The relentless demand for advanced optical technologies, from high-speed data transmission to sophisticated laser systems, is fundamentally constrained by the performance of nonlinear optical (NLO) materials. Developing novel, highly efficient NLO materials thus remains a critical scientific endeavor, with significant implications for next-generation devices [1]. Such materials are essential for applications spanning optical sensing [2,3], signal control [4,5], switching [3,6] and advanced laser devices [7]. Among various NLO material classes, organic compounds have garnered significant interest over their in Ref. [8], ultra-fast response times [9], and highlight damage thresholds [10]. Their remarkable electrical properties, tunable π-conjugation, and structural flexibility further position them as a crucial focus for high-performance NLO material synthesis and design [11,12].

Chemically stable antiaromatic compounds are rare. However, fusing anti-aromatics within larger π-conjugated systems is an appealing strategy to stabilize these inherently reactive motifs, creating structures with distinct aromatic and antiaromatic regions [13]. The presence of antiaromatic regions in these systems often leads to unique electronic configurations, conferring exceptional optical and electrochemical characteristics crucial for tailored optoelectronic performance and high nonlinear optical (NLO) response. This has driven significant research into the aromatic/antiaromatic properties of planarized conjugated cyclooctatetraenes as a key model system [14]. These efforts have led to the realization of diverse π-architectures, ranging from fully conjugated systems to those featuring discrete, non-fused antiaromatic units [15,16]. Building on the study of planarized antiaromatic systems, [n]circulenes represent a fascinating class of π-conjugated architectures. While carba[n]circulenes such as corannulene (n = 5) and coronene (n = 6) have attracted interest in molecular electronics and materials science [17,18], recent attention has turned toward hetero[n]circulenes. In particular, aza- and oxa[8]circulenes possess a planar eight-membered core with intrinsic antiaromatic character, prompting fundamental investigations into their electronic structure [19]. Diazadioxa[8]circulene derivatives have demonstrated encouraging optical and electronic properties, including blue emission in organic light-emitting diodes, making them attractive candidates for functionalization toward optoelectronic and nonlinear optical applications [[20], [21], [22]].

The first hyperpolarizability (β), a key parameter in evaluating second-order NLO activity, directly correlates with the strength of a material's nonlinear optical response [23,24]. Despite growing interest in alkali-metal-enhanced nonlinear optical materials, no prior study has investigated the NLO properties of alkali metal-doped diazadioxa[8]circulene (M@C8, M = Li, Na, K) complexes. Alkali metals (Li, Na, K) were selected as dopants due to their high electropositivity and low ionization potential, facilitating efficient charge transfer to the π-conjugated framework. Their gradual increase in atomic size enables systematic tuning of interaction strength and NLO response in circulene-based systems. In this study, we employ density functional theory (DFT) to systematically investigate the structural, electronic, optical, and nonlinear optical properties of M@C8 complexes. We assess their thermal stability, examine charge-transfer characteristics, and quantify the enhancement in hyperpolarizability introduced by alkali metal adsorption. Our findings provide key insights into the design of high-performance organic NLO materials based on antiaromatic circulene scaffolds [25]. Recent studies have highlighted the importance of molecular environment and dopant engineering in optimizing NLO performance, including the effects of donor-acceptor interactions [26], alkali-metal doping [27], and charge redistribution [28].