Density Functional Theory Used in Structure Determinations and Raman Band Assignments for Pseudoisocyanine and Its Aggregate

Abstract

We have performed quantum chemical calculations for the optimized structure and Raman vibrational frequenciessbased on density functional theory at the B3LYP level and using the 6-31G(dp) basis setsfor the cyanine dye cation 1,1′-diethyl-2,2′-quinocyanine, also referred to as pseudoisocyanine (PIC). We have ascertained that the equilibrium structure of ground-state PIC has near C 2 symmetry, with a 46°twist between the planes of the two quinoline moieties that are positioned about the central methine carbon. Vibrational mode analysis of the calculated Raman spectrum suggests that many of the experimentally observed Raman bands between 500 and 1800 cm -1 for monomeric PIC are associated with totally symmetric in-plane deformations of phenyl and/or pyridyl rings, while several weak bands below 500 cm -1 are attributed to out-of-plane doming and ruffling of quinoline macrocycles. We further have noted that (1) upon comparison of the Raman spectrum of PIC monomer with the Raman spectrum measured for aggregated PIC, under nonresonant condition, changes reveal that aggregation results in enhanced scattering for specific vibrational modes that contain principal contributions from in-plane deformation of the phenyl ring in the quinoline moiety; (2) with resonant excitation for the aggregate, observed vibrational modes associated with out-ofplane distortions of the quinoline macrocycle are found to exhibit even greater enhancement. Analysis of our Raman measurements for monomeric and aggregated PIC provides details about the structure of the molecular aggregate. Additionally, calculation of charge distribution, utilizing the Mulliken population analysis approach, indicates that positive and negative charges are alternately and symmetrically distributed over a conjugated ring system, and the positive charges among the peripheral hydrogens and those in the N-ethyl side chains are asymmetrically distributed.

Optical and Resonant Non-Linear Optical Properties of J-Aggregates of Pseudoisocyanine Derivatives in Thin Solid Films

“…The charge distribution in the ground state of the cation PIC is shown in part 8 of this chapter. It coincides with the charge distribution calculated using the theory of the functional density method used in Guo et al (2002) on the qualitative level. The charge redistribution on the atoms of the PIC molecule takes place at the excitation.…”

Whispering gallery modes microcavities with J-aggregates and plasmonic hot spots

“…Moreover, due to better spectral resolution both Raman spectra (curve 3 and 4) show the fine structure of the peaks centered at 569.4 and 573.9 nm, which is not resolved in spectrum 1. An important point is that spectral positions of all peaks in spectra 3 and 4 exactly match the positions of the Raman peaks of PIC Jaggregates reported elsewhere [33]. Three lines forming group B in these spectra are associated with the totally symmetrical in-plane deformations of individual phenyl and pyridyl rings, which are constituents of PIC molecule.…”

Whispering gallery mode resonators with J-aggregates

“…These deformations give major contribution to the Raman optical activity spectra of PIC J-aggregates and therefore these Raman modes are most pronounced. Four lines which form the group A are associated with various (not necessarily totally symmetrical) deformations involving both phenyl and phridyl rings [33]. The spectra presented in Fig.…”