5) with an increase in conjugation at donor groups and heteroaromatic donor substituents have been studied by density functional theory and time-dependent density functional theory methodologies. In these molecules, croconines always have absorption nearly 100-nm red shift than its corresponding squaraines (it is in consistent with C-C bonding, near infrared absorbing squaraine and croconines). The reason behind this drastic red shift, by changing the central acceptor of 4-membered squarate ring with 5-membered croconate ring, has been analyzed by considering the concept of diradical character and variation in central C-C-C angle. It is also observed that within the same series of molecules (either in squaraine or in croconines), with an increase in donor capacity (conjugation), absorption increases towards longer wavelength region because of destabilization of HOMO and stabilization of LUMO levels. A small blue shift was observed for heteroaromatic donor groups when compared with aromatic donor group.

Potential Use of Squarates and Croconates as Singlet Fission Sensitizers

“…Relative stability between the two resonant forms depends on the substituents (R) in the ring . Due to the existent similarities between the backbone of both croconate and squarate derivatives and the oxyallyl diradical, some diradical character was expected in these molecules, as it has been shown in previous works, where a broad range of diradical characters were computationally obtained ,,,. In general, croconates exhibit stronger tendency towards the diradical form compared with their squarate counterparts, which may be related to the experimentally observed redshifted absorption maxima on croconates in comparison with the correspondent squarates.…”

“…Thus, it is observed a general decrease of the S 0 →S 1 and S 0 →T 1 transition energies in croconates with respect squarates that can be related to systematically larger diradical characters (see Figure , bottom left and right). Experimentally, it has been observed a redshift of around 0.5 eV (100 nm) in the first visible excited state of croconates with respect to the corresponding squarate . The shift of these transition energies to lower energies due to the enhancement of y 0 may be used to tune the optoelectronic properties, to include a broader range of wavelengths.…”

“…Besides, four additional derivatives, R 12 ‐R 15 , may also be considered, as the C1 condition is nearly fulfilled. It must be emphasized that R 2 ‐R 4 , R 6 , R 21 and R 22 have been already synthesized ,. Nevertheless, the E[S 1 ] values are, in general, lower than required for an optimal match with the silicon energy gap, and only the R 6 , R 7, R 9 , and R 10 show E[S 1 ] >2 eV.…”

“…[29] Due to the existent similarities between the backbone of both croconate and squarate derivatives and the oxyallyl diradical, some diradical character was expected in these molecules, as it has been shown in previous works, where a broad range of diradical characters were computationally obtained. [29,30,31,32] In general, croconates exhibit stronger tendency towards the diradical form compared with their squarate counterparts, which may be related to the experimentally observed redshifted absorption maxima on croconates in comparison with the correspondent squarates. This redshift points to a smaller HOMO-LUMO energy difference, a feature that favors the formation of open-shell singlet or triplet states and, therefore, the enhancement of the diradical character.…”

“…In this sense, squarates and croconates appear as potential candidates to undergo SF efficiently. Some authors such as Tripathi et al . or Thomas et al .…”