Journal of Theoretical & Computational Science
Open Access

Predictions of the Rate Constants of the Radiative Energy Dissipation and the Intersystem Crossing of Some Coumarin Laser Dyes

MSA Abdel-Mottaleb^{* *}Correspondence: MSA Abdel-Mottaleb, Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt, Tel: 72724240122, Email:

Author info »

Introduction

Coumarin is one of the very famous organic fluorescent materials. Coumarin derivatives have been widely applied in many fields, such as optical brightening agents, photobiological energy transfer processes, Light-Emitting Devices (LEDs) with the advantages of a sizeable conjugated system and rigid planar structure), laser dyes, medicine, and bio/ chemosensors [1-3].

Previous work noticed that we still lack theoretically predicted spectroscopic characteristics of efficient laser dyes [4]. This paper aims to simulate the missing excited-state dynamic parameters such as fluorescence, ISC, and phosphorescence rates of some coumarin laser dyes induced by different substituents, identified in Figure 1 and Table 1. The varying photophysical properties of coumarin and its various derivatives are sources of motivation that prompted us to study the architecture of the coumarin derivatives, which imparts changes in the electronic distribution within the molecular skeleton. And the effect of substituents on the excited state dynamic rates will be investigated by predicting the rates of fluorescence (kflu), intersystem crossing (kISC), and phosphorescence (kPho) in the laser dyes and related molecule (2) given in Table 1. The current study results will pave the way to highlight the molecular architecture that enables designing efficient laser dyes.

Table1: Dyes studied, ChemSpider ID, and the natural charge on the oxygen atom in position 2 (see Figure 1).

Figure 1:The structure of the coumarins (1,3,4,5) and related molecule (2) (see Table 1).

Computations

All calculations were carried out using the ORCA 4.2 (parallel) software package [5]. Structures were optimized using the B3LYP functional and the def2-TZVP (-F) basis set as recommended. The rates and spectra were calculated using the same methods recommended and detailed in the literature [6,7]. We considered the triplet spin-sublevels (1,0, or -1). For the molecules under investigation, we were predicted the kISC as the mean of the sum of the individual kISC (T1), kISC T2), and kISC (T3). We employed the well-known CPCM model of solvation [5] using ethanol as a solvent. An example of calculating the ISC rate for molecule 1 is given below:

!B3LYP DEF2-TZVP(-F) ESD(isc) TIGHTSCF GRID5 GRIDX5 RIJCOSX RI-SOMF(1X) CPCM(ETHANOL)

%TDDFT NROOTS 5

SROOT 1

TROOT 1

TROOTSSL 1

DOSOC TRUE

END

%MaxCore 1024

%scf ConvForced false END

%SCF MAXITER 300 END

%GEOM MAXITER 300 END

%ESD ISCISHESS "CARS7-ES.hess"

ISCFSHESS "CARS7-TS.hess"

USEJ TRUE

DOHT true

DELE 28643.09

END

%pal nprocs 35 end

Results and Discussion

The predicted spectra of dye 3 (coumarin 120) are depicted in Figure 2 as an example. The theoretically simulated photophysical parameters are summarized in Table 2.

Table 2: Some predicted radiative (S1 S0, and T1 S0) and non-radiative (ISC) transitions of the dyes 1-5. Phosphorescence lifetime (τ) in ms is given between parentheses. The percent of the Hertzberg-Teller coupling (HT%) due to vibronic coupling is also p.

Figure 2: The simulated absorption, fluorescence, and phosphorescence spectra of the laser dye 3 (coumarin 120) in ethanol. Similar spectra were obtained for the other molecules studied.

Referring to Figure 1 and Table 1, architecturally, three important modified positions are observed in the coumarin skeleton for controlling the photophysical properties. One is the 1-position (X in Figure 1), and the second is 7-position (Y in Figure 1), and the third is the 4-position (Z in Figure 1). In this framework, the molecular engineering of these three positions by sensible substituent effect to understand the dominant deactivation channel and the laser efficiency of coumarin derivatives is addressable. Herein, two members of coumarin derivatives with O and N atoms substituted at the X-position were selected. For now, the rates of deactivation channels were successfully tuned by introducing substituents with variouselectronic properties at the Y-position (OH and NH2, Figure 1) and two-electron donor (CH3) electron acceptor (CF3) functional groups at the Z-position.

