Nature of Linear Spectral Properties and Fast Electronic Relaxations in Green Fluorescent Pyrrolo[3,4-c]Pyridine Derivative

Abstract

The electronic nature of 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT) was comprehensively investigated in liquid media at room temperature using steady-state and time-resolved femtosecond transient absorption spectroscopic techniques. The analysis of the linear photophysical and photochemical parameters of HPPT, including steady-state absorption, fluorescence and excitation anisotropy spectra, along with the lifetimes of fluorescence emission and photodecomposition quantum yields, revealed the nature of its large Stokes shift, specific changes in the permanent dipole moments under electronic excitation, weak dipole transitions with partially anisotropic character, and high photostability. Transient absorption spectra of HPPT were obtained with femtosecond resolution and no characteristic solvate relaxation processes in protic (methanol) solvent were revealed. Efficient light amplification (gain) was observed in the fluorescence spectral range of HPPT, but no super-luminescence and lasing phenomena were detected. The electronic structure of HPPT was also analyzed with quantum-chemical calculations using a DFT/B3LYP method and good agreement with experimental data was shown. The development and investigation of new pyrrolo[3,4-c]pyridine derivatives are important due to their promising fluorescent properties and potential for use in physiological applications.

Keywords: femtosecond transient absorption spectroscopy; linear spectral properties; pyrrolo[3,4-c]pyridine derivative; quantum chemical analysis.

Figure 1

Normalized steady-state absorption (1), fluorescence (1′), and excitation (2) spectra of HPPT in methanol (a) and water (b). Excitation anisotropy spectrum of HPPT in glycerol ((a), curve 3) and 3D fluorescence maps for HPPT in methanol (c) and water (d).

Figure 2

Fluorescence decay traces for HPPT in water (1), methanol (2), and instrument response function (3).

Figure 3

Consecutive changes in the main absorption band of HPPT in methanol (a) and water (b) under irradiation at ≈400 nm with average irradiance ≈ 40 mW/cm² and irradiation times: 0 min (1), 4 min (2), 8 min (3), 12 min (4), and 16 min (5).

Figure 4

Transient absorption profiles $\Delta D=f({\tau }_D)$ for HPPT in methanol: (a,b) ${\lambda }_{pr}$ = 470 nm (1), 510 nm (2), 550 nm (3); (c,d) ${\lambda }_{pr}$ = 620 nm (1) and 560 nm (2).

Figure 5

Transient absorption spectra of HPPT in methanol for ${\tau }_D$ = 1 ps (1), 5 ps (2), 30 ps (3), and 80 ps (4).

Figure 6

Optimized molecular geometry of HPPT in the ground (S₀) and excited (S₁) electronic states.

Figure 7

Calculated frontal and nearest molecular orbitals of HPPT.

Figure 8

Molecular structure of the ammonium salt of HPPT.