Characterization of Electronic Properties in Complex Molecular Systems: Modeling of a Micropolarity Probe

Abstract

Quantitative characterization of quantum states in complex molecular systems is a rather complicated task because of the necessity of maintaining the pure quantum definition of a state interacting with a configurationally complex molecular environment. Unfortunately, many of the "observables" that are of interest for a chemist, typically dealing with "complex objects", belong to the above class and their theoretical modeling may represent a hard task. In this respect, we have developed a new theoretical methodology, "perturbed matrix method", essentially based on the perturbation theory whose main aim is the characterization of the quantum states of a predefined portion of a complex molecular system, e.g., a solute, classically interacting with the environment, e.g., the solvent. This method has been used in this study to systematically characterize, for the first time and in conjunction with experimental observations, the intrinsic nature of pyrene whose vibrational and electronic states are highly sensitive to the nature of molecular environment. More precisely, pyrene shows a strong alteration of spectral intensities upon modification of polarity of the solvent. This property has been extensively used in many experimental studies and has been interpreted in the present study by characterizing pyrene electronic states as fluctuating states strictly connected to the polarity and the fluctuations of the surrounding medium. A correct theoretical modeling has been also obtained and commented for the vertical transitions in different media and also for the vibronic structure for the first transition in water.

Theoretical modeling of UV-Vis absorption and emission spectra in liquid state systems including vibrational and conformational effects: The vertical transition approximation

“…The parametrization of the force field as well as the definition of the RC's for the QC excited state MD simulation (employed to obtain the emission spectrum) is more complex, in principle requiring a polarizable QC and the use of different unperturbed electronic states to obtain at each MD frame the proper quantum vibrational modes minimum energy positions to be used in the corresponding RC. 25,27 However, for 1PN the first perturbed spectroscopic accessible excited state, as it is often the case, corresponds essentially to the unperturbed first excited state, as shown analyzing the electronic excitations obtained from the ground state simulation (data not shown). Moreover, such QC first unperturbed excited state is very similar to the unperturbed ground state as far as the bond lengths, bond angles, and atomic point charges are concerned thus allowing, in line with previous studies, 26,27 to use the same procedure associated with the ground state simulation to obtain the excited state force field and RC's definition with the only modifications of employing different dihedral angle potential energy and quantum vibrational coordinates minimum energy positions, with the latter in this case obtained from the first unperturbed electronic excited state.…”

“…In these cases, the huge dimension of the configurational space spanned by the chromophore and the perturbing environment as well as the relevant effects of the fluctuating perturbation onto the chromophore quantum properties, require computational approaches able to properly account for the complexity of the whole system and, for the emission signal, the related longtimescale relaxations. 21 As a part of our continuous interest in the study of the quantum properties in complex molecular systems, [22][23][24][25] we describe in this study a general and efficient methodology, based on the perturbed matrix method and molecular dynamics simulations, to model UV-Vis absorption and emission spectra of complex molecular systems including vibrational and conformational effects. As a case study we have selected the UV-Vis absorption-fluorescence spectra of 1-phenyl-naphthalene (1PN, Figure 1) in solution.…”

Functions of Natural Organic Matter in Changing Environment

“…Optical methods attract the attention of researchers because of their simplicity, sensitivity, ability of in vivo and real-time analysis (also in a remote mode). Experimentally this conclusion is confirmed by the data of Nakajima (1971) and Aschi et al (2010)-it can be seen that s01 decreases and s02 increases with a decrease in environmental polarity. It relies on the well-known fact that the fine structure of pyrene fluorescence band depends strongly on the environmental polarity-this phenomenon underlies the py scale that implies the use of the I 1 /I 3 ratio, where I 1 is the first (at 373 nm) and I 3 is the third (at 383 nm) vibronic peak intensities (Kalyanasundaram and Thomas 1977).…”

Experimental evidence of incomplete fluorescence quenching of pyrene bound to humic substances: implications for Koc measurements

“…Indeed, the extinction coefficient of the S 0 → S 2 transition increases along with a decrease in the solvent polarity. 31,32 At the same time, a lower value of the absorption cross-section σ 01 is observed in the less polar environments. For instance, a factor of four decrease was observed in the σ 01 value upon replacing acetonitrile with cyclohexane.…”

“…As a result, vibronic coupling between the S 1 and S 2 electronic states leads to an intensity borrowing effect, which is manifested as an enhancement of the weak S 0 → S 1 transition intensity by vibrational coupling to the neighboring S 2 state due to oscillator strength exchange. 28 Given that any change in the pyrene microenvironment polarity leads to alteration in the intensity borrowing degree and in mixing between its electronic states, [29][30][31] it will be manifested in opposite changes in extinction coefficients of the S 0 → S 2 and S 0 → S 1 transitions. This is because an increase in the intensity of S 0 → S 1 transition due to oscillator strength exchange with the S 0 → S 2 transition brings about a decrease in the transition cross-sections σ 01 and σ 02 .…”