A glance at Figure 3 and 4 reveals the effect of substituents on the nonradiative transition rate kISC and the radiative kflu. Interestingly, a structural change in Y significantly influenced fluorescence, ICS, and phosphorescence rates (see Table 2). Substituting the OH group in dye 5 by the more electrondonating NH2 group (dye 3) enhances kflu and kpho and slightly depresses kICS. The triplet state lifetime is most importantly shortened significantly, decreasing light loss by T T absorption. In other words, shortening of a triplet-state lifetime due to the replacement of the OH group by the more electron-donating NH2 group prevents or limits excessive Triplet-State Losses (TSLs.) and enhances laser dye efficiency. Moreover, replacing the CH3 group in 3 with the more electronwithdrawing CF3 group in 4 further decreases the kICS value and renders dye 4 less efficient phosphorescent with a longer triplet lifetime dye, enabling considerable TLS.

Figure 3: Predicted ISC and fluorescence rates for various molecules at 77 K.

Astonishingly, the N heteroatom in the X position, as in the carbostyryl 7 (dye 1), significantly depresses both kflu and kpho, noticeably enhances the kISC value relative to that of dye 3. Consequently, dye 1 should be a less efficient laser dye than dye 3. Molecule 2 behaves similarly to dye 1, indicating that its CF3 group is of minor influence in this case.

Noteworthy mentioning that the experimentally available photophysical data in the literature [4,8,9-12] match our theoretically predicted parameters (see Table 2). To the best of our knowledge, the triplet state characteristics of all the molecules studied are not reported in the literature [11, 12]. Previous experimental research [11, 12] rolled out the possibility of ISC based on the absence ofexperimental proofs. However, it was generally assumed that the ISC process is partially responsible for the fast nonradiative deexcitation channel for the fluorescent state of many dyes in nonpolar solvents.

Conclusion

This paper aimed to simulate the missing excited-state dynamic parameters such as fluorescence, ISC, and phosphorescence rates of some extensively reported coumarin laser dyes induced by different substituents. Structural substituents that influence the simulated spectroscopic rates of coumarin laser dye derivatives for an efficient operation have been highlighted.

We found that all the NH2-based dyes (3 and 4) show a much higher fluorescence rate with short-lived triplet-state than those of OH-(dye 5) or -N- heteroatom and NH2-based molecules (1 and 2). The results obtained pave the way to enable designing new efficient laser dyes.

Conflict of Interest

There is no conflict of interest.

References

1. Song D, Wang CM, Ye ZP, Xia PJ, Deng ZX, Xiao JA,etal. Visible-light-driven, photoredox-catalyzed cascade of ortho-hydroxycinnamic esters to access 3-fluoroalkylated coumarins. J organic chem. 2019;84(11):7480-7487
2. Feng Z, Yu Y, Yang X, Zhong D, Song D, Yang H, et al. Isomers of coumarin-based cyclometalated Ir (III) complexes with easily tuned phosphorescent color and features for highly efficient organic light-emitting diodes. Inorganic chem. 2019;58(11):7393-7408.
3.  Cao D, Liu Z, Verwilst P, Koo S, Jangjili P, Kim JS, et al. Coumarin-based small-molecule fluorescent chemosensors. Chemical reviews. 2019;119(18):10403–10519.
4.  Pavlopoulos TG. Scaling of dye lasers with improved laser dyes, Progress in Quantum Electronics. 2002;26:193-224.
5. Pavlopoulos TG. Scaling of dye lasers with improved laser dyes, Progress in Quantum Electronics. 2002;26:193-224.
6. Neese F. The ORCA program system Wiley interdisciplinary Reviews - Computational Molecular Science. 2012;2:73-78.
7. Souza de B, Neese F, Izsák R. On the theoretical prediction of fluorescence rates from first principles using the path integral approach. J Chem Phys. 2018;148.
8. De Souza B, Farias G, Neese F, Izsák R. Predicting phosphorescence rates of light organic molecules using time-dependent density functional theory and the path integral approach to dynamics. J Chem theory comput. 2019;15(3):1896–1904,
9. Krystkowiak E, Dobek K, Burdzinski G, Maciejewski A. Radiationless deactivation of 6-aminocoumarin from the S 1-ICT state in nonspecifically interacting solvents. Photochem Photobiol Sci. 2012;11(8):1322-1330.
10.  Abdel-Mottaleb MS, El-Sayed BA, Abo-Aly MM, El-Kady MY. Fluorescence-properties and excited state interactions of 7-hydroxy-4-methylcoumarin laser dye. J Photochem Photobiol A: Chem. 1989;46(3):379-390
11. Li LD, Yang SZ. Room temperature phosphorescence properties of 27 coumarin derivatives on filter paper. Analytica chimica acta. 1994;296(1):99-105.
12.  Nad S, Pal H. Unusual photophysical properties of coumarin-151. J Phys Chem A. 2001 Feb 22;105(7):1097-1106.