Impact of Donor–Acceptor Functionalization on the Properties of Linearly π-Conjugated Oligomers: Establishing Quantitative Relationships for the Substituent and Substituent Cooperative Effect Based on Quantum Chemical Calculations

Abstract

To understand better the impact of donor-acceptor substitution on the properties of linearly -conjugated compounds, we performed a computational study on a series of variably substituted transpolyacetylenes, polyynes, and polythiophenes. The focus of this work is on how rapidly the impact of a given substituent or a given combination of substituents vanishes along the -conjugated chain. The response of the structural (bond-length alternation, rotational barrier) and molecular properties ((hyper)polarizability, chemical shift) to substitution is analyzed using different protocols, including a superposition model for the evaluation of the cooperative effect of substituents in homo-and heterosubstituted oligomers. With the exception of the (hyper)polarizability, the impact of donor-acceptor substitution is found to vanish following an exponential. The rate of decay of the substituent impact is found to be characteristic for each backbone, whereas the choice of substituent determines the absolute value of the respective property. The combination of substituents is shown to determine whether the substituent cooperative effect on a property is of an enhancing or damping nature. The rate of decay of the cooperative effect on most properties, including the (hyper)polarizability, is also found to follow an exponential law. With the exception of the (hyper-)polarizability, the impact of donor-acceptor substitution is found to vanish following an exponential. The rate of decay of the substituent impact is found to be characteristic for each backbone, whereas the choice of substituent determines the absolute value of the respective property. The combination of substituents is shown to lead whether the substituent cooperative effect on a property is of enhancing or damping nature. The rate of decay of the cooperative effect on most properties, including the (hyper-)polarizability, is also found to follow an exponential law.

A large enhancement of (hyper)polarizabilities of polyyne capped by Cu

“…For comparison, the corresponding BLAs of HC 2n H (n = 1-7) based on the MP2/cc-pVTZ optimized geometries are included in Table 2. Note that the Dr is an important quantity for understanding the relationship between the molecular geometry and (hyper)polarizability [1,15,30]. For a smaller Dr, one can generally obtain a larger (hyper)polarizability due to a higher delocalized p-conjugated backbone or a more homogeneous electronic potential in the molecule [36].…”

“…The (hyper)polarizabilities of the linear p-conjugated organic systems, such as polyene and polyyne oligomers, have attracted much attention for more than 30 years owing to its importance in the design of materials with large nonlinear optical responses [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The organic nonlinear optical materials have many merits, such as relatively low cost, ease of design, and optical susceptibilities comparable to or exceeding those of inorganic crystals [1,6,8,10,14,16,17].…”

“…For example, the triisopropylsilyl endcapped polyyne oligomers with up to 10 consecutive AC"CA units have strong responses in nonlinear optical measurements [11]. The long linear p-conjugated organic oligomers may have a tunable electron delocalization by selecting different end-capping units (atoms or groups) [15,18].…”

“…For the long chain conjugated polyynes end-capped by the hydrogen atom, people focus on the length dependence of properties such as the length dependent linear and nonlinear optical polarizabilities because the end-capping H atom was considered to have no significant effect on the properties of molecule. However, this is not the case for the non-hydrogen units [11,15]. It was suggested that the end-capping units could participate in the p-delocalization of the linear carbon chain framework and this participation could significantly affect the properties of molecule [12].…”

“…We find that the cap of Cu largely enhance the nonlinear optical responses for the short chain (n < 4). As an increase of n, however, this enhancement rapidly decreases and trends to vanish at the effective conjugation length [15,28]. The natural bond orbital (NBO) analysis shows that the participation of Cu in the p-delocalization results in a large nonlinear optical polarizability.…”