Context <p>Carbon-based nanostructures have attracted considerable attention because of their tunable physicochemical properties and promising applications in nanoelectronics and catalysis. In particular, transition-metal doping of aromatic hydrocarbon frameworks has emerged as an effective approach for tailoring their structural, electronic, and magnetic characteristics. In this work, a comprehensive first-principles investigation is performed to examine the structural, vibrational, and electronic properties of pristine and Mn-doped nanographene fragments. The results indicate that Mn incorporation induces only slight structural distortions while preserving the intrinsic aromatic framework, thereby confirming the structural stability of the doped system. Vibrational analysis, supported by infrared (IR) spectra, further verifies the dynamical stability of both pristine and Mn-doped configurations, with noticeable shifts in characteristic vibrational modes after doping. Electrostatic potential analysis reveals significant charge redistribution localized around the Mn impurity site, demonstrating the strong influence of transition metal incorporation on the electronic environment. Furthermore, electronic structure calculations show that Mn doping effectively modulates the physicochemical properties of C₁₈H₁₂ by introducing localized electronic states and enhanced electronic activity improving its potential for nanoelectronic applications. These findings provide valuable insights into the functionalization of carbon-based nanostructures through transition-metal doping.</p> Methods <p>All calculations were performed within the framework of density functional theory (DFT). The exchange correlation effects were treated using the generalized gradient approximation (GGA) supplemented with the Hubbard U correction (GGA + U) to accurately describe the localized d-electrons of the Mn atom. Structural optimizations were carried out to obtain the ground-state configurations of both pristine and Mn-doped C₁₈H₁₂ systems. Vibrational properties were investigated through phonon and infrared (IR) spectral calculations to confirm dynamical stability. The electronic properties, including band structure and charge distribution, were analyzed to understand the effect of Mn doping on the electronic behavior. Electrostatic potential calculations were performed to examine charge redistribution within the system. All simulations were conducted using a first-principles computational package based on DFT, employing appropriate basis sets and convergence criteria for total energy, force, and charge density.</p>